• Séminaire nano-électronique quantique
• Personne à contacter : Alexandre Assouline ()
• Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé : The massive number of spectra required for high-resolution quasiparticle interference of low-dimensional quantum materials motivates the development of faster point spectroscopies. While the advent of parallel spectroscopy and compressive sensing enhancements has provided welcome speed boosts, these come at a cost. The application of a sinusoidal voltage on the nonlinearities in the current-voltage characteristics of a tunneling junction generates a frequency comb of higher order current-harmonics. While their parallel measurement enables faster tunneling spectroscopy, it unfortunately averages longest where the currents are largest, leading to poor signal-to-noise ratios for smaller signals associated with features close to the Fermi level. After examining the key concepts of QPI, parallel spectroscopy, and compressive sensing, we introduce a multifrequency excitation mode that increases the averaging time for small currents, enabling fast and high-resolution spectroscopy. Additionally, the AC excitation of our method can be used to dramatically increase the dynamical current range by exactly and deliberately suppressing the large amplitude, low order harmonics that would otherwise saturate the preamplifier stage.

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter : Nicolas Roch ()
• Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé : Electrons confined to solid neon surfaces manipulated through circuit quantum electrodynamics (circuit QED) architecture represent a promising quantum computing platform with demonstrated impressive coherence times. I will present two interconnected investigations critical to advancing this technology. First, we examine how resonator and substrate surface properties influence the formation of electron-on-neon (eNe) charge states and their coupling to microwave resonators. Our experimental observations reveal that shallow-depth etching maximizes coupling strength, while comparisons of trapping statistics across devices with varied surface roughness elucidate the crucial role of fabrication-induced surface features in forming strongly coupled eNe states. These findings motivate our second investigation: characterizing the growth kinetics and film properties of neon on microchips. Using high-Tc YBCO microwave resonators, we analyze the uniformity, conformality, and solidification dynamics of both quench-condensed and liquid-grown neon thin films. Our microwave measurements between 4-30K reveal novel film thickness dynamics near neon's triple point, providing a comprehensive picture of neon deposition behavior on microwave resonators. Together, these studies address fundamental challenges in surface engineering and material growth essential for developing scalable electron-on-neon quantum computing systems.

Liens :Kater MurchWashington University, St. Louis, Missouri

• Séminaire théorie
• Personne à contacter : Serge Florens ()
• Lieu : Salle Roger Maynard G-421

Résumé : Flat band systems have been the focus of much interest due to the rich exotic physics they display such as non-conventional superconductivity. To study such systems, we developed a multi-band mean field method and showed excellent agreement with exact density matrix renormalization group (DMRG) calculations over a very wide range of parameters and reveal the limits of validity of the simple BCS approach. We design flat band models with tunable topological properties to study the effect of topology, and illustrate the strong dependence of the superconducting density on the compact localized states, rather than being determined by the quantum metric. In addition, we examine the enhancement of superconductivity arising from the band touchings between the flat band and the next dispersive band. Lastly, we investigate superconducting transport in the DC field induced Wannier-Stark flat bands in the presence of interactions. We systematically characterize the superconducting behavior on these flat bands by studying the effect of the DC field and attractive Hubbard interaction strengths on the wavefunction, correlation length, pairing order parameter and the superfluid weight. In particular, we find that the superfluid weight is dramatically enhanced at an optimal value of the interaction strength and weak DC fields.

Liens :

• Séminaire interne LPMMC
• Personne à contacter : Anna Minguzzi ()
• Lieu : Salle Roger Maynard G-421

Résumé : In the last few years the topic of quantum fluids of light has extensively been studied both with theoretical approaches and experimental measurements, giving unique results and raising new questions. One of the most promising examples of nonlinear quantum optical system is the polaritonic cavity where photons (light) and excitons (matter) are coupled. They can interact strongly with each other giving rise to nonlinear optical phenomena such as bistability, defined as the occurrence of two equilibrium stable states for the same observable quantity with the same set of control parameters. Under coherent pumping, it is possible to trigger a bistable transition, causing a far from equilibrium phase transition. In this work, we identify an optimal working point coming from the interplay between the integrated gain of photons and the jump probability: this is an interesting step ahead because we carried the simulation during the jump process. In this framework, the quadratic Bogoliubov approximation for the fluctuations becomes unreliable. Large quantum fluctuations require us to go beyond linear contributions and solve the full Lindblad's master equation. We use a cascade approach in order to study the evolution of the polaritons' number in the system under an external perturbation.

Liens :LPMMC

• Séminaire nano-électronique quantique
• Personne à contacter : Alexandre Assouline ()
• Lieu : Salle Rémy Lemaire K223, Institut Néel

Annulé
The airport is closed to the escalation bewtween Israel and Iran.

Liens :

• Séminaire théorie
• Personne à contacter :
• Lieu : Salle Roger Maynard G-421

Résumé : As quantum devices continue to grow in scale while remaining affected by noise, it is essential to understand when and how they can offer a practical advantage over classical methods. Recently, a new family of classical algorithms based on Pauli propagation has emerged as a leading approach for simulating near-term quantum devices, raising the bar for demonstrating practical quantum advantage. Pauli propagation methods approximate the evolution of quantum operators in the Pauli basis and provide rigorous performance guarantees across a broad class of both noisy and noiseless circuits. More broadly, they have proven effective in simulating key proposals for near-term quantum applications, including Quantum Convolutional Neural Networks and noisy implementations of Trotter-based quantum simulations. In this talk, we will present recent theoretical advances in Pauli propagation and discuss how these methods can be used in conjunction with near-term quantum hardware to tackle practical tasks such as finding low-energy states of physical Hamiltonians or probing the properties of quantum many-body systems.

Liens :

• Séminaire interne LPMMC
• Personne à contacter : Vincent Rossetto ()
• Lieu : Salle Roger Maynard G-421

Résumé : Simulating precisely the non-equilibrium dynamics of quantum impurities is generally challenged by a complexity wall at long times. Exploiting time-dependent orbital rotations of matrix product states, we propose a numerically exact algorithm that reveals a hidden few-body and low entanglement structure of the quantum state vector for all times. This method provides drastic improvements both to the accuracy and to the running time of dynamical computations. As an application, we investigate the spatio-temporal formation of the Kondo screening cloud following a quantum quench.

Liens :Institut Néel

• Séminaire nano-électronique quantique
• Personne à contacter : Alexandre Assouline ()
• Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé : The quantum coherence of single electronic states in mesoscopic system is usually very fragile. This fragility can be harnessed to create hyper-sensitive quantum sensors such that a single electron can detect a few photons excitation. In this work, we demonstrate a quantum sensor that exploits the variations of the phase of a single electron wavefunction upon its interaction with a classical time-dependent electric field. We use a Fabry-Perot interferometer in the quantum Hall regime to extract the phase of the wave packet as a function of time. Such device ensures a fast and sensitive detection as are able to detect a signal equivalent to a few microwave photons with a resolution of 50 ps. Moreover, by measuring both the phase and contrast of the interferometer, it is possible to obtain information on both the amplitude of the electromagnetic field and its fluctuations. This opens possibilities for on-chip detection of non-classical radiation such as squeezed states or Fock states. [1] Bartolomei, Hugo, et al. "Time-resolved sensing of electromagnetic fields with single-electron interferometry." Nature Nanotechnologies https://rdcu.be/edRw8 (2025)

Liens :

• Séminaire interne LPMMC
• Personne à contacter : Léonie Canet ()
• Lieu : Salle Roger Maynard G-421

Résumé : The trace‐preserving, completely positive time evolution of an open quantum system is governed by the Lindblad master equation. Numerically probing bosons in such systems is particularly challenging because of their infinite‐dimensional Hilbert space. In this talk, we first investigate the spectral properties of the one‐dimensional driven‐dissipative Bose–Hubbard model with one-body incoherent pumping and loss. We compare these open-system spectra with (i) the ground-state excitation branches (Lieb I and II) and (ii) finite-temperature spectral functions. In the driven-dissipative regime, we find a nontrivial renormalization of the decay rate of the retarded Green’s function at large hopping amplitudes and strong interactions. Next, employing random-matrix theory, we demonstrate that the XX model (hard-core bosons on a lattice)— its continuum limit, the Tonks-Girardeau gas, known for its integrability even at finite temperature and under quenched time evolution—becomes non-integrable upon the introduction of homogeneous Markovian one-body pumping and loss. We then use perturbation theory to reveal emergent structures in the full Lindblad spectrum. Finally, we show how tensor-network methods enable access to larger system sizes and higher bosonic occupations. By adding two-body loss to the driven-dissipative Bose–Hubbard model, a quasi-condensate can emerge whose phase fluctuations in the one-dimensional thermodynamic limit, depending on the parameter regime, fall into the Kardar–Parisi–Zhang universality class. We complement the picture by investigating the spectrum of the full quantum evolution at strong but finite interactions.

Liens :LPMMC

• Séminaire nano-électronique quantique
• Personne à contacter : Loïc Herviou ()
• Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé : Quantum computers have the potential to solve computational problems that are beyond the reach of classical computers. Superconducting circuits with nonlinear Josephson junctions are a leading platform for quantum computing, where qubits are encoded in microwave photons. However, microwave quantum signals are highly susceptible to thermal noise, necessitating cryogenic environments to maintain coherence. This challenge hinders the transmission of quantum signals over microwave cables at room temperature. A promising solution for long-range quantum communication without loss of coherence is to convert the microwave quantum state into optical photons, enabling the interconnection of remote superconducting processors. At IBM Research Europe – Zurich, we explore electro-optomechanical and electro-optical integrated photonic circuits for microwave-optical quantum transduction. Our investigation focuses on a triply resonant transducer composed of low-loss barium titanate (BaTiO3) waveguides integrated with a niobium superconducting microwave resonator. BaTiO3 has the potential to surpass currently used thin film platforms based on aluminum nitride or lithium niobate in terms of efficiency and noise reduction, thanks to its strong Pockels coefficient. The device demonstrates bidirectional microwave-optical transduction with total off-chip efficiencies of 1×10−6 using pulsed pumping. The used device concept allows in-situ poling of the ferroelectric material without introducing excess microwave loss. Additionally, we investigate optically induced heating, revealing fast thermalization and quasiparticle resilience of the microwave resonator.

Liens :

• Séminaire interne LPMMC
• Personne à contacter : Vincent Rossetto ()
• Lieu : Salle Roger Maynard G-421

Résumé : Sarah will talk about "Exploring Spin-Orbit Coupling in Quantum Hall Systems" and Élodie about "Thermodynamics of SU(N) Fermi-Hubbard Model with Semi-Standard Young Tableaux"

Liens :LPMMC

• Séminaire théorie
• Personne à contacter : Serge Florens ()
• Lieu : Salle Roger Maynard G-421

Résumé : The non-perturbative and non-Markovian dynamics of open quantum systems are highly challenging to simulate, due to the extensive quantum correlations that appear in both time and space between the observable systems and their environments. However, understanding and exploiting these rich - and often cooperative - physics could provide novel ways to optimise dissipative processes, particularly in the intrinsically ‘open' conditions related to energy harvesting and transduction. In this talk I will present a numerically exact approach to such problems based on the Time-Evolving Density with Orthogonal Polynomials (TEDOPA) technique, as it is currently implemented in the open source code MPSDynamics.jl [1,2]. Through a series of examples covering photosynthetic light-harvesting, organic photovoltaic materials and molecular photphysics, I’ll demonstrate how this approach provides direct insights into the microscopic, multi-scale interplay of dissipative processes that drive advanced functionalities in such systems, and suggest how these tools and emerging concepts could be exploited in designing future quantum devices. [1] MPSDynamics.jl: Tensor network simulations for finite-temperature (non-Markovian) open quantum system dynamics. Thibaut Lacroix, Brieuc Le Dé, Angela Riva, Angus J. Dunnett, Alex W. Chin J. Chem. Phys. 161, 084116 (2024) [2] https://github.com/shareloqs/MPSDynamics.jl

Liens :

• Séminaire théorie
• Personne à contacter : Adolfo Grushin ()
• Lieu : Salle Roger Maynard G-421

Résumé : The concept of quantum geometry has been at the forefront of condensed matter physics, starting from how quantized Berry curvature leads to quantized Hall conductivity, anomalous velocities in Dirac metals, or other topological responses in a growing list of topological materials. Recently, the real part of the quantum geometric tensor - the quantum metric - has also been suggested to play an important role, both in response and in the tendency for materials to assume correlated ground states at low temperatures. In this talk, I will give a local picture of quantum geometry to create an intuition about what it is and when it is essential, relating it to how bonds are formed in infinite lattices.

Liens :

• Séminaire théorie
• Personne à contacter : Adolfo Grushin ()
• Lieu : Salle Roger Maynard G-421

Résumé : Herbertsmithite is the leading candidate to host a quantum spin liquid -- a long sought state of matter featuring long-range quantum entanglement and fractionalised 'spinon' excitations. However, despite two decades' work, definitive evidence remains lacking. One complicating factor is that the material features significant disorder in the form of magnetic impurities. Here we utilise these impurities as 'witnesses' to probe the quantum spin liquid. Using spin noise spectroscopy to measure witness magnetization fluctuations, we find unusual 1/f noise develop below a cusp in DC magnetic susceptibility at 260mK. Ageing effects confirm witness spin glass formation. I will present a microscopic model of witness interactions mediated by a Z2 quantum spin liquid. Despite having only one free parameter, the model gives a quantitative match to all experiments, including a spin glass transition, the temperature dependence of the susceptibility, the temperature and frequency dependence of the noise spectrum, the Curie Weiss temperature, and the previously observed neutron scattering intensity as a function of momentum transfer.

Liens :

• soutenance de thèse
• Personne à contacter : Denis Basko ()
• Lieu : Salle Roger Maynard G-421

Résumé : The performance of any physical qubit is limited by spurious noises, which makes extremely challenging the goal of building a quantum computer. To address this challenge, the concept of quantum error correction (QEC) has emerged. QEC consists in using multiple physical qubits to encode information, combined into a single logical qubit with sufficiently low error rates. It has been shown that the use of superconducting Schrodinger cat qubits can significantly improve the QEC thresholds. Cat qubits are driven quantum systems and differ significantly from conventional superconducting qubits, making their study interesting from the fundamental point of view, in addition to their importance for quantum technology. In this thesis, we conduct an extensive study of errors in cat qubits, with a particular focus on Bogolyubov quasiparticles. These quasiparticles are known to exist in superconducting devices even at the lowest temperatures. Unlike Cooper pairs, Bogolyubov quasiparticles can cause dissipation and degrade the performance of superconducting qubits. They are difficult to mitigate because they are intrinsic to the superconducting circuit. Thus, it is important to quantify their effect on cat qubits. The thesis covers three related projects. In the first project, I developed perturbation theory for the dissipative cat qubit, which can predict cat qubit lifetimes based on the strength of various noise types. In particular, I calculated the exponentially suppressed bit-flip rate of a cat qubit, and my analytical expressions show good agreement with numerical simulations. In the second project, I investigated the impact of Bogolyubov quasiparticles on cat qubits, including both the driven Kerr and driven-dissipative variations. I derived error rates caused by quasiparticles and found that, due to the driven nature of cat qubits, some error rates differ significantly from those in conventional superconducting qubits. I also examined the potential overheating of quasiparticles due to the drive. In the third project, I studied the pure dephasing rate induced by Bogolyubov quasiparticles. The noise caused by these quasiparticles exhibits long-time correlations, which prevents the determination of the pure dephasing rate using standard approaches perturbative in the coupling strength. By using the low quasiparticle concentration as a small parameter, I determined the pure dephasing rate and showed that the fermionic nature of the quasiparticles bath plays a crucial role.

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter : Loïc Herviou ()
• Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé : Superconducting quantum devices usually operate Josephson junctions in the superconducting state as nonlinear inductors to form anharmonic oscillators. Strong anharmonicity is used for qubits while weak anharmonicity in used for amplification to parametrically convert pump photons into signal and idler photons. In this talk I will show that Josephson junctions can also yield useful quantum devices when operated in the voltage state. In this configuration, called Josephson photonics, Cooper pairs, tunneling through a junction biased at a voltage V below the gap, can transfer their energy 2eV into one or more photons in the harmonic modes of the circuit in which the junction is embedded (1). In Josephson photonics, the junction, therefore, acts as a nonlinear drive, doing away with the pump tones that are usually required to operate quantum devices, thereby removing the hardware overhead required for generating, routing, and filtering of these tones. I will show how we use Josephson photonics to implement quantum-limited linear amplifiers (2) and photon-number amplifiers (3), which amplify photon number while deamplifying phase and promise to be useful for counting itinerant microwave photons. (1) Hofheinz et al., Phys. Rev. Lett. 106, 217005 (2011) (2) Martel et al., Appl. Phys. Lett. 126, 074001 (2025) (3) Albert et al., Phys. Rev. X 14, 011011 (2024)

Liens :

• Séminaire théorie
• Personne à contacter : Adolfo Grushin ()
• Lieu : G421

Résumé : Topological phases of matter are distinguished by robust properties that are insensitive to perturbations. These include quantized responses, perfectly conducting interfaces, and exactly zero energy modes, with wide-ranging technological applications. Recent years saw the complete classification of topological band structures, revealing an abundance of topological crystalline insulators. Many questions about the robustness of these phases to disorder are, however, still open. We develop new theoretical and numerical methods based on local topological markers and effective momentum-space Hamiltonians to study topological phases in disordered systems. We introduce symmetry indicators for amorphous systems, and we also demonstrate that it is possible to construct a fully isotropic and inversion-symmetric three-dimensional medium where time-reversal symmetry is systematically broken. We propose an amorphous system with scalar time-reversal symmetry breaking, implemented by hopping through chiral magnetic clusters along the bonds. The average spatial symmetries alone protect a statistical topological insulator phase in this system, analogous to crystalline mirror Chern insulators. We also show the expected transport properties of a three-dimensional statistical topological insulator, which remains critical on the surface for odd values of the invariant.

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter : Loïc Herviou ()
• Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé : Quantum reservoir engineering leverages dissipative processes to achieve desired behavior, with applications ranging from entanglement generation to quantum error correction. Thereby, a structured environment thereby acts as an entropy sink for the system and no time-dependent control over the system is required. In this work, we focus on an active approach to reservoir engineering, where time-dependent control over a quantum system is used to manipulate its environment. In this case, the system may act as an entropy sink for the environment. We develop a theoretical description for active reservoir engineering that captures the dynamical interplay between system and environment, and provides an intuitive picture of how finite-size effects and system-environment correlations allow for manipulating the environment by repeated initialization of the quantum system. We illustrate our results with two examples: a superconducting qubit coupled to an environment of two-level systems and a semiconducting quantum dot coupled to nuclear spins. In both scenarios, we find qualitative agreement with previous experimental results. By providing a theoretical framework for active reservoir engineering, our work opens new avenues to control open quantum systems.

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter :
• Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé : Adiabatic passage is a powerful control technique atomic physics which is gaining interest also in the solid-state realm since it implements quantum operations which are very robust against parametric fluctuations. We exploit the application of coherent techniques as coherent transport by adiabatic passage (CTAP) or stimulated Raman adiabatic passage (STIRAP) in quantum architectures where the robustness of the protocols may determine key advantages for selected tasks(1,2). As a first example we discuss quantum operation for modular computing in ultrastrongly coupled structures of artificial atoms (3) studying the design of ultrafast operations between different QPUs. Faster dynamics has a cost since USC breaks the symmetry associated with the conservation of the number of excitations, leading to a series of fundamental physical effects but detrimental for quantum state processing. In particular the highly entangled nature of the eigenstates, dressed by a potentially very large number of virtual photons, leads to leakage of excitation via the dynamical Casimir effect (DCE). We show that CTAP-like manipulation ensure the suppression of unrecoverable errors due to the dynamical Casimir effect and can be crafted in a way that other errors due to couterrotating interaction can be corrected. We study state-trandfer between QPUs and protocolls performing SWAP gates and N-qubit entanglement sharing. We address quantification of performaces by quantum communication theory (4). A second example is noise classification in multilevel quantum structures where we propose a STIRAP-based supervised learning procedure to recognize energy-correlations of noise and their relation to the Markovianity of the environment (5). (1) Jonathon Brown et al 2021 New J. Phys. 23 093035 (2) L. Giannelli, et al. 2024 Phys. Rev. Research 6, 013008 (3) L. Giannelli et al. Il Nuovo Cimento C 45-6, 171 (2022); G. Falci et al., in preparation (2025). (4) S. Cordovana et al., to be published in EPJ-ST (2025). (5) S. Mukherjee, et al., Machine Learning: Science and Technology 5, 045049 (2024) arXiv:2405.01987

Liens :Pino Falci

• Séminaire théorie
• Personne à contacter : Serge Florens ()
• Lieu : Salle Roger Maynard G-421

Résumé : Many-body correlations in quantum materials can generate various complex and intriguing phenomena such as Hund metallicity, magnetism and unconventional superconductivity. We use the union of density functional theory and dynamical mean-field theory (DFT, DMFT), a first-principles framework with very few adjustable parameters, to numerically investigate these phenomena in various materials. In particular, we will focus on strontium ruthenate, a material for which the normal state obtained using DFT et DMFT yields an impressive agreement with experiments. However, despite three decades of intensive research on this material, a consensus on the symmetry of its order parameter remains elusive. We will discuss different methods to interrogate a normal-state regarding potential low-temperature superconducting instabilities. For this specific material, the leading symmetry appears to be a generalized d-wave, while for the second leading instability, the calculations suggest odd-frequency pairing.

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter :
• Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé : The physics of a carbon vacancy in graphene has been the subject of intense research since the early days of graphene. It was studied from various points of views: zero energy state (1), bound charge and its fractionalization (2,3), magnetism (4,5) and unusual Friedel oscillations (6). And yet the interrelation of all these is still not completely clear. In this presentation, I will review these results and explain how our recent discovery of a vortex in the bond order near the vacancy (7) (so-called Kekulé vortex) provides a unified framework to understand these seemingly disparate aspects of the carbon vacancy physics. I will show how that, in addition, this research has led to the recognition that the local density of states measured near defects is a topological observable. (1) Pereira, V. M., Guinea, F., Lopes dos Santos, J. M. B., Peres, N. M. R. Castro Neto, A. H. Disorder Induced Localized States in Graphene. Phys. Rev. Lett. 96, 036801 (2006). (2) Ducastelle, F. Electronic structure of vacancy resonant states in graphene: a critical review of the single-vacancy case. Phys. Rev. B 88, 075413 (2013) (3) Ovdat, O., Don, Y. & Akkermans, E. Vacancies in graphene: Dirac physics and fractional vacuum charges. Phys. Rev. B 102, 075109 (2020) (4) Yazyev, O. V. & Helm, L. Defect-induced magnetism in graphene. Phys. Rev. B 75, 125408 (2007). (5) González-Herrero, H. et al. Atomic-scale control of graphene magnetism by using hydrogen atoms. Science 352, 437–441 (2016). (6) Dutreix, C. et al. Measuring the Berry phase of graphene from wavefront dislocations in Friedel oscillations. Nature 574, 219–222 (2019). (7) Guan, Y et al. Observation of Kekulé vortices around hydrogen adatoms in graphene. 15, 2927 (2024)

Liens :

• Séminaire théorie
• Personne à contacter : Serge Florens ()
• Lieu : Salle Roger Maynard G-421

Résumé : Hilbert-space shattering is usually presented as a delicate phenomenon, wherein generic perturbations quickly restore ergodicity. In this work we introduce a large class of models exhibiting robust ergodicity breaking in quantum dynamics. The shattering in these models is topological in the sense that it persists to all orders of perturbation theory. This means that ergodicity is not restored, at least for timescales exponentially long in the perturbation strength. The models have crisp connections to gauge theories, and generalize Kitaev's quantum double to infinite groups. We also argue that in some group-based models, within a region of parameter space, each Krylov sector hosts a lowest-energy state that remains absolutely stable to generic perturbations for times that diverge with system size. We discuss how this conjectured absolute stability depends on the input group. In three dimensions we argue that this ergodicity breaking even survives coupling to a nonzero-temperature heat bath.

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter : Loïc Herviou ()
• Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé : Long-distance interactions are crucial for scaling quantum dot spin qubit devices. Silicon presents a promising platform for these advancements, as it enables the simultaneous integration of photonic circuits and colour centres (such as T-centers), strongly coupled to telecom light, as well as gate-defined SiMOS quantum dot spin qubits. We propose a scheme that leverages T-centers to mediate the interaction between quantum dot spin qubits and optical light. This approach can be extended to achieve either ultra-long-distance entanglement between two quantum dot spins or be adapted for fast and all-optical quantum measurement. Additionally, we estimate fidelities for prepared Bell states under realistic experimental conditions.

Liens :

• Séminaire maths-physique
• Personne à contacter : Loïc Herviou ()
• Lieu : Salle Roger Maynard G-421

Résumé :

Space-time velocity correlation functions in turbulence

Dynamics of vortices in two-dimensional or axisymmetric flows

Liens :

• Séminaire LPMMC
• Personne à contacter : Denis Basko ()
• Lieu : Salle Roger Maynard G-421

Résumé : Full spectrum of a large quantum system (composed e.g. of spins 1/2) is of macroscopic width, proportional to the number of spins N. However, usual lowest-lying excitations compose a band of some width W that is size-independent. I will report theoretical results describing the probability of decay of high-energy local excitation with energy N W >> E >> W into the many-body continuum. For such a decay to occur, a generation of a large number of excitations is necessary, so the problem becomes a truly many-body one. I will provide results of analytical theory and numerics, both of them demonstrating the behavior of the decay rate Γ that is qualitatively different between a 1-dimensional integrable spin chain and a generic 2-dimensional spin array. While in the integrable case -lnΓ ~ (E/W)², a generic behavior of Γ(E) is found to be a simple exponential: -lnΓ ~ (E/W).

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter :
• Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé : Semiconductor spin qubit is well recognized as a promising platform for scalable fault-tolerant quantum computers (FTQCs) because of relatively long spin coherence time in solid state devices and high-electrical tuneability of the quantum states (1). Indeed, the single qubit and two-qubit gate operation with high fidelity exceeding fault-tolerance threshold have been demonstrated (2,3,4). These achievements have ignited research for scaling up the qubits towards million qubit systems operating based on quantum error correction. The strategy for scaling-up to million qubits is not simply arranging qubits in neighbor to each other but building quantum networks by intermediate quantum state transfer or optical networking are considered as more effective routes to develop FTQCs. In this talk, we present the photon-polarization to spin quantum interface for connecting quantum computers to the optical quantum networks (5,6), a high-fidelity spin qubit operation in GaAs triple QD, and the acceleration of the adiabatic spin state transfer bringing a concept to shortcut to adiabaticity into the intermediate spin state transfer for middle range spin quantum link (7). (1) G. Burkard et al., Rev. Mod. Phys. 95, 025003 (2023). (2) J. Yoneda et al., Nature Nanotechology 13, 102 (2018). (3) A. Noiri et al., Nature 601, 338 (2022). (4) X. Xue et al., Nature 601, 343 (2022). (5) T. Fujita et al., Nature Commun. 10, 2991 (2019). (6) K. Kuroyama et al., Phys. Rev. B 10, 2991 (2019). (7) X.-F. Liu et al., Phys. Rev. Lett. 132, 027002 (2024).

Liens :

• Séminaire théorie
• Personne à contacter : Adolfo Grushin ()
• Lieu : G421

Résumé : QuantAlps Lecture II: In the previous lecture, I discussed non-interacting photonic Chern insulators with chiral edge states.  In this lecture I'll consider the case when we turn on interactions between the photons at the mean-field level, or in other words, optical nonlinearity.  Topics will include bulk and edge state solitons, as well as integer and fractional quantization of transport in photonic Thouless pumps. Note: Coffee and pastries will be served at the break.

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter :
• Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé : As one of the few group IV materials with the potential to host superconductor–semiconductor hybrid devices, planar germanium hosting proximitized quantum dots is a compelling platform to achieve and combine topological superconductivity with existing and new qubit modalities. We demonstrate a quantum dot in a Ge/SiGe heterostructure proximitized by a platinum germanosilicide (PtSiGe) superconducting lead, forming a superconducting lead–quantum dot–superconducting lead junction. We show tunability of the coupling strength between the quantum dot and the superconducting lead, and gate control of the ratio of charging energy and the induced gap, and we tune the ground state of the system between even and odd parity. Furthermore, we characterize critical magnetic field strengths, finding a critical out-of-plane field of 0.90 ± 0.04 T. Finally, we explore sub-gap spin splitting, observing rich physics in the resulting spectra, that we model using a zero-bandwidth model in the Yu–Shiba–Rusinov limit. Our findings open up the physics of alternative spin and superconducting qubits, and the physics of Josephson junction arrays, in germanium.

Liens :Anasua Chatterjee

• Séminaire théorie
• Personne à contacter : Adolfo Grushin ()
• Lieu : G421

Résumé : QuantAlps Lecture I: When non-interacting electrons are free to move in two dimensions under the influence of a magnetic field, their conductance is sharply quantized, even in the presence of disorder.  This can be understood in terms of electronic states that lie at the edge of the sample and are immune scattering by defects.  A central goal of topological photonics has been to realize such states for photons - rather than electrons - to be used as highly robust waveguides.  The difficulty has been that since photons have no charge, they don't respond to a magnetic field.  In this lecture series I will present a selected history of the field from the perspective of the use of Floquet temporal modulation to get around this problem and realize robust chiral edge states for light. Note: Coffee and pastries will be served at the break.

Liens :

• Séminaire théorie
• Personne à contacter : Adolfo Grushin ()
• Lieu : G421

Résumé : In this talk I will demonstrate that Andreev modes that propagate along
a transparent Josephson junction have a perfect transmission at the point
where three junctions meet. The chirality and the number of quantized
transmission channels is determined by the topology of the Fermi surface
and the vorticity of the superconducting phase differences at the
trijunction. We expect that this Chiral Adiabatic Transmission (CAT) is
observable in nonlocal conductance and thermal transport measurements.
Furthermore, because it does not rely on particle-hole symmetry, CAT is
also possible to observe directly in metamaterials.
https://arxiv.org/abs/2311.17160

Liens :

• Séminaire interne LPMMC
• Personne à contacter : Vincent Rossetto ()
• Lieu : Salle Roger Maynard G-421

Résumé : Single electron spins in silicon quantum dots offer a promising pathway to scalable quantum computing, leveraging compatibility with semiconductor technologies and the high tunability of electrons. However, the susceptibility of these qubits to noise remains a critical challenge. This seminar explores ongoing research into understanding and mitigating noise in spin qubit hardware, focusing on strategies to enhance qubit fidelity in a scalable manner.

Liens :LPMMC

• Séminaire maths-physique
• Personne à contacter : Robert Whitney ()
• Lieu : Salle Roger Maynard G-421

Résumé : A pair of seminars on the mathematics and theoretical physics of tensor networks.

Liens :

• Séminaire LPMMC
• Personne à contacter : Loïc Herviou ()
• Lieu : Salle Roger Maynard G-421

Résumé : In quantum or classical wave systems, some properties of wave systems are known to be topologically protected. Due to their increased robustness, such properties have attracted much interest in the past decades. The most studied case is the existence of unidirectional edge states in the quantum Hall effect and, more generally, the existence of protected states at the edges of topologically insulators. An important result is then the bulk-edge correspondence that links the existence of topological edge states to a topological index defined in the volume of the material. This is not the only case studied in topological physics and different, yet similar, results have been obtained for topological semimetals, higher order insulators or continuous wave systems. In this talk I will explain how all these results can be understood in a unifying theory using the mode-shell correspondence formalism which relates the existence of isolated topological modes in phase space, to a topological invariant defined in the surface which encloses these modes in phase space. Invariant that reduces to Chern or winding numbers in the semiclassical limit.

References

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter :
• Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé : Non-Abelian states of matter retain memory of the order in which their quasiparticles are exchanged, presenting an intriguing possibility in condensed matter physics. While some fractional quantum Hall states are expected to host non-Abelian quasiparticles, they have been notoriously difficult to probe due to the narrow energy range over which they are realized. In this talk, I will report the quantitative determination of fractional quantum Hall energy gaps in bilayer graphene using both thermally activated transport and direct measurements of the chemical potential. These experiments establish bilayer graphene as a robust platform for probing the non-Abelian anyons expected to arise as the elementary excitations of even-denominator fractional quantum Hall states. Additionally, I will show that chemical potential measurements can be used to extract the entropy of quantum Hall phases, providing new insights into the nature of their excitations.

Liens :

• Séminaire théorie
• Personne à contacter : Adolfo Grushin ()
• Lieu : G421

Résumé : The imposition of crystalline symmetries is known to lead to a rich variety of insulating and superconducting topological phases. These include higher-order topological phases and obstructed atomic limits. We classify such topological crystalline phases (TCPs) in the presence of disorder that preserves the crystalline symmetry on average. We find that, while clean TCPs evade a general bulk-boundary principle, disordered TCPs admit a complete bulk-boundary correspondence. While the boundary signatures of most disordered TCPs are similar to their clean counterparts, the addition of disorder to certain TCPs results in higher-order statistical topological phases, in which zero-energy hinge states have critical wavefunction statistics, while remaining protected from Anderson localization.

Liens :

• Séminaire QuantAlps
• Personne à contacter : Anna MINGUZZI ()
• Lieu : Bâtiment Green-Er Amphithéâtre 2A003

Résumé : Scale invariance, a concept initially introduced in high-energy physics, has gained numerous applications in the physics of quantum fluids. It is applicable to strongly interacting Fermi gases, two-dimensional Bose gases, as well as few-body systems that exhibit the "Efimov effect." In the presentation, I will illustrate how scale and conformal invariance emerge in cold atomic gases. I will use various examples ranging from thermodynamics to molecular physics to specific structures with periodic time evolution called "breathers".

Liens :Jean Dalibard

• Séminaire LPMMC
• Personne à contacter : Vincent Rossetto ()
• Lieu : Salle Roger Maynard G-421

Résumé : Rigidity Percolation (RP) attracted much attention in soft matter, as an elegant framework to understand the non-trivial emergence of solidity in media that not present any long-range structural order. The solidification of amorphous systems like gels, fiber networks or living tissues can indeed be understood by focusing on locally rigid structures --clusters, that form and grow until one eventually percolates the whole system, ensuring macroscopic mechanical stability. As a statistical model, Rigidity Percolation is defined from the concept of graph rigidity. I will explain that RP possesses a unique non-local character, which makes it fundamentally different (and much more difficult) than the usual Connectivity Percolation problem. Inspired by the great success of conformal field theory to understand critical phenomena, I have recently examined conformal invariance in 2D Rigidity Percolation. I will present a first work where I gave numerical evidence of conformal invariance from properties of the connectivity functions --the probabilities that points belong to the same rigid clusters. I will present a subsequent work where I used a rigorous mathematical framework, Schramm-Loewner Evolution, to provide additional numerical evidence that the interfaces of rigid clusters are conformally invariant random fractal curves. This allows notably to obtain a new relation between two of the critical exponents of RP. A lot remains to be understood about Rigidity Percolation, and I will end with my favourite perspectives.

References

• Phys. Rev. Lett. 130, 268201 (2023)
• Phys. Rev. Lett. 132, 018201 (2024)

Liens :

• Séminaire QuantAlps
• Personne à contacter : Anna MINGUZZI ()
• Lieu : Bâtiment Green-Er Amphithéâtre Ampère

Résumé : Solitons are wave packets that maintain their shape during free propagation, thanks to a non-linearity that prevents the usual spreading. In quantum fluids, this non-linear element is provided by interactions, which can be described by a Gross-Pitaevskii-type equation. In this way, bright and dark solitons are observed for attractive and repulsive interactions, respectively. After describing some of these results, I will turn to mixtures of quantum gases which lead to a richer physics, with the emergence of magnetic solitons and liquid-like states, both of which show rather counterintuitive behaviors.

Liens :Jean Dalibard

• Séminaire QuantAlps
• Personne à contacter : Anna MINGUZZI ()
• Lieu : Bâtiment Green-Er Amphithéâtre Bergès

Résumé : Macroscopic quantum coherence emerges in fluids such as Bose-Einstein condensates, where a significant fraction of the particles share the same wave function. Superfluidity describes the rigidity of a gas or a liquid when we try to set it in motion. Although the two phenomena are often linked, they are not equivalent. In this lecture, I will use recent experiments with quantum gases to show that we can have a superfluid system with no long-range coherence, and conversely a fully coherent system with only a small superfluid fraction.

Liens :Jean Dalibard

• Séminaire interne LPMMC
• Personne à contacter : Vincent Rossetto ()
• Lieu : G421

Liens :LPMMC

• Séminaire QuantAlps
• Personne à contacter : Adolfo Grushin ()
• Lieu : Bâtiment Green-Er Amphithéâtre Bergès

Résumé : We will review the beginning of experimental and theoretical studies of moire systems and their evolution up to present. We will show how thousands of p orbitals in a moire unit cell of graphene can create single Heavy fermion at moire scale, and how the interaction between such fermions can lead to a perfect quantum simulator of an Anderson model. We will then present a catalogue of possible twistable materials and show how a huge variety of strongly interacting models can be realized in twisted homo and hetero twiste bilayers and multilayers of these materials.

Liens :

• Soutenance de thèse
• Personne à contacter : Robert Whitney ()
• Lieu : Salle Roger Maynard G-421

Liens :

• Séminaire théorie
• Personne à contacter : Serge Florens ()
• Lieu : G421

Résumé : Quantum paramagnets represent intriguing quantum phases that evade ordering even at absolute zero temperature. While detecting their presence is relatively straightforward, unraveling their fundamental nature can be a challenging task. In this talk, I will present our recent work [1] on the Lieb-Schultz-Mattis (LSM) constraints which prohibit certain quantum paramagnets from being a “trivial” one. I will illustrate the use of these results through two examples: (1) the prediction of a Dirac spin liquid in the triangular lattice compound NaYbO2, and (2) the characterization of U(1) quantum spin liquids in a pyrochlore S=1/2 antiferromagnet. I will highlight the topological response theory underlying the LSM constraints that we developed, containing information about symmetry, excitations, and lattice defects, applicable to all 3D quantum paramagnets. [1] Liu & Ye, arXiv:2410.03607.

Liens :

• Séminaire interne LPMMC
• Personne à contacter : Robert Whitney ()
• Lieu : G421

Résumé : In recent years, quantum thermometry has emerged as a promising approach for achieving precise temperature measurements at the nanoscale, where classical thermometers fail to perform effectively. Quantum probes, such as single- and two- qubit systems, offer a powerful method for accurately measuring the temperature of a bosonic bath. In this talk, I will discuss how the precision of temperature estimation can be improved through the use of quantum Fisher information and the quantum signal-to-noise ratio. A key result is that introducing an ancilla qubit, which mediates the interaction between the probe and the thermal environment, enhances the thermometric sensitivity by encoding temperature information into the coherence of the probe. We further investigate the use of two interacting qubits, either entangled or separated initially, as quantum probes in various environmental configurations.

The findings demonstrate that thermometric precision improves as the system approaches a steady state, governed by the interaction between the qubits. This provides a pathway for efficient low-temperature estimation by tuning the qubit-qubit interaction. The study raises important questions about the role of energy dissipation and resource efficiency in quantum thermometry, which is crucial for developing quantum devices that operate near the quantum limit. These results connect to the broader challenge of optimizing energy consumption in quantum technologies, especially in cryogenic environments where precise temperature control is critical for performance.

Reference

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter : Jeremie Viennot ()
• Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé : I will present our recent efforts towards realizing the photoelectric detection of single microwave photons, a key missing element in microwave quantum optics. I will start first by explaining our approach to realize an efficient and continuous microwave photon-to-electron converter with large quantum efficiency and low dark current. I will insist on the fact that these unique properties were enabled by the use of a high kinetic inductance disordered superconductor, granular aluminium, to enhance light-matter interaction and the coupling of microwave photons to electron tunnelling processes. As a consequence of strong coupling, we could observe both linear and nonlinear photon-assisted processes where two, three, and four photons are converted into a single electron at unprecedentedly low light intensities. I will then proceed by explaining the implementation of a charge-based detection technique that allows to sense individual microwave photons as a result of photoelectron conversion.

Liens :

• Soutenance de thèse
• Personne à contacter : Léonie Canet ()
• Lieu : Salle Roger Maynard G-421

Résumé : Dans cette thèse, nous étudions la dynamique critique des systèmes hors-équilibre en une dimension. Dans ce contexte, un rôle majeur est joué par l’équation de Kardar-Parisi-Zhang (KPZ), une équation aux dérivées partielles stochastique qui décrit la dynamique d’interfaces croissantes, et qui constitue un modèle paradigmatique de la physique statistique hors-équilibre. Dans une première partie, nous mettons en évidence le point fixe nommé `Inviscid Burgers' (IB), qui émerge dans la limite de non-linéarité infinie de l'équation KPZ. Ensuite, nous examinons l’émergence du point fixe IB dans les systèmes hors-équilibre décrits par l’équation de Ginzburg-Landau complexe, dans le régime connu comme turbulence de phase. Dans ce contexte, nous trouvons que ce point fixe gouverne les propriétés statistiques aux échelles intermédiaires entre les patterns chaotiques issus de l'instabilité modulationnelle et le comportement à grande échelle qui est décrit par le point fixe KPZ. Dans une deuxième partie, nous présentons une étude de la dynamique macroscopique des condensats d'excitons-polaritons unidimensionnels. Ces systèmes quantiques hors-équilibre représentent une plateforme d'intérêt majeur pour l'observation de phénomènes critiques émergents, notamment les fluctuations de phase appartiennent à l’universalité KPZ. Nous caractérisons le diagramme de phases de ces condensats, dans lesquels nous montrons comment la dynamique KPZ peut être détruite par la prolifération de défauts topologiques et solitons, tout en discutant les implications expérimentales.

Liens :LPMMC

• Séminaire nano-électronique quantique
• Personne à contacter :
• Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé : Using a nano-Josephson junction, of only 20 nm cross-section, and superconducting granular Aluminum films with critical field above 5 Tesla, we operate a flux qubit in magnetic field of up-to 1.2 Tesla. Remarkably, the spectrum and coherence of the qubit remain stable in this entire range, and we observe the freezing of a paramagnetic spin ensemble coupled to the qubit.

Liens :

• Séminaire théorie
• Personne à contacter : Serge Florens ()
• Lieu : G421

Résumé : When an impurity is immersed in a many-body background, it is dressed by the excitations of the bath, and forms "a polaron". As a result, the injection spectrum of the impurity carries the hallmarks of the correlations present in the bath. This physics is relevant for excitons optically injected in a few layer heterostructure, or for cold atomic mixtures. In this talk, we will first review the basic theoretical framework and recent experimental progress. Then, we will theoretically analyze a few cases of correlated many-body states: the impurity injection spectra are predicted to display peculiar features, that allow to distinguish whether the bath features BCS pairing, charge density waves, topological phases, the BKT transition, etc.

Liens :

• Séminaire théorie
• Personne à contacter : Serge Florens ()
• Lieu : G421

Résumé : Typical models for many-body localization (MBL) can be represented as tight-binding models over the many-body Hilbert space (Fock space), where sites correspond to non-interacting basis states, and possible hoppings depend on interaction. In this representation, the disordered model is fully specified by the joint distributions and correlations for on-site energies as well as hopping matrix elements.

In this talk, I will present our conclusions regarding the role of these correlations in the scaling of the MBL critical disorder W_c(n). For this purpose, we study five different disordered spin models which share the same distributions of diagonal (energy) and off-diagonal (hopping) Fock-space matrix elements but differ by their Fock-space correlations. These include a quantum-dot (QD) and one-dimensional (1D) MBL models, their modifications (uQD and u1D models) with removed correlations of off-diagonal matrix elements, as well a quantum random energy model (QREM) with no correlations at all. QD (resp. uQD) and 1D (resp. u1D) models differ essentially by the structure of their many-body energy correlations, which reflect the real-space spatial structure in the latter case. On the other hand, the absence of hopping correlations in the uQD and u1D models makes them "maximally chaotic". In full consistency with our predictions from analytical arguments, our numerical results from exact diagonalization indicate W_c ∼ n for the QD model, W_c(n) ∼ const for the 1D model, W_c ∼ n ln n for the uQD and u1D models without off-diagonal correlations, and W_c ∼ n ^1/2ln n for QREM, where n is the system size. A cross-comparison of these results for all five models demonstrate that the scaling of W_c(n) for MBL transitions is governed by a combined effect of Fock-space correlations of diagonal and off-diagonal matrix elements.

Finally, we extend our investigation to dynamical aspects of the MBL transition. By numerically computing the time-evolution of the imbalance for all models at various disorder, we extract their relaxation rates in the vicinity of the MBL transition, and observe the critical slowing down of the dynamics with increasing disorder strength. Comparing the scaling of these relaxation rates with increasing system size for all models reveals the intricate relationship between Fock-space correlations and the breaking down of ergodicity in MBL systems.

References

• [[1]] T. Scoquart, I. Gornyi and A. Mirlin, Role of Fock-space correlations in many-body localization, Physical Review B 109 (21), 214203 (2024)
• [[2]] T. Scoquart, I. Gornyi and A. Mirlin, unpublished (2024)

Liens :

• Séminaire LPMMC
• Personne à contacter : Anna Minguzzi ()
• Lieu : G421

Résumé : Ultracold atoms confined in one dimension with strongly repulsive interactions behave like a gas of impenetrable particles, whose wave function vanishes when two particles approach each others and change its sign according to particle-exchange symmetry. In this regime, the so-called Tonks-Girardeau regime, these systems are exactly solvable via a Bose-Fermi mapping, which can be generalized to arbitrary confining geometries and include spin degrees of freedom. The latter are described by a spin-chain model with nearest-neighbor interactions. In this talk, I will focus on two specific aspects of the dynamical properties of these systems: the role played by an interaction imbalance [1] and the finite-size effects due to the presence of a box trap [2, 3]. Both aspects are analyzed in particular by looking at the momentum distribution of the particles, an observable easily accessible in ultracold experiments.

In the first part of the talk, I will present the comparison of the time evolution of an initially symmetry-mixed state of two-component bosons dictated by the strongly interacting Hamiltonian with equal inter- and intraspecies interactions and that with different interaction strengths, which breaks the SU(2) symmetry. While the evolution of the total momentum distribution is (quasi)constant if the symmetry is unbroken, it shows large oscillations for the symmetry-breaking case. These symmetry oscillations are related to the oscillations of the symmetry indicator, which can be identified using group theory. In the second part of the talk, I will discuss the dynamics of an initial state of two-component fermions prepared in a single spin-domain configuration and confined in a box potential. For finite systems, the presence of nonsmooth borders of the trap induces the appearance of high-momentum oscillating tails in the momentum distribution. These tails depend on a nonlocal high-order spin correlation, whose initial growth contains information about the chosen initial spin configuration and its later evolution follows that of the induced magnetization.

References

• [1] S. Musolino, M. Albert, A. Minguzzi, and P. Vignolo, PRL 133, 183402 (2024);
• [2] G. Aupetit-Diallo, S. Musolino, M. Albert, and P. Vignolo, PRA 107, L061301 (2023);
• [3] S. Musolino, G. Aupetit-Diallo, M. Albert, P. Vignolo, and A. Minguzzi, arXiv:2410.23716 (2024).

Liens :

• Soutenance de thèse
• Personne à contacter : Serge Florens ()
• Lieu : G421

Résumé : The significant advancements in atomic, molecular and optical physics characterizing the last few decades have brought us a proliferation of highly controllable many-body quantum systems. These are promising platforms to realize quantum processors, i.e. quantum systems that can be programmed to simulate the dynamics of a target model, or to solve challenging computational problems. As they hold the promises of large-scale quantum simulations and fault-tolerant quantum computing, this new generation of quantum processors also bring a new set of theoretical challenges. In my thesis I focus on two key matters: (1) programming a quantum system with fixed interactions to realize quantum circuits and Hamiltonian dynamics, and (2) learning the structure of the state of a quantum processor from measurement data. In the first part we present a protocol which allows to engineer arbitrary Hamiltonian dynamics in a variety of programmable quantum simulators, based on the local driving of their degrees of freedom. We illustrate concrete applications of this protocol through examples in the contexts of quantum simulation, quantum error correction and quantum optimization. Finally, inspired by recent experimental works, we consider the effects of several experimental imperfections, demonstrating how these pulse sequences are naturally robust. This makes our protocol well-suited for near-term quantum experiments, resulting in a promising approach to realize universal quantum processing in systems which do not allow for a direct control of their interactions. In the second part we develop new methods to characterize the quantum state of a quantum processors using the recently introduced framework of randomized measurements. Among the work done in this direction, we discuss how properties of one-dimensional tensor network mixed states (i.e. matrix-product density operators) allow us to prove an approximate local factorization of fidelities between such states. This implies that these states can be benchmarked on quantum processors with polynomially many local measurements, which are available in several experiments. Leveraging on this insight, we introduce a provably scalable learning algorithm to reconstruct matrix-product operators from randomized measurement data. These results extend the scope of randomized measurements, and uncover new applications in structured quantum states of many qubits.

Liens :

• Séminaire théorie
• Personne à contacter : Serge Florens ()
• Lieu : G421

Résumé : Topological band theory has celebrated various successes over the last few years, such as the recent classifications of crystalline materials based on their space group symmetry. We are currently witnessing a drive to generalise this theory to the case where interactions between electrons become relevant, with much work focused on ground states. As an alternative direction, we here study the topology of interaction-induced excitations, specifically excitons in semiconductors. In my talk, I will give a pedagogical introduction to the classification and bulk-boundary correspondence of exciton band structures based on inversion symmetry.

Liens :

• Séminaire LPMMC
• Personne à contacter : Cécile Repellin ()
• Lieu : G421

Résumé : Thermal Hall conductivity has recently emerged as an experimentally accessible property of materials, especially insulators. Theoretical understanding thereof has remained a challenge, in particular since the breaking of time-reversal symmetry by neutral particles is nontrivial and can emerge from multiple mechanisms (semiclassical dynamics, skew-scattering, etc). In a first part, I will present a general formulation of skew-scattering of energy-carrying bosons by other collective excitations. Specializing to phonon-magnon interactions, I will show that a phonon thermal Hall effect from skew-scattering in antiferromagnets is allowed by magnetoelastic and spin-orbit couplings. In a second part, I will focus on the free semiclassical dynamics of neutral bosons, and present a systematic derivation of their kinetic equation, incorporating the topological dynamics of wavepackets in the form of Berry curvatures (generalized to phase space). This makes it possible to treat inhomogeneous systems, including boundaries, textures, etc., in a compact and natural manner. If time permits, I will eventually mention some ongoing work on skyrmion lattices.

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter : Jeremie Viennot ()
• Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé : Fabry–Pérot interferometers have attracted significant attention in recent years as powerful tools to study anyon braiding and explore exotic quantum states. This talk will be divided into two parts. In the first part, I will discuss novel electron pairing and tripling observed in the integer quantum Hall (IQH) regime. Contrary to the conventional understanding that IQH effects can be described by non-interacting theories, I will present evidence from graphene-based quantum Hall FPIs, revealing electron pairing at a bulk filling factor of filling factor 2. More intriguingly, I have also discovered an unexpected emergence of electron tripling, characterized by a fractional Aharonov–Bohm flux period of h/3e at filling factor 3. To unveil these phenomena, we developed a novel plunger-gate spectroscopy, demonstrating that electron pairing (tripling) involves correlated charge transport across two (three) entangled quantum Hall edge channels. This spectroscopy reveals a quantum interference flux periodicity determined by the sum of the phases acquired by distinct edge channels with slightly different interfering areas. In the second part, I will address the “phase jump,” often considered a hallmark of anyon braiding. Interestingly, phase jumps appear in the IQH regime, where anyon is not expected. I will discuss the implications of these findings and their relevance to our understanding of quantum Hall physics.

Liens :

• Séminaire théorie
• Personne à contacter :
• Lieu : G421

Résumé : Geometrical frustration is a simple yet profound concept in condensed matter physics that can give rise to exotic quantum matters like spin liquid. In this talk, I will study a new source of geometrical frustration from the orbitals instead of the spins of the electrons. A concrete platform for realizing such orbital geometrical frustration is the twisted bilayer graphene (TBG) at fractional filling, where we can map the configurations of electrons to the tiling patterns of trimers. This deceptively simple model of trimers exhibits a rich suite of complex phases, including unusual excitations exhibiting the physics of fractionalization and fractons. I will discuss both the effect of thermal and quantum fluctuations. The finite-temperature phase diagram reveals a novel polar fluid and an ordered brick-wall phase characterized by fractionally charged e/3 excitations with subdimensional lineonic dynamics. Notably, we identify a critical trimer liquid phase. Its e/3 monomers are fractionalized bionic excitations: they carry a pair of emergent gauge charges, as evidenced by algebraic correlations with two distinct exponents. These field theoretical predictions offer theoretical grounds for numerical observations of critical exponents. By mapping the basis of the constrained Hilbert space to tiling patterns, we find an emergent exact global symmetry for the effective quantum model, dubbed ``tile-invariant symmetry”. Quantum fluctuation prefers to spontaneously break the tile-invariant symmetry, giving rise to an exotic Mott insulator, with a gapless spectrum. Our study highlights the triangular trimer model as a new key platform for investigating fractionalization and emergent symmetry and calls for experimental investigations of this physics in twisted van der Waals materials and a broader class of systems with intermediate-range interactions. Refs: DM, et al. Phys. Rev. Lett. 131, 106801 Tian, H., Codecido, E., DM, et al. Dominant 1/3-filling correlated insulator states and orbital geometric frustration in twisted bilayer graphene. Nat. Phys. 20, 1407–1412 (2024). Zhang.K, DM, et al. https://arxiv.org/abs/2410.00092

Liens :Dan Mao

• Séminaire QuantAlps
• Personne à contacter :
• Lieu : Salle de Séminaire du LNCMI

Résumé : When electrons moving in a two-dimensional plane are subject to a perpendicular magnetic field they move in circles called cyclotron orbits as a result of the Lorentz force. Treated quantum mechanically, these orbits become quantized like the orbitals of an atom, forming highly degenerate states called Landau levels. In this colloquium, I will show how we used strain to make photons "feel" a magnetic field and thus form Landau levels in a photonic crystal, despite the fact that photons carry no charge and thus cannot experience the Lorentz force. This increases the strength of interaction between light and matter, which has implications in quantum optics and integrated photonics. Time permitting, I will discuss the related topic of how edge states in a "Chern insulator" photonic crystal can be used to slow down light in a photonic chip over a wide bandwidth.

Liens :Mikael Rechtsman

• Séminaire théorie
• Personne à contacter : Serge Florens ()
• Lieu : G421

Résumé : Topology leads to many robustly quantized physical phenomena ranging from particle physics to condensed matter physics. In solids, many topological phenomena appear in bands of nearly free electrons moving in a periodic potential. Recently, periodic media composing metamaterials enable photons carrying modulated momenta. These systems have highly tunable band structures and their dynamics are generically described by non-Hermitian matrices when dissipative effects are considered. The level crossings between such photonic bands are known as exceptional points. In this presentation, I will give a brief introduction on these nodal points, demonstrating they are described by non–Abelian topological charges. This leads to non-commuting operations of moving them in the reciprocal space and violation of the famous fermion-doubling Nielsen-Ninomiya theorem. I will show how these are verified experimentally. Then I will introduce parity-time symmetric systems, where the system can transit between Hermitian and non-Hermitian regimes by spontaneous symmetry breaking. This system has the great potential of exploring topological bands as the gauge structures are tunable through spontaneous symmetry breaking. This leads to some new ideas of preparing topological states via dissipation.

Liens :

• Séminaire théorie
• Personne à contacter : Serge Florens ()
• Lieu : G421

Résumé : Limit-cycle or time-crystal phases, in which the order parameter retains a periodic motion in time, are non-equilibrium phases. They are therefore expected to display phenomena that fall beyond equilibrium classifications. We study such phenomena in paradigmatic O(N) models. Once suitably driven out of equilibrium, they develop instabilities towards limit-cycle long-range orders, which emerge in driven quantum systems and solid-state platforms, as well as nonreciprocal active matter. I will discuss realization in existing platforms, but also new implementations in driven magnets. I will describe the complete phase diagram beyond mean-field, which hosts new type of phase transitions towards limit cycles, associated to new non-equilibrium universality classes. Finally, I will discuss why such phases generically display the Kardar-Parisi-Zhang physics and generalization of it.

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter : Jeremie Viennot ()
• Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé : Josephson traveling-wave parametric amplifiers (J-TWPA) are a promising platform for new quantum optics experiments in the microwave domain. Recent experiments showed that a J-TWPA could generate broadband two-mode squeezing when excited by a strong classical pump. This indicates that it is possible to create non-classical states of light in these devices, but only when the non-linearity is enhanced by pumping with a large number of photons. The possibilities offered by superconducting circuits to engineer dispersion and non-linear interactions of such devices are interesting, as they could lead to single-photon non-linear interactions in traveling-wave devices. In this discussion, we will explore recent quantum optics experiments that aim to understand the generation of non-classical states of light in a J-TWPA. We will also highlight how these results offer exciting possibilities for the continuous generation of non-classical states of light in traveling-wave devices.

Liens :

• Séminaire QuantAlps
• Personne à contacter :
• Lieu : Salle de Conférence du CNRS Bât. A

Résumé : Most magnetic materials, phenomena and devices are well described in terms of their constituent magnetic dipoles. There is mounting evidence, however, that higher-order magnetic multipoles can lead to intriguing magnetic behaviors, which are often attributed to "hidden order" since they are difficult to characterize with conventional probes. In this talk I will focus on the existence and relevance of the so-called magnetoelectric multipoles, which form the next-order term, after the magnetic dipole, in the multipolar expansion of the energy of a magnetization density in a magnetic field. I will describe how magnetoelectric multipoles underlie multiferroic behavior and dominate the magnetic response to applied electric fields, then discuss signatures of hidden magnetoelectric multipolar order and possibilities for its direct measurement. I will argue that all is not lost if your material lacks magnetoelectric multipoles, and that hidden magnetic octupoles and even triakontadipoles also cause fascinating physics, including the currently rather fashionable "altermagnetism". Finally, I will show that ferroic ordering of these higher-order magnetic multipoles results in a magnetization at the surface of a sample, even in materials with no net magnetization in their bulk and with apparently compensated surface dipoles.

Liens :Nicola Spaldin

• Séminaire théorie
• Personne à contacter : Serge Florens ()
• Lieu : G421

Résumé : The experimental realization of increasingly complex quantum states underscores the pressing need for new methods of state learning and verification. In one such framework, quantum state tomography, the aim is to learn the full quantum state from data obtained by measurements. Without prior assumptions on the state, this task is prohibitively hard; and only a few classes of states are currently known to be efficiently learnable. In this talk, I will present our new algorithm to learn fermionic states on n modes, prepared by any number of Gaussian (free-fermionic) and at most t non-Gaussian (interacting) gates. The algorithm is based exclusively on single-copy measurements and produces a classical representation of a state, which is guaranteed to be accurate. I will show why the runtime of our algorithm — poly(n,2^t) — is essentially optimal for this task, under common assumptions from theory of cryptography. In addition to the outputs of quantum circuits, I will demonstrate how our tomography algorithm can be efficiently applied to learn target states which arise in the physics of impurity models. Beyond tomography, I would like to review an improved circuit compilation strategy, which is also enabled by our results. This talk is based on the work done in collaboration with Antonio Anna Mele (FU Berlin), which can be accessed at https://arxiv.org/abs/2402.18665

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter : Jeremie Viennot ()
• Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé : The dispersive readout scheme enables quantum non-demolition measurement of superconducting qubits. An increased readout power can shorten the readout time and reduce the state discrimination error but can promote qubit transitions into higher noncomputational states. These measurement-induced transitions present a severe obstacle to improving the qubit readout. The physics of the effect has been only studied in the context of transmon qubits thanks to approximations unique to this qubit type. In the talk, I will introduce the problem and review the existing experimental and theoretical results. Building upon the concept of dressed coherent states, I will explain how one can think about these transitions in an arbitrary qubit system with few degrees of freedom. Taking advantage of the presented approach, I will show how to predict the number of microwave photons causing such transitions using a fluxonium qubit as an example system.

Liens :

• Séminaire théorie
• Personne à contacter : Serge Florens ()
• Lieu : G421

Résumé : Quantum fluctuations can inhibit long-range ordering in frustrated magnets and potentially lead to quantum spin liquid (QSL) phases. A prime example are gapless “Dirac” QSLs with emergent U(1) gauge fields which may emerge on triangular-lattice J1-J2 Heisenberg models. Despite several promising candidate materials, however, a complicating factor for their realisation is the presence of other degrees of freedom, as well as the absence of unambiguous thermodynamic or spectral signatures. In this talk, I will present our recent work which predicts that the U(1) DSL exhibits a spin-Peierls instability to valence bond solid order upon coupling the the lattice. We investigate the stability for realistic systems using both field-theoretical and various computational methods, and argue that emergent monopoles drive the spin-lattice transition. Finally, I will discuss the prediction of this system’s spectral properties including the effects of strong spinon interactions and gauge field fluctuations. We highlight that both spinon-bound states and monopoles contribute strongly to the spin structure factor as well as the phonon self-energy.

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter : Jeremie Viennot ()
• Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé : Although qubits are prime examples of quantum systems, single qubits can only host semi-classical spin coherent states. Qudits, on the other side, can exhibit non-classical states, which cannot be described by a classical probability distribution. Here we detect quantumness in the time evolution of a uniformly precessing 8-dimensional nuclear qudit. In this experiment quantumness is certified by asking how often the x-coordinate of a uniformly precessing state is positive [1]. A violation of a classical bound in this protocol indicates the absence of a classical probability distribution that can explain the observed data, thus confirming quantumness [2]. We present our experimental demonstration of this protocol focusing on a family of non-classical states in a spin-7/2 antimony nucleus, implemented within a silicon nano-electronic device [3]. Our results reveal a significant violation of the classical protocol bound, surpassing it by 19 standard deviations for the spin-cat state. These findings underscore our ability to prepare arbitrary nonclassical resource states with high fidelity in a single atomic-scale qudit, with potential applications in quantum information processing, sensing and quantum error correction. Ref.: [1] Tsirelson, B. How often is the coordinate of a harmonic oscillator positive? https://doi.org/10.48550/arXiv.quant-ph/0611147 (2006). [2] Zaw, L. H., Aw, C. C., Lasmar, Z. & Scarani, V. Detecting quantumness in uniform precessions. Phys. Rev. A 106, 032222 (2022). [3] Yu, X. et al. Creation and manipulation of Schrodinger cat states of a nuclear spin qudit in silicon. Preprint at https://doi.org/10.48550/arXiv.2405.15494 (2024).

Liens :

• Séminaire théorie
• Personne à contacter : Serge Florens ()
• Lieu : G421

Résumé : We study a Josephson junction in a Kitaev chain with particle-hole symmetric nearest-neighbor interactions. When the phase difference across the junction is π, we show analytically that the full many-body spectrum of the interacting system is fourfold degenerate up to corrections that vanish exponentially in the system size. The Majorana bound states at the ends of the chain are known to survive interactions. Our result proves that the same is true for the zero-energy quasiparticle localized at the junction. We further study finite-size corrections numerically, and show how repulsive interactions lead to stronger end-to-end correlations than in a noninteracting system with the same bulk gap.

Liens :

• Séminaire théorie
• Personne à contacter : Adolfo Grushin ()
• Lieu : G421

Résumé : In this talk, I will argue that the topology of an insulator can be defined even when all eigenstates of the system are localized — an extreme case of Anderson insulators that we call ultra-localized. I will describe their bulk-boundary correspondence and show that ultra-localized systems are in many instances phases of matter not described by the known classification of topological insulators.

Liens :

• Séminaire théorie
• Personne à contacter : Adolfo Grushin ()
• Lieu : G421

Résumé : In this talk I introduce the chiral anomaly in the context of magnetic Weyl semimetals with domain walls. Weyl semimetals serve as a platform to explore the chiral anomaly---the nonconservation of chiral charge due to applied parallel electric and magnetic, and/or parallel axial electric and axial magnetic, fields. Axial electromagnetic fields are emergent fields which couple with opposite sign to fermions with opposite chirality. We consider a magnetic Weyl semimetal which contains two Weyl fermions of opposite chirality separated in momentum space. Introducing a dynamic domain wall in the Weyl node separation generates axial electromagnetic fields, leading to the chiral anomaly. Via the chiral magnetic effect, the anomaly generates a current, resulting in electromagnetic radiation, which if detected, measures the axial anomaly. In reverse, the anomaly influences the domain wall dynamics, enabling electric control of the chirality of domain walls and improving the domain wall dynamics. Measuring the electric field mediated changes in the domain wall chirality would constitute a direct proof of the chiral anomaly. Refs: Hannukainen, Ferreiros, Cortijo, Bardarson, Phys. Rev. B 102, 241401(R) (2020), Hannukainen, Bardarson, Cortijo, Ferreiros ,SciPost Phys. 10, 102 (2021)

Liens :

• Séminaire théorie
• Personne à contacter : Adolfo Grushin ()
• Lieu : G421

Résumé : In between the periodic systems and random systems, there is an intermediate system so-called quasiperiodic system. These systems do not have periodicity but possess long-range orders. Recently, these systems are attracting attention, because many fascinating phenomena including unconventional superconductivity have been reported by experimental groups. In addition, there is a special system called quasicrystal. These systems possess both quasiperiodicity and crystalline nature. In Ref. (1), we combined a typical model of topological charge pumping (Rice-Mele model) and Fibonacci quasicrystal to make the Fibonacci-Rice-Mele model which possesses topological charge pumping while conserving quasicrystalline nature. Since this model is quasicrystal, the lattice also has a fractal structure. Accompanying this nature, the electronic band structure has fractal energy gaps, and we can induce and characterize multilevel topological charge pumping. (1) M. Yoshii, S. Kitamura and T. Morimoto, Phys. Rev. B 104, 155126 (2021).

Liens :

• Séminaire interne LPMMC
• Personne à contacter : Vincent Rossetto ()
• Lieu : G421

Liens :LPMMC

• Séminaire théorie
• Personne à contacter : Serge Florens ()
• Lieu : G421

Résumé : Rare-earth crystals offer a rich variety of fascinating phenomena whose understanding relies on confronting theories and experiments to account for the complex interactions of their atomistic and emergent constituents. Motivated by the detailed information from the crystalline environment for the magnetostatics of rare-earth (RE) ions and for the spin-dynamics of their effective models, this talk shall emphasise on the role that magneto-crystalline anisotropies play in materials with strong spin-orbit coupling. Focussing on RE pyrochlores and RE garnets, we shall discuss the relevance of the crystalline symmetries for the f-electrons of the rare-earth ions driving the unconventional magnetism. We shall present models and calculations for system with challenging non-collinear anisotropies, and thus of interest for neutron and x-ray scattering. This theoretical approach ought to contribute to advancements in theory and experiments on quantum magnetism and spintronics.

Liens :

• Séminaire LPMMC
• Personne à contacter : Sergey Skipetrov ()
• Lieu : G421

Résumé : Periodically-driven quantum systems make it possible to reach stationary states with new emerging properties. However, this process is notoriously difficult in the presence of interactions because continuous energy exchanges generally boil the system to an infinite temperature featureless state. Here, we describe how to reach nontrivial states in a periodically-kicked Gross-Pitaevskii disordered system. One ingredient is crucial: both disorder and kick strengths should be weak enough to induce sufficiently narrow and well-separated Floquet bands. In this case, inter-band heating processes are strongly suppressed and the system can reach an exponentially long-lived prethermal plateau described by the Rayleigh-Jeans distribution. Saliently, the system can even undergo a wave condensation process when its initial state has a sufficiently low total quasi-energy. These predictions could be tested in nonlinear optical experiments or with ultracold atoms.

Liens :

• Séminaire LPMMC
• Personne à contacter : Denis Basko ()
• Lieu : G421

Résumé : Scaling up superconducting quantum processors with optimized performance requires a sufficient flexibility in the choice of operating points for single and two qubit gates to maximize their fidelity and cope with imperfections. Flux control is an efficient technique to manipulate the parameters of tunable qubits, in particular to activate entangling gates. At flux sensitive points of operation, the ubiquitous presence of 1/f flux noise however gives rise to dephasing by inducing fluctuations of the qubit frequency. We show how two-tone modulation of the flux bias, a bichromatic modulation, gives rise to a continuum of dynamical sweet spots where dephasing due to slow flux noise is suppressed to first order for a wide range of time-averaged qubit frequencies. The qubits can be operated at these dynamical sweet spots to realize protected entangling gates and to avoid collisions with two-level-system defects.

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter : Romain Maurand ()
• Visio-conférence Zoom hybride.
  En présentiel : Salle Rémy Lemaire K223, Institut Néel
  Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 91808901596
  

Résumé : The recent discovery of fractionalized anomalous quantum Hall effect in twisted Molibdenium ditelluride (tMoTe2) is a landmark in condensed matter physics [(1-2-3)]. Electrical transport and optical spectroscopy measurements have shown that devices where two layers of MoTe2 are twisted by around 4 degrees exhibit fractionally quantized resistance, in the abscence of a magnetic field. These physics occur in the valence flat bands of the system, that arise from the moiré superlattice of the 2D crystals. Additionally, these flat bands feature berry curvature at the origin of the quantized anomalous Hall effect. Since this non-trivial topology is intimatey tied to the microscopic details of the moiré superlattice, we use scanning tunneling microscopy (STM) measurements of high quality tMoTe2 devices to investigate these microscopic details. I will explain the fabrication of these devices as well as decribe how STM/S measurements are performed reliably on this semiconductor. By having access to the density of states, we are able to investigate the energy dependent polarization of the wavefunctions. Our results (4) help shed light on the origins of the FQAH physics and inform future studies of this system. (-1-) Haldane, F. D. M. Model for a quantum Hall effect without Landau levels: condensed matter realization of the “parity anomaly”. Phys. Rev. Lett. 61, 2015–2018 (1988) (-2-) Joon, H. et al, Observation of fractionally quantized anomalous Hall effect, Nature 622, 74-79 (2023). (-3-) Xu, F. et. al., Observation of integer and fractional quantum anomalous Hall states in twisted bilayer MoTe2, Phys. Rev. X 13, 031037 (2023). (-4-) Thompson, E. et. al., Visualizing the microscopic origins of topology in twisted molybdenum ditelluride, arXiv:2405.19308 (2024).

Liens :

• Séminaire interne LPMMC
• Personne à contacter : Benoît Vermersch ()
• Lieu : G421

Liens :LPMMC

• Séminaire nano-électronique quantique
• Personne à contacter : Jeremie Viennot ()
• Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé : Topology, like symmetry, is a fundamental concept in understanding general properties of physical systems. In condensed matter systems, non-trivial topology may manifest itself as singular features in the energy spectrum or the quantization of observable quantities such as electrical conductance and magnetic flux. Using microwave spectroscopy, we show that a superconducting circuit with three Josephson tunnel junctions in parallel can possess energy degeneracies indicative of intrinsic non-trivial topology. We identify three topological invariants, one of which is related to a hidden quantum mechanical supersymmetry. Depending on fabrication parameters, devices are gapless or not, and fall on a simple phase diagram which is shown to be robust to perturbations including junction imperfections, asymmetry, and inductance. Josephson tunnel junction circuits, which are readily fabricated with conventional microlithography techniques, allow access to a wide range of topological systems which have no condensed matter analog. Notable spectral features of these circuits, such as degeneracies and flat bands, may be leveraged for applications in quantum information and sensing.

Liens :

• Séminaire QuantAlps
• Personne à contacter : Michele Filippone ()
• Lieu : Bât. GreEn-Er Amphitéâtre Bergès

Résumé :

Naive attempts to put together relativity and quantum measurements lead to signaling between space-like separated regions. In QFT, these are known as impossible measurements. We show that the same problem arises in non-relativistic quantum physics, where joint nonlocal measurements (i.e., between systems kept spatially separated) in general lead to signaling, while one would expect no-signaling (based for instance on the principle of no-nonphysical communication). This raises the question: Which nonlocal quantum measurements are physically possible? We review and develop further a non-relativistic quantum information approach developed independently of the impossible measurements in QFT, and show that these two have been addressing virtually the same problem. The non-relativistic solution shows that all nonlocal measurements are localizable (i.e., they can be carried out at a distance without violating no-signaling) but they (i) may require arbitrarily large entangled resources and (ii) cannot in general be ideal, i.e., are not immediately reproducible.

We find all joint quantum measurements on 2 qubits that are localizable with 1 and 3 e-bits. Interestingly, the Elegant Joint Measurement appears naturally in such a structuring of all joint measurements.

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter : Jeremie Viennot ()
• Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé : Extensive efforts have been undertaken to combine superconductivity and the quantum Hall effect so that Cooper-pair transport between superconducting electrodes in Josephson junctions is mediated by one-dimensional edge states. This interest has been motivated by prospects of finding new physics, including topologically-protected quasiparticles, but also extends into metrology and device applications. So far it has proven challenging to achieve detectable supercurrents through quantum Hall conductors. I will show that domain walls in minimally twisted bilayer graphene support exceptionally robust proximity superconductivity in the quantum Hall regime, allowing Josephson junctions to operate in fields close to the upper critical field of superconducting electrodes. The critical current is found to be non-oscillatory and practically unchanging over the entire range of quantizing fields, with its value being limited by the quantum conductance of ballistic, strictly one-dimensional electronic channels residing within the domain walls. The system described is unique in its ability to support Andreev bound states at quantizing fields and offers many interesting directions for further exploration. Reference: J. Barrier et al, Nature 628, 741-745 (2023)

Liens :

• Séminaire théorie
• Personne à contacter :
• Lieu : G421

Résumé : The classical simulation of quantum field theories is a challenging problem. We will present a new method for the study of 1 et 1D quantum field theory models, built upon Hamiltonian truncation and tensor network techniques, which is suitable for the study of low-energy eigenstates and out-of-equilibrium dynamics. Applications of the new method to the sine-Gordon and massive Schwinger model demonstrate that, despite its relatively high computational cost, it dramatically improves precision compared to exact diagonalisation results, allowing the study of so far unexplored regimes of the models’ phase diagram and of the emergence of equilibrium after a quantum quench.

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter : MAURAND Romain 240302 ()
• Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé : Atomic scale Josephson junctions are formed in a scanning tunneling microscope (STM) between a superconducting tip and sample. The investigated junctions show hysteretic behavior with respect to the current sweep direction. In the presence of single magnetic adatoms non-reciprocal features emerge in the VI-characteristics. Magnetic adatoms on superconducting surfaces couple to the superconducting condensate and induce Yu-Shiba-Rusinov (YSR) states. YSR states can be probed by either electrons or holes with different tunneling probabilities resulting in asymmetric quasiparticle currents inside the superconducting gap. We find that this electron-hole asymmetry causes the non-reciprocity of the STM Josephson junctions. Furthermore, the phase coherence of the Cooper-pair tunneling processes is investigated by exposing the pristine Pb-Pb junction to high frequency irradiation. Phase diffusion is enhanced in the presence of the irradiation. Additionally, step-like features emerge as energy is absorbed by the tunneling Cooper pairs. The observed steps experience hysteretic behavior depending on the bias sweep direction. Ref: Diode effect in Josephson junctions with a single magnetic atom, M. Trahms et al., Nature 615, 628 (2023).

Liens :

• Séminaire théorie
• Personne à contacter : Serge Florens ()
• Lieu : G421

Résumé : Monitored quantum systems undergo Measurement-induced Phase Transitions (MiPTs) stemming from the interplay between measurements and unitary dynamics. When the detector readout is post- selected to match a given value, the dynamics is generated by a Non-Hermitian Hamiltonian with MiPTs characterized by different universal features. Here, we derive a partial post-selected stochastic Schrodinger equation based on a microscopic description of continuous weak measurement. This formalism connects the monitored and post-selected dynamics to a broader family of stochastic evolution. We apply the formalism to a chain of free fermions subject to partial post-selected monitoring of local fermion parities. Within a 2-replica approach, we obtained an effective bosonized Hamiltonian in the strong post-selected limit. Using a renormalization group analysis, we find that the universality of the non-Hermitian MiPT is stable against a finite (weak) amount of stochasticity. We further show that the passage to the monitored universality occurs abruptly at finite partial post-selection, which we confirm from the numerical finite size scaling of the MiPT. Our approach establishes a way to study MiPTs for arbitrary subsets of quantum trajectories and provides a potential route to tackle the experimental post-selected problem.

Liens :

• Séminaire QuantAlps
• Personne à contacter : Michele Filippone ()
• Lieu : Bât. GreEn-Er Amphitéâtre Ampère

Résumé :

In this talk, I will review more than 10 years of research activities of concerning the exploration of the potential of two-dimensional materials and van der Waals heterostructures for spintronic applications. Starting from the first demonstration of long spin diffusion length in graphene to most recent breakthroughs of active spin devices, we will discuss the progress achieved within the Spintronics workpackage of the Graphene Flagship (from 2013-2023), highlighting the main milestones of the field and the current perspective and challenges concerning the use of 2D materials for spintronic applications as recently presented by a consortium of leading groups in the field (H. Yang et al. Nature 606 (7915), 663-673 (2022)).

I acknowledge the European Union Seventh Framework Program under Grant Agreement No. 881603 Graphene Flagship.

Liens :Stephan Roche

• Séminaire nano-électronique quantique
• Personne à contacter : Jeremie Viennot ()
• Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé : For more than thirty years, experimental analysis of quantum phase transitions (QPTs) has been largely focused on finding critical exponents and universality classes of studied systems. This approach emphasizes scale-invariance of QPTs and ignores the fact that system response also depends on two non-universal length scales: microscopic “seeding” scale of the correlation length and the dephasing length. Correcting this deficiency, we have developed a phenomenological model of QPTs based on conjecture that the dephasing length is set by a distance travelled by a system-specific semi-classical elementary excitation over the Planckian time, and that the scaling function assumes the generic exponential form predicted by the scaling theory of localization (the figure shows some examples). Using this model, we have quantitatively explained QPTs in eighteen systems including: magnetic-field-driven QPT in superconducting films, nanowires, La1.92Sr0.08CuO4 and Josephson junction chains; QPT in Ising and Heisenberg spin chains, the Mott transition in 2d cold atomic gases and moiré superlattices; and QPT between the states of quantum Hall and other topological insulators. The model illuminates the universal microscopic nature of many-body gapless state of matter emerging near QPTs. Surprisingly, the only system deviating from the trend is doped Si : P, where metal-insulator transition is explained by the non-interaction version of the model. We anticipate that shifting emphasis from critical exponents to the microscopic parameters of a phase transition will be a fruitful approach for many systems beyond equilibrium condensed matter physics. Ref. : [1] A. Rogachev, Microscopic scale of quantum phase transition: from doped semiconductors to spin chains, cold gases and moiré superlattices, arXiv:2309.00749. [2] A. Rogachev and K. Davenport, Microscopic scale of pair-breaking quantum phase transitions in superconducting films, nanowires and La1.92Sr0.08CuO4, arXiv:2309.00747. [3] A. Rogachev, Quantum phase transitions in quantum Hall and other topological systems: role of the Planckian time, arXiv:2309.00747.

Liens :

• Séminaire QuantAlps
• Personne à contacter : Michele Filippone ()
• Lieu : Nevill Mott - D420 - 3rd Floor

Résumé : In the original Matrix movie, the bulk of the human population lives not in the real world but inside a computer simulation called the Matrix. They are unable to detect this situation, except for the fact that certain agents can transcend the normal rules of physics. In this talk, I will explain how this is eerily similar to the world we live in. Certain people (quantum physicists) can transcend the normal rules by using entangled particles to do things that "should be" impossible. This makes the world very puzzling place, even for quantum physicists. These “super-powers” are also central to the emerging field of quantum information technology. Finally, I will explain very recent work by myself and co-workers that ties all of this together in order to show that the world is even more puzzling than we had thought. Much like the latest Matrix movie.

Liens :

• Séminaire théorie
• Personne à contacter : Adolfo Grushin ()
• Lieu : G421

Résumé : Amorphous solids, i.e., systems which feature well-defined short-range properties but lack long-range order, constitute an important research topic in condensed matter. While their microscopic structure is known to differ from their crystalline counterpart, there are still many open questions concerning the emergent collective behavior in amorphous materials. This is particularly the case in the quantum regime, where the numerical simulations are extremely challenging. In this talk, we instead propose to explore amorphous quantum magnets with an analog quantum simulator. To this end, we first present an algorithm to generate amorphous quantum magnets, suitable for Rydberg simulators of the Ising model. Subsequently, we use semiclassical approaches to get a preliminary insight of the physics of the model. In particular, we calculate mean-field phase diagrams, and use the linear-spin-wave theory to study localization properties and dynamical structure factors of the excitations. Finally, we outline an experimental proposal based on Rydberg atoms in programmable tweezer arrays, thus opening the road towards the study of amorphous quantum magnets in regimes difficult to simulate classically.

Liens :

• Séminaire interne LPMMC
• Personne à contacter : Vincent Rossetto ()
• Lieu : G421

Résumé : Tensor networks can be used to describe a plethora of physically interesting many-body states in one dimension, from thermal states of local Hamiltonians to the output of noisy quantum circuits. In this talk we're going to discuss how properties of one-dimensional tensor network states result in an approximate local factorization of fidelities. This implies that such tensor network states can be verified on quantum processors with a polynomial measurement budget, using randomized measurements. Furthermore, I will show that a similar factorization occurs for the gradient of the fidelity with respect of one tensor, allowing for an efficient state learning algorithm. I will then explain such algorithm for pure and mixed states, and present numerical results for ground and thermal states of local Hamiltonians. Finally, I will briefly explain possible verification procedures for the learning algorithm.

Liens :LPMMC

• Séminaire interne LPMMC
• Personne à contacter : Denis Basko ()
• Lieu : G421

Résumé : Quantum simulation of the single-particle tunneling problem by means of the Suzuki-Trotter approximation (STA) is analyzed. The target system describes a particle hopping across a chain of sites with position-dependent potential profile. The latter is assumed to be smooth and to posses several local minima separated by a potential barrier, arranging a tunneling problem between the localized states in different minima. The conducted analysis confirms the naive criteria of applicability max{T,P} ≪ 1/δt (with T, P being the typical scales of kinetic and potential terms, respectively), while also revealing the structure of error and its actual behavior with system parameters. Notably, in certain cases we find an exponential acceleration of tunneling, while other configurations lead to a complete suppression of the latter. Analysis of the case of large Trotter step is also performed, with the main result being the reconstruction of the low-energy spectrum due to coupling between states with energy difference close to 2π/δt. The connection of the obtained results with the rigorous upper error bounds on the STA error is discussed, with particular emphasis on the reasons for the fact that these rigorous bounds are not always saturated. We also point out that the proposed problem can be directly implemented on existing quantum devices arXiv:2012.00921. The talk is based on the recent paper arXiv:2312.04735.

Liens :LPMMC

• Séminaire nano-électronique quantique
• Personne à contacter :
• Lieu : Salle Rémy Lemaire, Institut Néel

Résumé : Graphene possesses exceptional properties that, since its discovery, have attracted wide attention from the scientific and engineering communities. In this talk, I will present a series of experiments via two different approaches, i.e., proximity effect and twist angle design, to induce superconductivity and strong correlations in graphene-based systems—two phenomena that do not intrinsically occur in this material. In the first part of my talk, graphene is flanked by two superconductors and inherits their superconducting properties by proximity effect. A superconductor-graphene-superconductor junction is coupled to a superconducting circuit to create and manipulate the first graphene-based transmon qubit. In the second part of my talk, the electronic properties of graphene-based systems are engineered by controlling the relative twist angle between the atomic planes. In particular, when two graphene sheets are stacked on top of each other near the “magic angle” ≈ 1.1°, nearly flat bands develop, featuring superconductivity and correlated insulating states. I will show that local electrostatic control over the different electronic phases of magic-angle twisted bilayer graphene (MATBG) enables the creation of versatile hyper-tunable quantum devices. I will also present transport, local electronic compressibility, and nano-optics experiments to demonstrate the emergence of exotic electronic phases in MATBG. Last, I will discuss studies of novel 2D moiré systems beyond MATBG.

Liens :Daniel Rodan-Legrain

• Séminaire interne LPMMC
• Personne à contacter : Vincent Rossetto ()
• Lieu : G421

Résumé : Dissipative cat qubits have attracted a lot of interest in the recent years. Their main feature is that their unperturbed dynamics is described by a Lindbladian, not a Hamiltonian. This complicates the analysis of the noise effects on them. In this paper we develop perturbation theory on top of the dissipative cat qubit Lindbladian and evaluate the qubit relaxation rate due to the single photon loss up to the second-order in the perturbation strength. We analytically show that the second-order contribution to the slowest decay rate can become parametrically larger than the first-order contribution. Also, we analyze the detuning perturbation up to the second-order using the same procedure.

Liens :

• Séminaire interne LPMMC
• Personne à contacter : Vincent Rossetto ()
• Lieu : G421

Résumé : We study the phase turbulence of the one-dimensional complex Ginzburg-Landau equation, in which the defect-free chaotic dynamics of the order parameter maps to a phase equation well approximated by the Kuramoto-Sivashinski model. In this regime, the behaviour of the large wavelength modes is captured by the Kardar-Parisi-Zhang equation, which yields universal scaling and statistical properties. We present numerical evidence of the existence of an additional scale-invariant regime, with dynamical scaling exponent $z=1$, emerging at scales which are intermediate between the microscopic, intrinsic to the modulational instability, and the macroscopic ones. We argue that this new regime is a signature of the universality class corresponding to the inviscid limit of the Kardar-Parisi-Zhang equation.

Liens :LPMMC

• Séminaire nano-électronique quantique
• Personne à contacter :
• Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé : The persistence of a magnetic order in a monolayer of van der Waals magnetic material has been established in 2016, offering the perspective to embed a magnetic degree of freedom in heterostructures made of other bidimensional materials such as graphene or light-emitting transition metal dichalcogenides. The physical properties of van der Waals materials can be easily tuned by perturbations like strain or doping, inviting to the exploration of magnetism in two dimensions and its exploitation in novel ultrathin devices. Our approach is to suspend these magnetic materials forming micrometric drum-like resonators. We probe magnetic phase transitions in homo- and heterostructures based on FePS3 and NiPS3, two materials from the transition metal thiophosphates family displaying a zigzag antiferromagnetic order, combining nano-optomechanics to optical spectroscopies. The tuning by strain of their light emission and magnetic properties is also investigated, in particular the photoluminescence of NiPS3. This work opens to the study of proximity effects in van der Waals magnetic heterostructures and their control by strain.

Liens :Arnaud Gloppe

• Cours
• Personne à contacter :
• Lieu : G421

Liens :

• Séminaire théorie
• Personne à contacter : Serge Florens ()
• Lieu : G421

Résumé :

Superconducting (SC) fluctuations, discovered in the late 1960s, have constituted an important research area in superconductivity as they are manifest in a variety of phenomena. Indeed, the underlying physics of SC fluctuations makes it possible to elucidate the fundamental properties of the superconducting state. The interest in SC fluctuation phenomena was further enhanced with the discovery of cuprate high-temperature superconductors (HTSs). In these materials, superconducting fluctuations appear over a wide range of temperatures due to the superconductors extremely short coherence lengths and low effective dimensionality of the electron systems. These strong fluctuations lead to anomalous properties of the normal state in some HTS materials. Within the framework of the phenomenological Ginzburg-Landau theory, and more extensively in the diagrammatic microscopic approach based on BCS theory, SC fluctuations as well as other quantum contributions (weak localization, etc.) enabled a new way to investigate and characterize disordered electron systems, granular metals, Josephson structures, artificial superlattices, and others. The characteristic feature of SC fluctuations is its strong dependence on temperature and magnetic field in the vicinity of the superconducting phase transition. This dependence allows the separation of fluctuation effects from other contributions and provides information about the microscopic parameters of a material, in particular, the critical temperature and the zero-temperature critical magnetic field. As such, SC fluctuations are very sensitive to the relaxation processes that break phase coherence and can be used as a versatile characterization instrument for SCs: Fluctuation spectroscopy has emerged as a powerful tool for studying the properties of superconducting systems on a quantitative level. Here the physics of SC fluctuations is reviewed, commencing from a qualitative description of thermodynamic fluctuations close to the critical temperature and quantum fluctuations at zero temperature in the vicinity of the second critical field. The analysis of the latter allows us to present fluctuation formation as a fragmentation of the Abrikosov lattice. This review highlights a series of experimental findings followed by microscopic description and numerical analysis of the effects of fluctuations on numerous properties of superconductors in the entire phase diagram and beyond the superconducting phase.

1. A. A. Varlamov, A. Galda, and A. Glatz,” Fluctuation spectroscopy: From Rayleigh-Jeans waves to Abrikosov vortex clusters”, Rev. Mod. Phys. 90, 015009 (2018).

2. Anatoly Larkin, Andrei Varlamov, “Theory of Fluctuations in Superconductors” OUP Oxford, (2009), 432 pages.

Liens :

• Séminaire QuantAlps
• Personne à contacter : Laetitia Marty ()
• Lieu : G-1A002 - Ampère Amphitheater, GreEn-ER Building, 21 Avenue des Martyrs

Résumé :

I will discuss: how heat propagates in media, what is the difference between pizza baking in wood oven and in the electric one, why tastes of the boiled meet and the grilled one are so different, how scientifically calculate cooking time of the soft-boiled duck egg and spaghetti, why cin-cin with crystal glasses filled by sparkling wine is not accompanied by nice canorous sound, why barrista varies the coffee beans grounding depending on weather.

1. L.G. Aslamazov, A.A.Varlamov, ”The Wonders of Physics”, WSPC, Singapore, 2012.

2. A. Rigamonti, A.A. Varlamov, J. Villain, "Le Kaleidoscope de la Physique”, Belin, Paris, 2014.

The organizers: Michele Filippone, Laëtitia Marty

Liens :

• Cours
• Personne à contacter :
• Lieu : G421

Résumé : In this informal course, I intend to give an introduction to Tensor Networks and their use for numerical simulations. The class is targeted towards PhD students and researchers with no background on the subject. The goal is for the audience to be able to concretely use TNs in their research at the end of the course. The first session will start with a quick global introduction to tensor networks, and then focus on the one-dimensional Matrix Product States (Tensor Trains). We will go through their properties, strengths and limitations, and give a few explicit examples. We will then study a few concrete time-evolution and groundstate-search algorithms for finite and infinite systems.

Liens :

• Séminaire théorie
• Personne à contacter : Serge Florens ()
• Lieu : G421

Résumé : Generic many-body quantum evolution encodes local information, making them practically inaccessible. The effective information loss underlies thermalisation. Meanwhile, when a quantum object is observed by a macroscopic apparatus, the opposite happens: some classical information becomes widely accessible. Many observers can independently retrieve it from small fractions of the environment. In this talk we shall advocate that the two distinct behaviours are “phases of quantum information”, of which a quantitative definition will be proposed for a general open-system setup. We shall then present toy models that exhibit sharp transitions between those phases. At last we shall discuss speculatively how hard it is to observe these transitions, and conceptual implications of our findings. Refs: https://arxiv.org/abs/2312.04284 https://arxiv.org/abs/2305.03694

Liens :Laboratoire de Physique de l'ENS

• Séminaire interne LPMMC
• Personne à contacter : Vincent Rossetto ()
• Lieu : G421

Résumé : Superconducting circuits allow for the experimental realization of quasi-thermodynamical baths coupled to few non-linear quantum degrees of freedom. Usually one looks at the impact of the bath on the small system, all while neglecting the back-action of the system on the bath. In this work we demonstrate that a driven impurity, namely a small Josephson junction, can induce non-trivial dynamics of the bath, implemented as a chain of 5000 Josephson junctions operating in the harmonic regime. Namely inelastic Cooper pair tunneling in the small junction can populate the bath with a high number of excitations. We show that this is reminiscent of the Joule effect that occurs in usual electrical circuits where a current flowing through a resistor produces heat.

Liens :Institut Néel

• Séminaire interne LPMMC
• Personne à contacter : Vincent Rossetto ()
• Lieu : G421

Résumé : Presentations or seminars on topology often employ the familiar illustration of a mug being transformed into a donut to explain this concept. While this image is effective in conveying how topology concerns itself with global properties of objects, it prompts a lurking question: how does this relate to physics? I aim to address this inquiry by introducing a topological invariant that does not necessitate translation invariance for computation. Additionally, I will provide an intuitive explanation for what transpires in this context: why is it an integer? Why does it become nonzero when the system is topological? And what does it mean for a system to be topological? This discussion will focus on a simple model—the Haldane model (PRL 61, 1988)—and will later extend to another tight-binding model that I have recently been investigating.

Liens :LPMMC

• Séminaire nano-électronique quantique
• Personne à contacter : Jeremie Viennot ()
• Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé : Cavity quantum electrodynamics (QED) uses a cavity to engineer the mode structure of the vacuum electromagnetic field such as to enhance the interaction between light and matter. Exploiting these ideas in solid-state systems has lead to circuit QED which has emerged as a valuable tool to explore the rich physics of quantum optics and as a platform for quantum computation. Despite the demonstration of quantum processor units with hundreds of qubits, there remains significant knowledge to be gained from simpler configurations, such as the one qubit plus one cavity system, as exemplified in the subsequent two experiments presented here. In part I, we introduce a simple approach to further engineer the light-matter interaction in a driven cavity by controllably decoupling a qubit from the cavity\'s photon population, effectively cloaking the qubit from the cavity. This is realized by driving the qubit with an external tone tailored to destructively interfere with the cavity field, leaving the qubit to interact with a cavity which appears to be in the vacuum state. Our experiment demonstrates how qubit cloaking can be exploited to cancel the ac-Stark shift and measurement-induced dephasing, and to accelerate qubit readout. In addition to qubit readout, applications of this method include qubit logical operations and the preparation of non-classical cavity states in circuit QED and other cavity-based setups. In part II, we will look into the quantum thermodynamics of driving a qubit and exploit it to demonstrate a qubit engine. Resonantly driving a qubit introduces a Rabi angle, changing the state of the qubit but also its internal energy. Indeed, there is an energy transfer between the qubit and the ingoing and outgoing fields and thermodynamic work can be gained in the outgoing field. Here, we exploit these energy transfers to demonstrate and characterize a single-qubit quantum engine fueled by qubit-state measurements. Thanks to quantum-non-demolition qubit measurement and real-time active feedback, a thermodynamic cycle is realized, iteratively extracting work from the qubit into the outgoing field and measuring and resetting the qubit to repeat the work extraction process.

Liens :

• Soutenance de thèse
• Personne à contacter :
• Lieu : salle G-421, 3e étage du bâtiment G , campus CNRS

Résumé : The content of the thesis is focused on the exchange of both momentum and angular momentum between the electromagnetic field and matter in the presence of external fields. External fields induce electric and magnetic moments in the medium that can modify the propagation of light, and as a result, the matter can undergo a force as well as a torque. Despite the fact that this field of research has been studied for more than 120 years, there still exist numerous gray areas. I will discuss some of these aspects in this defense.
   I will first focus on the exchange of momentum between matter and the electromagnetic quantum vacuum. In the presence of matter and external fields, the electromagnetic quantum vacuum momentum can be modified, resulting in a force on matter. Such a force has not been observed for now because it is beyond experimental accuracy. We compute the aforementioned force on a Rydberg atom, and we show encouraging results for future experimentations.
   In a second part, I will discuss the exchange of angular momentum when a light source is placed in a disordered magneto-birefringent environment. We show that in this configuration, the system can emit light carrying angular momentum. We study both the back reaction on matter as well as the nature (spin or orbital) of the emitted angular momentum for different regimes of scattering. Finally, we discuss the impact of the detuning of the source with respect to the resonance frequency of the scatterers.
The jury is composed of the following members:

• Marcel Filoche ESPCI, Institut Langevin (Reviewer)
• Carlo Rizzo Laboratoire National des Champ Magnétiques Intenses (LNCMI) Toulouse (Reviewer)
• Léonie Canet LPMMC and Université Grenoble Alpes (Examiner)
• Nicolas Cherroret Laboratoire Kastler-Brossel (Examiner)
• Patrick Bruno European Synchroton Radiation Facility (ESRF) (Examiner)
• Willem Vos University of Twente (UT) (Examiner)

• Marcel Filoche ESPCI, Institut Langevin (Rapporteur)
• Carlo Rizzo Laboratoire National des Champ Magnétiques Intenses (LNCMI) Toulouse (Rapporteur)
• Léonie Canet LPMMC and Université Grenoble Alpes (Examinateur)
• Nicolas Cherroret Laboratoire Kastler Brossel (Examinateur)
• Patrick Bruno European Synchroton Radiation Facility (ESRF) (Examinateur)
• Willem Vos University of Twente (UT) (Examinateur)

Liens :

• Séminaire théorie
• Personne à contacter : Léonie Canet ()
• Lieu : G421

Résumé : Since about 30 years ago, an approximation scheme called "Derivative Expansion" (DE) has been developed within the framework of the Functional Renormalization Group (which is a modern version of the Wilson Renormalization Group). This approximation scheme does not assume that coupling constants are small and has been extremely successful in the study of various problems in several branches of physics, particularly in Statistical Mechanics. In spite of its various empirical successes, this approximation scheme has been repeatedly questioned, being usually labeled as an "uncontrolled" scheme, lacking of an expansion parameter to ensure that successive corrections tend to be smaller. In this talk, relatively recent advances will be presented showing that the success of DE is not accidental but is associated with a "small parameter" (of the order of 1/4) of a very robust and general nature that suppresses successive orders of such an approximation scheme. Such an advance allows, in particular, to establish a priori error bars estimates of the successive orders of the approximation scheme.

Liens :

• Séminaire interne LPMMC
• Personne à contacter : Vincent Rossetto ()
• Lieu : G421

Résumé : We compute the effect of a d-wave magnetization (altermagnetism) on the spectrum of bound states (Andreev levels) in a junction between two s-wave superconductors (gap $\Delta_0$, phase difference $\phi$). Compared to a nonmagnetic junction, the $\phi$-dependence of the spectrum is shifted by an offset $\pm\delta\phi$, dependent on the spin direction, so that the Andreev levels become spin-polarized. In a planar junction, oriented along the crystalline axis of $d_{xy}$-wave symmetry, the excitation energies are determined by the normal-state transmission probability $T$ according to $E=\Delta_0\sqrt{1-T\sin^2\tfrac{1}{2}(\phi\pm\delta\phi)}$. We calculate the corresponding Josephson energy and supercurrent, recovering the 0--$\pi$ transition of related studies.

Liens :LPMMC

• Séminaire LPMMC
• Personne à contacter : Anna Minguzzi ()
• Lieu : G421

Résumé : So far, most of the work aimed at understanding the many-body localization (MBL) problem has focused on the regime of relatively strong interactions. In this talk I will show that the overlooked regime of weak interactions turns out to be quite interesting, in other words: how the Anderson localization affects the MBL problem.

Liens :

• Séminaire QuantAlps
• Personne à contacter : Michele Filippone ()
• Lieu : Salle conférences CNRS Bât A

Résumé :

Applications of learning algorithms using deep neural networks have developed considerably recently, often with spectacular results. The physics of complex quantum systems is no exception, with multiple applications that constitute a new field of research. Examples include the representation and optimization of wave functions of quantum systems with large numbers of degrees of freedom (neural quantum states), the determination of wave functions from measurements (quantum tomography), and applications to the electronic structure of materials, such as the determination of more precise density functionals or the learning of force fields to accelerate molecular dynamics simulations. I will survey some of these applications, with an emphasis on neural quantum states.

The lecture will last 2 hours with a coffee break in beetween (end lecture at ca 12h30).

Liens :

• Séminaire QuantAlps
• Personne à contacter : Michele Filippone ()
• Lieu : Salle conférences CNRS Bât A

Résumé :

Materials in which electrons `work as a team’ in a strongly correlated manner display emergent collective behavior at the macroscopic scale, such as superconductivity, magnetism, or metal-insulator transitions. Materials synthesis is constantly surprising us with new families of such materials. The `standard model’ of solid-state physics, based on electrons occupying bands of individual energy levels, must be seriously revised for strongly correlated materials. Instead, a description accounting for both localized atomic excitations and delocalized wave-like quasiparticles is required. I will review how Dynamical Mean-Field Theory (DMFT) – and more broadly `Quantum Embedding’ methods - fulfills this goal and provides an original physical perspective on strongly correlated electron materials. Thanks to the contributions of a large community over almost three decades, the theory now provides a practical framework to understand and predict the properties of quantum materials starting from their structure and chemical composition. I will discuss in particular how DMFT allowed to identify `Hund metals’ as a large family of materials in which strong correlations emerge for different reasons than in the classic Mott and heavy fermion compounds.

Coffee will be offered at the end of the colloquium in the Salle de convivialité.

Liens :

• Séminaire QuantAlps
• Personne à contacter : Michele Filippone ()
• Lieu : Salle conférence CNRS Bat A

Résumé :

Simplified as it is, the Hubbard model embodies much of the complexity of the `strong correlation problem’ and has established itself as a paradigmatic model in the field. These lectures will emphasize that several key aspects of its physics in two dimensions can now be established beyond doubt, thanks to the development of controlled and accurate computational methods (such as quantum embedding, tensor networks and various forms of quantum Monte Carlo). These methods implement different and complementary points of view on the quantum many-body problem. Along with pushing forward each method, the community has recently embarked into a major effort to combine and critically compare these approaches, and in several instances a consistent picture of the physics has emerged as a result. I will review in this perspective our current understanding of the emergence of a pseudogap in both the weak and strong coupling regimes. I will present recent progress in understanding how the pseudogap phase evolves into an ordered phase at low temperature. The next nearest neighbor hopping t’ plays a key role, with low t’/t favoring stripe order and larger t’/t favoring d-wave superconductivity.

The lecture will last 2 hours with a coffee break in beetween (end lecture at ca 12h30).

Liens :

• Séminaire LPMMC
• Personne à contacter : Sergey Skipetrov ()
• Lieu : G421

Résumé : During the time evolution of many-body systems entanglement spreads rapidly, limiting exact simulations to small-scale systems or small timescales. Quantum information tends, however, to flow towards larger scales without returning to local scales, such that its detailed large-scale structure does not directly affect local observables. This allows for the removal of large-scale quantum information in a way that preserves all local observables and gives access to large-scale and large-time quantum dynamics. To this end, we use the recently introduced information lattice to organize quantum information into different scales, allowing us to define local information and information currents which we employ to systematically discard long-range quantum correlations in a controlled way. Our approach relies on decomposing the system into subsystems up to a maximum scale and time evolving the subsystem density matrices by solving the subsystem von Neumann equations in parallel. Importantly, the information flow needs to be preserved during the discarding of large- scale information. To achieve this without the need to make assumptions about the microscopic details of the information current, we introduce a second scale at which information is discarded while using the state at the maximum scale to accurately obtain the information flow. The resulting algorithm, which we call local-information time evolution (LITE), is highly versatile and suitable for investigating many-body quantum dynamics in both closed and open quantum systems with diverse hydrodynamic behaviors. In this talk, I will present results for the energy transport in the mixed-field Ising model, where we accurately determine the power-law exponent and the energy diffusion constant. Furthermore, I will sketch how we used the LITE approach to reproduce and interpret the results found in a recent experiment with NV centers in diamonds. Finally, I will briefly mention how the information lattice can be employed to obtain insightful results about the spatial structure of entanglement in generic many-body quantum states. References:

• [1] T. Klein-Kvorning, L. Herviou, and J. H. Bardarson, Time-evolution of local information: Thermalization dynamics of local observables, SciPost Phys. 13, 080 (2022).
• [2] C. Artiaco, C. Fleckenstein, D. Aceituno, T. Klein-Kvorning, and J. H. Bardarson, Efficient Large-Scale Many- Body Quantum Dynamics via Local-Information Time Evolution, arXiv:2310:06036.
• [3] K. Harkins et al., Nanoscale engineering and dynamical stabilization of mesoscopic spin textures, arXiv:2310:05635.

Liens :

• Cours
• Personne à contacter : Michele Filippone ()
• Lieu : campus CNRS (Polygone) G-421

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter : Romain Maurand ()
• Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé : The electronic structure of organic molecules can be efficiently tailored through a rational design of molecular size, shape, and atomic structure of the edges. Simple geometrical considerations can also give rise to magnetism in organic molecules. As opposed to the localized nature of magnetic moments in transition metal atoms, unpaired electrons in organic molecules reside in delocalized molecular orbitals, which promotes efficient, tunable, and robust intermolecular magnetic coupling. Combining rational design principles with on-surface chemistry, I will show the generation and characterization of magnetism in individual polyaromatic hydrocarbons by scanning probe microscopy 1, 2. I will further demonstrate the bottom-up fabrication of molecular S = 1 antiferromagnetic quantum spin chains 3. Herein, a systematic study of length-dependent magnetic excitations in both open-ended and cyclic spin chains reveal gapped spin excitations in the bulk, with the gap saturating for sufficiently long spin chains, and fractional S = 1/2 edge states at the chain termini, which manifest as Kondo resonances. It will be shown that these spectral features ratify Haldane’s conjecture for integer-spin antiferromagnetic chains 4, 5, which has not been conclusively demonstrated in experiments. Lastly, I will move toward the study of magnetic molecules on insulating surfaces. Here, the tip-induced synthesis of an elusive organic molecule, namely indeno[1,2-a]fluorene, will be demonstrated on ultrathin insulating NaCl films on (111) coinage metal surfaces 6. The molecule exhibits ground state bistability, wherein it can be stabilized either in a magnetic or a non-magnetic state, and can be switched between the two states simply by changing its adsorption site on NaCl. References : - 1 S. Mishra et al. Nature Nanotechnology 15, 22–28 (2020) -2 S. Mishra et al. Nature Chemistry 13, 581–586 (2021)- 3 S. Mishra et al. Nature 598, 287–292 (2021) -4 F. D. M. Haldane, Physical Review Letters 50, 1153–1156 (1983) - 5 I. Affleck et al. Physical Review Letters 59, 799–802 (1987) 6 S. Mishra et al. Nature Chemistry (2023, in press).

Liens :

• Cours
• Personne à contacter : Michele Filippone ()
• Lieu : campus CNRS (Polygone) G-421

Résumé : Recent experimental developments in areas - ranging from cold atomic gases over light driven quantum materials to NISQ platforms - move systems into the focus. These systems realize instances of driven open quantum matter: Coherent and driven-dissipative quantum dynamics occur on an equal footing, and they are operated in the thermodynamic limit. They are thus located on the interface of quantum optics, condensed matter physics and statistical mechanics. In the first lecture, we will develop the tools to understand such systems based on a field theory approach to the many-body Lindblad equation, which is particularly well suited to perform the transition from microscopic physics to macroscopic observables, thereby distilling universal aspects of such setups. In the second lecture, we will argue that drive and dissipation need not to act destructively on quantum mechanical correlations such as phase coherence, entanglement or topological order, but can even be used to create them. We will concentrate on topology, and apply the field theory framework to demonstrate

• (i) the universality of topological response, being identical in ground- and dissipatively induced topological states, and
• (ii) a dynamical fine structure in the Altland-Zirnbauer symmetry classification, distinguishing equilibrium from non-equilibrium generators of dynamics.

Liens :

• Séminaire théorie
• Personne à contacter : Adolfo Grushin ()
• Lieu : G421

Résumé : Random vacancies in a graphene monolayer induce defect states that are known to form a narrow impurity band centered around zero energy at half-filling. We use a space-resolved formulation of the quantum metric and establish a strong enhancement of the electronic correlations in this impurity band. The enhancement is primarily due to strong correlations between pairs of vacancies situated on different sublattices at anomalously large spatial distances. We trace the strong enhancement to both the multifractal vacancy wave functions, which ties the system exactly at the Anderson insulator transition for all defect concentrations, and preserving the chiral symmetry.

Liens :

• Séminaire LPMMC
• Personne à contacter : Vincent Rossetto ()
• Lieu : G421

Résumé : Below the so-called spin-glass temperature, the free state of the Ising model on a regular tree is not extremal. Moreover, its extremal decomposition is a continuous measure, supported on (uncountably many) inhomogeneous extremal states, which have some kind of « glassy » feature. I will present some ideas behind a proof of this result, which provides explicit concentration bounds on ”branch overlaps”, playing the role of an order parameter for typical extremals. The proof extends quite generally to ferromagnetic finite-spin models, even slightly asymmetric (i.e. where small inhomogeneous field terms are added), which shows that the above behaviour is generic on regular trees. This is a joint work with Christof Külske (Bochum) and Arnaud Le Ny (Paris-Est Créteil).

Liens :Loren CoquilleInstitut Fourier, Grenoble

• Séminaire nano-électronique quantique
• Personne à contacter : Jeremie Viennot ()
• Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé : Bosonic modes provide a hardware-efficient alternative to qubit-based quantum information processing. However, achieving universal control on bosons requires access to a nonlinearity, or to resourceful non-Gaussian quantum states like cubic phase states. Superconducting microwave circuits offer such strong nonlinearities but face other challenges, like parasitic state distortion due to the Kerr effect and shorter coherence times. In this talk, we will demonstrate how these difficulties can be overcome. We harness the 3rd order non-linearity of a SNAIL (Superconducting Nonlinear Asymmetric Inductive eLement) dipole terminated resonator through simultaneous flux and charge pumping to obtain the desired cubic state, 45 times faster than decoherence. In parallel, we minimize the 4th order Kerr effect by adjusting the flux DC bias. Achieving this required meticulous pulse calibration and circuit modeling. We will delve into the details of these processes and discuss how our simulation efforts shed light on the primary causes of infidelity in our current experimental setup.

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter : Jeremie Viennot ()
• Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé : In the past decade superconductor-semiconductor hybrids have been studied intensively, with significant efforts towards studying Majorana bound states (MBSs). In this talk I will discuss a relatively new approach to realize MBSs using quantum dot-superconductor hybrids. I will focus on how MBSs can be systematically and reliably engineered in a two-dimensional electron gas by tuning the relative strengths of the different kinds of couplings between the quantum dots [1,2]. Ref.: [1] Triplet correlations in Cooper pair splitters realized in a two-dimensional electron gas; https://www.nature.com/articles/s41467-023-40551-z [2] Engineering Majorana bound states in coupled quantum dots in a two-dimensional electron gas; https://arxiv.org/abs/2311.03208

Liens :

• Séminaire QuantAlps
• Personne à contacter : Michele Filippone ()
• Lieu : Salle de Conférence du LNCMI

Résumé :

Triangular lattices are uncomfortable stages for interacting electrons, however, which bring about emergent states. In half-filled band systems, antiferromagnetically interacting spins are strongly frustrated and may exhibit a quantum spin liquid (QSL). In quarter-filled band systems, Coulomb interacting electrons fail to form a Wigner crystal on a triangular lattice but may freeze into a charge glass (CG) state, which even may quantum　melt. The organic k-ET salts and q-ET salts are good model systems for the former and latter subjects, respectively. I present our updated results on these two issues.

For the issue of QSL, I summarize the present experimental status on the QSL candidate, k-(ET)₂Cu₂(CN)₃, and also show unconventional properties (non-Fermi liquidity, quantum criticality, BEC-like superconductivity) in the doped QSL candidate, k-(ET)₄Hg_2.89Br₈. For the issue of CG, I show quantum-classical dual properties of glasses exhibited by a series of q-ET₂X.

Liens :

• Séminaire théorie
• Personne à contacter : Serge Florens ()
• Lieu : G421

Résumé : We study the effect of quantum fluctuations on the parametric instability of a degenerate parametric oscillator at threshold. Relying on a weak nonlinearity, we identify an (effective) universal Liouvillian consistent with the symmetries of the system that captures the slow (long-time) dynamics. We find that all cumulants exhibit universal power-law scaling with the nonlinearity, with the Fano factor showing a maximum near the threshold. For a voltage biased Josephson junction, the method of third quantization method is used to identify the slow modes and to derive the effective dynamics by adiabatic elimination of the fast modes. In this way, the parameters entering the effective model can be linked to the microscopic parameters of the experimental platform. Our findings offer insights into the oscillator\'s behavior and provide a foundation for understanding and predicting the parametric instability at threshold.

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter : Jeremie Viennot ()
• Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé : Beyond their applications in quantum computing, superconducting qubits are a powerful platform to probe various quantum phenomena in the context of hybrid quantum systems [1]. However, most of them are confined to the GHz frequency domain, limiting the class of systems they can interact with. Building upon the heavy fluxonium architecture introduced by ref. [2], we have developed a superconducting qubit with an unprecedentedly low transition frequency of 1.8 MHz [3]. Notably, we have demonstrated a qubit with a coherence time exceeding 30 μs, a sideband cooling scheme to prepare the qubit in a pure state with 97.7% fidelity, and single-shot readout capability. Moreover, by detecting a weak charge modulation by repeated qubit interrogation, we demonstrate the high-sensitivity of this qubit architecture to a nearly resonant AC-charge drive, proving its potential in a hybrid circuit scenario. We will finally present our recent efforts to achieve the strong coupling regime between this qubit and an ultra-coherent softly-clamped mechanical membrane. Ref.: [1] Y. Chu et al. Nature 563, 666 (2018). [2] H. Zhang et al. Physical Review X 11, 011010 (2021). [3] Najera et al. arXiv:2307.14329 (2023). In review at PRX.

Liens :

• Séminaire théorie
• Personne à contacter : Serge Florens ()
• Lieu : G421

Résumé : The strong coupling picture of twisted bilayer graphene provides a framework for understanding the correlated insulators observed in experiments. In this picture, the insulators are generalised quantum Hall ferromagnets which break certain symmetries. However, this picture is known to be incomplete and certain features are incompatible with experiments. In particular, recent scanning tunnelling have observed a state that breaks moiré translational symmetry. I will show that incorporating strain into a Hartree-Fock study of twisted bilayer graphene results in precisely the state observed in experiment. On the other hand, a state that respects moiré translational symmetry has been observed in low strain devices. Reconciling this observation with the model requires an additional ingredient, namely a specific phonon mode, the so-called K-phonon. I will investigate whether this phonon mode could be responsible for superconductivity.

Liens :Zurich University

• Séminaire théorie
• Personne à contacter : Adolfo Grushin ()
• Lieu : G421

Résumé : Prof. Bardarson, invited CPTGA professor, will deliver a two session lecture touching on contemporary topics in disorder, topological and out-of equilibrium phenomena. This is the announcement of the second session, which will be split into two lectures, 45 min each, with a 15 min break in between. Coffee and pastries will be served during the break, and food and refreshments will be served to conclude the lecture series. The format is mainly directed to PhD students and postdocs, but senior researchers are of course welcome.

Liens :

• Séminaire LPMMC
• Personne à contacter : Bart van Tiggelen ()
• Lieu : G421

Résumé :

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter : Robert Whitney ()
• Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé : Bosonic codes encoded using superconducting circuits offer a promising approach towards quantum error correction. Using continuous-variable systems, such as a harmonic oscillator realized in superconducting circuits, these codes compactly encode the quantum information in multi-photon states of a memory mode. Specifically, we use cat qubits for which the logical states 0 and 1 are two coherent states of a harmonic oscillator with opposite phases. These states are stabilized by leveraging dissipation to our advantage so that photon exchanges between the harmonic mode and its environment predominantly occur in pairs. In this way, "bit-flip" errors are exponentially suppressed as a function of the number of photons contained by the mode, at the modest cost of a linear increase in "phase-flip" errors. These errors could then be corrected by an additional layer of correction, such as a repetition code of cat qubits. At the heart of this thesis work is the introduction of a self-parametric superconducting circuit that non-linearly couples a mode containing the cat qubit to a dissipative mode whose frequency is set to twice that of the cat mode. Unlike previous implementations, this passive coupling does not require a parametric pump and achieves a high two-photon dissipation rate of around 2.16 MHz. Bit-flip errors are then avoided for a characteristic period of up to 0.3 s, with a moderate impact on phase-flip errors. In addition, we demonstrate universal control of this qubit using the two-photon dissipation to implement X, Y, and Z logic gates of arbitrary angles.

Liens :

• Soutenance de thèse
• Personne à contacter :
• Lieu : G421

Liens :

• Séminaire théorie
• Personne à contacter : Adolfo Grushin ()
• Lieu : G421

Résumé : Prof. Bardarson, invited CPTGA professor, will deliver a two session lecture touching on contemporary topics in disorder, topological and out-of equilibrium phenomena. This is the announcement of the first session, which will be split into two lectures, 45 min each, with a 15 min break in between. Coffee and pastries will be served during the break. The format is mainly directed to PhD students and postdocs, but senior researchers are of course welcome. Second lecture: Friday 17/11/23

Liens :

• Séminaire LPMMC
• Personne à contacter : Pierre Nataf ()
• Lieu : G421

Résumé : Extreme value theory is best known to predict disasters, for example in hydrology to anticipate floods or in epidemiology to quickly identify emerging diseases. Here, I would like to describe how it gives insight into some non-trivial effects in random spin chains. Indeed, despite a very good understanding of single-particle Anderson localization in one-dimensional quantum disordered systems, many-body effects are still full of surprises, a famous example being the interaction-driven many-body localization (MBL) [1, 2] in the random-field Heisenberg chain, whose non-interacting limit already shows non-trivial multiparticle physics, which allows to probe some general mechanisms using large-scale exact diagonalization. Here, I will focus in particular on a chain breaking mechanism occurring in the XX and the Heisenberg spin chains in a random field from an extreme value statistics perspective [3]. Supported by state-of-the-art numerical simulations at infinite temperature, this analysis leads to the striking observation of a sharp "extreme-statistics transition" in the Heisenberg chain as the disorder changes, which may coincide with the recently debated MBL transition.

References

1. F. Alet and N. Laflorencie, Many-body localization: An introduction and selected topics, Comptes Rendus Physique 19, 498 (2018).
2. D. A. Abanin, E. Altman, I. Bloch, and M. Serbyn, Many-body localization, thermalization, and entanglement, Rev. Mod. Phys. 91, 021001 (2019).
3. J. Colbois and N. Laflorencie, Breaking the chains: extreme value statistics and localization in random spin chains, arXiv:2305.10574 (2023).

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter : Jeremie Viennot ()
• Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé : Variational quantum algorithms and quantum machine learning are considered to be promising candidates for achieving a quantum advantage on noisy intermediate-scale quantum (NISQ) computers, which are limited by decoherence and gate errors. In such a setting, the quality of the result of an algorithm strongly depends on the fidelity and duration of the applied quantum logic gates, but also on the so-called circuit depth, i.e., the number of subsequent gates required to execute the algorithm. A versatile hardware-native gate set, including, e.g., a continuous set of two-qubit gates, can thus improve the performance of noisy intermediate-scale quantum computing by reducing the circuit depth [1]. After revisiting some fundamentals of superconducting transmon qubits, this talk presents a hardware-efficient implementation of a continuous set of controlled-phase gates, parameterized by the conditional phase. The approach is an extension of the controlled-phase gate from [2] and is based on the resonant interaction between two flux-tunable transmons. In this implementation, an arbitrary conditional phase can be achieved by tuning a single pulse parameter, and the vanishing time integral of the employed net-zero control pulses [2] provides robustness against memory effects stemming from long-time distortions in flux control lines. Furthermore, by activating the gate via flux control of both qubits, we demonstrate that the gate can be performed between far-detuned qubits, strongly suppressing residual interactions when the gate is off. We characterize the continuous gate set with cross-entropy benchmarking for fixed values of the conditional phase and for phases randomly drawn from a uniform distribution, confirming a consistently high gate fidelity over the full range of conditional phases. Ref.: [1] Lacroix et al., PRX Quantum 2020. [2] Negirneac et al., PRL 2021.

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter : Jeremie Viennot ()
• Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé : What are the thermodynamic laws governing the energy flows and fluctuations of light interacting with quantum systems? Despite the widespread use of light in science, the thermodynamics of the fluctuations of photons at the nanoscale is still ill understood. A powerful framework to study energy fluctuations in small, complex, and far from equilibrium systems has emerged in the past two decades, stochastic thermodynamics (1), which was successfully used for electronic, colloidal and biological systems. One of the main achievements of stochastic thermodynamics is the derivation of fluctuation theorems, expressed as a symmetry relating entropy or energy fluctuations generated during a given forward process with the fluctuations of the time reversed process. Fluctuation theorems imply that, remarkably, regardless of the complexity of the system, the dynamics are constrained to obey a universal symmetry. The even more recent field of quantum stochastic thermodynamics aims at extending these ideas to quantum systems, in order to account for coherent effects and entanglement, but little has been done for light-matter interactions. In this talk, I will present our recent results, in the direction of building a framework for the thermodynamics of light-matter interactions at the fluctuating level. Central to the theory of stochastic thermodynamics is the notion of thermodynamic consistency: every thermodynamic quantity obtained from the stochastic dynamics should have a thermodynamic interpretation which is justified at the microscopic scale. On the other hand, a widespread approach to open quantum systems is to derive quantum master equations describing the reduced dynamics of a quantum system by tracing out its environment. A thermodynamically consistent quantum master equation should therefore preserve the symmetries of the fluctuation theorems. The first main result (2) of the talk is the identification of a new generalized quantum detailed balance condition which quantum master equations must satisfy in order to be thermodynamically consistent. We then focus of the case of coherent energy exchanges between lasers and atoms. The second main result (3) is the derivation of a fluctuation theorem for the energy transferred from a laser to an atom, valid even in the strong light-matter coupling regime. We then examine the thermodynamic consistency of master equations describing the dynamics of an atom coherently driven by a laser – the optical Bloch master equation and the Floquet master equation. We find that the Floquet master equation is fully consistent, i.e., satisfies the first and second laws of thermodynamics at the fluctuating level, while the Bloch equation is only consistent at the average level. Ref.: (1) U. Seifert. “Stochastic thermodynamics, fluctuation theorems and molecular machines”. Reports on progress in physics, 75(12):126001 (2012) (2) A. Soret, V. Cavina, M. Esposito. “Thermodynamic consistency of quantum master equations”, Phys. Rev. A, 106:062209 (2022) (3) A. Soret, M. Esposito. “Thermodynamics of coherent energy exchanges in quantum optics”. (draft in preparation).

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter : Jeremie Viennot ()
• Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé : In a tunnel junction, electrons cross randomly the insulating barrier in very short times. This generates a broadband electromagnetic field. Quantum mechanics adds an ingredient to this: the voltage bias V imposed on the electrons translates into a correlation time h/eV in the current fluctuations and the generated field. With a time-resolved experiment we show how these correlations occur within a single cycle of an ac excitation at 4 GHz. This allows to generate squeezing and entanglement between far apart frequencies in the 10GHz range. In a broadband signal however, frequency is not a very relevant concept. We have developed an experiment to analyze the statistics of bicolor or broadband photons. From this we show that a tunnel junction excited at 12 GHz generates squeezed vacuum over the 3-9 GHz bandwidth. Our setup could be used to analyze any signal in the 1-10GHz range.

Liens :

• Séminaire LPMMC
• Personne à contacter : Léonie Canet ()
• Lieu : G421

Résumé : Soft matter systems are often governed by processes on multiple different time and length scales. Bridging the gap between these scales and numerically study emergent behavior of many-body systems thus usually requires the usage of coarse-grained models. In general situations in which the separation of time scales is incomplete, such coarse-grained models will feature non-Markovian memory effects and colored noise, as suggested by the Mori-Zwanzig formalism. In equilibrium systems, a plethora of methodologies have been derived to construct non-Markovian coarse-grained models. Their applicability, however, becomes questionable in non-equilibrium systems. In this talk I will give an introduction to non-Markovian modeling and provide examples for successful applications. Subsequently, I will explain why the presented methodology cannot be directly applied to non-equilibrium systems. At the end, I will present my personal roadmap towards developing theoretical foundations and data-driven coarse-grained models for out-of-equilibrium soft matter systems.

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter : Jeremie Viennot ()
• Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé : [Beware of the unusual day/time: Monday 11am !]. The recent discovery of intrinsic supercurrent diode effect (1), and its prompt observation in a rich variety of systems, has shown that nonreciprocal supercurrents naturally emerge when both space- and time-inversion symmetries are broken. I will report on both dc and ac manifestations of the Josephson diode effect in the non-linear inductance in planar Josephson junctions, based on a ballistic Al/InAs-heterostructure that is exposed to an in-plane magnetic field Bp (2). At low Bp a non-reciprocal term is found in the inductance that is linear in Bp. At higher Bp a sign reversal of the magnetochiral term is observed that can be traced back to a 0-pi-like transition in the current-phase relation (3). Different avenues for a theoretical interpretation are discussed. As pronounced date tunability of both the phi_0 shift and the diode efficiency in an asymmetric SQUID device demonstrates that Rashba spin-orbit interaction provides a substantial contribution to the Josephson diode effect (4). Ref.: [1] F. Ando et al., Nature 584, 373 (2020). [2] C. Baumgartner et al., Nature Nanotech. 17, 39 (2022). [3] C. Baumgartner et al., Nature Nanotech. (2023). [4] S. Reinhardt et al., arXiv.2308.01061

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter : Robert Whitney ()
• Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé : Proximity induced superconductivity in a normal conductor is a rich field of experimental and theoretical investigations in many systems. In the last decade, it has been particularly at the heart of the quest for realizing topological modes in hybrid superconductor-nanowire nanodevices. Yet surprisingly, it turns out that there was a clear lack of investigations in simple systems. We therefore took on to investigate an elemental nanowire in the 1D limit (an ultra-clean carbon nanotube) coupled to a superconducting lead. We observe for the first time a long standing prediction of random matrix theory (RMT), dating back to 2001, that mesoscopic fluctuations of the mini-gap in a conductor follow a universal distribution (1). The statistical distribution of the mini-gap recorded over 60 consecutive charge states in our device shows a universal behavior with a transition when time reversal symmetry is broken, as predicted by RMT. Interestingly, mesoscopic fluctuations of the minigap were precisely predicted to lead to ubiquitous nontopological edge states clustering towards zero energy. We do indeed observe ubiquitous and robust zero bias conductance peaks in our device that cannot host topological modes by design, as expected by RMT. The RMT predictions that we confirm are very general and must be present in any system showing disorder, even if it is weak. It therefore unambiguously calls for alternative probes than transport measurement to identify Majorana modes in 1D systems. Microwave photons in a cavity are a promising powerful platform (2) that I will discuss. Ref.: (1) L. C. Contamin et al., Nature Communications 13, 6188 (2022). (2) L. C. Contamin et al., Npj Quantum Inf. 7, 171 (2021).

Liens :

• Séminaire théorie
• Personne à contacter :
• Lieu : G421

Résumé : Self-binding in Bose-Bose mixtures has received lots of theoretical [1] and experimental [2] attention in the recent years, and a few studies also discussed Bose-Fermi droplets [3]. Fermi-Fermi mixtures with zero-range interspecies attraction, however, are not expected to display self-bound states, since the fermions of one species should overcome a strong Pauli pressure to bind the fermions of the other. This repulsion is, in fact, the fundamental mechanism that provides stability of Fermi mixtures along the BCS-BEC crossover, in which the dimers repel and do not form larger clusters [4]. In our work [5], we find that a 1D Fermi-Fermi mixture with sufficiently large mass imbalance can form a self-bound state in the thermodynamic limit. This result elaborates our previous few-body analyses [6], and is based on a mean-field theory in which the heavy fermions are described within the Thomas-Fermi approximation, which is exact in the limit of large mass ratios. We are also extending our theory towards the 2D case, which is complicated by the same scaling with length of the kinetic energy and of the interaction energy of the system [7]. Our work sets the basis for understanding liquid-like states in fermionic gases.

References

• [1] D. S. Petrov, Quantum Mechanical Stabilization of a Collapsing Bose-Bose Mixture, Phys. Rev. Lett. 115, 155302 (2015).
• [2] C. R. Cabrera, et al., Quantum liquid droplets in a mixture of Bose-Einstein condensates, Science 359, 301 (2018).
• [3] D. Rakshit, et al., Self-bound Bose–Fermi liquids in lower dimensions, New J. Phys. 21, 073027 (2019); T. Karpiuk, et al., Bistability of Bose–Fermi mixtures, New J. Phys. 22, 103025 (2020).
• [4] D. S. Petrov, C. Salomon, and G. V. Shlyapnikov, Weakly bound dimers of fermionic atoms, Phys. Rev. Lett. 93, 090404 (2004).
• [5] J. Givois, A. Tononi, and D. S. Petrov, Self-binding of one-dimensional fermionic mixtures with zero-range interspecies attraction, arXiv:2207.04742 , accepted for publication on SciPost Physics.
• [6] A. Tononi, J. Givois, and D. S. Petrov, Binding of heavy fermions by a single light atom in one dimension, Phys. Rev. A 106, L011302 (2022).
• [7] J. Givois, A. Tononi and D. S. Petrov, in preparation (2023).

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter : Romain Maurand ()
• Lieu : Rémy Lemaire K223, Institut Néel

Résumé : I will report on an extensive search of the AC Josephson effect of superconducting junctions and weak links obtained from various materials in a low-ohmic environment allowing for DC bias. The materials are two-dimensional graphene, Al proximitized InAs quantum wells, the Dirac semimetal cadmium arsenide, the Weyl semimetal tungsten telluride WTe2, the 3D topological insulator HgTe, InAs nanowires and carbon nanotubes, as well as conventional Al-based reference Josephson junctions. We have studied AC Josephson emission, Shapiro steps, current-phase relations and Fraunhofer patterns to address the current distribution. We can observe missing Shapiro steps, but there is no AC Josepshon signal appearing concurrently at frequency eV/h, as expected for topological junctions. However, we find strong higher order Josephson terms that go with frequency as 2neV/h with n=1,2,3... These terms stem from simultaneous inelastic tunneling of n Cooper-pairs. The relation to the skewness of the CPR will be addressed. I would also like to stress that we observe missing Shapiro steps even in standard conventional Al-Al-oxide-Al Josephson junctions. Hence, missing odd Shapiro steps cannot serve as a signature for topological superconductivity, although this signature has been used multiple of time for the in the concurrent literature.

If time permits, I would also like to discuss current-phase relation measurements (CPR) in WTe2, which has been predicted to be a higher-order topological insulator. We show that one has to be careful in the interpretation of seemingly 4pi-periodic multivalued CPR data, since also this signature can be generated by spurious, but large, inductances that notoriously appear in contact regions due to alloying effects.

This work was done in collaboration with many colleagues over the last six years. I am grateful to them:(alphabetic order of group members participating in these studies) A. Baumgartner, C. Ciaccia, M. Endres, G. Fülöp, R. Haller, D. Indolese, R. Jha, M. Jung, A. Kononov, P. Karnatak, P. Kumbhakar, F. Oppliger, M. Osterwalder, J. Ridderbos, C. Schönenberger, D. Sufra, L. Wang, D. Weiss, H. Zheng.

Liens :

• Séminaire LPMMC
• Personne à contacter : Vincent Rossetto ()
• Lieu : G421

Liens :

• Séminaire théorie
• Personne à contacter :
• Visio-conférence Zoom hybride.
  En présentiel : G 421
  Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 99884767935
  

Résumé : Recent experimental studies of transport in strongly disordered Indium Oxide films revealed an unusual first-order phase transition between superconducting and insulating state, with the jump of kinetic inductance of the system at the transition. In the present work we propose interpretation of this transition as the transition between superconducting state built on top of Cooper pairs localized due to disorder present in the system, and a Coulomb-glass type insulator with developed Coulomb gap. To describe this transition analytically, we present a theoretical model formulated in terms of Anderson pseudospins. Utilizing mean-field Parisi replica-symmetry breaking scheme, we demonstrate the existence of a region in the parameter space where both phases are locally stable w.r.t. small fluctuations, which is the necessary condition for the first order phase transition. Finally, we provide estimates on the free energy of both phases and the position of the transition itself. The talk will start from an extensive exposition of the SIT subject, which has a long history. The seminar can also via followed via zoom https://univ-grenoble-alpes-fr.zoom.us/j/99884767935?pwd=dmpSVGVFZ2I5WkQyNEFLN2t1dVhEQT09 (Meeting ID: 998 8476 7935, Passcode: 630179)

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter : Robert Whitney ()
• Lieu : F418, Institut Néel

Liens :

• Séminaire théorie
• Personne à contacter : Serge Florens ()
• Lieu : G421

Liens :

• Séminaire interne LPMMC
• Personne à contacter : Robert Whitney ()
• Lieu : G421

Résumé : A round-table discussion about academic careers in theoretical physics, led by members of the LPMMC staff. It is intended for PhDs/Postdocs of LPMMC, but theoretical physics PhDs/postdocs from other labs are welcome. The discussion will start with a presentation on the practical aspects: when to apply for CNRS and University jobs (France and abroad), what juries are looking for, etc. It will then move on to discuss how you judge if a academic career is suitable for you, and how to maximize your chances.

Liens :Robert WhitneyLPMMC

• Séminaire théorie
• Personne à contacter : Adolfo Grushin ()
• Lieu : G 421

Résumé : Almost two decades after the theoretical formulation of the quantum spin Hall insulating phase (QSHI), the number of real two-dimensional (2D) QSHI materials which have been confirmed experimentally is relatively limited, often exhibiting poor performances. Indeed, the intrinsic robustness advocated by topological protection can still suffer by the presence of defects or temperature effects, as most real QSHI can be easily perturbed into a metallic or trivial state. In addition, theoretical predictions of QSHIs can be very sensitive to the accuracy of the electronic-structure methods employed. Hence, fundamental research and potential technological applications of QSHIs are hindered both by the rarity of high-performance topological materials and by the lack of predictive modelling in complex settings, ranging from strong correlations to disorder, to finite temperature. In this talk, I will first present some achievements in the discovery and design of novel QSHI with first-principles simulations. In particular, I will discuss our discovery of jacutingaite, a naturally-occurring dual topological insulator made by potentially-exfoliable monolayers. 2D jacutingaite realizes graphene’s Kane-Mele model with a large band gap and a nice interplay between spin-orbit coupling, crystal-symmetry breaking, and dielectric response. Beyond materials discovery, I will provide an example of materials design and discuss our prediction of robust ferroelectric QSHI states in van-der-Waals heterobilayers made by two non-topological monolayers.  Finally, I will present more recent efforts towards ab-initio modelling of disorder and temperature in QSHIs. In particular, I will introduce single-point and space-resolved frameworks to calculate the Z2 topological invariant and other geometrical quantities for non-crystalline systems. These latest efforts are being released in a dedicated software package, SPInv, which is designed to work both with model Hamiltonians and first-principles simulations, operating in the Wannier function software ecosystem.

Liens :

• Séminaire LPMMC
• Personne à contacter : Pierre Nataf ()
• Lieu : G421

Résumé : Over the last decades, tensor network methods have emerged as the one of the most powerful numerical methods for tackling the many-body problem in quantum physics. In this talk, we will review the core principles of tensor network and their applications in condensed matter physics. We will focus on two dimensional systems and discuss simulating strongly correlated systems at thermal equilibrium. In such systems, implementing efficient imaginary time evolution and tensor network contraction proves challenging due to the requirement of very large bond dimensions to maintain physical relevance at low temperatures. In addition, it is essential to preserve good numerical accuracy to avoid unphysical symmetry breakings that may hide important physics. The solution for both issues is to incorporate non-abelian symmetry directly at the tensor level, which yields substantial improvements in performance and precision. To illustrate the potential of this approach, we will present results on the spin-1/2 Heisenberg model on the square lattice with nearest-neighbor coupling J1 and next-nearest coupling J2 (J1-J2 model) at finite temperature [1]. We will introduce several ways to detect the thermal phase transition occurring in this model and generalize them to related models.

Reference

Liens :Olivier GauthéCMTC, EPFL

• Séminaire nano-électronique quantique
• Personne à contacter :
• Visio-conférence Zoom hybride.
  En présentiel : Salle Nevill Mott, D420, Institut Néel
  Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 91808901596
  

Résumé : The Kondo effect, deriving from a local magnetic impurity mediating electron-electron interactions, constitutes a flourishing basis for understanding a variety of intricate many-body problems. Its experimental implementation in tunable circuits has made possible advances through well-controlled investigations. However, these have mostly concerned transport properties, whereas thermodynamic observations - notably the fundamental measurement of the spin of the Kondo impurity - remain elusive in test-bed circuits. In this talk, I will present how we directly observe the state of the impurity and its progressive screening with a novel combination of a "charge" Kondo circuit and a charge sensor. We establish the universal renormalization flow from a single free spin to a screened singlet, the associated reduction in the magnetization, and the relationship between scaling Kondo temperature and microscopic parameters.

Liens :

• Séminaire théorie
• Personne à contacter :
• Lieu : G421

Résumé : Superconductivity in the limit of a vanishing bandwidth in isolated bands is a classic example of a non-perturbative problem, where BCS theory does not apply. What sets the superconducting phase stiffness, and relatedly the transition temperature, in this limit is of both fundamental and practical interest. This question has become especially relevant with the discovery of superconductivity in moiré materials. I will begin by examining critically the relevance of various proposed bounds on the superconducting transition temperature and propose a non-perturbative upper bound on the integrated optical spectral weight for partially filled electronic flat bands with generic density-density interactions. I will also present numerically exact results for the interplay between superconductivity and various competing orders in models of interacting flat-bands.

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter :
• Visio-conférence Zoom hybride.
  En présentiel : Salle Rémy Lemaire K223, Institut Néel
  Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 91808901596
  

Résumé : Diodes are key elements for electronics, optics, and detection. Their evolution towards low dissipation electronics has lead to the hybridization with superconductors (S) and the realization of non-reciprocal transport of both quasiparticles and Cooper pairs. That occurs when both spatial inversion and time-reversal symmetries are broken. Here, we review both effects comparing their efficiencies and basic principles. The quasi-particle diode is a superconducting tunnel junction with zero conductance in only one direction. The directionselective propagation of the charge has been obtained through the broken electron-hole symmetry induced by the spin selection of a ferromagnetic tunnel barrier made of a EuS thin film separating a superconducting Al and a normal metal Cu layer. It achieves a large rectification of up to 40%. On the other hand, supercurrent diodes made with hybrid S/spinorbit/ S Josephson Junctions or with two-dimensional Rashba superconductors have been demonstrated to show zero resistance in only one direction. We describe the equation of the supercurrent diode effect in a generic formalism that may inspire novel devices based on helical magnetism induced in conventional superconductors. lien zoom: https://univ-grenoble-alpes-fr.zoom.us/j/91808901596?pwd=UWZ2cml2N1VBOEZBenk0d3RJek9rdz09

Liens :Maria SpiesCNR NANO (Istituto Nanoscienze)

• Séminaire QuantAlps
• Personne à contacter : Michele Filippone ()
• Lieu : Salle René Pauthenet J229, bâtiment J, LNCMI

Résumé : Since the beginnings of high temperature superconductivity in the cuprates, a key theoretical question has been whether the single band Hubbard model and its cousin, the t-J model, describe the superconductivity at least qualitatively. While initially this seemed like a simple question, the physics of both the cuprates and the models is now known to be much more complicated. A key complication is the presence of spontaneously formed striped arrangements of holes, which have been argued (by different people!) to either enhance or to suppress superconductivity. The models are very challenging to simulate, but even harder to treat analytically. In this talk I will present new simulation results from both DMRG and quantum Monte Carlo which are closing in on the ground state phase diagrams of these iconic models.

Liens :

• Séminaire interne LPMMC
• Personne à contacter : Vincent Rossetto ()
• Lieu : G421

Résumé : External driving fields acting on qubits have been proven a useful tool in quantum simulation, both to improve coherence and to effectively engineer hamiltonian evolutions not accessible with the underlying quantum hardware. We propose a protocol to engineer 1) arbitrary two-body hamiltonians and 2) a universal set of gates with long-range connectivity, using a qubit system subject to a static dipolar Hamiltonian and local driving fields. Our approach relies on the circuit decomposition of the quantum dynamics, and the parametrization of the time dependence of the driving fields with Walsh functions, which allows to sequentially implement the various hamiltonian terms on specific subsystems. Furthermore, it can easily incorporate global pulse sequences to reduce decoherence, for which robustness criterion have been studied. Our proposal can be readily implemented in various quantum technology platforms, e.g. in trapped ions, or dipolar Rydberg atom arrays with qubits encoded in the hyperfine levels. We illustrate this toolbox with two examples: 1) a quantum simulation protocol for spin models and 2) the realization of surface and toric codes.

Liens :LPMMC

• Séminaire nano-électronique quantique
• Personne à contacter : Jeremie Viennot ()
• Lieu : E424

Résumé : Hybrid superconductor-semiconductor nanowires came into focus due to their promise for realizing Majorana modes. They were also tried in quantum circuits such as transmon qubits and parametric amplifiers. The same factors that make them interesting for these applications are also associated with a host of interesting Josephson effects. The structures tend to be ballistic or quasi-ballistic, come with gate-voltage control, exhibit strong spin-orbit coupling, large g-factors and transparent interfaces. Because of this, we took to studying higher-order Josephson effects, Josephson phi0-junctions, the combination of these that leads to Josephson diode phenomena, as well as supercurrents through single one-dimensional subbands. We also keep and eye on effects related to Majorana and topology such as spin-polarized supercurrents and fractional Josephson effects, but so far have not found those.

Liens :

• Séminaire théorie
• Personne à contacter :
• Lieu : G421

Résumé : Graphene and other Dirac (semi)metals provide an experimentally accessible playground for a host of (pseudo) relativistic phenomena that would be hard (or impossible) to observe in nature. This discussion reviews some of those peculiar behaviors that can be probed in transport, tunneling, photoemission, and other measurements

Liens :

• Séminaire interne LPMMC
• Personne à contacter : Vincent Rossetto ()
• Lieu : G421

Résumé : The Kardar-Parisi-Zhang (KPZ) is a non-linear stochastic equation describing the growth of an interface. Although it has been thoroughly studied in one dimension, recent numerical simulations unveiled a new unexpected scaling regime in the tensionless limit, when the diffusion term vanishes. This new regime is characterized by a dynamical exponent z=1, in contrast of its value in the conventional KPZ regime (z=3/2) and Edward-Wilkinson (EW) regime (z=2) . In this work, we study this tensionless regime as an ultra-violet fixed point of the renormalisation group (RG). A numerical study of the flow to the infrared allows us to observe the scaling function associated to this fixed point when the effective coupling goes to infinity. In addition, we obtain the z=1 dynamical exponent using a large momentum expansion of the RG flow equation.

Liens :LPMMC

• Séminaire nano-électronique quantique
• Personne à contacter :
• Visio-conférence Zoom hybride.
  En présentiel : Salle Rémy Lemaire K223, Institut Néel
  Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 91808901596
  

Résumé : The fractional quantum Hall effect is one of the most intriguing phenomena of condensed matter physics, where electronic interactions in a two-dimension electron gas subjected to a strong magnetic field lead to the emergence of highly exotic states with highly unusual properties. Among these, the existence of neutral edge modes, carrying only energy along the edges of the sample in a direction upstream to that of charge transport, has driven more than three decades of research. Their charge neutral nature has made them singularly challenging to probe, such that they were only first observed in 2010. Since then, many works have addressed the thermal transport properties of neutral modes, in particular whether they exchange energy with their neighboring counterpropagating charged edge modes. Significant progress was recently made on this topic, but an important question remained unanswered: can upstream neutral modes exchange energy and thermalize with integer¬-charged edge modes located up to several hundreds of nanometers away from them? This question is far from trivial, as it can profoundly change our understanding of the quantum Hall effect in terms of independent transport channels, and affect the realization of future experiments seeking to explore and exploit the remarkable properties of fractional quantum Hall states. We present heat transport measurements in quantum Hall states of graphene demonstrating that the integer channels can strongly equilibrate with the fractional ones, leading to markedly different regimes of quantized heat transport that depend on edge electrostatics. Our results allow for a better comprehension of the complex edge physics in the fractional quantum Hall regime. G. Le Breton, R. Delagrange, Y. Hong, M. Garg, K. Watanabe, T. Taniguchi, R. Ribeiro-Palau, P. Roulleau, P. Roche, and F. D. Parmentier, Phys. Rev. Lett. 129, 116803 (2022).

Liens :

• Séminaire théorie
• Personne à contacter : Adolfo Grushin ()
• Lieu : G 421

Résumé : Statistical mechanics mappings provide key insights on quantum error correction. However, existing mappings assume incoherent noise, thus ignoring coherent errors due to, e.g., spurious gate rotations. We map the surface code with coherent errors, taken as X- or Z-rotations (replacing bit or phase flips), to a two-dimensional (2D) Ising model with complex couplings, and further to a 2D Majorana scattering network. Our mappings reveal both commonalities and qualitative differences in correcting coherent and incoherent errors. For both, the error-correcting phase maps, as we explicitly show by linking 2D networks to 1D fermions, to a Z2-nontrivial 2D insulator. However, beyond a critical rotation angle, instead of a Z2-trivial insulator as for incoherent errors, coherent errors map to a Majorana metal. This critical angle is the theoretically achievable storage threshold. We numerically find the angle 0.14Ï€. The corresponding bit-flip rate exceeds the known incoherent threshold.

Liens :

• Séminaire interne LPMMC
• Personne à contacter : Bart van Tiggelen ()
• Lieu : G421

Résumé : Since many decades electromagnetic forces on globally neutral matter are controversial [1]. This controversy is imputed to the non unicity of the decomposition of stress-energy tensor into matter part and field part. More recently, experiment have been built to solve this controversy [2,3]. Thanks to these experiments, the controversy has been partially solved but there are still gray areas. On the theoretical side, different works have shown that electromagnetic forces could also get radiative corrections [4,5]. Rikken and V. Tiggelen measured the controversial Abraham force but couldnÂ’t exclude or support the existence of QED corrections to it [3]. In our work we show that Rydberg atoms are good candidates to observe QED corrections to the Abraham force. In a second time, we demonstrate the existence of an electromagnetic force on a Rydberg atom which have no classical equivalent to our knowledge and which is in reach of experiments.

References

• [1] I. Brevik, Phys.Rep. 52, 133 (1979)
• [2] G. B. Walker, D. G. Lahoz, and G. Walker, Can. J. Phys. 53, 2577 (1975)
• [3] G. L. J. A. Rikken and B. A. Van Tiggelen, Phys. Rev. Lett. 107, 170401 (2011)
• [4] A. Feigel, Phys. Rev. Lett. 92, 020404 (2004)
• [5] B. A. Van Tiggelen, D. Kawka, and G. L. J. A. Rikken, Eur. Phys. J. D 66, 272 (2012)

Liens :LPMMC

• Séminaire nano-électronique quantique
• Personne à contacter : Jeremie Viennot ()
• Visio-conférence Zoom hybride.
  En présentiel : Salle Rémy Lemaire K223, Institut Néel
  Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 91808901596
  

Résumé : Cavity electromechanics relies on parametric coupling between microwave and mechanical modes to manipulate the mechanical quantum state, and provide a coherent interface between different parts of hybrid quantum systems. High coherence of the mechanical mode is of key importance in such applications, in order to protect the quantum states it hosts from thermal decoherence. After an extended introduction to the field of optomechanics, we present the characterisation at millikelvin temperatures of a microwave electro-mechanical system featuring an ultra-coherent phononic-crystal membrane. The mechanical dissipation rate is measured down to 30 mK reaching a Q-factor of 1.5 billion, at 1.485 MHz mode frequency. Then we perform resolved sideband cooling on the mechanical mode, cooling it to its motional ground state nmin = 0.76 ± 0.16. We thus show the operation of an electromechanical system in the quantum regime, where its coherence time is estimated to be ~100 ms. We also show microwave-induced mechanical broadening up to 630 Hz, reaching manipulation speeds on the order of state-of-the-art superconducting qubits coherence times making our device a candidate for microwave quantum memories.

Liens :

• Séminaire QuantAlps
• Personne à contacter : Michele Filippone ()
• Lieu : Nevill Mott D420

Résumé : Moire superlattices in two dimensional semiconductors have enabled the observation of a wealth of phenomena driven by strong electronic correlations, ranging from Mott-Wigner states to quantum anomalous Hall effect. In this talk, I will describe magnetic properties of van der Waals heterostructures forming a frustrated triangular lattice in the vicinity of Mott-insulator states of electrons. By directly measuring electronic magnetization through the strength of the polarization-selective attractive polaron resonances, we find that when the Mott state is electron doped, the system exhibits ferromagnetic correlations in agreement with Nagaoka model. Our observations, which are in agreement with DMRG calculations, provide a direct evidence for itinerant magnetism with a kinetic origin.

Liens :Atac Imamoglu

• Séminaire interne LPMMC
• Personne à contacter : Vincent Rossetto ()
• Lieu : G421

Résumé : Understanding mechanisms of qubit decoherence is a crucial prerequisite for improving the qubit performance. The conventional theory of superconducting qubit decoherence by residual Bogoliubov quasiparticles [1,2] was constructed for qubits in equilibrium. However, the novel cat qubits of dissipative and Kerr types [3,4] are operated under non-equilibrium conditions. Namely, an external microwave drive is needed to stabilize the so-called "cat states", given by superpositions of coherent degenerate eigenstates of the effective stationary Lindbladian in the rotating frame. We quantify the effect of the quasiparticles on such driven-dissipative qubits by introducing additional dissipators which act on the density matrix of the cat qubit. We also account for the effect of the external drive on the quasiparticles along the lines of Ref. [5].

References

• [1] G. Catelani, R. J. Schoelkopf, M. H. Devoret, and L. I. Glazman, Phys. Rev. B 84, 064517 (2011).
• [2] G. Catelani et al., Phys. Rev. B 86, 184514 (2012).
• [3] Z. Leghtas et al., Science 347, 853 (2015).
• [4] A. Grimm et al., Nature 584, 205 (2020).
• [5] G. Catelani and D. M. Basko, SciPost Phys. 6, 013 (2019).

Liens :LPMMC

• Séminaire nano-électronique quantique
• Personne à contacter : Jeremie Viennot ()
• Visio-conférence Zoom hybride.
  En présentiel : Salle Rémy Lemaire K223, Institut Néel
  Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 91808901596
  

Résumé : Transmons are ubiquitously used in superconducting quantum information processing architectures. Strong drives are required to realize fast high-fidelity gates and measurements, including parametrically activated processes. We show that even off-resonant drives, in regimes routinely used in experiments, can cause strong modifications to the structure of the transmon spectrum rendering a large part of it chaotic. Chaotic states, often neglected through the hypothesis that the anharmonicity is weak, strongly impact the lifetime of the computational states. Chaos-assisted quantum phase slips greatly enhance band dispersions. In the presence of a readout resonator, the onset of chaos correlates with high transmon-resonator entanglement, and an average resonator response centered on the bare resonator frequency. We define a photon number threshold to characterize the appearance of chaos-induced quantum demolition effects during strong-drive operations, such as dispersive qubit readout. More generally, chaos-induced phenomena such as the ones studied here are expected to be present in all circuits based on low-impedance Josephson junctions. Ref.: Joachim Cohen, Alexandru Petrescu, Ross Shillito, and Alexandre Blais, arXiv:2207.09361 ; Ross Shillito, Alexandru Petrescu, Joachim Cohen et al., Phys. Rev. Appl. 18, 034031 (2022)

Liens :

• Séminaire théorie
• Personne à contacter :
• Lieu : G421

Annulé

Liens :

• Séminaire interne LPMMC
• Personne à contacter : Vincent Rossetto ()
• Lieu : G 421

Résumé : Exciton-polariton are bosonic quasi-particles that arise from the strong coupling between light and matter. They are typically formed in a quantum well embedded in an optical microcavity, from the interaction between quantum well excitons and cavity photons. Under non-resonant pumping, it is shown that exciton-polariton can form a Bose-Einstein condensate (BEC) [1]. This out of equilibrium BEC is sustained in a stationary state by the competition between continuous laser driving and losses coming from the leakage of cavity photons. Recent studies focused on the coherence properties of such driven-dissipative condensates and established connections with the Kardar-Parisi-Zhang (KPZ) universality class [2]. In particular, it is now known that the variance of the phase of one-dimensional polariton condensates follows the KPZ scaling in space and in time [3]. Chemical potential corrections are extensively studied for equilibrium BECs [4]. However, a description of these corrections is still lacking in driven-dissipative condensates. In the defect-free KPZ phase [5] of a 1D polariton BEC, we investigate the blueshift stochastic fluctuations, analogue of beyond mean field chemical potential corrections.

References

• [1] Kasprzak J., Richard M., Kundermann S. et al. Nature 433 (2006)
• [2] Altman E., Sieberer L.M., Chen L., et al. PRX 5 (2015).
• [3] Fontaine Q., Squizzato D., Baboux F. et al. Nature 608 (2022).
• [4] Lee T. D., Huang K., Yang C. N. Physical Review 106 (1957).
• [5] He L., Sieberer L. M., Diehl S PRL 118 (2017).

Liens :LPMMC

• Séminaire nano-électronique quantique
• Personne à contacter : Jeremie Viennot ()
• Visio-conférence Zoom hybride.
  En présentiel : Salle Rémy Lemaire K223, Institut Néel
  Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 91808901596
  

Liens :

• Séminaire théorie
• Personne à contacter :
• Lieu : G421

Résumé : Flat-band materials may have relatively high superconducting transition temperature. Several systemswere proposed to host nearly flat bands such as, for example, multilayer graphene with rhombohedral stacking, twisted bilayer graphene, and semimetals with high pseudospin quasiparticles. However, the effect of the flat band on superconductivity can be twofold. Despite favoring Cooper pair formation, its nearly dispersionless nature can be a serious impediment to pair condensation. The Cooper pairs formation and their condensation may occur at different temperatures.

I will talk about the conditions for the emergence of the preformed Cooper pairs in materials hosting flat bands. As a particular example, a semimetal with a pair of three-band crossing points at which a flat band intersects with a Dirac cone will be considered. It will be shown that the flat band promotes local Cooper pair formation so that the system may be modeled as an array of superconducting grains. The delocalized states give rise to the phase-coherent superconductivity at low temperatures. The transition temperature between the preformed Cooper pair state and the phase-coherent state will be discussed.

Liens :

• Séminaire LPMMC
• Personne à contacter : Vincent Rossetto ()
• Lieu : G 421

Résumé : The nature of the experimentally-measured fractional conductance plateau at filling 5/2 in Quantum Hall states remains an open question, with several candidate states presenting slightly different topological order. After a decade of debate, the theoretical consensus settled on the non-Abelian Antipfaffian state, Nonetheless, recent experimental results measured a quantized thermal conductance of 5/2, incompatible with the theoretical proposal. Our work revisits the theoretical approaches by pushing the expansion of the effective Hamiltonian of the 5/2 quantized Hall state to third-order in the Landau level mixing, the parameter controlling the interaction between different Landau levels. I will present the challenges behind this expansion, and our main results: the third-order expansion shows an inversion of the gaps at mixings well below the experimental regime, indicating that either the gaps are much smaller than previously expected, or that a quantum phase transition occurs. Our work also emphasizes the role of frozen spin degrees of freedom. Finally, I will discuss what is needed to give a definite answer to this long standing problem, if it is at all possible.

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter : Jeremie Viennot ()
• Visio-conférence Zoom hybride.
  En présentiel : Salle Rémy Lemaire K223, Institut Néel
  Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 91808901596
  

Résumé : Remarkably, complex assemblies of superconducting wires, electrodes, and Josephson junctions are compactly described by a handful of collective phase degrees of freedom that behave like quantum particles in a potential. Almost all these circuits operate in the regime where quantum phase fluctuations are small—the associated flux is smaller than the superconducting flux quantum—although entering the regime of large fluctuations would have profound implications for metrology and qubit protection. The difficulty arises from the apparent need for circuit impedances vastly exceeding the resistance quantum. Independently, exotic circuit elements that require Cooper pairs to form pairs in order to tunnel have been developed to encode and topologically protect quantum information. In this work, we demonstrate that pairing Cooper pairs magnifies the phase fluctuations of the circuit ground state. In a first experiment, we measure a tenfold suppression of flux sensitivity of the first transition energy only, implying a twofold increase in the vacuum phase fluctuations and showing that the ground state is delocalized over several Josephson wells. In a second experiment, we demonstrate that Cooper-pair pairing mediates high order photon-photon interactions, resulting in some peculiar spectral properties. Ref.: W. C. Smith et al., Phys. Rev. X 12, 021002

Liens :

• Séminaire théorie
• Personne à contacter :
• Lieu : G 421

Annulé
L'orateur ne peut pas venir à Grenoble à cause de la grève SNCF. Le séminaire est reporté au 27 avril.

Liens :

• Séminaire interne LPMMC
• Personne à contacter : Vincent Rossetto ()
• Lieu : G 421

Résumé : ractional Chern Insulators (FCIs, which are lattice analogs of fractional quantum Hall states) have been the subject of intensive research in the last decades, not only for the interest we have in understanding the nature of topological phases of matter, but also thanks to the possible applications in quantum computing. Cold atoms in optical lattices can host these topologically ordered phases, and hallmark signatures have already been detected experimentally in two-boson systems [arXiv:2210.10919 (2022)]. We address the following question: how can we probe the edge spectrum of FCIs experimentally? We propose to subjet the atomic FCI ground state to a Laguerre- Gauss laser, creating edge excitations through a transfer of angular momentum and energy, following a similar proposal in integer Chern insulators [Phys. Rev. Lett. 108, 255303 (2012)], and to subsequently measure the excitation fraction through local density measurements. We numerically test this protocol in a model of strongly-interacting bosons in the Hofstadter lattice. We use the variation of density profile to extract the transition frequencies and consequently rebuild the excitation spectrum. We find a chiral edge branch, indicative of topological order in systems with as few as 2 bosons. Finally, we use our tool to show the progressive opening of an edge gap in the limit of very dense systems.

Liens :LPMMC

• Séminaire nano-électronique quantique
• Personne à contacter : Jeremie Viennot ()
• Visio-conférence Zoom hybride.
  En présentiel : Salle Rémy Lemaire K223, Institut Néel
  Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 91808901596
  

Résumé : A promising path to reduce gate errors in superconducting quantum processors consists in developing highly anharmonic circuits with some degree of protection from prevailing decoherence sources. At present, properly designed single highly anharmonic fluxonium qubits can have over 1 ms coherence time -- about tenfold or more compared to regular transmons superconducting qubits. In this talk, I will compare the different approach to superconducting quantum computation and I will describe recent implementations of high-fidelity single and two-qubit gates in fluxonium circuits. This includes a fast microwave-activated controlled-Z gate completed in less than 9 qubit Larmor cycles (about 60 ns) with a fidelity of 99.2%, which is on-par with the best microwave-activated gates reported on several other platforms. Finally, I will discuss the prospects of extending the system size to large scale quantum processors and simulators.

Liens :

• Séminaire théorie
• Personne à contacter :
• Lieu : G 421

Résumé : The identification of universal properties from minimally processed datasets is one goal of machine learning techniques. Both in supervised or unsupervised settings, “making sense” of hitherto unseen raw data is defined at the outset, byencoding the task (regression, classification, etc.) in an objective function. This turns learning and inference into an optimisation problem. Here, starting from data-sets sampled from classical partition functions and one-dimensional quantum models, we build networks (graphs) by drawing links between the points according to a cutoff distance that is determined by the data structure and the choice of metric. Remarkably, this enables a transfer of methods and concepts from disconnected fields that allow us to tackle in an agnostic way the study of phase transition in several models. We observe how the minimum number of variables needed to accurately describe the important features of a data-set, the intrinsic dimension Id, behaves in the vicinity of phase transitions. We show how the finite-size analysis of the Id allows us to identify critical points with an accuracy comparable to methods that rely on apriori identification of order parameters. We review previous works [Physical ReviewX 11 (1), 011040] and elaborate on the topic with new results in case of ground states of one-dimensional quantum systems.

Liens :

• Séminaire interne LPMMC
• Personne à contacter : Benoît Vermersch ()
• Lieu : G 421

Résumé : The quantum Fisher information (QFI) is a fundamental quantity of interest in many areas from quantum metrology to quantum information theory. It can in particular be used as a witness to establish the degree of multi-particle entanglement in quantum many-body-systems. As the QFI is a macroscopic property of the quantum state, it has been a challenge till date to measure it in state-of-art quantum platforms. To address this problem, I will firstly discuss the randomized measurement (RM) toolbox which has emerged as a good candidate to estimate properties associated to the density matrix [Nature Reviews Physics volume 5, (2023)]. This framework motivated us to formulate the QFI in terms of a converging series of lower bounds that can then be estimated using the RM toolbox [Phys. Rev. Lett. 127, 260501, (2021)]. Lastly I will show some preliminary experimental estimations of the QFI on a superconducting device that implement recent error mitigation and post-processing methods.

Liens :LPMMC

• Séminaire nano-électronique quantique
• Personne à contacter : Romain Maurand ()
• Visio-conférence Zoom hybride.
  En présentiel : Salle Rémy Lemaire K223, Institut Néel
  Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 91808901596
  

Résumé : Quantum computing holds the potential to solve key tasks exponentially faster than classical algorithms. Semiconductor spins are among the leading candidates for a highly scalable qubit platform. Hole spins offer many advantages such as all-electrical spin manipulation without micromagnets or antennas, as well as decoupling from the nuclear spins, and offer novel physics such as direct Rashba spin-orbit coupling, exchange anisotropy and longitudinal coupling which could be developed into valuable assets for quantum computing. In this seminar, I will present recent progress on two different hole spin qubit platforms: Ge/Si core/shell nanowires and Si fin field-effect transistors (FinFETs). The Ge/Si nanowires offer exceptionally strong yet electrically tunable spin-orbit interaction, the direct-Rashba SOI, allowing unprecedented control of key qubit parameters. We have recently identified sweet spots with strongly enhanced coherence, can operate the qubits at temperatures up to 2 K, are implementing an exchange based CROT 2Q gate and present dispersive qubit readout using STO varactors. Si FinFETs have championed classical transistor scaling for a decade, integrating billions of FinFETs on a chip. We have demonstrated 1Q gate fidelities at the fault-tolerance threshold at 1 K and can operate up to 5 K, potentially allowing in-situ integration of the control electronics. We have implemented a CROT gate with spin-orbit induced anisotropic exchange interaction, opening the door to high fidelity and fast 2Q gates. Recently, we have also observed phase driving of such qubits at radio frequencies.

Liens :Dominik Zumbuhl

• Séminaire théorie
• Personne à contacter :
• Visio-conférence Zoom hybride.
  En présentiel : G 421
  Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 94573011629 code d'accès: 471269
  

Résumé : We study the one-dimensionless free Fermionic model subject to random strong projective measurements of site occupation numbers, and derive an effective R-replica Keldysh non-linear sigma-model (NLSM) to study fluctuations of the entanglement entropy and fluctuations of number of particles in such a model, with the unusual replica limit R -> 1. Treatment of the NLSM within Gaussian approximation suggests logarithmic behavior of the fluctuations in the limit of rare measurements or weak monitoring, with the fluctuations growing as measurement frequency increases. The results for the transition and frequent measurements are yet to be obtained.

Liens :

• Séminaire interne LPMMC
• Personne à contacter : Denis Basko ()
• Lieu : G 421

Liens :LPMMC

• Séminaire LPMMC
• Personne à contacter : Cécile Repellin ()
• Visio-conférence Zoom hybride.
  En présentiel : G 421
  Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 94573011629
  

Résumé : The study of two-dimensional droplets of electrons in a strong magnetic field lies at the heart of the quantum Hall effect. In this talk, I present recent results on geometric deformations of such droplets, resulting from variations of the underlying spatial metric and/or confining potential. Time-dependent variations give rise to Berry phases that can remarkably be written in closed form despite the fact that the underlying parameter space is infinite-dimensional. In particular, I argue that a large class of deformations that generalize squeezing and shearing probe the edge modes of the system, including their topological central charge. (Based on 2212.12935 and 2301.01726 , ongoing work)The study of two-dimensional droplets of electrons in a strong magnetic field lies at the heart of the quantum Hall effect. In this talk, I present recent results on geometric deformations of such droplets, resulting from variations of the underlying spatial metric and/or confining potential. Time-dependent variations give rise to Berry phases that can remarkably be written in closed form despite the fact that the underlying parameter space is infinite-dimensional. In particular, I argue that a large class of deformations that generalize squeezing and shearing probe the edge modes of the system, including their topological central charge. (Based on arXiv:2212.12935 and arXiv:2301.01726 et ongoing work)

Liens :

• Séminaire théorie
• Personne à contacter :
• Lieu : G 421

Résumé : Research and development in novel energy technologies must deal with the optimization of extremely complex, often disordered, materials. On one hand, optimization calls for the development of new chemistries and materials engineering concepts; on the other hand, complexity obliges us to deeply understand fundamental physical processes, some related to a hierarchical multiscale system organization, down to nanometric sizes. At these scales, the granularity of matter cannot be put aside and the presence of interfaces, confining environments, fluctuations, crucially modify well established behaviors in the bulk. Here computer simulation and statistical mechanics approaches really help. In the talk I will overview our work in this direction, discussing a few scientific cases ranging from nanoconfined fluids [1], through polymer electrolytes [2], to superlattices and glasses [3]. Based on Molecular Dynamics simulations and artificial neural network algorithms, and also referring to data coming from advanced scattering experiments with Neutrons and X-Rays, I will describe our work to improve the understanding of disordered materials organization at the nanoscale, clarify how spatial constraint modify phase properties, and suggest how to control and optimize materials functions by tailoring the confining environments. [1] “Re-entrant phase transitions and dynamics of a nanoconfined ionic liquid”, [10.1103/PhysRevX.8.031062] [2] “From Ionic Surfactants to Nafion through Convolutional Neural Networks”, with L. Dumortier, [10.1021/acs.jpcb.0c06172] [3] “Beating the amorphous limit in thermal conductivity by superlattices design”, with H. Mizuno and J.-L. Barrat, [10.1038/srep14116]

Liens :Stefano Mossa

• Séminaire interne LPMMC
• Personne à contacter : Léonie Canet ()
• Lieu : G 421

Résumé : Exciton-polariton condensates represent a promising playground to investigate the physics of driven-dissipative quantum fluids. In 1d systems, the coherence of such fluids has been shown to exhibit critical behaviour, related to the Kardar-Parisi-Zhang (KPZ) universality class. Further intriguing features (phase defects, proliferation of dark solitons) have been suggested to populate the spectrum of possible regimes of exciton-polaritons. We aim to determine the phase diagram obtained via the stochastic Gross-Pitaevski equation which models the condensate at mean-field level, guided by realistic experimental tunability in the choice of the leading parameters: intensity of pumping and noise strength.

Liens :LPMMC

• Séminaire théorie
• Personne à contacter : Serge Florens ()
• Lieu : G 421

Résumé : Amperean superconductivity is an exotic phenomenon stemming from attractive effective electron-electron interactions (EEEIs) mediated by a transverse gauge field. Originally introduced in the context of quantum spin liquids and high-Tc superconductors, Amperean superconductivity has been recently proposed to occur at temperatures on the order of 1-20 K in two-dimensional, parabolic-band, electron gases embedded inside deep sub-wavelength optical cavities. I will first generalize the microscopic theory of cavity-induced Amperean superconductivity to the case of graphene and then argue that this superconducting state cannot be achieved in the deep sub-wavelength regime. In the latter regime, indeed, a cavity induces only EEEIs between density fluctuations rather than the current-current interactions which are responsible for Amperean pairing.

Liens :

• Séminaire LPMMC
• Personne à contacter : Vincent Rossetto ()
• Lieu : G 421

Résumé : We identify a mapping between two-dimensional (2D) electron transport in a minimally twisted graphene bilayer and a 1D quantum walk, where one spatial dimension plays the role of time. In this mapping a magnetic field B perpendicular to the bilayer maps onto an electric field. Bloch oscillations due to the periodic motion in a 1D Bloch band can then be observed in purely DC transport as magnetoconductance oscillations with periodicity set by the Bloch frequency. Ref : Phys. Rev. B 105, L241408 (2022)

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter : Jeremie Viennot ()
• Visio-conférence Zoom hybride.
  En présentiel : Salle Rémy Lemaire K223, Institut Néel
  Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 91808901596
  

Résumé : When the strength of Coulomb interaction between itinerant electrons in a two-dimensional system becomes significantly larger than the kinetic energy, the electrons start to develop strong correlations. A paradigm phase that is expected to emerge in this regime is an electronic Wigner crystal, in which the electrons spontaneously form a periodic lattice mimicking that of atoms in the real crystals. However, in order for this electronic crystallization to occur, the actual ratio of the above energy scales must exceed 30, which turns out to be notoriously difficult to fulfill in conventional semiconductors (e.g., GaAs). Recently, atomically-thin transition metal dichalcogenides (TMDs) have emerged as a highly-tunable experimental platform that unlocks the access to uncharted territories of strongly correlated electron physics. This is due to reduced dielectric screening and large carrier effective masses, which endow TMD monolayers with excellent optical properties and give rise to strong inter-electron interactions enabling to reach Coulomb-to-kinetic energy ratios being more than an order of magnitude larger than that for the GaAs at comparable electron densities. In this talk, I will review our recent optical investigations of landmark correlated phases in charge-controlled TMD-based van der Waals heterostructures. In particular, I will present our novel spectroscopic technique enabling us to detect the Wigner crystal in a TMD monolayer through the periodic potential it generates for the excitons (1). In the presence of this potential, the excitons Bragg scatter off the Wigner crystal, which gives rise to the emergence of a Bragg-umklapp resonance in the reflectance spectrum that heralds the presence of an electronic lattice. Our observation of this resonance provides the first unequivocal evidence for the formation of the Wigner crystal that has been thus far probed only by indirect methods in two-dimensional systems. In the second part of the talk, I will also show how the Rydberg excitons in a TMD monolayer can be exploited to optically probe the formation of correlated electronic phases in an adjacent graphene layer (2), which is otherwise optically inaccessible owing to the lack of a robust energy gap. I will demonstrate that this approach allows for sensing fractional quantum Hall effect in graphene with a similar sensitivity to that of state-of-the-art transport tools. References: (1) T. Smolenski, P. E. Dolgirev, C. Kuhlenkamp, A. Popert, Y. Shimazaki, P. Back, X. Lu, M. Kroner, K. Watanabe, T. Taniguchi, I. Esterlis, E. Demler, and A. Imamoglu, Nature 595, 53-57 (2021). (2) A. Popert, Y. Shimazaki, M. Kroner, K. Watanabe, T. Taniguchi, A. Imamoglu, and T. Smolenski, Nano Letters 22, 7636 (2022)

Liens :

• Séminaire LPMMC / Néel
• Personne à contacter : Vincent Rossetto ()
• Lieu : G 421

Résumé : Numerical simulations in the field of quantum nanoelectronics are often restricted to devices of finite size that are connected to the macroscopic world with quasi-one-dimensional electrodes. I will present a new numerical method, based on the Green’s function formalism, that lifts some of these restrictions and allows simulations of systems that are infinite in 1, 2 or 3 dimensions and mostly invariant by translation. I will illustrate these method by computing transport properties in a disorder Weyl semimetal. In a rather unrelated subject, I will talk about my work at Isterre, modelling the Earth's magnetic field and liquid iron core using geomagnetic data, i.e. satellite observations of the magnetic field. Data is cleaned from internal and external contributions, and then assimilated using an ensemble Kalman filter. Predictions of the flow are compared to changes in the length of day and show good agreement down to interannual frequencies.

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter :
• Lieu : Salle Rémy Lemaire K223, Institut Néel

Résumé : The advent of Van der Waals technology has allowed the development of many materials that did not exist before and has led to the observation of many exciting new physical phenomena in these materials due to the unique electronic, optical, and mechanical properties of 2D atomic crystals. For instance, tuning twist angle allows altering electronic bands of 2D materials by a moiré pattern induced between 2D layers. Control of the stacking order, on the other hand, provides an alternative approach to program quantum properties, and without the need for a moiré superlattice. In this talk, I will discuss how interlayer stacking order can be used for deterministic control of the properties of van der Waals materials. In particular, controlling stacking order in multilayer graphite films allowed us to discover the quantum Hall effect in hexagonal graphite and to find strong electronic correlations in rhombohedral graphite films.

Liens :Artem Mishchenko

• Séminaire QuantAlps
• Personne à contacter : Michele Filippone ()
• Lieu : Nevill Mott - D420 - 3rd Floor

Résumé :

Machine-learning-based approaches, routinely adopted in cutting-edge industrial applications, are being increasingly adopted to study fundamental problems in science. Many-body physics is very much at the forefront of these exciting developments, given its intrinsic "big-data" nature. In this seminar I will present selected applications to the quantum realm. First, I will discuss how a systematic, and controlled machine learning of the many-body wave-function can be realized. This goal is achieved by a variational representation of quantum states based on artificial neural networks [1]. I will then discuss recent applications in diverse domains, focusing on prototypical open problems in many-body quantum physics. I will especially focus on the problem of accurately describing interacting fermions, in Condensed Matter [2], Chemistry [3], and Nuclear Matter [4] — where these approaches have significantly improved over previous variational descriptions.

Liens :Colloquium Giuseppe Carleo

• Séminaire nano-électronique quantique
• Personne à contacter : Jeremie Viennot ()
• Visio-conférence Zoom hybride.
  En présentiel : Salle Rémy Lemaire K223, Institut Néel
  Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 91808901596
  

Résumé : Semiconductor spin qubits based on spin-orbit states stand as promising candidates in view of developing a quantum processor. Owing to their strong spin-orbit interaction, hole spins in silicon (1) and germanium (2) are responsive to electric field excitations, allowing for practical, fast and potentially scalable qubit control. As a drawback, spin electric susceptibility renders these qubits generally vulnerable to electrical noise, which limits their coherence time. Here we report on the operation and readout of a single hole spin in natural silicon, made from a semi-industrial 300 mm CMOS foundry. We demonstrate the existence of a preferential magnetic field orientation, at which the qubit is decoupled from charge noise while keeping an efficient electrical control. We first realize spin single-shot readout (3) of the first hole accumulated in a silicon quantum dot. Subsequently, we characterize the hole spin gyromagnetic tensor and its susceptibility to electric fields by coherent ma- nipulation techniques. We evidence a strong dependence on the external magnetic field orientation, and reveal optimal operation points at which the longitudinal spin-electric susceptibility is minimal. At these sweet spots, we measure a Hahn-Echo decay time in the order of 100 μs while maintaining Rabi frequencies in the MHz range. This work opens new perspectives for quantum processing based on spin-orbit qubits. References: (1) Piot, N., Brun, B., et al. A single hole spin with enhanced coherence in natural silicon. Nature Nanotechnology (2022). (2) Hendrickx, N. W. et al. A four-qubit germanium quantum processor. Nature 591, 580–585 (2021). (3) Elzerman, J. M. et al. Single-shot read-out of an individual electron spin in a quantum dot. Nature 430, 431–435 (2004).

Liens :

• Séminaire théorie
• Personne à contacter : Benoit Vermersch ()
• Lieu : G 421

Résumé : The concept of causality, stating that physical actions cannot propagate in space at an arbitrary speed, can be captured for qudit systems by the notion of Quantum Cellular Automata (QCA), defined as unitary maps preserving locality of observables. In this talk, I will show that QCA can be identified, in any dimension and geometry, with special tensor network operators, yielding a general connection between causality and bounds on entanglement production in the form of area laws. I will stress the importance of unitarity, by discussing generalizations of our results for different classes of non-unitary quantum channels. Finally, I will mention how the set of QCA can be extended to a larger class of deterministic maps via LOCC (local operations and classical communication) and illustrate implications on state-preparation protocols and classification of phases of matter.

Liens :

• Séminaire LPMMC
• Personne à contacter : Sergey Skipetrov ()
• Lieu : G 421

Liens :TU Wien

• Séminaire nano-électronique quantique
• Personne à contacter : Jeremie Viennot ()
• Visio-conférence Zoom hybride.
  En présentiel : Salle Rémy Lemaire K223, Institut Néel
  Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 91808901596
  

Résumé : Recent years superconductor-semiconductor hybrid materials have been established as an essential platform for quantum devices, notably used in the search for novel bound states that may serve as qubits (1). In this talk we firstly focus on the excitations and correlations arising in short chains of coupled quantum dots and superconducting islands (2). We then take a look beneath the surface of these nanowire devices, addressing advances in materials science where in situ MBE fabrication and various superconductors have been implemented in order to expand the available parameter space for hybrid devices (3,4,5). Ref.: (1) E. Prada et al., From Andreev to Majorana bound states in hybrid superconductor-semiconductor nanowires, Nature Reviews Physics 2, 575 (2020). (2) J.C. Estrada Saldana et al., Excitations in a superconducting Coulombic energy gap, Nature Comm. 13, 2243 (2022); Two Bogoliubov quasiparticles entangled by a spin, arxiv:2203.00104 (3) T. Kanne et al., Epitaxial Pb on InAs nanowires for quantum devices, Nature Nanotechnology 16, 767 (2021); T. Kanne et al., Double nanowires for hybrid quantum devices, Advanced Functional Materials 32, 2107926 (2021). (4) D. Carrad et al., Shadow Epitaxy for In Situ Growth of Generic Semiconductor/Superconductor Hybrids, Advanced Materials 32, 1908411 (2020). (5) J. Sestoft et al., Scalable Platform for Nanocrystal-Based Quantum Electronics, Advanced Functional Materials 32, 2112941 (2022)

Liens :Jesper Nygard

• Séminaire théorie
• Personne à contacter :
• Lieu : G 421

Résumé : Topological insulating (TI) phases were originally highlighted for their disorder-robust bulk responses, such as the quantized Hall conductivity of 2D Chern insulators. With the discovery of time-reversal- (T-) invariant 2D TIs, and the recognition that their spin Hall conductivity is generically non-quantized, focus has since shifted to boundary states as signatures of 2D and 3D TIs and symmetry-enforced topological crystalline insulators (TCIs). However, in T-invariant (helical) 3D TCIs such as bismuth, BiBr, and MoTe2 - termed higher-order TCIs (HOTIs) - the boundary signatures manifest as 1D hinge states, whose configurations are dependent on sample details. It is hence desirable to elucidate bulk and surface signatures of helical TCIs, and their relationship to sample-independent experimental observables. Using a range of theoretical and numerical probes, including a newly introduced principle of spin-resolved topology, we fully characterize the bulk topological properties of inversion- and T-protected helical HOTIs. We discover that helical HOTIs realize one of three spin-resolved phases with distinct responses that are quantitatively robust to large deformations of the bulk spin-orbital texture: 3D quantum spin Hall insulators, "spin-Weyl" semimetal states with gapless spin spectra, and T-doubled axion insulator (T-DAXI) states with nontrivial “partial” axion angles indicative of a 3D spin-magnetoelectric bulk response. We provide experimental signatures of each spin-stable regime of helical HOTIs, including surface Fermi arcs in spin-Weyl semimetals under strong Zeeman fields, and half-quantized 2D TI states on the gapped surfaces of T-DAXIs originating from a partial parity anomaly. References: [1] K.-S. Lin, Palumbo, Z. Guo, Blackburn, Shoemaker, Mahmood, Z.-J. Wang, Fiete, Wieder, Bradlyn, arXiv:2207.10099 (2022) [2] Schindler, Tsirkin, Neupert, Bernevig, Wieder, Nature Communications (2022)

Liens :

• Séminaire théorie
• Personne à contacter : Serge Florens ()
• Lieu : G-421

Résumé : Whereas a conventional bulk superconductor is perfectly well described by the mean-field Bardeen-Cooper-Schrieffer theory of superconductivity, in the limit of ultra-small grains of superconducting material a more appropriate description is in terms of the interacting Richardson's (picket-fence) model and its extensions. This charge-conserving Hamiltonian consists of a kinetic energy term and an interacting pairing term between all orbitals, usually with a constant coefficient, in which case the model is integrable and its complete solution can be reduced to numerically solving a system of coupled algebraic equations. In the presence of an impurity carrying a local magnetic moment, the integrability is broken by the exchange scattering that splits the Cooper pairs. We show that the corresponding Hamiltonian admits a compact representation in terms of small matrix product operators, which makes it possible to solve this class of problems accurately and rather efficiently using the density matrix renormalization group (DMRG) implemented using the tensor-network formalism. I will discuss the pros and cons of the Richardson-model description of systems of coupled superconducting island and quantum dots, the effects of the Coulomb repulsion (charging) term on the superconducting island on the phase diagrams and on the nature of the (subgap) states, the basic properties of the two-channel version of the problem, as well as other possible extensions of the model (level-dependent pairing, spin-orbit coupling, two impurities).

Liens :Rok Zitko

• Séminaire QuantAlps
• Personne à contacter : Michele Filippone ()
• Lieu : Nevill Mott - D420 - 3rd Floor

Résumé : For the last twenty years, physicists have been learning to manipulate individual quantum objects: atoms, ions, molecules, quantum circuits, electronic spins... It is now possible to build "atom by atom" a synthetic quantum matter. By controlling the interactions between atoms, one can study the properties of these elementary many-body systems: quantum magnetism, transport of excitations, superconductivity... and thus understand more deeply the N-body problem. More recently, it has been realized that these quantum machines could perhaps find applications in industrial fields, such as finding the solution of combinatorial optimization problems.
This seminar will present an example of a synthetic quantum system, based on laser-cooled ensembles of individual atoms trapped in microscopic optical tweezer arrays. By exciting the atoms into Rydberg states, we manage to make them interact, even at distances of about ten micrometers. In this way, we study the magnetic properties of an ensemble of more than a hundred interacting ½ spins, in a regime in which simulations by usual numerical methods are already impossible. Some aspects of this research have led to the creation of the startup PASQAL.

Liens :

• Séminaire théorie
• Personne à contacter :
• Lieu : G 421

Liens :

• Séminaire LPMMC
• Personne à contacter : Sergey Skipetrov ()
• Visio-conférence Zoom Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 91808901596
  

Résumé : We theoretically analyse the ground state of the Frenkel-Kontorova model when the particles interact with repulsive power-law interactions. For periodic boundary conditions we show that the classical ground state can be mapped to the one of a long-range Ising model. We then show that the classical ground state has the properties of a complete devil staircase as a function of the discommensuration and determine the parameters for which it is stable. We analyse the low-energy spectrum and discuss the effect of quantum fluctuations, focussing on experimentally relevant regimes. This work sets the stage for a full quantum mechanical description of the Aubry transition.

Liens :

• Séminaire théorie
• Personne à contacter : michele filippone ()
• Lieu : G 421

Résumé : Gauge theories are the cornerstone of our understanding of fundamental interactions among particles. Describing the evolution of these strongly coupled systems is a formidable challenge for classical computers, and represents one of the key open quests for quantum simulation approaches to particle physics phenomena. In this talk, I will first introduce a method to realize a quantum simulation of U(1) lattice gauge theories coupled to matter, utilizing alkaline-earth(-like) atoms in state-dependent optical lattices. I will then illustrate how this quantum simulator can be used to probe various phenomena, including particle collisions and confinement.

Liens :

• Séminaire LPMMC
• Personne à contacter : Robert Whitney ()
• Lieu : G 421

Résumé : Various transport and optical properties of quantum devices require an account of significant coupling to their electronic or photonic environments. This makes approximation strategies necessary that go beyond the successful but limited weak-coupling, memoryless master equations (Lindblad dynamical semigroups). However, even the perturbative calculation of corrections in the environment coupling are already quite involved for stationary properties and fraught with dangers for strong local interactions. Dynamical properties further complicate the matter by requiring an account of "memory effects". In the first part of my talk I will introduce and review some of these issues from the general perspective of open-system dynamics. I will highlight how insights from quantum information about density operators can clarify, guide and motivate the application of statistical field theoretical techniques to compute their real-time evolution. In the second part of my talk I will discuss the particular situation that the same open quantum system can be equivalently described by two exact, but fundamentally different equations of motion. This puzzling issue is resolved by a surprisingly simple "fixed-point" relation between the system's memory-kernel and its generalized, time-dependent Lindblad generator. This result allows a series of long-standing problems to be solved, suggests a new general picture of memory in quantum dynamics, and provides an intriguing new iterative approach to account for memory effects.

Reference

Liens :

• Séminaire QuantAlps
• Personne à contacter : Michele Filippone ()
• Lieu : Salle de conférences CNRS Bâtiment A

Résumé : Progress in developing analog quantum simulation platforms is reflected in increasing control of engineered many-body Hamiltonians, and the ability to perform single-site and single-shot readouts. This defines a new generation of programmable quantum simulators which combine a certain amount of programmability with scalability to large particle numbers. The focus of this talk is to report work from a theory-experiment collaboration with trapped ion platforms with up to fifty qubits/spins, with the goal to develop and demonstrate quantum protocols, addressing questions from the fundamental to the practical. Examples to be discussed include measurement protocols revealing the entanglement structure of the many-body wavefunction, and implementing `optimal' quantum metrology with variational quantum circuits, where quantum simulators act as `programmable quantum sensors'. The event will be followed by a coffee in the "Salle de convivialité" close by.

Liens :

• Soutenance HDR
• Personne à contacter : Vincent Rossetto ()
• Visio-conférence Zoom hybride.
  En présentiel : G 421
  Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 99152726338
  

Résumé : It is easy to invent quantum systems that look like they would violate the second law of thermodynamics. Yet careful analyses — such as those reviewed here for nanoelectronic systems — have shown no such violations. Thus, to-date, cracks in the laws of thermodynamics that have sometimes been glimpsed, have always turned out to be illusory. Here I review a small subset of the domain of quantum thermodynamics by reconsidering a very old thought-experiment, usually known as Smoluchowski's trapdoor. This thought experiment was first posed by Maxwell in the 1860s, in the context of his musing on ways to violate the laws of thermodynamics; musings that led to what became known as Maxwell's demon. Smoluchowski analysed much of the physics of this trapdoor, and made a huge step towards explaining why it would not violate the laws of thermodynamics. However, I will argue that his explanation was incomplete, and that it makes violations of the laws of thermodynamics unlikely but not impossible. Now is the time to revisit this issue, because now we can use quantum dots to build an experimental nanoelectronic system that acts like a trapdoor. I analysis such a nanoelectronic trapdoor using the methods of modern quantum thermodynamics, and thereby prove that (as expected) such a trapdoor always obeys the laws of thermodynamics. The analysis of the trapdoor system is similar to that of many other nanoelectronic devices in the literature. This makes it a nice pedagogical example to guide the reader through some of the ideas and methods within quantum thermodynamics, while working on a problem that follows directly in the footsteps of the greatest thermodynamicists of the 19th century.

Liens :Robert WhitneyLPMMC

• Séminaire nano-électronique quantique
• Personne à contacter : Jeremie Viennot ()
• Visio-conférence Zoom hybride.
  En présentiel : Salle Rémy Lemaire K223, Institut Néel
  Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 91808901596
  

Résumé : Recently, much attention is being paid to the study of superconducting systems whose transport properties depend on the direction of the supercurrents. Non-reciprocal transport effects have been proposed and observed in various structures motivated by creating a perfect superconducting diode. Besides possible applications, the physics of non-reciprocal effects is very rich, in particular in systems in which superconductivity coexists with spin-dependent fields. In this talk, I will discuss the superconducting diode effect and its connection with other closely related phenomena. These include the anomalous current and phi-Josephson junctions, the helical phase of Rashba superconductors, and magnetoelectric effects induced by the spin-orbit coupling.

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter : Cecile Repellin ()
• Visio-conférence Zoom hybride.
  En présentiel : Salle Rémy Lemaire K223, Institut Néel
  Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 91808901596
  

Résumé : One very successful approach to photonic systems engineering is based on controlling ensembles of coupled nonlinear photonic resonators. Fascinating properties emerge at the confluence between nonlinear optics and condensed matter physics: light can undergo Bose-Einstein condensation, behave as a superfluid, or propagate along edge channels in topological lattices. One ingredient at the heart of the physics of the system is the driving field, which is used to maintain a non-equilibrium steady-state. Drive and dissipation constitute important knobs to control the physics of the system. In this talk, I will focus on two situations where the driven-dissipative nature of the system plays a key role in the physics of polariton lattices. I will start with a general introduction to polariton physics and polariton lattices [1]. I will then present two recent experiments realized at C2N in 1D lattices. In the first experiment, we investigated the non-linear optical properties of a 1D topological lattice under resonant excitation, and found pumping conditions where the nonlinear steady-state triggers the emergence of an edge state in the Bogoliubov excitation spectrum [2]. In the second experiment, we explored the physics of out-of-equilibrium Bose-Einstein condensates and evidenced universal scaling laws related to the Kardar–Parisi–Zhang universality class [3]. References: [1] C. Ciuti and I. Carusotto, Quantum fluids of light, Rev. Mod. Phys. 85, 299 (2013). [2] N. Pernet et al., Gap solitons in a one-dimensional driven-dissipative topological lattice, Nature Physics 18, 678 (2022). [3] Q. Fontaine et al., Observation of KPZ universal scaling in a one-dimensional polariton condensate, arXiv:2112.09550 (2021).

Liens :

• Séminaire LPMMC
• Personne à contacter : Bart van Tiggelen ()
• Lieu : G 421

Résumé :

Liens :

• Séminaire théorie
• Personne à contacter :
• Lieu : G-421 (nouvelle salle au bâtiment G

Résumé : Weyl semimetal is a solid material with isolated touching points between conduction and valence bands in its Brillouin zone---Weyl points. Low energy excitations near these points exhibit a linear dispersion and act as relativistic massless particles. Weyl points are stable topological objects robust with respect to most perturbations. We study effects of weak disorder on the spectral and transport properties of Weyl semimetals in the limit of low energies. We use a model of Gaussian white-noise potential and apply dimensional regularization scheme near three dimensions to treat divergent terms in the perturbation theory. In the framework of self-consistent Born approximation, we find closed expressions for the average density of states and conductivity. Both quantities are analytic functions in the limit of zero energy. We also include interference terms beyond the self-consistent Born approximation up to the third order in disorder strength. These interference corrections are stronger than the mean-field result and non-analytic as functions of energy. Our main result is the dependence of conductivity (in units $e^2/h$) on the electron concentration $sigma = sigma_0 - 0.891 n^{1/3} et 0.115 (n^{2/3}/sigma_0) ln|n|$.

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter :
• Visio-conférence Zoom hybride.
  En présentiel : Salle Rémy Lemaire K223, Institut Néel
  Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 91808901596
  

Résumé : Recent years superconductor-semiconductor hybrid materials have been established as an essential platform for quantum devices, notably used in the search for Majorana zero modes and other bound states that may serve as qubits (1). In this talk we briefly discuss hybrid quantum dot physics and then look beneath the surface of the nanowire devices, addressing the advances in materials science and nanofabrication. We will describe how in-situ fabrication (2,3,5) and various superconductors (3,4) have been implemented in order to expand the available parameter space for hybrid quantum devices. The work is mainly based on Molecular Beam Epitaxy growth of III-V nanowires, high resolution electron microscopy and low temperature electron transport experiments. References: (1) E. Prada et al., From Andreev to Majorana bound states in hybrid superconductor-semiconductor nanowires, Nature Reviews Physics (2020) (2) T. Kanne et al., Double nanowires for hybrid quantum devices, Advanced Functional Materials (2021) (3) D. Carrad et al., Shadow Epitaxy for In Situ Growth of Generic Semiconductor/Superconductor Hybrids, Advanced Materials (2020) (4) T. Kanne et al., Epitaxial Pb on InAs nanowires for quantum devices, Nature Nanotechnology (2021) (5) J. Sestoft et al, Scalable Platform for Nanocrystal-Based Quantum Electronics, Advanced Functional Materials (2022)

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter : Jeremie Viennot ()
• Visio-conférence Zoom hybride.
  En présentiel : Salle Rémy Lemaire K223, Institut Néel
  Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 91808901596
  

Résumé : High impedance quantum circuits hold great potential for protected quantum bits and in general for coherent elements required for superconducting quantum processors. To reach the high impedance regime, we use granular Aluminum (grAl), a disordered superconductor which can be understood as a self-assembled Josephson junction array (1,2). One illustration of grAl's utility in quantum circuit design is the remarkable resilience of grAl fluxonium qubits (3,4) to photons populating its dispersively coupled readout resonator. This resilience allows single shot QND measurements (5) and quantum state preparation via active feedback with fidelity exceeding 90% even without using a parametric amplifier (6). An outstanding challenge is the mitigation of quasiparticle bursts (7) and long lived two level systems in the qubits' environment (8). References: (1) Maleeva et al. Nature Comm. 9, 3889 (2018) (2) Winkel et al. Phys. Rev. X 10, 031032 (2020) (3) Grunhaupt, Spiecker et al. Nature Materials 18, 816-819 (2019) (4) Rieger, Gunzler et al. arXiv:2202.01776 (5) Takmakov, Winkel, et al. Phys. Rev. App. 15, 064029 (2021) (6) Gusenkova, Spiecker, et al. Phys. Rev. App. 15, 064030 (2021) (7) Cardani, Valenti et al. Nat. Comm. 12, 2733 (2021) (8) Spiecker et al. arXiv:2204.00499

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter :
• Lieu : Salle Rémy Lemaire K223, Institut Néel

Liens :Artem Mishenko

• Séminaire nano-électronique quantique
• Personne à contacter : Denis Basko ()
• Visio-conférence Zoom hybride.
  En présentiel : Salle Rémy Lemaire K223, Institut Néel
  Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 91808901596
  

Résumé : Heavily used in classical signal processing, surface acoustic waves (SAWs) have also been proposed as a means to coherently couple distant solid-state quantum systems. Several groups have already reported the coherent coupling of standing SAWs modes to superconducting qubits (1) In this seminar, I will describe our progress in coupling superconducting qubits to propagating SAWs. We can controllably release and capture individual itinerant photons, demonstrating that quantum state transfer as well as remote entanglement generation between superconducting qubits using phonons can be realized. Going a step further, I will show how two-phonon entanglement can also be generated and used to realize a fundamental quantum optics experiment, quantum erasure (2), using phonons (3). In a second part, I will also briefly mention my current project on how superconducting circuits can also be used for detecting spins in samples coming from condensed matter or chemical or biological applications. References: (1) M. V. Gustafsson, et al, Science, 346, 207-211, 2014 (2) M. O. Scully and K. Druhl, Phys. Rev. A 25, 2208, 1982 (3) A. Bienfait et al., Phys. Rev X 10, 021055, 2020

Liens :

• Séminaire LPMMC
• Personne à contacter : Sergey Skipetrov ()
• Lieu : Amphithéâtre, maison des Magistères

Résumé : A salient feature of Weyl and Dirac materials is the possibility to realize the chiral anomaly due to their relativistic-like electronic spectra and nontrivial topology. In this seminar, I will present my recent results related to the manifestations of the chiral anomaly in sound attenuation and electromagnetic field penetration. Due to the interplay of intra- and inter-node scattering processes as well as screening, an external magnetic field generically reduces the sound absorption. A nontrivial dependence on the relative direction of the magnetic field and the sound wave vector, i.e., the magnetic sound dichroism, can occur in materials with nonsymmetric Weyl nodes. Also, I will demonstrate that the current response to an electromagnetic field in a Weyl or Dirac semimetal becomes nonlocal due to the chiral anomaly even under the conditions of the normal skin effect. Signatures of this nonlocality may be found in the transmission of electromagnetic waves.

Liens :

• Séminaire LPMMC
• Personne à contacter : Anna Minguzzi ()
• Lieu : Amphithéâtre, maison des Magistères

Résumé : Hot atomic vapors are widely used in non-linear and quantum optics due to their large Kerr non-linearity. This non-linearity induces effective photon-photon interactions allowing light to behave as a fluid displaying quantum properties such as superfluidity. In this presentation, I will show that we have full control over the Hamiltonian that drives the system and that we can engineer an analogue simulator with light. In particular, I will discuss 2 experiments:

1. Observation of a pre-thermal state in a 2D quantum fluid and the analogy with non-equilibrium BKT physics
2. Emergence of isotropy and inverse energy cascade in a 2D turbulent fluid light

Liens :Laboratoire Kastler Brossel

• Séminaire nano-électronique quantique
• Personne à contacter :
• Visio-conférence Zoom hybride.
  En présentiel : Salle Rémy Lemaire K223, Institut Néel
  Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 91808901596
  

Résumé : In this seminar, we will address the difficulties encountered in defining the internal energy of a quantum system when studying energy exchanges in open quantum systems. This is an essential aspect of the so-called quantum thermodynamics, which claims the formulation of theoretical machinery that could be consistent for both the system of interest and its environment. Here, we will see that there is a formulation in which such consistency is naturally present and recovers the usual thermodynamic aspects of internal energy, e.g., its additivity.

Liens :

• Séminaire LPMMC
• Personne à contacter : Benoît Vermersch ()
• Lieu : Amphithéâtre, maison des Magistères

Résumé : In the first part of the talk, we discuss the usage of near-term quantum computers in solving classical tasks. The noisy-intermediate scale quantum computers are composed of a small number of qubits, and can faithfully run only short circuits. This puts many proposed approaches for quantum machine learning beyond currently available devices. We address the problem of compressing classical data into efficient representations on quantum devices. Our proposed methods allow both the required number of qubits and depth of the quantum circuit to be tuned. We achieve this by using a correspondence between matrix-product states and quantum circuits, and further propose a hardware-efficient quantum circuit approach, which we benchmark on the Fashion-MNIST dataset. Finally, we demonstrate that a quantum circuit-based classifier can achieve competitive accuracy with current tensor learning methods using only 11 qubits. In the second part of the talk, we focus on the task of classifying the quantum phases of matter using a quantum convolutional neural network (QCNN). We describe a model-independent protocol to train the QCNN. We show that similar to the definition of quantum phases, the fixed-point wavefunctions together with the unitary representation of the symmetry group of the system provide sufficient information for the QCNN to learn the structure of the phases. We test the trained QCNN on several interacting and non-interacting spin chains exhibiting trivial, symmetry-breaking, and symmetry-protected topological order. We show that the location and topology of the phase boundary are accurately predicted. Our method provides a hardware-efficient and scalable way to perform quantum phase classification on a quantum processor. Furthermore, it opens up new ways to study the quantum phases and their symmetry by exploiting classical or quantum machine learning.

Liens :

• Séminaire théorie
• Personne à contacter : Serge Florens ()
• Lieu : Amphithéâtre, maison des Magistères

Résumé : Topological one-dimensional superconductors can sustain in their extremities zero energy modes that are protected by different kinds of symmetries. The observation of these excitations in the form of Majorana fermions is one of the most intensive quests in condensed matter physics. Their study is not only interesting in itself, but also because they have promising applications in the area of quantum computation. In this work we are interested in another class of one dimensional topo- logical systems, namely topological insulators. These also present symmetry protected end modes with robust properties and do not require the low temperatures necessary for topological super- conductivity. We consider the simplest kind of topological insulators, namely chains of atoms with hybridized sp orbitals. We study the transport properties of these chains in the trivial, non-trivial topological phases and at the quantum topological transition. We use a simple device consisting of two semi-infinite hybridized sp-chains connected to a quantum dot and obtain the thermoelectric properties of this system as a function of temperature and distance to the topological transition. We show that the electrical conductance and the Wiedemann-Franz ratio of the device at the topological transition have universal values at very low temperatures. The conductance and thermopower give direct evidence of fractional charges in these systems.

Liens :

• Séminaire interne LPMMC
• Personne à contacter : Vincent Rossetto ()
• Lieu : Amphithéâtre, maison des Magistères

Liens :LPMMC

• Séminaire interne LPMMC
• Personne à contacter : Vincent Rossetto ()
• Lieu : Amphithéâtre, maison des Magistères

Résumé : Recent theoretical work has shown that the competition between coherent unitary dynamics and stochastic measurements, performed by the environment, along wavefunction trajectories can give rise to novel measurement-induced phase transitions (MITs). The latter are characterized by a change in the scaling law for the entanglement entropy along quantum trajectories. First instances of such transitions have been discovered in quantum circuits of random unitaries interspersed with measurements, and subsequently also in the dynamics of other open and monitored quantum systems. Often, the competition arises between a coherent unitary evolution via a Hamiltonian or quantum gates and destructive dissipative dynamics, such as projective measurements. There, the unitary evolution drives the system towards highly entangled states while the local projective measurements partially collapse the system wavefunction and thereby reduce the entanglement. In our work, we consider a new and complementary scenario, in which it is engineered dissipative dynamics that drives the system to an entangled state, while competing Hamiltonian dynamics tends to reduce the entanglement. In this framework, we establish the existence of a MIT, and characterize its properties. As a key finding, we show that the scaling of the entanglement entropy indicates a log-to-area law transition.

Liens :Thomas BotzungInstitute for Quantum Information, RWTH Aachen University

• Séminaire nano-électronique quantique
• Personne à contacter : Jeremie Viennot ()
• Visio-conférence Zoom hybride.
  En présentiel : Salle Rémy Lemaire K223, Institut Néel
  Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 91808901596
  

Résumé : The possibility to operate massive mechanical oscillators in the quantum regime has become central in fundamental sciences. Optomechanics, where photons are coupled to mechanical motion, provides the tools to control mechanical motion near the fundamental quantum limits. Our setup (1) consists of a magnetic field sensitive cavity coupled to a magnetic cantilever, a beam equipped with a magnet on its tip, leading to a position dependent magnetic field. A SQUID embedded in our superconducting cavity provides the sensitivity to magnetic fields. In this magneto-mechanical system, we achieve single photon coupling strength, which are among the highest in the field and more than a factor of ten larger compared to other electro-mechanical systems. Despite working at cryogenic temperatures, macroscopic mechanical objects (i.e. the cantilever) are in highly excited thermal states and need to be cooled close to the ground state in order to investigate quantum phenomena. We demonstrate a novel cooling scheme (2) by using the intrinsic nonlinearity of the cavity induced by the SQUID. We show, that the non-linearity has to be included in describing the back action and demonstrate a one order of magnitude improvement in the cooling compared to a linear system with comparable parameters. With our system it seems to be possible to overcome the back-action limit, which limits the cooling performance in linear cavities. References: (1) D. Zöpfl et al., Phys. Rev. Lett. 125, 023601 (2020); https://doi.org/10.1103/PhysRevLett.125.023601 (2) D. Zoepfl et al., arxiv:2202.13228 (2022)

Liens :

• Séminaire LPMMC
• Personne à contacter : Sergey Skipetrov ()
• Lieu : Amphithéâtre, maison des Magistères

Résumé : Concept of diffusion is widely used to describe propagation of light through multiple scattering media such as clouds, interstellar gas, colloids, paint, biological tissue, etc. Such media are often called random. This terminology is, however, misleading. Notwithstanding its complexity, the process of wave propagation is entirely deterministic – uniquely defined by the exact positions of scattering centers and the shape of the incident wavefront – making it possible to deduce the precise pattern of wave field throughout the system. Technological advances over the last decade enabled one to synthesize an arbitrary wavefields opening new frontier in light control inside strongly scattering media. Feasibility of the coherent control necessitates a general framework for predicting and understanding the ultimate limit for a targeted energy delivery into a diffusive system. In this talk, we will discuss such scientifically and technologically important questions as “How can one systematically find the incident wavefront that optimally deposits energy into a target region of arbitrary size and shape, deep inside a diffusive medium?” and “What is the ultimate limit on the energy enhancement in a region?” Predictable energy delivery opens the door to numerous applications, e.g., optogenetic control of cells, photothermal therapy, as well as probing and manipulating photoelectrochemical processes deep inside nominally opaque media.

Liens :Alexey Yamilov

• Séminaire LPMMC
• Personne à contacter : Vincent Rossetto ()
• Lieu : Amphithéâtre, maison des Magistères

Résumé : For atoms in their ground state, the interatomic forces and the interaction of atoms with light are well described with stationary quantum theory and semiclassical approaches. However, when atoms are excited, either by a pulse or by an incoherent pump, a fully quantum time-dependent approach becomes necessary. Within that framework, we will reveal the apparent violation of the action-reaction principle in the interaction between excited atoms, as well as the existence of non-conservative forces. In addition, we will show how to tailor the scattering properties of an atom with gains so as to obtain a PT-symmetry condition for null extinction.

References

• M. Donaire, R. Guérout, and A. Lambrecht, Phys. Rev. Lett. 115, 033201 (2015)
• M. Donaire, Phys. Rev. A94, 062701 (2016)
• M. Donaire, Phys. Rev. A104, 043704 (2021)
J. Sánchez-Cánovas, M. Donaire, arXiv:2111.12828

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter :
• Visio-conférence Zoom Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 91808901596
  

Résumé : We recently discovered a new kind of a superconducting qubit, the unimon, that can be fabricated using standard materials and techniques out of a single Josephson junction and a superconducting resonator, yet having higher anharmonicity than the transmon and resilience against charge and flux noise. Our first experiments on the unimon demonstrate single-qubit-gate fidelity of 99.9% stable for several hours without recalibration. In addition, we have developed qubit readout, reset, and control electronics that operates at millikelvin temperatures and can be integrated with the unimon in the future. These results have been obtained by the Quantum Computing and Devices (QCD) group in collaboration with several other groups. See https://www.aalto.fi/en/department-of-applied-physics/qcd-media for highlighted published results and https://arxiv.org/abs/2203.05896 for the preprint on the unimon. ATTENTION! Séminaire en visioconférence uniquement!

Liens :

• Séminaire théorie
• Personne à contacter :
• Lieu : Amphithéâtre, maison des Magistères

Résumé : Proximity-induced superconductivity in fractional quantum Hall edges is a prerequisite to proposed realizations of parafermion zero-modes. A recent experimental work (Gül et al., arXiv: 2009.07836) provided evidence for such coupling, in the form of a crossed Andreev reflection signal, in which electrons enter a superconductor from one chiral mode and are reflected as holes to another, counter-propagating chiral mode. Remarkably, while the probability for cross Andreev reflection was small, it was stronger for $ u=1/3$ fractional quantum Hall edges than for integer ones. We theoretically explain these findings, including the relative strengths of the signals in the two cases and their qualitatively different temperature dependencies. An essential part of our model is the coupling of the edge modes to normal states in the cores of Abrikosov vortices induced by the magnetic field, which provide a fermionic bath. We find that the stronger crossed Andreev reflection in the fractional case originates from the suppression of electronic tunneling between the fermionic bath and the fractional quantum Hall edges.

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter :
• Lieu : Salle Rémy Lemaire K223, Institut Néel et en visio: https://univ-grenoble-alpes-fr.zoom.us/j/91808901596?pwd=UWZ2cml2N1VBOEZBenk0d3RJek9rdz09#success

Résumé : Most quantum computers are built as lattices of qubits with nearest-neighbor couplings. This has several advantages: these machines are readily scaled and are well suited to error correction via surface codes. However, when operated as computers this architecture imposes a substantial overhead in implementing algorithms, as gates between distant qubits require swapping states across the lattice until they reach neighboring sites. These SWAP operations can easily dominate the gate count of the circuit, and thus limit the computational power of the quantum computer. In this talk, I will discuss our efforts to construct an alternative modular architecture for superconducting QCs via parametric gates and controls. Our scheme is based on a so-called SNAIL device whose three-wave couplings we exploit to controllably couple quantum modes. In this talk I will review our recent experimental efforts, especially our realization of four transmon all-to-all quantum modules and a quantum state router [1] which can link four modules with highly coherent operations, as well as the prospects for scaling to larger modular quantum processors. 1. A modular quantum computer based on a quantum state router C. Zhou, P. Lu, M. Praquin, T.-C. Chien, R. Kaufman, X. Cao, M. Xia, R. Mong, W. Pfaff, D. Pekker, M. Hatridge. arXiv:2109.06848 (2021).

Liens :Michael Hatridge

• Cours
• Personne à contacter :
• Lieu : Salle MAG2, maison des Magistères

Résumé : Resource theories - magic and non stabilizerness - Stabilizer Renyi Entropy (SRE) - SRE and quantum chaos - measurement of magic

Liens :

• Séminaire théorie
• Personne à contacter :
• Lieu : Amphithéâtre, maison des Magistères

Résumé : Quenched disorder, in the sense of random imperfections in a system, is both a blessing and a curse for scientists — it can give rise to a host of novel phenomena, but it also tends to impede transport and communication. Here we cover examples of both from our work on quantum dynamics, and in doing so touch on topics ranging from chaos to computing. In the first part, we discuss tunneling processes in “rugged energy landscapes”, of which the classic optimization problems from computer science (such as Traveling Salesman and Satisfiability) are examples. After describing how many such problems share essential features with mean-field spin glasses — long-range interactions, disorder, frustration — we summarize our understanding of the quantum dynamical phases in the latter. In the second part, we consider the opposite extreme of 1D nearest-neighbor spin chains. We describe how “Lieb-Robinson bounds” have proven to be an invaluable tool for studying both many-body dynamics and constraints on quantum information protocols. We then cover our recent work developing Lieb-Robinson bounds tailored to disordered spin chains.

Liens :

• Séminaire théorie
• Personne à contacter :
• Lieu : Amphithéâtre, maison des Magistères

Résumé : The plenty of available data on various strongly correlated electronic materials calls for the development of a general theory of the non-Fermi-liquid ground state(s) of an interacting many-body fermionic system. The recently proposed Sachdev-Ye-Kitaev (SYK) model of interacting fermions provides a new and fresh view of this old problem. It has recently attracted a lot of attention as a possible boundary theory of a two-dimensional gravitational bulk. This model, also, can be considered a nonlinear generalization of usual random-matrix Hamiltonians. In my talk, I will make an introduction to the SYK model. I will describe the mean-field solutions, its asymptotic symmetries and discuss the role of fluctuations. I also briefly show my results related to the stability of the SYK model with respect to perturbation. The last part of the talk covers transport and chaotic properties of the SYK-based model.

Liens :

• Cours
• Personne à contacter :
• Lieu : Salle MAG2, maison des Magistères

Résumé : Stabilizer formalism - Clifford group - simulability of quantum states and gates

Liens :

• Séminaire interne LPMMC
• Personne à contacter :
• Lieu : Amphithéâtre, maison des Magistères

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter :
• Visio-conférence Zoom hybride.
  En présentiel : Salle Remy Lemaire, Insitut Néel
  Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 91808901596
  

Résumé : Future quantum computers capable of solving relevant problems will require a large number of qubits that can be operated reliably(1). However, the requirements of having a large qubit count and operating with high-fidelity are typically conflicting. Spins in semiconductor quantum dots show long-term promise but demonstrations so far use between one and four qubits and typically optimize the fidelity of either single- or two-qubit operations, or initialization and readout (2,3,4,5,6,7,8). Here (9) we expand the number of qubits and simultaneously achieve respectable fidelities for universal operation, state preparation and measurement. We design, fabricate and operate a six-qubit processor with a focus on careful Hamiltonian engineering, on a high level of abstraction to program the quantum circuits and on efficient background calibration, all of which are essential to achieve high fidelities on this extended system. State preparation combines initialization by measurement and real-time feedback with quantum-non-demolition measurements. These advances will allow testing of increasingly meaningful quantum protocols and constitute a major stepping stone towards large-scale quantum computers. In this talk I will briefly review electron spin qubits and explain the results described above. 1. Vandersypen, L. M. K., et al., npj Quantum Information, vol. 3.1, pp. 1-10, 2017. 2. Veldhorst, M., et al, Nature nanotechnology, vol. 9.12, pp. 981-985, 2014. 3. Yoneda J., et al., Nature Nano, vol. 13, pp. 102-106, 2018. 4. Xue X., et al, Nature 601, 343–347, 2022 5. Noiri, A.et al., Nature 601, 338–342, 2022 6. Mills, A.et al., arXiv:2111.11937, 2021 7. Takeda K., et al., Nature Nano, pp. 1-5, 2021. 8. Hendrickx N. W., et al., Nature, vol. 591, pp. 580–585, 2021 9. Philips S., MÄ…dzik M, et al., https://arxiv.org/abs/2202.09252

Liens :

• Cours
• Personne à contacter :
• Lieu : Salle MAG2, maison des Magistères

Résumé : Introduction - Mathematical preliminaries - layers of quantum mechanical behavior: entanglement and speed up - quantum circuits and channels - universality

Liens :

• Colloque CPTGA
• Personne à contacter : Aurélien Barrau ()
• Lieu : Amphithéatre, Maison des Magistères, CNRS, Grenoble

Résumé : Thermodynamics provides a robust conceptual framework and set of laws that govern the exchange of energy and matter. Although these laws were originally articulated for macroscopic objects, nanoscale systems also exhibit “thermodynamic­-like” behavior – for instance, biomolecular motors convert chemical fuel into mechanical work, and single molecules exhibit hysteresis when manipulated using optical tweezers. To what extent can the laws of thermodynamics be scaled down to apply to individual microscopic systems, and what new features emerge at the nanoscale? I will describe some of the challenges and recent progress – both theoretical and experimental – associated with addressing these questions. Along the way, my talk will touch on non-equilibrium fluctuations, “violations” of the second law, the thermodynamic arrow of time, nanoscale feedback control, strong system-environment coupling, and quantum thermodynamics.

Liens :CPTGA 10 juin (Café

• Séminaire théorie
• Personne à contacter :
• Lieu : Amphithéâtre, maison des Magistères

Résumé : Extracting important information from a quantum system as efficiently and tractably as possible is an important subroutine in most quantum technologies. We present an efficient method for constructing an approximate classical description of a quantum state using very few measurements of the state. This description, called a classical shadow, can be used to predict many different properties. The required number of measurements is independent of the system size and saturates information-theoretic lower bounds [arXiv:2002.08953]. These quantum-to-classical converters pave the way for new synergies between (near-term) quantum computing and classical machine learning [arXiv:2106.12627]. Conversely, instances where they fail constitute promising candidates for new types of quantum advantage [arXiv:2112.00778].

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter :
• Visio-conférence Zoom hybride.
  En présentiel : Salle Rémy Lemaire K223, Institut Néel
  Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 91808901596
  

Liens :

• Séminaire théorie
• Personne à contacter :
• Lieu : Amphithéâtre, maison des Magistères

Résumé : Light-matter interaction represents the fundamental tool to probe and actively manipulate the properties of matter. In most cases, the investigation of strong light-matter interaction relies on coherent light sources to excite microscopic degrees of freedom. Recently, several proposals have suggested an alternative route based on the exploitation of the enhanced 'vacuum fluctuations' in confined geometries such as optical cavities. Despite the significant potential of this approach, the predictions based on simplified models of cavity QED can sometimes lead to contradictory results. In contrast to that, the properties of the light-matter interaction beyond simplified models remain poorly explored. In this seminar, I will discuss the general properties of the light-matter interaction as derived from the quantum many-body theory of photons coupled to the microscopic degrees of freedom in a solid-state system. I will consider the renormalization of electronic properties, the ground-state properties of light, and their dependence on cavity confinement. Eventually, I will compare classical and quantum aspects of the strong coupling regimes of the light-matter interaction.

Liens :

• Séminaire interne LPMMC
• Personne à contacter : Thierry Champel ()
• Lieu : Salle de lecture 2, maison des Magistères

Résumé : Cavity quantum electrodynamics has considerably developed recently thanks to the technological progress. Among the paradigmatic models of cavity quantum electrodynamics, the Dicke model, which describes the coupling of N atoms to the same cavity mode, can give rise, under some conditions, to the « superradiant » quantum phase transition (SQPT). Such a transition is predicted for a relatively high value of the light-matter coupling, which has actually been reached in the last five years in some systems, such as superconducting circuits and Landau polaritons. But while the technology is ready, the SQPT has never been observed at equilibrium because it requires also a diamagnetic energy smaller than a given threshold. I will develop a theory of cavity quantum electrodynamics for a two-dimensional electron gas in the presence of Rashba spin-orbit and Zeeman couplings and perpendicular magnetic field, coupled to a spatially nonuniform quantum photon field. I will show that the SQPT, can in principle occur through a pure in-plane Zeeman coupling, but it requires extremely small (unrealistic) quantum well widths or extremely fine tuning of the effective Landé factor which makes two Landau levels coincide. Landau level crossings can also be induced by the Rashba spin-orbit coupling and they promote the SQPT which can be obtained for certain values of the effective Landé factor and filling factors. In this case, the SQPT can occur for quantum well widths in the nanoscale.

Liens :LPMMC

• Séminaire nano-électronique quantique
• Personne à contacter : Jeremie Viennot ()
• Visio-conférence Zoom hybride.
  En présentiel : Salle Rémy Lemaire K223, Institut Néel
  Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 91808901596
  

Résumé : Recently developed semiconducting InAs nanowires with epitaxial superconducting Al and ferromagnetic insulator EuS shells display induced superconductivity with Zeeman-like splitting at zero external magnetic field (1). The intricate interplay between spin-orbit coupling, magnetic domains, and superconducting coherence gives rise to unique ground states and corresponding electrical properties. In this talk, I will discuss our latest experiments on spin-polarization of the induced superconductivity (2,3). References: (1) Y. Liu, et al., Nano Lett. 20, 456 (2020). (2) S. VaitiekÄ—nas, et al., Phys. Rev. B 105, L041304 (2022). (3) D. Razmadze, et al., arXiv:2204.03202 (2022).

Liens :

• Séminaire théorie
• Personne à contacter :
• Lieu : Amphithéâtre, maison des Magistères

Résumé : Tensor network states are used extensively as a mathematically convenient description of physically relevant states of many-body quantum systems. Those built on regular lattices, i.e. matrix product states (MPS) in dimension 1 and projected entangled pair states (PEPS) in dimension 2 or higher, are of particular interest in condensed matter physics. In this talk, I will try to answer the following general question: which features of MPS and PEPS are generic and which are, on the contrary, exceptional? Or to rephrase it: given an MPS or PEPS sampled at random, what are the features that it displays with either high or low probability? One property which we will focus on is that of having either rapidly decaying or long-range correlations. In a nutshell, the main result I will state is that translation-invariant MPS and PEPS typically exhibit exponential decay of correlations, at a provably high rate. I will show two distinct ways of getting to this conclusion, depending on the dimensional regime under consideration. Both yield intermediate results which are of independent interest, namely: the parent Hamiltonian and the transfer operator of such MPS and PEPS typically have a large spectral gap. If time allows, I will also present on-going attempts at quantifying the amount of genuinely multipartite entanglement in such random MPS and PEPS. The talk will be based mainly on a joint work with David Perez-Garcia, available at arXiv:1906.11682, and on some work in progress with Ion Nechita.

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter : Jeremie Viennot ()
• Visio-conférence Zoom hybride.
  En présentiel : Salle Rémy Lemaire K223, Institut Néel
  Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 91808901596
  

Résumé : Electrons, elementary particles of nonzero charge, spin, and mass, have long been perceived of as paradigmatic local quantum information carriers. Despite superior controllability and configurability, their practical performance as qubits via either motional or spin states depends critically on their material environment. I will discuss recent collaborative work where we have successfully trapped single electrons on a solid surface of neon in vacuum. By integrating an electron trap in a circuit quantum electrodynamics architecture, we achieve strong coupling between the motional states of a single electron and an on-chip microwave resonator. We further tune the system into a regime of dispersive coupling where we utilize microwave pulses to perform qubit gate operations and state readout, allowing us to characterize the coherence of this new qubit architecture. I will further discuss our plans for the next steps with his new qubit platform where we will couple to the electron’s spin which is expected to have coherence times measured in seconds.

Liens :

• Séminaire théorie
• Personne à contacter :
• Lieu : Amphithéâtre, maison des Magistères

Résumé : In this talk I will introduce an approach to study the non-equilibrium dynamics of extended quantum many-body systems, inspired by the Feynman-Vernon influence functional description of quantum baths. We take an open-quantum-system viewpoint and describe evolution of a local subsystem in terms of an influence matrix (IM) - an operator acting on the space of temporal trajectories of the subsystem. The IM fully encodes the effects of the many-body system on its local subregions, and thus characterizes its ability (or failure) to behave as an efficient bath. I will show that this complementary angle of attack on quantum many-body dynamics offers many advantages, both conceptually and practically. In one spatial dimension, space-time duality allows to write an exact linear self-consistency equation for the IM. This equation possesses remarkable solutions in a class of maximally chaotic quantum circuits corresponding to perfect Markovian dephasing dynamics of subsystems. Away from such special points, quantum many-body systems exert a non-Markovian influence on subsystems, associated with temporal entanglement (TE) in the IM. Analyzing a wide range of models with analytical methods and numerical matrix-product-state computations, we study the scaling of TE in several dynamical regimes, ranging from strongly chaotic to (quasi-)integrable and many-body localized. Based on recent works with Dmitry Abanin and Michael Sonner.

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter : Romain Maurand ()
• Visio-conférence Zoom hybride.
  En présentiel : Salle Rémy Lemaire K223, Institut Néel
  Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 91808901596
  

Résumé : Single-electron tunneling enables coupling mechanical vibrations to electrons by a large amount. In this talk, I will show how to use this coupling to create a nonlinear mechanical oscillator approaching the quantum regime, where the resulting quantum energy levels of the mechanical oscillator are no longer evenly spaced. We achieve this using carbon nanotube electromechanical resonators in the ultrastrong coupling regime, where the single-electron single-phonon electromechanical coupling can be up to 20 times larger than the mechanical frequency. Using mechanical nanotubes hosting multiple quantum dots, we expect that our approach may enable the realization of a mechanical qubit [1] and a quantum simulator of quantum matters featuring strong electron-phonon correlations [2]. [1] F. Pistolesi, A. N. Cleland, and A. Bachtold, Phys. Rev. X 11, 031027 (2021) [2] U Bhattacharya, T Grass, A Bachtold, M Lewenstein, F Pistolesi, Nano Lett. 21, 9661 (2021)

Liens :Adrian Bachtold

• Séminaire LPMMC
• Personne à contacter : Sergey Skipetrov ()
• Lieu : Amphithéâtre, maison des Magistères

Résumé :

Liens :

• Séminaire LPMMC
• Personne à contacter : Bart van Tiggelen ()
• Lieu : Amphithéâtre, maison des Magistères

Résumé : Anderson localization has been a intriguing phenomenon to physicists and mathematicians for now more than 6 decades. In particular, although firmly believed, the existence in 3D (and above) of a transition between localized and delocalized states has never been rigorously proven. Even more, the exact location of this transition (also called the "mobility edge") remains elusive. In this talk, we will show that a theoretical tool, the "localization landscape", casts a new light on the localization induced by a disordered potential. We will introduce the essential concepts and results obtained thanks to this tool, and show that the aforementioned mobility edge can be related to a percolation transition of the effective potential deduced from the localization landscape. We will finally present how these results can be connected to actual experimental measurements, especially in the context of cold atoms.

Liens :

• Séminaire théorie
• Personne à contacter :
• Lieu : Amphithéâtre, maison des Magistères

Résumé : The interplay between measurements and chaotic many-body systems has recently attracted extended interest within the quantum community in the context of measurement-induced phase transitions (MIPS). MIPS describes a phase transition of the entanglement entropy from a volume law to an area law when the measurement rate exceeded a certain critical value. The vast majority of the studies concerning MIPS were done for quantum systems but in principle, the interplay of measurements and chaotic dynamics could also lead to interesting phenomenology within the classical realm. In this talk, I will present one of the simplest models illustrating this interplay between an internal chaotic dynamics and a measurement process, explicitly a single random walker on the lattice undergoing continuous measurements. After presenting the model, I will show that in the limit of weak-measurement, the stochastic dynamics of the probability distribution can be mapped to the stochastic heat equation with in turn implies that the log probability follows a discrete KPZ equation via the Cole-Hopf transform. Finally, I will show numerical evidence that at higher measurement rates, a second growth regime of the width of the log probability emerges.

Liens :

• Séminaire théorie
• Personne à contacter : Adolfo G. Grushin ()
• Lieu : Amphithéâtre, maison des Magistères

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter :
• Visio-conférence Zoom hybride.
  En présentiel : https://univ-grenoble-alpes-fr.zoom.us/j/91808901596?pwd=UWZ2cml2N1VBOEZBenk0d3RJek9rdz09
  
  Identifiant de réunion : Salle Rémy Lemaire K223 code d'accès: Institut Néel
  

Résumé : Ever since the discovery of ultra-high mobility when placed onto an h-BN substrate, emerging physical phenomena (such as fractional quantum Hall effect, fractal Landau spectrum, and etc.) have been continuously found in graphene up to now. And along with the physical properties, the technique of transferring and stacking van der Waals layers itself has led to a new direction of research of moiré superlattice in recent years. Except for h-BN, researchers have been working on finding new substrates for graphene, in order to trigger exciting new physics in the resulting heterosystems. However, very few materials can be as potent as h-BN to serve as a substrate of graphene. In this talk, we will introduce our recent progresses in the finding of new substrates for graphene: by bring mono-layer or bernal-stacked bilayer graphene into contact with a few-layered antiferromagnetic insulator CrOCl, the resulted vertical heterostructures can give rise to an extraordinarily robust quantum Hall phase in monolayer graphene [1], and an exciton-enhanced insulator in bilayer graphene [2], which are attributed to the subtle coupling of graphene-CrOCl interface. Such interfacial coupling can be a simple yet very powerful technique in effectively engineering the quantum electronic states. [1] arXiv preprint arXiv:2110.02899. [2] arXiv preprint arXiv:2110.02921.

Liens :

• Séminaire LPMMC
• Personne à contacter : Benoît Vermersch ()
• Lieu : Amphithéâtre, maison des Magistères

Résumé : We present experimentally accessible quantities that can be used to identify different families of random entanglement states. In particular, we consider a ratio between low-order moments of the partially transposed reduced density matrix. We find that this ratio takes well-defined values in the thermodynamic limit for various families of entangled states. This allows to sharply distinguish each of these phases, in a way that can be understood from a quantum information perspective based on the spectrum of the partial-transpose density matrix. We analyze in particular the entanglement phase diagram of Haar random states, and the differences with respect to Clifford, matrix-product states, and fermionic Gaussian states. Our results can be used to experimentally test the mixed-state entanglement structure of quantum states formed in quantum computers and programmable quantum simulators.

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter :
• Visio-conférence Zoom hybride.
  En présentiel : Salle Rémy Lemaire K223, Institut Néel
  Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 91808901596
  

Résumé : Single-photon counters are essential for detecting weak incoherent electromagnetic radiation. In the optical domain, they are widely used to detect spontaneous emission from individual quantum systems, with applications in fluorescence microscopy, and in numerous areas of quantum technologies. In the microwave domain, operational single-photon counters have just recently been developed using superconducting quantum circuits (1), offering novel opportunities for detecting fluorescence or spontaneous emission at microwave frequencies. Here, we demonstrate the use of a microwave single-photon counter to detect the photons spontaneously emitted by a small ensemble of electron spins coupled to a superconducting micro-resonator (2). In this novel spin detection scheme, each click of the detector reveals the quantum jump of an individual spin from its excited to its ground state. Besides their fundamental interest, our results also constitute a novel methodology for Electron Spin Resonance spectroscopy, it paves the way toward the readout of individual electron spins for quantum sensing at the single molecule level and quantum computation with highly coherent electron spins (3) and their nuclear registers. (1) R. Lescanne, et al., Physical Review X, 10, 021038 (2020) (2) E. Albertinale, et al., Nature 600 7889, 434-438 (2021) (3) M. Le Dantec, et al., Science advances 7.51 (2021)

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter :
• Visio-conférence Zoom hybride.
  En présentiel : Salle Rémy Lemaire K223, Institut Néel
  Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 91808901596
  

Résumé : The achievement of high-quality epitaxial interfaces involving transition metal oxides gives a unique opportunity to engineer artificial materials where new electronic phases take place. The discovery of a high mobility two-dimensional electron gas (2-DEG) confined in a quantum well at the interface between two insulating oxides LaAlO3 and SrTiO3 is probably one of the most prominent examples in the field (1). Unlike more conventional semiconductor based quantum wells, conducting electrons at LaAlO3/SrTiO3 fill 3d-bands, which gives a favourable ground for the emergence of complex electronic phases. In particular, 2D superconductivity (2,3) and strong Rashba spin orbit coupling (4) have been reported in such interfaces. A key feature of these electronic systems lies in the possibility to control their carrier density by electric field effect, which results in gate-tunability of both superconductivity and Rashba spin-orbit coupling. In this talk, I will review some microwave transport measurements on LaAlO3/SrTiO3interfaces that evidence a transition from single-gap to two-gap s±-wave superconducting state driven by continuous and reversible electrostatic doping (5,6). I will also present the realization of top-gated LaAlO3/SrTiO3 devices whose physical properties, including superconductivity and Rashba spin-orbit coupling, can be tuned over a wide range of electrostatic doping, opening new perspectives for the realization of spintronics or mesoscopic devices. In particular, we have fabricated Quantum Point Contacts in an oxide interface, which exhibits a quantized conductance due to ballistic transport in a one-dimensional conducting channel. Finally, I will briefly discuss some recent experiments on the newly discovered superconducting 2-DEG in KTaO3 based heterostructures. (1) A. Ohtomo and H.Y. Hwang, Nature 427, 423 (2004). (2) A. Caviglia et al., Nature 456, 624–627 (2008). (3) J. Biscaras et al., Nature Communications 1, 89 (2010). (4) A. D. Caviglia et al., Phys. Rev. Lett. 104, 126803 (2010). (5) G. Singh et al., Nature Mat. 18, 948–954 (2019). (6) G. Singh et al., Phys. Rev. B 105, 064512 (2022). (7) A. Jouan et al. Nature Elec. 3, 201–206 (2020). (8) C. Liu,et al. Science 371, 716–721 (2021). Z. Chen et al. Science 372, 721–724 (2021).

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter : Jeremie Viennot ()
• Visio-conférence Zoom hybride.
  En présentiel : (ONLINE ONLY, not hybrid)
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  Identifiant de réunion : 91808901596
  

Résumé : Magnons enable the transfer of a magnetic moment or spin over macroscopic distances. In quantum Hall ferromagnets, it has been predicted that spin and charge are entangled, meaning that any change of the spin texture modifies the charge distribution. As a direct consequence of this entanglement, magnons should carry an electric dipole moment. Here we report evidence of this electric dipole moment in a graphene quantum Hall ferromagnet using a Mach-Zehnder interferometer. As magnons propagate across the insulating bulk, their electric dipole moment modifies the Aharonov-Bohm flux through the interferometer, affecting both the phase and visibility of the interference pattern. In particular, we relate the phase shift to the sign of this electric dipole moment, the loss of visibility to the flux of emitted magnons, and we show that the magnon emission is a Poissonian process. Finally, we probe the emission energy threshold of the magnons for transient states, between ν=0 and ν=1, and link them to the emergence of gapless mode predicted in the canted-antiferromagnetic phase at charge neutrality. The ability to couple the spin degree of freedom to an electrostatic potential is a property of quantum Hall ferromagnets that could be promising for spintronics.

Liens :

• Séminaire LPMMC
• Personne à contacter : Vincent Rossetto ()
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  Identifiant de réunion : 94573011629 code d'accès: 471269
  

Résumé : We consider the physical system of a polaron which is a model for a charged particle moving in a polarized crystal. Its quantum mechanical description is given by the Fröhlich model, introduced in 1937. We discuss the validity of the classical Landau-Pekar equations as an effective dynamics in the strong coupling limit. Moreover, we provide a definition of the effective mass of the classical polaron described by the Landau-Pekar equations. The resulting formula agrees with the prediction by Landau and Pekar in 1948. This is joint work with D. Feliciangeli, N. Leopold, D. Mitrouskas, B. Schlein and R. Seiringer.

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter : Jeremie Viennot ()
• Visio-conférence Zoom hybride.
  En présentiel : Salle Rémy Lemaire K223, Institut Néel
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  Identifiant de réunion : 91808901596
  

Résumé : Two-level systems and quasiparticles are believed to lie at the origin of noise and decoherence in superconducting quantum devices. For the development of quantum technologies, it is important to understand such systems, and possibly overcome the limitation they impose. A step further in this direction is unveiling their nature, which can be discerned in terms of how they interact with micro or nano-devices. To study the physics of two-level systems (TLSs) we make use of T-shaped biased resonators. Their resonance frequency and quality factor are probed simultaneously with a microwave interferometer while applying a dc or rf bias signal. Random telegraph variations are detected in both the resonators' frequencies and quality factors, allowing us to test their correlation, and investigate the time evolution of these parameters. We observe that a milli-Volt bias can tune the resonance frequency of TLSs, while a voltage of the order of few Volts completely changes the system dynamics. Using a rf bias we perform a two-tones spectroscopy analysis, and reveal a reduction of frequency fluctuations and quality factor at low frequency and high tone power. The effect of out of equilibrium phonons is analysed by using a gated superconducting nanowire. We study the suppression of superconductivity in sample geometries where the roles of electric field and electron-current flow can be clearly separated. Our results show that suppression of superconductivity does not depend on the presence or absence of an electric field at the surface of the nanowire, but requires a current of high-energy electrons. Our observations question existing interpretations and theories based on electric fields and contribute towards understanding the complex interactions between out-of-equilibrium phenomena in solids and performance of superconducting hardware.

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter : Cecile Repellin ()
• Visio-conférence Zoom hybride.
  En présentiel : Salle Rémy Lemaire K223, Institut Néel
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  Identifiant de réunion : 91808901596
  

Résumé : Fractional quantum Hall states are known to support quasiparticles that are fractions of electrons [1,2]. When combined with superconductivity, these are predicted to give rise to parafermionic zero modes — a fractional generalization of Majorana zero modes [3,4]. I will provide an introduction to the physics of parafermions, briefly describe how they can be useful for topologically protected quantum computation [5], and then discuss a parafermionic Kondo problem [6]. Parafermionic Kondo is a remarkable effect which on one hand is beautiful and counterintuitive, and on the other hand can provide strong signatures for parafermions without needing a proper quantum-computing-ready setup. [1] L. Saminadayar, D. C. Glattli, Y. Jin, and B. Etienne, “Observation of the $e/3$ Fractionally Charged Laughlin Quasiparticle,” Phys. Rev. Lett. 79, 2526–2529 (1997). [2] R. De-Picciotto, M. Reznikov, M. Heiblum, V. Umansky, G. Bunin, and D. Mahalu, “Direct observation of a fractional charge,” Nature 389, 162–164 (1997). [3] N. H. Lindner, E. Berg, G. Refael, and A. Stern, “Fractionalizing Majorana Fermions: Non-Abelian Statistics on the Edges of Abelian Quantum Hall States,” Phys. Rev. X 2, 041002 (2012). [4] D. J. Clarke, J. Alicea, and K. Shtengel, “Exotic non-abelian anyons from conventional fractional quantum Hall states,” Nat. Commun. 4, 1348 (2013). [5] K. Snizhko, R. Egger, and Y. Gefen, “Measurement and control of a Coulomb-blockaded parafermion box,” Phys. Rev. B 97, 081405 (2018). [6] K. Snizhko, F. Buccheri, R. Egger, and Y. Gefen, “Parafermionic generalization of the topological Kondo effect,” Phys. Rev. B 97, 235139 (2018).

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter : Cecile Repellin ()
• Visio-conférence Zoom hybride.
  En présentiel : Salle Rémy Lemaire K223, Institut Néel
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  Identifiant de réunion : 91808901596
  

Résumé : A system described by a non-Hermitian Hamiltonian will, in general, have complex energies and non-orthogonal eigenstates. The degeneracies of such a system are known as exceptional points. Near these degeneracies, the complex energies are described by Riemann manifolds whose topology enables new methods of control over the system. Using a superconducting circuit QED platform we employ dynamical control over an effective non-Hermitian Hamiltonian to utilize the topology of its complex energy surfaces to control quantum state vectors. If a quantum system is initialized in one eigenstate, and the Hamiltonian parameters are varied slowly such as to encircle an EP, returning to the initial parameters, the topology of the Riemann manifold predicts that adiabatic evolution will switch the state to a different eigenstate. I will describe experiments where we observe this quantum state transport and use a quantum phase reference to measure the chiral geometric phases accumulated after this dynamical control.

Liens :Kater Murch

• Séminaire LPMMC
• Personne à contacter : Vincent Rossetto ()
• Visio-conférence Zoom hybride.
  En présentiel : Salle de lecture 2, maison des Magistères
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  Identifiant de réunion : 94573011629 code d'accès: 471269
  

Résumé : Understanding thermalization in closed quantum mechanics remains a subtle problem. As the occupancies of the Hamiltonian eigenstates are conserved, thermalization occurs when eigenstates close in energy share similar local observables. In this talk, I will discuss thermalization through the point of view of information. I will introduce the concept of information lattice, which allows us to represent the information flow using locality as an additional dimension. Thermalization then corresponds to a flow of the state-specific information to a non-local range. Based on this observation, I will propose a tentative algorithm that allows to simulate large scale dissipative systems at long times.

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter :
• Lieu : Salle Rémy Lemaire K223, Institut Néel

Liens :Maria Luisa DellaLaboratoir Matériaux et Phénomènes Quantiques (MPQ)

• Séminaire LPMMC
• Personne à contacter : Vincent Rossetto ()
• Visio-conférence Zoom hybride.
  En présentiel : Salle de lecture 2, maison des Magistères
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Résumé : In this talk, I argue that adiabatic sequences of arbitrary area-preserving deformations, acting unitarily on a planar Hall droplet, produce Berry phases that admit a closed-form expression in terms of the many- body current and density. This result is new on its own, but also exhibits several familiar concepts pertaining to the quantum Hall effect. The leading term of the many-body phase is thus super-extensive (proportional to N^2 for N electrons) owing to Aharonov-Bohm phases. Removing the latter in a gauge-invariant way leaves out an extensive geometric phase that only measures the edge current. In particular, the Berry curvature per electron associated with linear deformations is a quantized multiple of a hyperbolic area form, providing a bulk-edge correspondence for Hall viscosity. [Based on upcoming work with Benoit Estienne.]

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter : Jeremie Viennot ()
• Visio-conférence Zoom hybride.
  En présentiel : Salle Rémy Lemaire K223, Institut Néel
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  Identifiant de réunion : 91808901596
  

Résumé : Full insight into the dynamics of a coupled quantum system depends on the ability to follow the effect of a local excitation in real-time. Here, we trace the free coherent evolution of a pair of coupled atomic spins by means of scanning tunneling microscopy. Rather than using microwave pulses, we use a direct-current pump-probe scheme to detect the local magnetization after a current-induced excitation performed on one of the spins. By making use of magnetic interaction with the probe tip, we are able to tune the relative precession of the spins. We show that only if their Larmor frequencies match, the two spins can entangle, causing angular momentum to be swapped back and forth (1). These results provide insight into the locality of electron spin scattering and set the stage for controlled migration of a quantum state through an extended spin lattice. (1) Free coherent evolution of a coupled atomic spin system initialized by electron scattering, L. M. Veldman, L. Farinacci, R. Rejali, R. Broekhoven, J. Gobeil, D. Coffey, M. Ternes, A. F. Otte, Science 372, 964 (2021).

Liens :

• Séminaire LPMMC
• Personne à contacter : Vincent Rossetto ()
• Visio-conférence Zoom hybride.
  En présentiel : Salle de lecture 2, maison des Magistères
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  Identifiant de réunion : 94573011629 code d'accès: 471269
  

Résumé : The violation of Bell's inequalities is probably the most striking manifestation of quantum entanglement, excluding all possible descriptions of a set of data in terms of a classical model (a "locally causal" model, in the words of J. S. Bell). While non-locality (that is: the violation of Bell's inequalities) has been demonstrated since the early 1980s for pairs of two-level systems, its exploration in more complex ensembles, and especially in quantum many-body systems, has remained quite unexplored until very recently. This is partly due to the lack of sufficiently scalable and flexible theoretical tools to prove the impossibility of any classical description of a given set of correlations. Yet, we will show in this talk that it is possible to devise such tools, resorting to an exact mapping between Bell's locally-causal models, and classical Ising models (or generalizations thereof). This leads to a constructive approach to probe Bell's non-locality in many-body systems, in which one tries to actively reproduce some set of observed correlations with a classical Ising model -- a so-called inverse Ising problem, well-known in many areas of data science. In contrast to typical data-science instances, however, it is the failure to find such a classical explanation to the data which ultimately leads to unveil its non-locality, in the form of new Bell's inequalities inferred from the data themselves. Both the general framework for this approach will be presented, as well as some specific results and extensions.

Liens :

• Séminaire LPMMC
• Personne à contacter : Denis Basko ()
• Lieu : Salle de lecture 2, maison des Magistères

Résumé : Dynamical localization is the quantum chaos analog of the Anderson localization of disordered systems, but in momentum space, as observed in the atomic quantum kicked rotor. Interactions tend to destroy (Anderson) localization, but strong enough disorder give rise to Many-Body Localization (MBL). Whether interactions destroy dynamical localization or not is addressed here. We discuss the effect of interactions on a one-dimensional kicked Bose gas in the strong interaction limit (Tonks regime), and show that dynamical localization is not destroyed by the interactions, in the sense that the energy of the system stays finite at long times. We show that this steady-state is ergodic, i.e. described by a thermal density matrix, with an effective temperature that depends on the kicking parameters and the number of particles. The one-body reduced density matrix of the gas decays exponentially at large distance, implying absence of coherence, while the momentum distribution's tail at large momenta is characterized by an effectively thermal Tan contact.

Liens :

• Séminaire LPMMC
• Personne à contacter : Vincent Rossetto ()
• Visio-conférence Zoom hybride.
  En présentiel : Salle de lecture 2, maison des Magistères
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  Identifiant de réunion : 94573011629 code d'accès: 471269
  

Résumé : Granular liquids are ubiquitous around us, from the physics of geological phenomena to the food processing industry. However, the description of their behaviour remains a challenge for fundamental physics. One of the most successful of such approaches is the so-called "µ(I)" law, a phenomenological law that describes with good accuracy experimental and numerical results, but still lacks theoretical support. In this seminar, I will present our recent works on the subject. From a set of fundamental equations, we managed to explain the µ(I) law from the competition between different time scales associated with fundamental processes within the granular flow. This shed a new light on the physics of these systems, and represents a first step towards the establishment of a complete theoretical framework to describe the physics of dense granular flows.

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter : Jeremie Viennot ()
• Visio-conférence Zoom hybride.
  En présentiel : Salle Rémy Lemaire K223, Institut Néel
  Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 91808901596
  

Résumé : Quantum Hall effects in 2D electron gas exemplify the earliest class of topological phases in solid state physics, characterized by an insulating bulk and gapless edge excitations. The quantum Hall effect has been a source of many concepts that have become essential in more general quantum many-body problems. The fractional quantum Hall states host fractional charges, neutral excitations, and unlike the integer quantum Hall states, they generally support counter-propagating chiral edge modes. The theoretical and the experimental search for such fractionalized particles continues to attract extensive attention. One reason for the enthusiasm is fundamental - as fractionalization can brilliantly exemplify the rich emergent long-range behavior that many-body systems can exhibit. Another reason is more pragmatic, as certain non-abelian fractionalized excitations form the basis of topological quantum computers that promise inherent immunity against errors. In this talk, I will present the role of neutral modes in the quantum Hall effect, as the highly sought after non-abelian excitations are often “charge-neutral.” Yet, there are bosonic neutral modes that plague the single-particle interferometers in the fractional quantum Hall regime.

Liens :

• Soutenance de thèse
• Personne à contacter : Vincent Rossetto ()
• Visio-conférence Zoom hybride.
  En présentiel : Amphithéâtre, maison Jean Perrin
  Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 94573011629 code d'accès: 471269
  

Liens :

• Séminaire LPMMC
• Personne à contacter : Vincent Rossetto ()
• Visio-conférence Zoom hybride.
  En présentiel : Salle de lecture 2, maison des Magistères
  Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 94573011629 code d'accès: 471269
  

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter : Jeremie Viennot ()
• Visio-conférence Zoom hybride.
  En présentiel : Salle Rémy Lemaire K223, Institut Néel
  Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 91808901596
  

Résumé : I will discuss two initial experiments in my group’s effort to probe quantum materials with circuit quantum electrodynamics. In the first experiment, we use coplanar waveguide resonators to probe superconductivity in a two-dimensional Al-InAs hybrid system. Our observations qualitatively agree with a theory of induced p±ip pairing that evolves into Bogoliubov-Fermi arcs at high magnetic field. In the second experiment, we study arrays of high-impedance Josephson junctions as a model system for the superconductor-insulator phase transition. Theory predicts that such chains should be insulating, but in experiment superconducting behavior is often observed. Fixing all parameters from microwave measurements, we find that apparent superconducting behavior can be quantitatively understood as the high-temperature fate of a melted insulator.

Liens :

• Séminaire LPMMC
• Personne à contacter : Bart van Tiggelen ()
• Visio-conférence Zoom hybride.
  En présentiel : Salle de lecture 2, maison des Magistères
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  Identifiant de réunion : 94573011629 code d'accès: 471269
  

Résumé :

Liens :

• Séminaire LPMMC
• Personne à contacter : Vincent Rossetto ()
• Visio-conférence Zoom hybride.
  En présentiel : Salle de lecture 2, maison des Magistères
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  Identifiant de réunion : 94573011629 code d'accès: 471269
  

Résumé : Recently, the booming field of quantum thermodynamics has been exploring the properties of heat engines involving phenomena specific to the quantum world, such as entanglement, coherent superpositions or quantum measurement. In particular, it has been noticed that the unavoidable perturbation induced by a quantum measurement can be seen as a thermodynamic transformation, generating a change of entropy and energy. A measuring apparatus can be seen as analogous to a heat bath, and engines able to convert the energy provided by the measurement into heat have been proposed. However, the performances of these engines cannot be completely understood without a deeper description of the energy flows involved in a quantum measurement. We perform such analysis in the case of a refrigerator fueled by a continuous measurement of charge in a nanoelectronic system. We show that for a well chosen range of parameters, the measurement-induced perturbation induces a heat flow from a cold to a hot electron reservoirs. By analyzing a realistic microscopic model for the measuring apparatus – based on an electronic tunnel junction – we compute the work and heat flows involved by the measurement and evaluate the thermodynamic efficiency of the refrigerator as a function of the parameters of the measuring apparatus. We show how the engine complies with Carnot efficiency, paving the road towards energetic optimization of quantum measurements and protocols that involve them.

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter :
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  Identifiant de réunion : 91808901596
  

Résumé : One of the main directions to realize the topological quantum bit is combining topological materials with conventional superconductors. The notion of topological phases has been extended to higher-order and has been generalized to different dimensions. In the last few years, our research demonstrates the possibility of realizing the topological superconductivity in engineered 3D Dirac semimetals [1,2] and their 1D hinge states. Particularly, Cd3As2 is predicted to be a higher-order topological semimetal, possessing three-dimensional bulk Dirac fermions, two-dimensional Fermi arcs [3], and one-dimensional hinge states [4]. These topological states have different characteristic length scales in electronic transport, allowing one to distinguish their properties when changing sample size. [1] Li, C. et al. Nat. Mater. 17, 875–880 (2018). [2] Wang, A. Q. et al. Phys. Rev. Lett. 121, 237701 (2018). [3] Li, C.-Z. et al. Nat. Commun. 11, 1150 (2020). [4] Li, C.-Z. et al. Phys. Rev. Lett. 124, 156601 (2020).

Liens :Twente University (NL)

• Séminaire théorie
• Personne à contacter : Adolfo Grushin ()
• Lieu : Amphithéâtre, maison des Magistères

Résumé : At strong repulsion, the triangular-lattice Hubbard model is described by S=1/2 spins with nearest-neighbor antiferromagnetic Heisenberg interactions and exhibits conventional 120° order. When decreasing the interaction, additional four-spin interactions are naturally generated from the underlying Mott-insulator physics of electrons. Although these interactions have historically been connected with a gapless ground state with emergent spinon Fermi surface (SFS), we find that at physically relevant parameters, they stabilize a chiral spin-liquid (CSL) of Kalmeyer-Laughlin (KL) type, clarifying observations in recent studies of the Hubbard model. We then present a self-consistent solution based on a mean-field rewriting of the interaction to obtain a Hamiltonian with similarities to the parent Hamiltonian of the KL state, providing a physical understanding for the origin of the CSL. Using a combination of the infinite density matrix renormalization group and exact diagonalization, we also study the wider phase diagram of the spin model, shedding light on the fate of the SFS state. Finally, we will comment on experimental systems in which triangular lattice Hubbard model physics might be observed. Ref.: Phys. Rev. Lett. 127, 087201 (2021).

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter : Cecile Repellin ()
• Visio-conférence Zoom hybride.
  En présentiel : Salle Rémy Lemaire K223, Institut Néel
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  Identifiant de réunion : 91808901596
  

Résumé : We review the conceprt of semiclassical topology in multiterminal superconducting nanostructures where the gapped states are characterzed by topological numbers. The topologicical protection requires these states to be separated by the gapless one: sometimes this work, sometimes not so a protection-unprotection transition can happen. We build up a general theoretical description of the transition vicinity in the spirit of Landau theory. We also discuss the interaction effects near the special points of the spectrum and prove a drastic effect of weak interaction.

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter : Romain Maurand ()
• Visio-conférence Zoom Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 91808901596
  

Résumé : Quantum many-body systems display rich phase structure in their low-temperature equilibrium states. However, much of nature is not in thermal equilibrium. Remarkably, it was recently predicted that out-of-equilibrium systems can exhibit novel dynamical phases that may otherwise be forbidden by equilibrium thermodynamics, a paradigmatic example being the discrete time crystal (DTC). Concretely, dynamical phases can be defined in periodically driven many-body localized systems via the concept of eigenstate order. In eigenstate-ordered phases, the entire many-body spectrum exhibits quantum correlations and long-range order, with characteristic signatures in late-time dynamics from all initial states. It is, however, challenging to experimentally distinguish such stable phases from transient phenomena, wherein few select states can mask typical behavior. Here [1] we implement a continuous family of tunable CPHASE gates on an array of superconducting qubits to experimentally observe an eigenstate-ordered DTC. We demonstrate the characteristic spatiotemporal response of a DTC for generic initial states. Our work employs a time-reversal protocol that discriminates external decoherence from intrinsic thermalization, and leverages quantum typicality to circumvent the exponential cost of densely sampling the eigen-spectrum. In addition, we locate the phase transition out of the DTC with an experimental finite-size analysis. Our results establish a scalable approach to study non-equilibrium phases of matter on current quantum processors. 1. Mi et al, arXiv:2107.13571

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter :
• Visio-conférence Zoom hybride.
  En présentiel : Salle Rémy Lemaire K223, Institut Néel
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  Identifiant de réunion : 91808901596
  

Résumé : Electrical resistance usually originates from lattice imperfections. However, even a perfect lattice has a fundamental resistance limit, given by the Landauer conductance of its discrete modes. This resistance, shown by Sharvin to appear at the contacts to electronic devices, sets the ultimate conduction limit of non-interacting electrons. Recent years have seen growing evidence of hydrodynamic electronic phenomena, prompting recent theories to ask whether liquid electrons can radically break this fundamental Landauer-Sharvin limit. Here, we use single-electron-transistor imaging of electronic flows in high-mobility graphene Corbino devices to answer this question. First, by imaging ballistic flows at low temperatures, we observe a Landauer-Sharvin resistance that does not appear at contacts but is instead distributed throughout the bulk. This underpins the phase-space origin of this resistance - as emerging from spatial gradients in the number of conduction modes. At elevated temperatures, we identify and account for the contribution of electron-phonon scattering and reveal the pure hydrodynamic flow. Strikingly, we find that hydrodynamic electron flow eliminates completely the bulk Landauer-Sharvin resistance. Finally, by adding small magnetic fields, we image swirling magneto-hydrodynamic flows, revealing the key emergent length-scale predicted by hydrodynamic theories – the Gurzhi length. These observations demonstrate that electronic fluids can dramatically overcome the limitations of ballistic electrons, with important implications for fundamental science and future technologies.

Liens :

• Séminaire interne LPMMC
• Personne à contacter : Vincent Rossetto ()
• Visio-conférence Zoom hybride.
  En présentiel : Salle de lecture 2, maison des Magistères
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  Identifiant de réunion : 94573011629 code d'accès: 471269
  

Résumé : Après un résumé sur les émissions de gaz à effet de serre et les objectifs de réduction annoncés, je vous présenterai les différentes rubriques du bilan carbone de l'Institut Néel (réalisé par le collectif "labo en transition"): missions, achats, déplacements domicile-travail, énergies, expériences sur les grands instruments, informatique etc. J'exposerai brièvement les politiques de réduction envisagées à court terme au laboratoire.

Liens :Institut Néel

• Séminaire nano-électronique quantique
• Personne à contacter : Romain Maurand ()
• Visio-conférence Zoom hybride.
  En présentiel : Salle Rémi Lemaire
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  Identifiant de réunion : 91808901596
  

Résumé : Recent years’ hunt for the Majorana fermions generated a genuine interest in superconducting subgap (Yu-Shiba-Rusinov or shortly Shiba) states both in the field of STM measurements and hybrid semiconductor-superconductor devices. The better understanding of the trivial Shiba states paves the way to engineering them into topological states. Previously, devices, quite similar to the hybrid Majorana devices, were investigated from the viewpoint of Cooper pair splitting. These works aimed to generate spatially separated entangled electron pairs in a controlled way. In my talk, after shortly introducing these research fields, I will present a recent work, where we studied the interplay of a Shiba state and the splitting process in a Cooper pair splitter device. Changing the tunnel coupling to one of the normal electrodes allows to tune the device from limit of Cooper pair splitting to the non-local spectroscopy of the Shiba state. First, focusing on the latter limit, I will demonstrate that we could observe the Shiba state at much larger distances than in recent STM measurements. Then, I will discuss how the presence of the Shiba state, producing a non-local signal similar to the pair splitting, affects splitting efficiency. Z.S. et al., Nature Communications 11, 1834 (2020) Z.S. et al., ArXiv: 2108.12155

Liens :

• Soutenance de thèse
• Personne à contacter :
• Visio-conférence Zoom hybride.
  En présentiel : Amphithéâtre, maison des magistères
  
  Identifiant de réunion : https://univ-grenoble-alpes-fr.zoom.us/j/4873487282
  

Résumé : This thesis is dedicated to the study of ground states of an anyon gas in the large number of particle limit. In two-dimensional space there are possibilities for quantum statistics continuously interpolating between the bosonic and the fermionic one. Quasi-particles obeying such intermediate statistics are called anyons. They can be described as ordinary bosons and fermions with interactions through long-range magnetic potentials, generated by magnetic charges carried by each particle. We study the almost-bosonic and almost-fermionic limit. We obtain rigorous results for the convergence of their ground state energies and the associated minimizers to those of effective models. The ground state of almost-bosonic anyons converges to the infimum of a Hartree-like functionnal and its minimizers to a convex superposition of factorized pure states. The particles behave like independent, identically distributed bosons interacting via a self-consistent magnetic field. The ground state energy of almost-fermionic anyons converges to the infimum of a Thomas-Fermi energy and its minimizers to measures associated with the corresponding semi-classical problem. More precisely, the ground state of our Hamiltonian converges to that of a classical modified Vlasov energy whose minimization leads to the Thomas-Fermi functional. The Vlasov energy is endowed with a self-consistent magnetic field, a landmark of anyonic statistics. The ground state of the Vlasov energy displays anyonic behavior in its momentum distribution.

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter : Cecile Repellin ()
• Visio-conférence Zoom hybride.
  En présentiel : Salle Rémy Lemaire K223, Institut Néel
  Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 91808901596
  

Résumé : Twisted bilayer graphene (tBG) and variants like twisted monolayer-bilayer graphene (tMBG) were proposed to be a platform for strongly correlated physics akin to the cuprate family. However, I will show that many of the observed interacting phenomena can be explained in terms of breaking of spin/valley symmetry. This can lead to a quantum anomalous Hall effect in the absence of a field, as I will show for tMBG [1]. In large magnetic fields the same spin- valley symmetry breaking leads to a series of Chern insulator states [2]. Finally, I will briefly discuss the possibility of genuine strong correlated physics in Moiré structures. [1] Rademaker, Protopopov, Abanin, Phys. Rev. Research, 2, 033150 (2020). [2] Saito, Ge, Rademaker, Watanabe, Taniguchi, Abanin, Young, Nature Physics, 108, 12233 (2021).

Liens :Louk Rademaker

• Séminaire
• Personne à contacter : Michele Filippone ()
• Lieu : Amphithéâtre, maison des Magistères

Résumé : The quantized magnetic excitations (magnons) of magnetic dielectrics can couple to microwave and optical photons, making such systems prospect hybrid quantum platforms. Prospect applications of the system includes quantum transduction between optics and microwaves, and generation of non-classical macroscopic magnetization states. Nevertheless, one of the limitations of the system is the weakness of the coupling between magnons and optical photons - a consequence of the weakness of the magneto-optical effects in dielectrics. In this talk, I present the concept of a novel platform for magnon-photon interaction based on a magnetic epsilon-near-zero (ENZ) medium, in which strong coupling can be achieved. In ENZ media, the diagonal components of the permittivity tensor vanish at specific frequencies, yielding an enhancement of non-linear effects and secondary responses, such as magneto-optical effects. After a brief review of magnon-photon interactions in magnetic dielectrics and the state-of-the-art optomagnonic systems, I will present a quantisation scheme that phenomenologically includes dispersion, which is the responsible for the ENZ behaviour. Within this framework, the enhancement of magneto-optical effects in ENZ media implies an enhancement of the magnon-photon coupling, which can surpass both magnon and photon decay rates, thus reaching the strong coupling regime. Finally, I discuss a possible measurement scheme for the strong magnon-photon coupling via the optics power spectrum, which exhibits multiple sidebands characteristic of the strong parametric coupling. The proposal of an ENZ-based magnon-photon platform pushes forward novel designs for magnonic systems, and can be the starting point for developing new magnon-based protocols.

Liens :Victor BittencourtMax-Planck Institute for the Physics of light

• Séminaire nano-électronique quantique
• Personne à contacter :
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Résumé : Over the last fifty years, the CMOS (Complementary-Metal-Oxide-Semiconductor) electronics industry has been continuously scaling down transistors in size, to increase performance and reduce power consumption. Nowadays, the smallest transistors in industry achieve 5nm features. As a result, those silicon structures tend to exhibit undesirable quantum effects for a classical transistor which appear to be new research opportunities for quantum information processing. In particular, it is nowadays possible to trap single electron spins in silicon quantum dots and perform high fidelity quantum gates (i). These demonstrations combined with the intrinsic properties of the silicon lattice (low spin orbit and hyperfine interaction) make CMOS device an excellent candidate for scalable quantum architectures. This presentation, will focus on the probing of electronic spins trapped in a CMOS device. In the first part I will present the basic characterization of a single electron spin and valley physics detected by standard charge sensing (ii). In the second part, I will show how we can operate a small array of quantum dot (iii) and how we can measure spin states inside. Finally, I’ll present the measurement of higher spin states by probing the quantum capacitance of the system (iv) and how we intend to scale up this measurement technique. (i) Veldhorst, M. et al. Nat. Nanotechnol. 9, 981 (2014). (ii) Spence, C. et al. In preparation (2021). (iii) Chanrion, E. et al. Phys. Rev. Appl. (2020). (iv) Lundberg, T. et al. Phys. Rev. X (2020).

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter : romain maurand ()
• Visio-conférence Zoom hybride.
  En présentiel : Salle Rémy Lemaire K223, Institut Néel
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  Identifiant de réunion : 91808901596
  

Résumé : In 2001, Gottesman, Kitaev and Preskill (GKP) proposed to encode a fully correctable logical qubit in grid states of a single harmonic oscillator. Although this code was originally designed to correct against shift errors, GKP qubits are robust against virtually all realistic error channels. Since this proposal, other bosonic codes have been extensively investigated, but only recently were the exotic GKP states experimentally synthesized and stabilized. These experiments relied on stroboscopic interactions between a target oscillator and an ancillary two-level system to perform non-destructive error syndrome measurements for the GKP code. In this talk, I will review the fascinating properties of the GKP code and the conceptual and experimental tools developed for trapped ions and superconducting circuits, which enabled quantum error correction of a logical GKP qubit encoded in a microwave cavity. I will describe ongoing efforts to suppress further logical errors, and in particular to avoid the apparition of uncorrectable errors stemming from the noisy ancilla involved in the GKP error syndromes detection.

Liens :

• Séminaire LPMMC
• Personne à contacter : Léonie Canet ()
• Visio-conférence Zoom hybride.
  En présentiel : Salle de lecture 2, maison des Magistères
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  Identifiant de réunion : 94573011629 code d'accès: 471269
  

Résumé : Methods from statistical physics have been applied to describe characteristics of cloud fields. Analogies to two-dimensional lattice models have been found, in particular for tropical convection. Some physical parametrisations of clouds make use of those ideas. But so far these parametrisations are applied in single model columns, such that horizontal correlations are mostly neglected. The analogy of cloud fields to percolation models might lend itself to describe those horizontal correlations as well as cloud size distributions. To apply those descriptions on the various scales on which physical parametrisations within weather and climate modelling are used, the models should be defined in a scale-aware manner. Here, the functional renormalization group might be a method to derive an effective model on a given scale, in order to arrive at a most consistent treatment of physical processes across models of different resolution.

Liens :

• Séminaire interne LPMMC
• Personne à contacter : Vincent Rossetto ()
• Visio-conférence Zoom Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 94573011629 code d'accès: 471269
  

Liens :LPMMC

• Séminaire interne LPMMC
• Personne à contacter : Vincent Rossetto ()
• Visio-conférence Zoom Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 94573011629 code d'accès: 471269
  

Liens :LPMMC

• Séminaire nano-électronique quantique
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  Identifiant de réunion : 91808901596
  

Résumé : There are many challenges to scaling the size of superconducting quantum computers. First and foremost, building processors with increasing numbers of qubits and longer coherence remains a daunting task. However, the success of superconducting quantum computing will also hinge on the development of many supporting technologies such as cryogenics, signal delivery, and microwave readout. Here I will discuss approaches to two of these bottlenecks, lying at the interface between quantum physics and engineering. First, I will discuss the challenges of wiring a million-qubit processor with coaxial lines, and how using photonic links can enable the use of optical fibers instead [1]. Second, I will discuss why superconducting quantum processors need nonreciprocal components, what are the limitations of the conventional microwave circulators, and how we intend to replace them with integrable nonreciprocal devices based on multimode parametric interactions [2]. [1] F. Lecocq, et al, Nature 591, 575-579 (2021) [2] F. Lecocq, et al, Phys. Rev. Lett. 126, 020502 (2021)

Liens :NIST

• Séminaire interne LPMMC
• Personne à contacter : Anna Minguzzi ()
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  Identifiant de réunion : 94573011629 code d'accès: 471269
  

Résumé : It is well known that a quantum circuit on n-qubits composed of Clifford gates with the addition of k non-Clifford gates can be simulated on a classical computer by an algorithm scaling polynomially in n and an exponentially in k. Our work in this direction focused on finding the number of resources needed to simulate a quantum chaotic behavior. In this talk, I will review some notions of simulability of quantum circuits and notions of Entanglement. I will show that the doping of a Clifford circuit with O(n) single qubit non Clifford resources is both necessary and sufficient to drive the transition to universal fluctuations of the purity.

Liens :University of Massachussetts

• Séminaire théorie
• Personne à contacter : Serge Florens ()
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  Identifiant de réunion : 95666934704
  

Résumé : As a consequence of the approximate spin-valley symmetry in graphene, the ground state of electrons in graphene at charge neutrality is a particular SU(4) quantum Hall ferromagnet. If only the Coulomb interaction is taken into account, this ferromagnet can appeal either to the spin degree of freedom or equivalently to the valley pseudo-spin degree of freedom. This freedom in choice is then limited by subleading energy scales that explicitly break the SU(4) symmetry, the simplest of which is given by the Zeeman effect that orients the spin in the direction of the magnetic field. In addition, there are also valley symmetry breaking terms that can arise from short-range interactions or electron-phonon couplings. Here, we build upon the phase diagram in order to identify the different skyrmions at filling $ u=0$ and spin waves at $ u=pm1$ that are compatible with these types of quantum-Hall ferromagnets. Similarly to the ferromagnets, the skyrmions at charge neutrality are described by the $ ext{Gr}(2,4)$ Grassmannian at the center, which allows us to construct the skyrmion spinors. The different skyrmion types are then obtained by minimizing their energy within a variational approach, with respect to the remaining free parameters that are not fixed by the requirement that the skyrmion at large distances from their center must be compatible with the ferromagnetic background. We show that the different skyrmion types have a clear signature in the local, sublattice-resolved, spin magnetization, which is in principle accessible in scanning-tunneling microscopy and spectroscopy. In addition to the skyrmions, we also describe the generalized spin waves at $ u=pm1$, where one encounters pure spin waves, valley-pseudospin waves as well as more exotic entanglement waves that have a mixed spin-valley character. Most saliently, the SU(4) symmetry-breaking terms do not only yield gaps in the spectra, but under certain circumstances, namely in the case of residual ground-state symmetries, render the originally quadratic (in the wave vector) spin-wave dispersion linear.

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter : Robert Whitney ()
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  Identifiant de réunion : 91808901596
  

Résumé : Quantum computing is receiving renewed interest with recent announcements from several players. The most obvious reason is that some quantum algorithms can solve in polynomial time the same tasks that are currently considered non-polynomial on classical computers. In recent years, analog quantum machines have appeared, of which the computers currently marketed by the Canadian company D-Wave are the first representatives, operating on a quantum accelerated annealing principle. From an abstract point of view, such a machine can be considered as an oracle specialized in solving an NP-hard optimization problem with an algorithm functionally analogous to simulated annealing but with quantum acceleration. In addition to the formal analogies between simulated annealing and quantum annealing, there is an analogy between the current state of the art and that of simulated annealing when it was introduced. The work carried out in the framework consists in understanding the functioning of such a machine and in determining to what extent it contributes to the solution of combinatorial problems. The challenge is to know whether or not there is an acceleration of a quantum nature in these machines compared to other computers. The idea is to provide a first study on the behavior of quantum annealing when confronted with a problem known to trap classical annealing. The bipartite matching problem was specifically chosen to be difficult to solve using simulated annealing. Comparing the performances between these two annealers gives a first benchmark and allows to better characterize their potentials.

Liens :

• Séminaire interne LPMMC
• Personne à contacter : Vincent Rossetto ()
• Visio-conférence Zoom Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 94573011629 code d'accès: 471269
  

Liens :LPMMC

• Séminaire nano-électronique quantique
• Personne à contacter :
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  Identifiant de réunion : 91808901596
  

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter : Cecile Repellin ()
• Visio-conférence Zoom Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 91808901596
  

Résumé : In the last five years, a new field has emerged at the intersection between Condensed Matter and Quantum Optics, denominated “Quantum Magnonics”. This field strives to control the elementary excitations of magnetic materials, denominated magnons, to the level of the single quanta, and to interface them coherently to other elementary excitations such as photons or phonons. The recent developments in this field, with proof of concept experiments such as a single-magnon detector, have opened the door for hybrid quantum systems based on magnetic materials. This can allow us to explore magnetism in new ways and regimes, has the potential of unraveling quantum phenomena at unprecedented scales, and could lead to breakthroughs for quantum technologies. A predominant role in these developments is played by cavity magnonic systems, where an electromagnetic cavity, either in the optical or microwave regime, is used to enhance and control the interaction between photons and magnons. In this talk, I will introduce the field and present some theoretical results from our group which aim to push the boundaries of the current state of the art.

Liens :Silvia Viola Kusminskiy

• Séminaire LPMMC
• Personne à contacter : Vincent Rossetto ()
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  Identifiant de réunion : 94573011629 code d'accès: 471269
  

Résumé : Abstract: In bosonic gases at thermal equilibrium, an external quadratic drive can induce a Bose-Einstein condensation described by the Ising transition, as a consequence of the explicitly broken U(1) phase rotation symmetry down to Z2. However, in physical realizations such as exciton-polaritons and nonlinear photonic lattices, thermal equilibrium is lost and the state is rather determined by a balance between losses and external drive. A fundamental question is then how nonequilibrium fluctuations affect this transition. Here, we show that in a two-dimensional driven-dissipative Bose system the Ising phase is suppressed and replaced by a nonequilibrium phase featuring Kardar- Parisi-Zhang (KPZ) physics. Its emergence is rooted in a U(1)-symmetry restoration mechanism enabled by the strong fluctuations in reduced dimensionality. Moreover, we show that the presence of the quadratic drive term enhances the visibility of the KPZ scaling, compared to two-dimensional U(1)-symmetric gases, where it has remained so far elusive.

Liens :Köln Universität

• Séminaire nano-électronique quantique
• Personne à contacter : Cecile Repellin ()
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  Identifiant de réunion : 91808901596
  

Résumé : In three-dimensional space, elementary particles are divided between fermions and bosons according to the properties of symmetry of the wave function describing the state of the system when two particles are exchanged. When exchanging two fermions, the wave function acquires a phase, φ=Ï€. On the other hand, in the case of bosons, this phase is zero, φ=0. This difference leads to deeply distinct collective behaviors between fermions, which tend to exclude themselves, and bosons which tend to bunch together. The situation is different in two-dimensional systems which can host exotic quasiparticles, called anyons, which obey intermediate quantum statistics characterized by a phase φ varying between 0 and Ï€ (1,2). For example in the fractional quantum Hall regime, obtained by applying a strong magnetic field perpendicular to a two-dimensional electron gas, elementary excitations carry a fractional charge (3,4) and have been predicted to obey fractional statistics (1,2) with an exchange phase φ=Ï€/m (where m is an odd integer). I will present how fractional statistics of anyons can be demonstrated in this system by implementing and studying anyon collisions at a beam-splitter (5). The collisions are first studied in the low magnetic field regime, where the elementary excitations are electrons which obey the usual fermionic statistics. It leads to the observation of an antibunching effect in an electron collision: electrons systematically exit in two different arms of the beam-splitter. The observed result is completely different in the fractional quantum Hall regime. Fractional statistics lead to a suppression of the antibunching effect and quasiparticles tend to bunch together in larger packets of charge in a single output of the splitter. This effect leads to the observation of negative correlations of the current fluctuations (5) in perfect agreement with recent theoretical predictions (6). (1) B. I. Halperin, Phys. Rev. Lett. 52, 1583–1586 (1984). (2) D. Arovas, J. R. Schrieffer, F. Wilczek, Phys. Rev. Lett. 53, 722–723 (1984). (3) R. de Picciotto et al., Nature 389, 162–164 (1997). (4) L. Saminadayar, D. C. Glattli, Y. Jin, B. Etienne, Phys. Rev. Lett. 79, 2526–2529 (1997) (5) H. Bartolomei, M. Kumar et al. Science 368, 173-177 (2020). (6) B. Rosenow, I. P. Levkivskyi, B. I. Halperin, Phys. Rev. Lett. 116, 156802 (2016).

Liens :

• Séminaire interne LPMMC
• Personne à contacter : Vincent Rossetto ()
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  Identifiant de réunion : 94573011629 code d'accès: 471269
  

Résumé : The use of Rydberg states in order to engineer strong correlations in atomic quantum simulators has reached huge popularity in recent times, because of its flexibility in addressing different interesting regimes of quantum matter, and its push towards new interesting problems in many-body physics. An important experiment in one of these regimes showed long-lived oscillations after a quench, leading to the discovery of weak ergodicity breaking in its effective hamiltonian, then resulting in the discovery of many-body quantum scars. In this seminar, stability properties under perturbation of these peculiar states are addressed with a perturbative version of the fidelity susceptibility. Furthermore, a larger class of models with constrained dynamics is considered, showing the presence of many-body quantum scars in a broader setting. In the second part of the seminar, the focus shifts to the possibilities for equilibrium physics in a novel class of Rydberg atoms quantum simulator, based on optical lattices. Similar systems have been shown to host unconventional Luttinger liquids and exotic criticality in one dimension, in their effective descriptions, known as clustering models. Since these new setups allow for dynamics on coupled chains, quantum phases of clustering models on ladders are addressed.

Liens :LPMMC

• Séminaire nano-électronique quantique
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  Identifiant de réunion : 91808901596
  

Résumé : Because of their atomically-thin structure, high surface to volume ratio, and reduced electric screening, new properties and functionalities are expected to emerge when exploiting the interactions of two dimensional (‘2D’) materials placed in contact with other nanomaterials such as zero dimensional (‘0D’) systems including clusters, nanocrystals and molecules. These so-called Mixed-dimensional van der Waals Heterostructures are now at the forefront of basic nanoscience and applied nanotechnology, providing new sets of possibilities to tailor device functions and novel physical properties.[1] Today, I will present some of our recent achievements and on-going works illustrating the possibilities offered by 2D-0D heterostructures in various fields of nanoelectronics including single-electron electronics [2], molecular electronics[3] and optoelectronics[4]. These 0D-2D devices take advantage of the functionalities of the 0D systems (electronic, magnetic, optic…) and of the specific properties of 2D materials such as : i) van der Waals interface, ii) high diffusion of metals enabling self-ordered growth of nanoclusters, iii) dual electric behavior combining in-plane charge transport with out-of-plane electric field transparency (they are thinner than the Debye screening length), iv) exacerbated surface/interface sensitivity. The works selected for this talk will allow me to introduce some of these concepts. References : [1] D. Jariwala et al., Nat. Mater. 2017, 16, 170 ; [2] Mouafo et al, Adv. Mater. 2018, 30, 1802478; Mouafo et al., Adv.Func.Mat. 2021, 31, 2008255 ; Godel et al., Adv. Mater. 2017, 29, 1604837; [3] Noumbe et al., ACS Nano 2020, 14, 4567 ; [4] Konstantinov, J. Mat. Chem. C. 2021, 9, 2712.

Liens :Jean-François DayenIPCMS, Strasbourg

• Séminaire nano-électronique quantique
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Résumé : Travelling wave parametric amplifiers (TWPAs) recently became crucial tools in superconducting quantum technologies since they allow broadband and near quantum-noise-limited microwave detection. The main obstacle in developing TWPAs is the attainment of the phase matching condition between thepump field, which provides the energy for the amplification, and the signal field, to be amplified. I will present a new experimental approach to solving the phase matching problem by exploiting a Josephson metamaterial with in-situ tunability and sign reversal of the Kerr nonlinearity: reversed Kerr phase matching. Such novel reversed Kerr TWPA (1), composed of a chain of asymmetric Josephson junction-based inductive elements, shows amplification performance superior to the ones of previous state of the art TWPAs. In this talk, I will present the design, fabrication, and amplification characterization focusing on the novel reversed Kerr phase matching method. In addition, I will show recent experimental results on the generation of two-mode squeezed statesand discuss the exciting perspective of using such a device as a source of multimode entangled states in microwave photonics. (1) Ranadive et al.A reversed Kerr traveling wave parametric amplifier(2021)http://arxiv.org/abs/2101.05815

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter : Romain Maurand ()
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  Identifiant de réunion : 91808901596
  

Liens :

• Séminaire LPMMC
• Personne à contacter : Vincent Rossetto ()
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  Identifiant de réunion : 94573011629 code d'accès: 471269
  

Résumé : We consider strong two-body losses in quantum gases trapped in one-dimensional optical lattices, starting from the bosonic case. We exploit the separation of time scales typical of a system in the many-body quantum Zeno regime to establish a connection with the theory of the time-dependent generalized Gibbs ensemble. Our main result is a simple set of rate equations that capture the simultaneous action of coherent evolution and two-body losses. This treatment gives an accurate description of the dynamics of a gas prepared in a Mott insulating state and shows that its long-time behaviour deviates significantly from mean-field analyses. The possibility of observing our predictions in an experiment with 174Yb in a metastable state is also discussed. We then move to the fermionic case, where experiments with 173Yb have already been performed: our theoretical approach pinpoints the importance of spin in determining the full dynamics of the system.

Liens :

• Séminaire théorie
• Personne à contacter : Adolfo Grushin ()
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  Identifiant de réunion : 95666934704
  

Résumé : Weyl semimetals derive their name from the Weyl equation, the relativistic wave equation for massless spin-1/2 particles - this equation emerges as an effective low-energy theory of certain solid state compounds. This mathematical resemblance between solid state and relativity entails a number of intriguing physical parallels. One idea that has been promoted for some time is the possibility to engineer Hamiltonians that can be mapped to space-times with black holes. This talks will shed some light on what that means, and use actual lattice models to explore how far this analogy carries. Going down this road will lead us to, optimistically speaking, a poor man's version of stimulated Hawking radiation.

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter : Romain Maurand ()
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  Identifiant de réunion : 91808901596
  

Résumé : Quantum computing's greatest challenge is scaling up. Several decades ago, classical computers faced the same problem and a single solution emerged: very-large-scale integration using silicon. Today's silicon chips consist of billions of field-effect transistors (FinFETs) in which current flow along the fin-shaped channel is controlled by wrap-around gates. The semiconductor industry currently employs fins of sub-10 nm width, small enough for quantum applications: at low temperature, an electron or hole can be trapped under the gate and serve as a spin qubit. An attractive benefit of silicon's advantageous scaling properties is that quantum hardware and its classical control circuitry can be integrated in the same package. This, however, requires qubit operation at temperatures greater than 1 K where the cooling is sufficient to overcome the heat dissipation. Here, we demonstrate that a silicon FinFET is an excellent host for spin qubits that operate even above 4K. We achieve fast electrical control of hole spins with driving frequencies up to 150 MHz and single-qubit gate fidelities at the fault-tolerance threshold. The number of spin rotations before coherence is lost at these hot temperatures already matches or exceeds values on hole spin qubits at mK temperatures. While our devices feature both industry compatibility and quality, they are fabricated in a flexible and agile way to accelerate their development. This work paves the way towards large-scale integration of all-electrical and ultrafast spin qubits.

Liens :

• Séminaire interne LPMMC
• Personne à contacter : Vincent Rossetto ()
• Visio-conférence Zoom Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 94573011629 code d'accès: 471269
  

Liens :LPMMC

• Séminaire nano-électronique quantique
• Personne à contacter : Romain Maurand ()
• Visio-conférence Zoom Depuis un navigateur internet : rejoindre le {seminaire}.
  Identifiant de réunion : 91808901596
  

Résumé : When twisted to angles near 1 degre, graphene multilayers provide a new window on electron correlation physics by hosting gate-tuneable strongly-correlated states, including insulators, superconductors, and unusual magnets. I will report the discovery of a new member of the family(1), a correlated electron-hole state, in double bilayer graphene (2,3). This state can be viewed as a density wave state and is intimately linked to equilibrium exciton condensation and superfluidity. The correlated states in moiré materials can be tuned without introducing chemical dopants, thus opening the door to a new class of devices. I will show the successful realization of an exemplary device, a gate-defined Josephson Junction in magic angle twisted bilayer graphene (4). By using multilayer gate technology we define the superconducting and insulating regions of a Josephson junction (JJ) in a single-crystal and observe tunable DC and AC Josephson. This work is an initial step towards devices where gate-defined correlated states are connected in single-crystal nanostructures with applications in superconducting electronics and quantum information technology. (1) P.Rickhaus, F. K. De Vries, E. Portolés, G. Zheng, T. Taniguchi, K. Wantanabe, A. H. Macdonald, T. Ihn, and K. Ensslin, ArXiv:2005.05373 (2020). (2) F. K. de Vries, J. Zhu, E. Portolés, G. Zheng, M. Masseroni, A. Kurzmann, T. Taniguchi, K. Watanabe, A. H. MacDonald, K. Ensslin, T. Ihn, and P. Rickhaus, PRL 125 (17) (3) P. Rickhaus, G. Zheng, J. L. Lado, Y. Lee, A. Kurzmann, M. Eich, R. Pisoni, C. Tong, R. Garreis, C. Gold, M. Masseroni, T. Taniguchi, K. Wantanabe, T. Ihn, and K. Ensslin, Nano Lett. 19, 8821 (2019). (4) F. K. de Vries, E. Portolés, G. Zheng, T. Taniguchi, K. Wantanabe, T. Ihn, K. Ensslin, and P. Rickhaus, arXiv:2011.00011, accepted in Nat. Nano

Liens :Peter RickhausETH Zurich

• Séminaire LPMMC
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  Identifiant de réunion : 94573011629 code d'accès: 471269
  

Résumé : We show that combining randomized measurement protocols with importance sampling allows for characterizing entanglement in significantly larger quantum systems and in a more efficient way than in previous work. A drastic reduction of statistical errors is obtained using classical techniques of machine-learning and tensor networks using partial information on the quantum state. In present experimental settings of engineered many-body quantum systems this effectively doubles the (sub-)system sizes for which entanglement can be measured. In particular, we show an exponential reduction of the required number of measurements to estimate the purity of product states and GHZ states.

Liens :LPMMC

• Séminaire nano-électronique quantique
• Personne à contacter :
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  Identifiant de réunion : 91808901596
  

Résumé : In the last years, van der Waals heterostructures have attracted an increasing attention due to their outstanding properties, resulting from combining different two dimensional materials in compact structures, that have led to the emergence of new phenomena not accesible in other platforms (1). Among them, graphene constitutes a promissing material for spintronics because it enables the transport of spin signals over larger distances compared to other systems (2). However, its low intrinsic spin-orbit coupling difficults spin signal manipulation, which has prevented the fast application of graphene for spintronic devices. In this seminar, I will describe two of our recent studies performed in graphene-based heterostructures, where we demonstrate that spin signals can be generated and manipulated by means of proximity effects induced by spin-orbit phenomena. In the first part, I will show how the imprinted spin texture in graphene interfaced with a transition metal dichalcogenide give rise to an anisotropic spin relaxation, where the spin lifetime for spins oriented out-of-plane is one order of magnitude larger than those oriented in-plane (3). In the second part, I will show how such proximity-induced effects can be used to generate spin signals in graphene that can also be controlled by electrical gatting with one of the highest efficiency reported to date at room temperature (4). These results provide the building blocks for development of ultra-compact devices made of two dimensional materials. (1) W. Savero Torres et al. MRS Bulletin 45(5), 357-365, (2020) (2) W. Savero Torres et al. 2D Mat. 4, 041008, (2017) (3) L. A. Benítez, J.F. Sierra, W. Savero Torres et al. Nat. Phys. 14, 303-308, (2018) (4) L. A. Benítez, W. Savero Torres et al. Nat. Mat. 19, 170-175, (2020)

Liens :ICN2, Barcelona

• Séminaire nano-électronique quantique
• Personne à contacter :
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  Identifiant de réunion : 91808901596
  

Résumé : Topological transition transforms common superconductivity into an exotic phase of matter, which holds promise for fault tolerant quantum computing. A hallmark of this transition is the emergence of Majorana states. While two dimensional semiconductor superconductor heterostructures are desirable platforms for topological superconductivity, direct phase measurements as the fingerprint of the underlying topological transition are conspicuously missing. On gate tunable Josephson junctions made on epitaxial Al InAs, we observe a closing and a reopening of the superconducting gap with increasing in plane magnetic field. Since our junctions are embedded into a phase sensitive SQUID, we are able to measure a pi jump in the superconducting phase across the junction coincident with the closing and reopening of the superconducting gap. Theoretical simulations confirm this transition is topological and compatible with the emergence of Majorana states while the magnetic field angle dependence of the transition further constrain this scenario. Remarkably, in each junction, this topological transition can be controlled by changing the gate voltage. These findings reveal versatile two dimensional platforms for scalable topological quantum computing.

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter : Laetitia Marty ()
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  Identifiant de réunion : 91808901596
  

Résumé : We experimentally and theoretically investigate the thermoelectric properties of gate-tunable single-quantum dot junctions at milliKelvin temperatures, with particular focus on the strong tunnel coupling regime, Γ > k_BT. In the spin-1/2 Kondo regime of the quantum dot, we have observed that the Seebeck coefficient displays characteristic sign changes with varying temperature and level depth, in good agreement with numerical renormalization group calculations [1]. We then move to the question of heat transport across a single quantum level. While the heat conductance of a sequentially coupled single quantum level is expected to be uniformly equal to zero, we show both experimentally and theoretically, that the inclusion of cotunnelling effects leads to restoring a finite heat conductance [2]. [1] B. Dutta et al., Nano Lett. 19, 506 (2019). [2] B. Dutta et al., Phys. Rev. Lett. 125, 237701 (2020).

Liens :Clemens WinkelmannInst. Neel

• Séminaire interne LPMMC
• Personne à contacter : Anna Minguzzi ()
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  Identifiant de réunion : 94573011629 code d'accès: 471269
  

Résumé : We study the dynamics of a two spin component one-dimensional Fermi gas trapped in an harmonic potential. We focus on the strongly interacting regime, performing a perturbative analysis starting from the regime of infinite interactions, where the model can be solved exactly. In this regime, spin and density degrees of freedom decouple and evolve independently in time. We consider an out-of-equilibrium initial state where the spin up and spin down particles are spatially separated in the trap. In this case, the dynamics of the particle density is trivial, while the single-spin component densities oscillate in time. We address the dynamics in the spin sector, performing numerical diagonalization of the Hamiltonian for different number of particles and comparing the different frequencies of the spin oscillations.

Liens :LPMMC

• Séminaire nano-électronique quantique
• Personne à contacter : Cecile Repellin ()
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  Identifiant de réunion : 91808901596
  

Résumé : In this seminar, I will discuss how quantum technologies are now able to unveil and investigate novel fundamental phenomena by simulating interacting quantum systems. In particular, we will ask : Is it possible to harness and preserve the quantum coherent properties of many-body systems?This project seems doomed to fail, as interactions in many-body systems generally lead to ergodicity, namely the inevitable loss of quantum coherence and memory about initial conditions. Nevertheless, the recent discovery of many-body localization (MBL) – a generalization of Anderson localization in the presence of interactions – has shown the possibility to circumvent ergodicity. I will illustrate an experiment in which an array of superconducting qubits probes the exotic dynamics of interacting and disordered bosons (1). Relying on real-time and interferometric probes, I will discuss how we could observe and characterize the mechanism of MBL. (1) https://arxiv.org/abs/1910.06024

Liens :

• Séminaire interne LPMMC
• Personne à contacter : Vincent Rossetto ()
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  Identifiant de réunion : 94573011629 code d'accès: 471269
  

Résumé : Using the standard fluctuational electrodynamics framework, we analytically calculate the radiative heat current between two thin metallic layers, separated by a vacuum gap. We analyse different contributions to the heat current (travelling or evanescent waves, transverse electric or magnetic polarization) and reveal the crucial qualitative role played by the dc conductivity of the metals. For poorly conducting metals, the heat current may be dominated by evanescent waves even when the separation between the layers greatly exceeds the thermal photon wavelength, and the coupling is of electrostatic nature. For well-conducting metals, the evanescent contribution dominates at separations smaller than the thermal wavelength and is mainly due to magnetostatic coupling, in agreement with earlier works on bulk metals.

Liens :LPMMC

• Séminaire LPMMC
• Personne à contacter : Cécile Repellin ()
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  Identifiant de réunion : 94573011629 code d'accès: 471269
  

Résumé : I consider a model of fluid particle motion closely related to the Korteweg-de Vries equation governing shallow water dynamics. Using the reformulation of this model as a geodesic in an infinite-dimensional group, the drift velocity of particles is shown to be an ergodic rotation number, sensitive to Berry phases produced by adiabatic spatial deformations. Along the way, I show that the topology of coadjoint orbits of wave profiles affects drift in a dramatic manner: orbits that are not homotopic to a point yield quantized rotation numbers. These arguments rely on the general structure of Euler equations, suggesting the existence of other similar applications of infinite-dimensional geometry to nonlinear waves.

Liens :Blagoje Oblakhttps://www.cpht.polytechnique.fr/

• Séminaire LPMMC
• Personne à contacter : Anna Minguzzi ()
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  Identifiant de réunion : 94573011629 code d'accès: 471269
  

Résumé : Experiments with quantum simulators (made of ultracold atoms, trapped ions, etc.) are not only a way to realize low-energy states of quantum matter, but they also offer an unprecedented set of tools to control and analyze quantum dynamics. For systems that are well isolated from their environment, such dynamics is fully determined by the many-body Hamiltonian. In our theoretical study we consider spin models with power-law couplings; for small systems we describe the dynamics exactly, while we employ the time-dependent Variational Monte Carlo technique to treat larger sizes. We first focus on Quench Spectroscopy, an approach to characterize low-energy excitations on top of the ground state. Starting from an uncorrelated state, spin-spin correlations appear and propagate during time evolution. Their spectral analysis (in momentum and frequency) shows signatures of low-energy excitations, like the quasiparticles described by linear spin waves. Quench Spectroscopy is an alternative to traditional spectroscopy approaches, as those implemented through inelastic neutron scattering or Bragg spectroscopy. As a second application, we look at the dynamical signatures of a subtle property of the energy spectrum, namely the presence of Anderson's tower of states. These states are connected to the eigenstates of a simpler model, consisting of a large-spin rigid rotor. We show how this link can be unveiled in the time evolution of collective spin variables, and how it gives us information about the generation of squeezing during the dynamics.

Liens :

• Séminaire théorie
• Personne à contacter : Adolfo Grushin ()
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  Identifiant de réunion : 95666934704
  

Résumé : The Sachdev-Ye-Kitaev (SYK) model is an interaction-only toy model for quantum chaos, holography and non-Fermi liquids. In its simplest form, Majorana fermions interact via structureless all-to-all four-body interactions. In this talk, I will demonstrate that this system belongs to one of eight (real) Altland-Zirnbauer symmetry classes set only by the number of Majorana fermions. We show that, depending on the symmetry class, the system may support exact many-body zero modes. These are manifestations of an intrinsic supersymmetry that requires no relations between couplings, in contrast to existing explicitly supersymmetric extensions of the model. The supersymmetry we uncover has a natural interpretation in terms of a one-dimensional topological phase supporting Sachdev–Ye–Kitaev boundary physics and has consequences away from the ground state, including in q-body dynamical correlation functions. Finally, I will briefly talk about a dynamical protocol based on Majorana zero modes that realize SYK physics.