Séminaires LPMMC 2022
Federica Surace (Berkeley )  Détails Fermer 
Quantum simulation of lattice gauge theories with ultracold atoms le vendredi 25 novembre 2022 à 11:00 

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 alkalineearth(like) atoms in statedependent optical lattices. I will then illustrate how this quantum simulator can be used to probe various phenomena, including particle collisions and confinement. Liens : 
Maarten Wegewijs (RWTH Aachen et FZ Jülich)  Détails Fermer 
How quantum evolution with memory is generated in a timelocal way le mercredi 23 novembre 2022 à 11:00 

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 weakcoupling, 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 opensystem 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 realtime 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 "fixedpoint" relation between the system's memorykernel and its generalized, timedependent Lindblad generator. This result allows a series of longstanding 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. ReferenceK. Nestmann, V. Bruch, M. Wegewijs, Phys. Rev. X 11, 021041 (2021)Liens : 
Peter Zoller (Center for Quantum Physics, University of Innsbruck  IQOQI, Austrian Academy of Sciences)  Détails Fermer 
`Programming' Quantum Simulators with Atoms and Ions le mardi 22 novembre 2022 à 14:30 

Résumé : Progress in developing analog quantum simulation platforms is reflected in increasing control of engineered manybody Hamiltonians, and the ability to perform singlesite and singleshot 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 theoryexperiment 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 manybody 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 : 
Robert Whitney (LPMMC)  Détails Fermer 
Illusory Cracks in the Second Law of Thermodynamics in Quantum Nanoelectronics le lundi 21 novembre 2022 à 14:00 

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, todate, 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 thoughtexperiment, 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 
Stefan Ilic (CSIC Donostia  San Sebastian)  Détails Fermer 
Magnetoelectric effects and nonreciprocal transport in superconducting structures le mardi 15 novembre 2022 à 14:00 

Résumé : Recently, much attention is being paid to the study of superconducting systems whose transport properties depend on the direction of the supercurrents. Nonreciprocal transport effects have been proposed and observed in various structures motivated by creating a perfect superconducting diode. Besides possible applications, the physics of nonreciprocal effects is very rich, in particular in systems in which superconductivity coexists with spindependent 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 phiJosephson junctions, the helical phase of Rashba superconductors, and magnetoelectric effects induced by the spinorbit coupling. Liens : 
Sylvain Ravets (C2N)  Détails Fermer 
Drivendissipative physics in Polariton lattices le mardi 25 octobre 2022 à 14:00 

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 BoseEinstein 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 nonequilibrium steadystate. Drive and dissipation constitute important knobs to control the physics of the system. In this talk, I will focus on two situations where the drivendissipative 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 nonlinear optical properties of a 1D topological lattice under resonant excitation, and found pumping conditions where the nonlinear steadystate triggers the emergence of an edge state in the Bogoliubov excitation spectrum [2]. In the second experiment, we explored the physics of outofequilibrium BoseEinstein 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 onedimensional drivendissipative topological lattice, Nature Physics 18, 678 (2022). [3] Q. Fontaine et al., Observation of KPZ universal scaling in a onedimensional polariton condensate, arXiv:2112.09550 (2021). Liens : 
Willem Vos (Université de Twente)  Détails Fermer 
Shaping waves to penetrate deep inside a forbidden gap le vendredi 21 octobre 2022 à 11:00 

Résumé : Liens : 
Pavel Ostrovsky (Max Planck Institute for Solid State Research, Stuttgart)  Détails Fermer 
Electron transport in weakly disordered Weyl semimetals le mercredi 19 octobre 2022 à 11:00 

Résumé : Weyl semimetal is a solid material with isolated touching points between conduction and valence bands in its Brillouin zoneWeyl 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 whitenoise potential and apply dimensional regularization scheme near three dimensions to treat divergent terms in the perturbation theory. In the framework of selfconsistent 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 selfconsistent Born approximation up to the third order in disorder strength. These interference corrections are stronger than the meanfield result and nonanalytic as functions of energy. Our main result is the dependence of conductivity (in units $e^2/h$) on the electron concentration $\sigma = \sigma_0  0.891 n^{1/3} et 0.115 (n^{2/3}/\sigma_0) \lnn$. Liens : 
Jesper Nygard (Niels Bohr Institute, Univ. of Copenhagen / LANEF)  Détails Fermer 
Superconductorsemiconductor dots and nanowires; in situ fabrication schemes and new materials for hybrid quantum devices le mardi 18 octobre 2022 à 14:00 

Résumé : Recent years superconductorsemiconductor 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 insitu 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 IIIV 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 superconductorsemiconductor 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 NanocrystalBased Quantum Electronics, Advanced Functional Materials (2022) Liens : 
Ioan Pop (KIT)  Détails Fermer 
High Impedance Quantum Circuits le mardi 11 octobre 2022 à 14:00 

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 selfassembled 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, 816819 (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 : 
Artem Mishenko (Manchester University) Annulé  Détails Fermer 
(titre non communiqué) le mardi 20 septembre 2022 à 14:00 

Liens :Artem Mishenko 
Audrey Bienfait (ENS Lyon)  Détails Fermer 
Superconducting circuits for phononic quantum erasure (and for detecting spins) le mardi 13 septembre 2022 à 14:00 

Résumé : Heavily used in classical signal processing, surface acoustic waves (SAWs) have also been proposed as a means to coherently couple distant solidstate 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 twophonon 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, 207211, 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 : 
Pavlo Sukhachov (Yale University)  Détails Fermer 
Anomalous sound attenuation and electromagnetic field penetration in Weyl and Dirac materials le vendredi 9 septembre 2022 à 14:00 

Résumé : A salient feature of Weyl and Dirac materials is the possibility to realize the chiral anomaly due to their relativisticlike 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 internode 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 : 
Quentin Glorieux (Laboratoire Kastler Brossel)  Détails Fermer 
Nonequilibrium physics in fluid of light: from BKT physics to turbulence le vendredi 9 septembre 2022 à 11:00 

Résumé : Hot atomic vapors are widely used in nonlinear and quantum optics due to their large Kerr nonlinearity. This nonlinearity induces effective photonphoton 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:
Liens :Laboratoire Kastler Brossel 
Frederico Borges de Brito (université de Sao Carlos, Brésil)  Détails Fermer 
The internal energy of quantum systems and its additivity (room K223 and online zoom) le mardi 30 août 2022 à 14:00 

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 socalled 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 : 
YuJie Liu (Munich Center for Quantum Science and Technology)  Détails Fermer 
Exploiting quantum machine learning in classical and quantum tasks le mercredi 24 août 2022 à 11:00 

Résumé : In the first part of the talk, we discuss the usage of nearterm quantum computers in solving classical tasks. The noisyintermediate 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 matrixproduct states and quantum circuits, and further propose a hardwareefficient quantum circuit approach, which we benchmark on the FashionMNIST dataset. Finally, we demonstrate that a quantum circuitbased 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 modelindependent protocol to train the QCNN. We show that similar to the definition of quantum phases, the fixedpoint 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 noninteracting spin chains exhibiting trivial, symmetrybreaking, and symmetryprotected topological order. We show that the location and topology of the phase boundary are accurately predicted. Our method provides a hardwareefficient 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 : 
Mucio Continentino (CBPFRio)  Détails Fermer 
Thermoelectric properties of topological chains coupled to a quantum dot le vendredi 22 juillet 2022 à 11:00 

Résumé : Topological onedimensional 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, nontrivial topological phases and at the quantum topological transition. We use a simple device consisting of two semiinfinite hybridized spchains 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 WiedemannFranz 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 : 
Valentin Lallemant (LPMMC)  Détails Fermer 
Étude d 'un processus de fragmentation le mercredi 13 juillet 2022 à 11:00 

Liens :LPMMC 
Thomas Botzung (Institute for Quantum Information, RWTH Aachen University)  Détails Fermer 
Engineered dissipation induced entanglement transition in quantum spin chains: from logarithmic growth to area law le mercredi 6 juillet 2022 à 11:00 

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 measurementinduced 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 logtoarea law transition. Liens :Thomas BotzungInstitute for Quantum Information, RWTH Aachen University 
Gerhard Kirchmair (Innsbruck University)  Détails Fermer 
Nonlinear MagnetoMechanics le mardi 5 juillet 2022 à 14:00 

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 magnetomechanical 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 electromechanical 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 nonlinearity 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 backaction 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 : 
Alexey Yamilov (Missouri University of Science & Technology)  Détails Fermer 
Coherent control of wave propagation inside scattering media le vendredi 1 juillet 2022 à 11:00 

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 
Manuel Donaire (Universidad de Valladolid et Institut Néel / CNRS)  Détails Fermer 
Excited atoms: nonreciprocal forces and optical response le mercredi 29 juin 2022 à 11:00 

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 timedependent approach becomes necessary. Within that framework, we will reveal the apparent violation of the actionreaction principle in the interaction between excited atoms, as well as the existence of nonconservative forces. In addition, we will show how to tailor the scattering properties of an atom with gains so as to obtain a PTsymmetry condition for null extinction. References
Liens : 
Mikko Möttönen (Aalto University )  Détails Fermer 
New superconducting qubit and millikelvin electronics to boost it le mardi 28 juin 2022 à 14:00 

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 singlequbitgate 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/departmentofappliedphysics/qcdmedia for highlighted published results and https://arxiv.org/abs/2203.05896 for the preprint on the unimon. ATTENTION! Séminaire en visioconférence uniquement! Liens : 
Noam Schiller (Weizmann Institute of Science)  Détails Fermer 
Superconductivity and fermionic dissipation in quantum Hall edges le vendredi 24 juin 2022 à 11:00 

Résumé : Proximityinduced superconductivity in fractional quantum Hall edges is a prerequisite to proposed realizations of parafermion zeromodes. 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, counterpropagating chiral mode. Remarkably, while the probability for cross Andreev reflection was small, it was stronger for $\nu=1/3$ 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 : 
Michael Hatridge (University of Pittsburgh)  Détails Fermer 
Modular quantum computing and parametric controls in superconducting quantum circuits le mardi 21 juin 2022 à 14:00 

Résumé : Most quantum computers are built as lattices of qubits with nearestneighbor 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 socalled SNAIL device whose threewave couplings we exploit to controllably couple quantum modes. In this talk I will review our recent experimental efforts, especially our realization of four transmon alltoall 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 
Alioscia Hamma (Universita di Napoli)  Détails Fermer 
Quantum Magic le vendredi 17 juin 2022 à 14:00 

Résumé : Resource theories  magic and non stabilizerness  Stabilizer Renyi Entropy (SRE)  SRE and quantum chaos  measurement of magic Liens : 
Christopher Lee Baldwin (University of Maryland)  Détails Fermer 
Quantum dynamics in disordered systems, in low and high dimensions le vendredi 17 juin 2022 à 11:00 

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 meanfield spin glasses — longrange 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 nearestneighbor spin chains. We describe how “LiebRobinson bounds” have proven to be an invaluable tool for studying both manybody dynamics and constraints on quantum information protocols. We then cover our recent work developing LiebRobinson bounds tailored to disordered spin chains. Liens : 
Aleksey Lunkin (Landau Institute for Theoretical Physics)  Détails Fermer 
Introduction to the SYK model and its nonFermi liquid properties le jeudi 16 juin 2022 à 11:00 

Résumé : The plenty of available data on various strongly correlated electronic materials calls for the development of a general theory of the nonFermiliquid ground state(s) of an interacting manybody fermionic system. The recently proposed SachdevYeKitaev (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 twodimensional gravitational bulk. This model, also, can be considered a nonlinear generalization of usual randommatrix Hamiltonians. In my talk, I will make an introduction to the SYK model. I will describe the meanfield 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 SYKbased model. Liens : 
Alioscia Hamma (Universita di Napoli)  Détails Fermer 
Quantum Magic le mercredi 15 juin 2022 à 14:00 

Résumé : Stabilizer formalism  Clifford group  simulability of quantum states and gates Liens : 
Martina et Félix (LPMMC)  Détails Fermer 
Présentations des stagiaires le mercredi 15 juin 2022 à 11:00 

Liens : 
Stephan Philips (TUDelft)  Détails Fermer 
Making quantum processors with spin qubits le mardi 14 juin 2022 à 14:00 

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 highfidelity are typically conflicting. Spins in semiconductor quantum dots show longterm promise but demonstrations so far use between one and four qubits and typically optimize the fidelity of either single or twoqubit 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 sixqubit 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 realtime feedback with quantumnondemolition measurements. These advances will allow testing of increasingly meaningful quantum protocols and constitute a major stepping stone towards largescale 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. 110, 2017. 2. Veldhorst, M., et al, Nature nanotechnology, vol. 9.12, pp. 981985, 2014. 3. Yoneda J., et al., Nature Nano, vol. 13, pp. 102106, 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. 15, 2021. 8. Hendrickx N. W., et al., Nature, vol. 591, pp. 580–585, 2021 9. Philips S., Mądzik M, et al., https://arxiv.org/abs/2202.09252 Liens : 
Alioscia Hamma (Universita di Napoli)  Détails Fermer 
Quantum Magic le vendredi 10 juin 2022 à 14:00 

Résumé : Introduction  Mathematical preliminaries  layers of quantum mechanical behavior: entanglement and speed up  quantum circuits and channels  universality Liens : 
Christopher Jarzynski (Université du Maryland)  Détails Fermer 
Scaling down the laws of thermodynamics le vendredi 10 juin 2022 à 11:00 

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 “thermodynamiclike” 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 nonequilibrium fluctuations, “violations” of the second law, the thermodynamic arrow of time, nanoscale feedback control, strong systemenvironment coupling, and quantum thermodynamics. Liens :Christopher Jarzynski 
Richard Kueng (Johannes Kepler University Linz, Austria)  Détails Fermer 
Classical shadows: efficient quantumtoclassical converters with many applications le jeudi 9 juin 2022 à 11:00 

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 informationtheoretic lower bounds [arXiv:2002.08953]. These quantumtoclassical converters pave the way for new synergies between (nearterm) 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 : 
Hélène Bouchiat (LPS Orsay)  Détails Fermer 
Singular orbital magnetism in Graphene with a moiré potential: diamagnetism and paramagnetism le mardi 7 juin 2022 à 14:00 

Résumé : A singular Landau orbital magnetism of graphene, with a sharp narrow diamagnetic peak at the Dirac point, was already predicted in 1956 by McClure. It is now understood as a fundamental signature of the characteristic Berry phase of graphene’s electronic wave functions. Using a highly sensitive giant magnetoresistance (GMR) sensor, we have measured the gatevoltage–dependent magnetization of a single graphene layer. The signal exhibits a sharp diamagnetic peak at the Dirac point whose magnetic field and temperature dependences agree with longstanding theoretical predictions. These measurements enables the investigation of orbital currents in 2D materials that cannot be detected in usual transport measurements. Among the predictions an intriguing orbital paramagnetism at saddle points of 2D materials is expected. In order to reveal this unusual orbital paramagnetism, we investigated graphene layers encapsulated between two hexagonal boron nitride (hBN) crystals, with among them, one nearly aligned with the graphene lattice giving rise to a large period moiré potential acting on graphene charge carriers. Beside the sharp diamagnetic peak at the Dirac point, followed by de Haasvan Alphen oscillations at larger doping, we detect extra diamagnetic peaks at the satellite Dirac peaks of the moiré lattice. We also find paramagnetic peaks surrounding these satellite diamagnetic peaks related to vanHove singularities in the density of states. These findings confirm the existence of paramagnetic orbital loops in 2D systems when the Fermi energy is tuned in the vicinity of saddle points. Ref: J. VallejoBustamante et al., Science 2021 (to be published). Liens : 
Giacomo Mazza (Université de Genève)  Détails Fermer 
Quantum and classical aspects of strong lightmatter coupling in cavity electrodynamics le vendredi 3 juin 2022 à 11:00 

Résumé : Lightmatter interaction represents the fundamental tool to probe and actively manipulate the properties of matter. In most cases, the investigation of strong lightmatter 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 lightmatter interaction beyond simplified models remain poorly explored. In this seminar, I will discuss the general properties of the lightmatter interaction as derived from the quantum manybody theory of photons coupled to the microscopic degrees of freedom in a solidstate system. I will consider the renormalization of electronic properties, the groundstate properties of light, and their dependence on cavity confinement. Eventually, I will compare classical and quantum aspects of the strong coupling regimes of the lightmatter interaction. Liens : 
Guillaume Manzanares (LPMMC)  Détails Fermer 
Superradiant Quantum Phase transition for Landau Polaritons with Rashba and Zeeman couplings le mercredi 25 mai 2022 à 11:00 

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 lightmatter 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 twodimensional electron gas in the presence of Rashba spinorbit 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 inplane 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 spinorbit 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 
Saulius Vaitiekenas (Niels Bohr Institute)  Détails Fermer 
Semisuperferro hybrids: A new platform for unconventional superconductivity le mardi 24 mai 2022 à 14:00 

Résumé : Recently developed semiconducting InAs nanowires with epitaxial superconducting Al and ferromagnetic insulator EuS shells display induced superconductivity with Zeemanlike splitting at zero external magnetic field (1). The intricate interplay between spinorbit 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 spinpolarization of the induced superconductivity (2,3). References: (1) Y. Liu, et al., Nano Lett. 20, 456 (2020). (2) S. Vaitiekėnas, et al., Phys. Rev. B 105, L041304 (2022). (3) D. Razmadze, et al., arXiv:2204.03202 (2022). Liens : 
Cécilia Lancien (Université GrenobleAlpes)  Détails Fermer 
Typical correlations and entanglement in random tensor network states le vendredi 20 mai 2022 à 11:00 

Résumé : Tensor network states are used extensively as a mathematically convenient description of physically relevant states of manybody 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 longrange correlations. In a nutshell, the main result I will state is that translationinvariant 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 ongoing 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 PerezGarcia, available at arXiv:1906.11682, and on some work in progress with Ion Nechita. Liens : 
Kater Murch (Washinton University)  Détails Fermer 
Trapping and manipulating singleelectron qubits on solid neon in a hybrid circuit quantum electrodynamics architecture le mardi 17 mai 2022 à 14:00 

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 onchip 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 : 
Alessio Lerose (Université de Genève)  Détails Fermer 
Influence matrix approach to quantum manybody dynamics le vendredi 13 mai 2022 à 11:00 

Résumé : In this talk I will introduce an approach to study the nonequilibrium dynamics of extended quantum manybody systems, inspired by the FeynmanVernon influence functional description of quantum baths. We take an openquantumsystem 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 manybody 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 manybody dynamics offers many advantages, both conceptually and practically. In one spatial dimension, spacetime duality allows to write an exact linear selfconsistency 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 manybody systems exert a nonMarkovian influence on subsystems, associated with temporal entanglement (TE) in the IM. Analyzing a wide range of models with analytical methods and numerical matrixproductstate computations, we study the scaling of TE in several dynamical regimes, ranging from strongly chaotic to (quasi)integrable and manybody localized. Based on recent works with Dmitry Abanin and Michael Sonner. Liens : 
Adrian Bachtold (ICFO)  Détails Fermer 
Manipulating mechanical resonators with singleelectron tunneling le mardi 10 mai 2022 à 14:00 

Résumé : Singleelectron 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 singleelectron singlephonon 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 electronphonon 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 
Bertrand Georgeot (Laboratoire de Physique Théorique IRSAMC, Toulouse)  Détails Fermer 
Multifractality and nonergodicity in complex quantum systems le mardi 10 mai 2022 à 11:00 

Résumé : Liens : 
Marcel Filoche (LPMC, Polytechnique)  Détails Fermer 
Is the mobility edge of the Anderson transition a percolation problem ? le vendredi 6 mai 2022 à 11:00 

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 : 
Tony Jin (Univerity of Geneva (NIGE))  Détails Fermer 
Classical random walker under continuous measurement le vendredi 29 avril 2022 à 11:00 

Résumé : The interplay between measurements and chaotic manybody systems has recently attracted extended interest within the quantum community in the context of measurementinduced 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 weakmeasurement, 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 ColeHopf 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 : 
Richard East (LIG / UGA)  Détails Fermer 
Formal diagrammatic reasoning for physics le vendredi 22 avril 2022 à 11:00 

Liens : 
Zheng Vitto HAN (Shanxi University)  Détails Fermer 
Substrates for graphene: a new interesting one? le mardi 12 avril 2022 à 14:00 

Résumé : Ever since the discovery of ultrahigh mobility when placed onto an hBN 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 hBN, 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 hBN 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 monolayer or bernalstacked bilayer graphene into contact with a fewlayered antiferromagnetic insulator CrOCl, the resulted vertical heterostructures can give rise to an extraordinarily robust quantum Hall phase in monolayer graphene [1], and an excitonenhanced insulator in bilayer graphene [2], which are attributed to the subtle coupling of grapheneCrOCl 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 : 
Matteo Votto (LPMMC)  Détails Fermer 
Revealing entanglement statistics of random manybody states via partial transpose moments le vendredi 8 avril 2022 à 11:00 

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 loworder moments of the partially transposed reduced density matrix. We find that this ratio takes welldefined 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 partialtranspose density matrix. We analyze in particular the entanglement phase diagram of Haar random states, and the differences with respect to Clifford, matrixproduct states, and fermionic Gaussian states. Our results can be used to experimentally test the mixedstate entanglement structure of quantum states formed in quantum computers and programmable quantum simulators. Liens : 
Emmanuel Flurin (CEA Saclay)  Détails Fermer 
Detecting spins by their fluorescence with a microwave photon counter le mardi 5 avril 2022 à 14:00 

Résumé : Singlephoton 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 singlephoton 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 singlephoton counter to detect the photons spontaneously emitted by a small ensemble of electron spins coupled to a superconducting microresonator (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, 434438 (2021) (3) M. Le Dantec, et al., Science advances 7.51 (2021) Liens : 
Nicolas Bergeal (ESPCI)  Détails Fermer 
Superconducting oxides interfaces le mardi 22 mars 2022 à 14:00 

Résumé : The achievement of highquality 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 twodimensional electron gas (2DEG) 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 3dbands, 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 gatetunability of both superconductivity and Rashba spinorbit coupling. In this talk, I will review some microwave transport measurements on LaAlO3/SrTiO3interfaces that evidence a transition from singlegap to twogap s±wave superconducting state driven by continuous and reversible electrostatic doping (5,6). I will also present the realization of topgated LaAlO3/SrTiO3 devices whose physical properties, including superconductivity and Rashba spinorbit 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 onedimensional conducting channel. Finally, I will briefly discuss some recent experiments on the newly discovered superconducting 2DEG 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 : 
Preden Roulleau (CEA (Saclay))  Détails Fermer 
Excitonic nature of magnons in a quantum Hall ferromagnet le mardi 8 février 2022 à 14:00 

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 MachZehnder interferometer. As magnons propagate across the insulating bulk, their electric dipole moment modifies the AharonovBohm 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 cantedantiferromagnetic 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 : 
Simone Rademacher (Institute for Science and Technology Austria)  Détails Fermer 
The polaron in the strong coupling limit le vendredi 4 février 2022 à 11:00 

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 LandauPekar 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 LandauPekar 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 : 
Nicolo Crescini (Néel)  Détails Fermer 
Effect of twolevel systems and phonons on superconducting quantum devices le mardi 1 février 2022 à 14:00 

Résumé : Twolevel 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 nanodevices. To study the physics of twolevel systems (TLSs) we make use of Tshaped 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 milliVolt 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 twotones 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 electroncurrent 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 highenergy electrons. Our observations question existing interpretations and theories based on electric fields and contribute towards understanding the complex interactions between outofequilibrium phenomena in solids and performance of superconducting hardware. Liens : 
Kyrylo Snizhko (CEA Grenoble)  Détails Fermer 
Parafermionic zero modes on quantum Hall edges, and parafermionic Kondo problem as their transport signature le mardi 25 janvier 2022 à 14:00 

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 quantumcomputingready 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. DePicciotto, 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: NonAbelian 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 nonabelian 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 Coulombblockaded 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 : 
Kater Murch (Washington University, St. Louis)  Détails Fermer 
Effective nonHermitian evolution of a superconducting qubit: harnessing the topology of a Riemann surface for quantum control le mardi 18 janvier 2022 à 14:00 

Résumé : A system described by a nonHermitian Hamiltonian will, in general, have complex energies and nonorthogonal 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 nonHermitian 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 