• Séminaire nano-électronique quantique
• Personne à contacter :
• Conférence Zoom : consulter le lien de connexion dans les emails d'annonce ou contacter l'organisateur

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 :
• Conférence Zoom : consulter le lien de connexion dans les emails d'annonce ou contacter l'organisateur

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.

• Séminaire nano-électronique quantique
• Personne à contacter : Laetitia Marty ()
• Conférence Zoom : consulter le lien de connexion dans les emails d'annonce ou contacter l'organisateur

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 Winkelmann][Inst. Neel]

• Séminaire interne LPMMC
• Personne à contacter : Anna Minguzzi ()
• Conférence Zoom : consulter le lien de connexion dans les emails d'annonce ou contacter l'organisateur

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 ()
• Conférence Zoom : consulter le lien de connexion dans les emails d'annonce ou contacter l'organisateur

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

• Séminaire interne LPMMC
• Personne à contacter : Vincent Rossetto ()
• Conférence Zoom : consulter le lien de connexion dans les emails d'annonce ou contacter l'organisateur

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 ()
• Conférence Zoom : consulter le lien de connexion dans les emails d'annonce ou contacter l'organisateur

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 Oblak][CPhT, École polytechnique]

• Séminaire LPMMC
• Personne à contacter : Anna Minguzzi ()
• Conférence Zoom : consulter le lien de connexion dans les emails d'annonce ou contacter l'organisateur

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.

• Séminaire théorie
• Personne à contacter : Adolfo Grushin ()
• Conférence Zoom : consulter le lien de connexion dans les emails d'annonce ou contacter l'organisateur

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.

Liens :[Jan Behrends][Cambridge University]

• Séminaire LPMMC
• Personne à contacter : Vincent Rossetto ()
• Conférence Zoom : consulter le lien de connexion dans les emails d'annonce ou contacter l'organisateur

Résumé : With current advances in state preparation, as well as gate and measurement operations, superconducting circuits are now a leading architecture for quantum information processing. As these systems are scaled up, strict requirements on the fidelity of operations required for computation and readout are imposed. In this talk we focus on the so-called “readout problem” in circuit quantum electrodynamics: several experiments have shown that qubit energy relaxation rates may become strongly dependent on the power of the measurement drive, even for moderate or weak drives; this hampers efforts to improve readout fidelity. To explain this, we devised a perturbation theory for driven-dissipative, weakly anharmonic, superconducting circuits based on a sequence of unitary transformations. Applied to a transmon qubit coupled to a readout resonator, this approach allows us to classify the nonlinear processes that enhance qubit relaxation in the presence of resonator photons. Among these, we are able to quantify changes to the Purcell rate, and to stimulated emission. Chiefly responsible for the dressing of relaxation rates are the counterrotating terms arising from the expansion of the Josephson potential, which are usually neglected in theories based on Kerr nonlinear oscillators. Time- permitting, we will discuss a general framework for quantizing driven superconducting circuits that arises from this study, with a concrete example in the accurate modeling of two-qubit gates.

Liens :[Université de Sherbrooke]

• Séminaire LPMMC
• Personne à contacter : Vincent Rossetto ()
• Conférence Zoom : consulter le lien de connexion dans les emails d'annonce ou contacter l'organisateur

Résumé : Many interesting physical phenomena are connected to strongly correlated systems, which, due to their complexity, cannot usually be studied analitically, making numerical approaches essential. The development of the latter and the study of the physical scenarios induced by strong correlations are therefore both of great importance: in my talk, I will present my results in the context of both these research directions. I will discuss my Path Integral Monte Carlo (MC) results about the equilibrium and out-of-equilibrium physics of a class of lattice bosonic models with extended-range interactions, relevant for experiments with cold Rydberg and Rydberg-dressed atoms, where exotic phenomena such as supersolid-supersolid transitions and (super)glass phases are induced by the formation of particle clusters in the medium- and strong-interaction regime. Furthermore, I will present my work on the ground-state properties of the fermionic t-J model, a candidate Hamiltonian to describe high-T_c superconductivity, in the presence of two mobile holes. Here, I employ Variational MC in conjunction with the Entangled Plaquette States (EPS) ansatz, a versatile and powerful trial wavefunction for the study of fermionic and frustrated many-body systems. My results confirm existing predictions with much higher accuracy and (unlike previously) to sizes large enough to approximate well the thermodynamic limit, and are foundational to prove the applicability of the EPS ansatz to other computationally challenging many-body problems.