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

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 :
• G 421

Résumé : Owing to a recent observation of the anomalous Hall effect in Mn3X (X = Ge, Sn, Ga), there has been an increased interest in the magnetic properties of compounds with a hexagonal AB-stacked kagome structure. Despite a relatively simple crystal structure, these compounds possess a plethora of various types of magnetic interactions [1]. Focusing first on the magnetic properties of Mn3X, we study the effects of magnetic anisotropy on the ground state and elementary excitations. We demonstrate that the single-ion and anisotropic exchange interactions can stabilize the same spin configurations but lead to distinct features in the spin-wave spectra. To further investigate the properties of our model, we consider a wider range of model parameters and construct the ground state phase diagrams [2]. We show that a minimal model with exchange and Dzyaloshinskii-Moriya interactions is sufficient to stabilize unusual magnetic structures with single and multiple magnetic ordering wavevectors. By determining the full spin symmetry of the model, we are able to identify a large set of self-duality transformations that provide an exact mapping between different parts of the phase diagram. Similar duality transformations have been exclusively studied in Kitaev-Heisenberg models on honeycomb and triangular lattices [3-5]. [1] A. Zelenskiy, T. L. Monchesky, M. L. Plumer, and B. W. Southern Phys. Rev. B 103, 144401 (2021) [2] A. Zelenskiy, T. L. Monchesky, M. L. Plumer, and B. W. Southern Phys. Rev. B 106, 144433 (2022) [3] Itamar Kimchi and Ashvin Vishwanath, Phys. Rev. B 89, 014414 (2014) [4] Jiří Chaloupka and Giniyat Khaliullin Phys. Rev. B 92, 024413 (2015). [5] Maksimov, P. A. and Zhu, Zhenyue and White, Steven R. and Chernyshev, A. L., Phys. Rev. X 9, 021017 (2019).

Liens :

• Séminaire QuantAlps
• Personne à contacter : Michele Filippone ()
• 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 : Serge Florens ()
• 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 théorie
• Personne à contacter :
• G 421

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter : Jeremie Viennot ()
• Salle Rémy Lemaire K223, Institut Néel et via https://univ-grenoble-alpes-fr.zoom.us/j/91808901596?pwd=UWZ2cml2N1VBOEZBenk0d3RJek9rdz09

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 : Benoit Vermersch ()
• G 421

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter : Jeremie Viennot ()
• Salle Rémy Lemaire K223, Institut Néel et via https://univ-grenoble-alpes-fr.zoom.us/j/91808901596?pwd=UWZ2cml2N1VBOEZBenk0d3RJek9rdz09

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 QuantAlps
• Personne à contacter : Michele Filippone ()
• Lieu à préciser

Liens :

• Séminaire nano-électronique quantique
• Personne à contacter :
• 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