Assembling and probing strongly correlated fluids of light

• Andrei Vrajitoarea (University of Chicago)

Learning how to create and manipulate highly-entangled many-body systems is a central challenge of modern quantum science, with promising applications from quantum computation to many-body physics and quantum-enhanced metrology. Analog quantum simulators provide a rich playground for exploring the emergent collective phenomena in synthetic quantum systems, and how to harness these many-body effects for future quantum technologies. In this talk, I will describe various approaches for assembling quantum matter from strongly interacting microwave photons. Our quantum circuit platform consists of an array of capacitively coupled transmon qubits acting as a Hubbard lattice for photons. I will highlight a novel approach to constructing low-entropy quantum fluids of light by employing particle-resolved assembly combined with adiabatic control of lattice disorder. Studying these fluids with site-resolved microscopy of entanglement and two-body correlations reveals how the photons delocalize and avoid one another, behaving as noninteracting fermions. The precise time- and space-resolved control of the lattice potential landscape presents another unique capability in our platform for investigating out-of-equilibrium quantum dynamics. Using controlled perturbations in the lattice potential, we can spectroscopically probe quasiparticle excitations, prepare superpositions of many-body eigenstates and observe the propagation of sound modes of light in the lattice. Towards developing practical tools for quantum computers, I will show how we can leverage many-body dynamics in these fluids for preparing highly-entangled states useful for quantum information processing and metrology.

Novel Non-equilibrium Phenomena in Quantum Fluids of Light

• Marzena Szymanska (University College London)

An open quantum safari: Criticality, Computing, and Chaos

• Fabrizio Minganti (EPFL)

Presentation of Institut Pascal

• Yves BALKANSKI (Institut Pascal - UPSaclay)

Gaussian quantum trajectories for open and closed systems

• Michiel Wouters

I will discuss the use of Gaussian states for the modeling of open quantum systems within the quantum trajectory framework, with applications to bosonic systems and spin systems: the to two-photon driven cavities and the dissipative XYZ model. Thanks to the dissipation, the quantum fluctuations typically remain small at all times, therefore improving the validity of the Gaussian states as compared to their use for the dynamics of closed systems. It is therefore suggested to introduce some fictitious dissipation to closed systems in order to improve their simulability.

Laser-driven superradiant ensembles

• Igor Ferrier-Barbut (CNRS, Institut d'Optique Graduate School, Université Paris Saclay)

I will present our recent studies on superradiance in dense clouds of ultracold atoms, that are indistinguishably coupled to a mode of the electromagnetic field, akin to cavity or waveguide QED systems, but here in free space. We are in particular interested in the case where the atomic ensemble is continuously driven by a resonant laser that leads to a competition between laser driving and collective spontaneous emission. We observed that this competition induces a transition to a steady-state superradiant phase. Beyond measuring the field intensity, we measure photon-photon correlations. This precious tool of quantum optics allows us to question wether atomic correlations can impart non-trivial photonic correlations. I will show that indeed photonic correlations emerge, and despite the system’s simplicity, their description poses a challenge to many-body theories.

Search as (quantum) selforganized process

• Giovanna Morigi (University of Saarland)

Efficient retrieval of information is a core operation in the world wide web, it is essential for the sustainance fof living organism. Search dynamics, moreover, is a paradigm for optimization algorithms: Searches permeate our everyday life. Inspired by the food search dynamics of a living organism, the Physarum polycephalum, we analyse the role of noise in finding the optimal path on a graph with multiple constraints and where the weight of the edges connecting the nodes is a dynamical variable. The network dynamics results from the interplay between a nonlinear function of the flow, dissipation, and Gaussian, additive noise. At a finite value the noise amplitude, the network selforganizes in the most robust topology with a resonant-like behavior. This specific topology maximizes the transport efficiency, it is reached with the maximal convergence rate, and it is not found by the noiseless dynamics. We argue that this dynamics is a manifestation of noise-induced resonances in network self-organization. Drawing from this knowledge, we then discuss the perspectives of designing quantum search algorithms that are assisted by stochastic dnamics.

Density wave ordering in a unitary gas with photon-mediated interactions

• Jean-Philippe Brantut (EPFL)

I will present the realization of a quantum degenerate Fermi gas interacting simultaneously via unitary-limited contact interaction and long-range, photon mediated interaction induced by an optical cavity. We observe the onset of density-wave order above a critical strength of the photon-mediated interaction, and characterize the phase diagram as a function of both interactions type. This system is very promising for the investigation of the interplay of charge-density wave order with superfluidity with a fully controlled microscopic Hamiltonian. I will then present briefly the perspectives open in the mid-term for the realization of the SYK interactions in quantum gases.

Phantom relaxation in local many-body Floquet systems

• Marko Znidaric (University of Ljubljana)

We would like to understand relaxation towards a long-time steady state under unitary pure-state evolution. Focusing on a bipartite entanglement or out-of-time-ordered correlations, one sometimes finds that relaxation is not a simple exponential with a fixed rate, but that the rate exhibits a jump at an extensive time. Studying some solvable cases of random circuits one finds that this two-step relaxation can be traced back to interesting non-Hermitian physics. Despite relaxation being described by a gapped Markovian matrix the rate is in the thermodynamic limit not given by the 2nd largest eigenvalue, but rather by a phantom eigenvalue -- an "eigenvalue" that is not in the spectrum. Resolution of this puzzle will lead to a pseudospectrum, Jacobi theta functions, and realization that when dealing with non-Hermitian matrices being exact can actually be wrong, while being slightly wrong is correct.

Driven-Dissipative Systems: two messy experiments and one developing theory

• Mohammad Hafezi

As an example of driven-dissipative systems, I discuss two recent experiments: (1) strongly interacting excitons in moire heterostructures, (2) orbital angular pumping of quantum Hall states of graphene with vortex light. In the end, I briefly discuss our recent progress in verifying measurement-induced phases by avoiding the post-selection barrier. https://arxiv.org/abs/2304.09731 https://arxiv.org/abs/2306.03417

Phase transitions at a many-body exceptional point

• Sebastian Diehl

Phases of quantum information on a noisy quantum processor

• Matteo Ippoliti (Stanford University)

Abstract: I will discuss the experimental realization of measurement-induced phases of quantum information on Google Quantum AI's superconducting processor. By using a hybrid quantum-classical order parameter, which correlates experimental data with simulation, we observe signatures of distinct entanglement structures up to 70 qubits. We further show that noise, an inevitable limitation of the hardware, can be exploited as an independent probe of the phases. Reference: https://arxiv.org/abs/2303.04792

Statistical mechanics insights into the complexity of tensor network calculations

• Romain Vasseur (University of Massachusetts Amherst)

Tensor network states offer memory-efficient representations of quantum many-body states, and play a key role in classical simulations of quantum materials, chemistry, and circuits. However, rigorous results show that exactly computing observables from a tensor network state is generically a computationally hard problem outside of special instances such as 1d matrix-product states. Yet, approximation schemes for computing properties of 2d projected entangled pair states (PEPS) have been widely and successfully used without encountering any sign of exponential hardness. Adopting the philosophy of random matrix theory, in this talk I will analyze the complexity of approximately contracting a 2d PEPS by exploiting an analytic mapping to an effective replicated statistical mechanics model that permits a controlled analysis at large bond dimension. Through this statistical-mechanics lens, I will argue that: although approximating individual wave-function amplitudes of a PEPS faces a computational-complexity phase transition above a critical bond dimension, one can generically efficiently estimate physical properties, such as correlation functions, for any finite bond dimension.

Observing the effect of many measurements

• Ehud Altman (University of California, Berkeley)

Magic Phase transition with Random Quantum Circuits

• Crystal Noel (Duke University)

Magic is a property of quantum states that enables universal fault-tolerant quantum computing using simple sets of gate operations. Understanding the mechanisms by which magic is created or destroyed is, therefore, a crucial step towards efficient and practical fault-tolerant computation. We observe that a random stabilizer code subject to coherent errors exhibits a phase transition in magic, which we characterize through analytic, numeric and experimental probes. Below a critical error rate, stabilizer syndrome measurements remove the accumulated magic in the circuit, effectively protecting against coherent errors; above the critical error rate syndrome measurements concentrate magic. A better understanding of such rich behavior in the resource theory of magic could shed more light on origins of quantum speedup and pave pathways for more efficient magic state generation.

Emergent hydrodynamics in constrained quantum matter

• Michael Knap (Technical University of Munich)

The far-from-equilibrium dynamics of generic interacting quantum systems is characterized by a handful of universal guiding principles, among them the diffusive transport of globally conserved quantities. Certain systems with kinetic constraints or constrained interactions, however, defy these expectations and exhibit anomalous transport instead. In this talk, we will discuss some of these exceptions. For example, systems with conserved, and sometimes hidden, spin patterns, including XNOR or tJz models, show anomalously slow spin relaxation dynamics. In these models, spin transport is governed by tracer diffusion, which describes the diffusion of a tagged particle with hard-core constraints. Another example are fracton systems, which conserve the dipole moment (or equivalently the center of mass). Fracton systems exhibit a localization transition as a function of the density separating an ergodic dynamical phase from a frozen one; a phenomena known as Hilbert space fragmentation. Even in the ergodic phase, transport is anomalously slow and exhibits sub-diffusive scaling. We will discuss relations and differences between the two cases and also draw connections to recent quantum simulation experiments with ultracold atoms.

Trapped-ion quantum state and process engineering using dissipation

• Jonathan Home (ETH Zürich)

I will discuss a number of results on quantum state engineering and dissipative control with trapped ions. Primarily this will relate to oscillator state control, where the engineering of open-system dynamics has allowed us to create a range of quantum steady-states, perform quantum error-correction using the Gottesmann-Kitaev-Preskill code, and to observe phase transitions in systems with competing dissipation. Furthermore I will outline how these methods might be extended to multi-partite systems, and present first results along these lines.

Replica field theories for measurement transitions and random tensor networks

• Adam Nahum (CNRS)

I will describe how continuum field theory can in some cases give exact results for dynamical phase transitions driven by repeated measurement, and also for entanglement transitions in random tensor networks. I will discuss both free and interacting systems.

Phase Transitions in Benchmarking and Mitigation of Noisy Quantum Circuits

• Michael Gullans (QuICS, NIST/University of Maryland, College Park)

Two central challenges in NISQ devices are the characterization and mitigation of the effects of noise. Originally introduced to analyze quantum random circuit sampling experiments, the linear cross-entropy benchmark (XEB) has emerged as a paradigmatic tool for characterizing noise in NISQ devices. A key question in the theory of XEB is whether it approximates the fidelity of the quantum state preparation. Previous works have shown that the XEB generically approximates the fidelity in a regime where the noise rate per qudit ε satisfies εN≪1 for a system of N qudits and that this approximation breaks down at large noise rates. Here, we show there is a breakdown of XEB as a fidelity proxy at a sharp phase transition at a critical value of εN. Next, we discuss the application of quantum error mitigation to reducing the effects of noise in current devices. Error mitigation relies on precise characterization of the noise, but the robustness of these techniques to noise characterization disorder are not well studied. We adapt an Imry-Ma argument to predict the existence of a disorder-driven error mitigation threshold for random spatially local circuits in two or more dimensions. We study these two noise-driven phase transitions using mappings of average two-copy quantities to statistical mechanics models of randomly driven quantum dynamics. We discuss implications of our results for tests of quantum computational advantage, fault-tolerant probes of measurement-induced phase transitions, and quantum algorithms in near-term devices.

Post-selection-free Measurement-Induced Phase Transition in Driven Atomic Gases with Collective Decay

• Rosario Fazio (The Abdus Salam International Centre for Theoretical Physics)

I will discuss the properties of a monitored ensemble of atoms driven by a laser field and in the presence of collective decay. By varying the strength of the external drive, the atomic cloud undergoes a measurement-induced phase transition separating two phases with entanglement entropy scaling sub-extensively with the system size. The critical point coincides with the transition to a superradiant spontaneous emission. This setup is implementable in current light-matter interaction devices, and most notably, the monitored dynamics is free from the post-selection measurement problem, even in the case of imperfect monitoring. G. Passarelli, X. Turkeshi, A. Russomanno, P. Lucignano, M. Schirò, R. Fazio https://arxiv.org/abs/2306.00841

Full counting statistics in integrable and chaotic systems

• Sarang Gopalakrishnan (Princeton University)

Quantum gas microscopy of Hubbard models in and out of equilibrium

• Johannes Zeiher

Neutral atoms trapped in optical lattices are a versatile platform to study many-body physics in and out of equilibrium. Quantum gas microscopes provide an excellent toolbox to prepare, control and detect such systems at the level of individual atoms. In the first part of my talk, I will present our recent work on implementing passively phase-stable square and triangular lattices for bosonic rubidium atoms. Combining these base lattices with fine-tuned local on-site blocking potentials, we realize several derived lattice configurations, among them Lieb and kagome lattices. As a first application of our system, we characterize the superfluid-to-Mott-insulator transition through a measurement of brane parity, a non-local observable derived from string correlators. Our measurements demonstrate that brane parity can act as an order parameter for the Mott-insulating state in two dimensions. In the second part of the talk, I will focus on recent experiments on characterizing spin transport in Heisenberg chains. Using a direct mapping between the two-component Bose-Hubbard model and the Heisenberg model, we study the relaxation of spin domain walls. We track the evolution of the magnetization transported across the initial location of the domain wall, from which we find that transport is superdiffusive. Furthermore, our microscopic detection sheds light on the fluctuations in the transported spin, which indicate that transport falls in the celebrated Kardar-Parisi-Zhang universality class.

Kardar-Parisi-Zhang universality in a one-dimensional polariton condensate

• Sylvain Ravets

Cavity polaritons are hybrid exciton-photon quasi-particles emerging from the strong coupling regime between photons confined in an optical cavity and excitons confined in quantum wells. They present physical properties reflecting their mixed nature. From the photon part, they inherit a small effective mass and can be confined in lattices with typical dimensions of the order of a few microns. Their excitonic part endows them with inter-particle interactions resulting in a giant Kerr non linearity. Cavity polaritons exhibit fascinating properties such as Bose Einstein condensation at elevated temperature, superfluidity, multistability... Importantly, the system is driven-dissipative in nature: the driving field maintains an out-of-equilibrium steady-state by compensating the constant leakage of photons. Recent theoretical works have shown that the phase dynamics of out-of-equilibrium condensates obeys the celebrated Kardar Parisi Zhang (KPZ) equation [1-3]. In this talk, I will present our recent experimental observation of universal KPZ scaling laws in the first order coherence of a 1D polariton condensate [4]. I will first explain how we generated highly elongated polariton condensates in a 1D polariton lattice. I will then describe our measurements of the condensate coherence. Finally, I will show that data points lying within a well-defined spatio-temporal window collapse onto a single scaling function, characteristic of the KPZ universality class.

Floquet Quantum East: Localization Transition in Thermodynamic Limit

• Tomaz Prosen

We define and study the Floquet quantum east circuit model, a kinetically constrained quantum dynamics in discrete space-time, exhibiting a localization transition in infinite volume. The localization is established using perturbative arguments as well as clearly convergent TEBD (time-evolving block decimation) method in real time.

Projective measurements in 
highly energetic but slightly entangled 
excited States

• Maurizio FAGOTTI (LPTMS)

Dynamics of scrambling of information, from shock waves to Fisher‐KPP

• Camille Aron

Quantum Transport and cold atomic gases

• Thierry Giamarchi (University of Geneva)

Quantum transport of a system which is between two reservoirs, at e.g. different chemical potentials, is one of the most common but also most important ways to put a quantum system out of equilibrium. Such a situation is relevant not only for charge transport but also for other transport properties such as spin transport or Hall transport for systems which are put under a magnetic field. I will discuss in this talk the recent progress done on the theoretical front, using in particular Keldysh technique, to deal with various situations such as the transport between two superconducting reservoirs with and without loss of particles, or to the transport in presence of a time dependent noise. These situations can be directly relevant for experimental situations encountered in cold atomic gases and I will present such realizations, and in particular recent experiments concerning Hall transport.

KPZ

Gas of wavepackets and an ab initio derivation of generalised hydrodynamics

• Benjamin Doyon (King's College London)

The hydrodynamic approximation is an extremely powerful tool to describe the behaviour of many-body systems such as gases. At the Euler scale, the approximation is based on the idea of local entropy maximisation: locally, within fluid cells, the system relaxes to a state that takes the Gibbs form. In conventional gases, these are thermal states, which include the few conserved quantities admitted by the model. In integrable systems, these are the so-called generalised Gibbs ensembles, which include the infinite set of conserved quantities, and the corresponding hydrodynamic theory is called generalised hydrodynamics (GHD). GHD applies for instance to experimentally realized one-dimensional interacting Bose gases described by the Lieb-Liniger model, and many more one-dimensional integrable systems, such as classical soliton gases and the hard-rod model. However, the local entropy maximisation is an assumption, and in general it is hard to establish the hydrodynamic equations from first principles (from the microscopic dynamics of the model). The GHD equations have a very specific structure: they can be interpreted as ``kinetic" equations for a phase-space density, in position-quasimomentum space. Thus suggests that there is a more kinetic way of deriving them. In this talk I will explain how to construct a gas of wavepackets in the Lieb-Liniger model, whose effective dynamics gives rise, without the assumption of local entropy maximisation, to the GHD equations. I will explain how this provides a blueprint for deriving the GHD equations from the Schroedinger equations. The main idea is a map to the scattering coordinates of the wavepackets' dynamics, and is similar in spirit to the techniques used in the known rigorous proof of the hydrodynamic equations for the hard-rod gas.

Quantum Work Statistics in Chaotic Fermion Systems

• Gergely Zarand (Budapest University of Technology and Economics)

We study the full distribution of quantum work in driven chaotic fermion systems by using a random matrix approach. We find that work statistics is generically non-Gaussian. At longer times, quantum work distribution is well-described in terms of a simple ladder model and a symmetric exclusion process in energy space, and bosonization and mean field methods provide accurate analytical expressions for the work statistics. At finite temperatures, a cross-over between diffusive and superdiffusive work statistics is found. The probability of adiabatic evolution crosses over from an exponential to a stretched exponential behavior. Our findings can be verified by measurements on nanoscale circuits and via single qubit interferometry, and have important implications for adiabatic quantum optimization.

On the dynamics of fractional quantum Hall clouds

• Iacopo Carusotto (Pitaevskii BEC Center, INO-CNR and Università di Trento)

I will first present recent results on the dynamics of anyonic molecules, namely composite objects emerging from the binding of a massive impurity with a quasi-hole excitation in a fractional quantum Hall fluid. In particular, I will highlight how the angular cross section for the scattering of such polarons gives direct information on the fractional statistics in the underlying fluid. I will then present a nonlinear chiral Luttinger liquid theory of the quantum Hall edge and I will discuss basic features of its quantum dynamics in response to external perturbations. The exciting perspectives towards extending the concepts of quantum nonlinear optics to chiral excitations on the quantum Hall edge will be finally sketched.

Fast rotation of a superfluid in a shell

• Hélène Perrin (LPL/CNRS/USPN)

Quantum gases provide us with a very convenient and widely tunable system for the study of superfluidity. In particular, they can be confined in a large variety of traps, enabling the study of superfluid dynamics with specific geometry. In this talk I will present the behaviour of a superfluid quantum gas confined at the surface of an ellipsoid: the atoms can move freely in directions parallel to the surface and are strongly confined in the transverse direction. The atoms initially at rest at the bottom of the shell -because of gravity- are set into rotation. At moderate rotation frequencies, a vortex lattice develops, and melts for large rotation speeds or low atom numbers. We explore the transition from a vortex crystal to a disordered vortex and eventually random phase fluctuations. At large rotation frequencies, the centrifugal force leads to the formation of a metastable dynamical ring which rotates with linear speeds widely exceeding the speed of sound in the gas.

Wigner dynamics for quantum gases under inhomogeneous gain and loss processes with dephasing

• Dragi Karevski (univ-lorraine)

We present a Wigner function-based approach for the particle density evolution in fermionic and bosonic open quantum many-body systems, including the effects of dephasing. In particular, we focus on chains of noninteracting particles coupled to Lindblad baths. The dissipative processes, described by linear and quadratic jump operators, are modulated by inhomogeneous couplings. Following a semiclassical approach, we find the differential equation governing the Wigner function evolution, which can be solved in closed form in some particular cases. We check the accuracy of the Wigner approach in different scenarios (ie, Gaussian jump rates), describing the density evolution and the transport phenomena in terms of classical quasiparticles. M Coppola, GT Landi, D Karevski Physical Review A 107 (5), 052213