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...
We build the quasiparticle picture for the tripartite mutual information (TMI) after quantum quenches in spin chains that can be mapped onto free-fermion theories. A nonzero TMI (equivalently, topological entropy) signals quantum correlations between three regions of a quantum many-body system. The TMI is sensitive to entangled multiplets of more than two quasiparticles, i.e., beyond the...
Superconducting circuits have recently emerged as a new platform to explore the physics of open many body systems using microwave photons in strongly non-linear media. In this talk, we will present some experimental results that were obtained in Orsay and in Grenoble where photons confined in a waveguide interact strongly through an impurity, here a Josephson junction. The system may be driven...
The success of Machine Learning owes to the development of neural-networks, variational approximators that can efficiently represent unknown functions living in high-dimensional spaces. Recently, those techniques have been ported to the field of numerical physics and used to approximate inherently high dimensional objects such as the Many-Body Wave-Function [1] or Density-Matrix [2] in an...
The experimental control of the coherent interaction between light and matter is one of the corner stones of the recent developments in the field of quantum technologies. In this context, cavity quantum electrodynamics has reached an important milestone in the last decade with the achievement of the ultrastrong coupling (USC) regime, where the coupling strength becomes comparable or even...
In this talk I will discuss the "entanglement" entropy growth dynamics in open spin models, comparing different matrix product representations of the many-body density matrix. Recently we discovered mechanisms behind a logarithmic growth of operator entanglement (OE) in XXZ model dynamics subjected to dephasing [1]. I will contrast this behavior to the growth of trajectory entanglement (TE),...
Superradiance of cold atoms in an optical cavity can be harvested to act as an optical frequency reference. By using an electronic transition much narrower spectrally than the cavity mode (i.e., by operating in the bad cavity limit), the frequency of the outcoming light is little affected by mirror position fluctuations – a signifiant limitation to short term stability in standard optical...
We will consider a parallel quantum dot as an example of an open quantum system that can feature a strong parity symmetry. For the dot, due to the presence of interactions, this symmetry results in the bistability characterised by distinct particle currents, while its explicit breaking leads to metastability. We will discuss when parameters of the dynamics can be estimated by continuously...
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...
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...
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...
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...
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...
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,...
Can we efficiently estimate transport coefficients (conductivities etc) in many-body quantum systems using classical computers? Drawing on lessons learned from studying scrambling and entanglement entropy dynamics in generic many-body systems, I propose an upper bound on the computational resources required to simulate transport at high temperatures: CPU time/memory $\sim...
In this work we characterize the metastable dynamics in the ferromagnetic quantum Ising chain with a weak longitudinal field subject to continuous monitoring of the local magnetization. To this end we exploit a numerical approach based on the combination of matrix product states with stochastic quantum trajectories which allows for the simulation of the trajectory-resolved non-equilibrium...
We study an exactly solvable model of monitored dynamics in a system of $N$ spin $1/2$ particles with pairwise all-to-all noisy interactions, where each spin is constantly perturbed by weak measurements of the spin component in a random direction. We make use of the replica trick to account for the Born's rule weighting of the measurement outcomes in the study of purification and other...
Local quantum measurements of many-body systems can induce phase transitions between volume and area law scaling of entanglement entropy.
Here we present a Gaussian fermionic model where continuous measurements of two non-commuting sets of observables induce a transition between area-law entanglement scaling phases of distinct topological order. We characterize the phase transition in terms...
Monitored Fermions provide a rich playground for the study of entanglement phase transitions in non-unitary quantum dynamics. We will discuss the phenomenology of entanglement transitions in several classes of monitored Hamiltonian systems and in fermion circuits and introduce effective theories describing both setups. We will then utilize adaptive feedback to reduce the configurational...
In this talk, I will discuss a variation of the standard framework of measurement-induced phase transitions, where the projective measurements are followed by control operations steering the system toward a pure absorbing state. In these dynamics, two types of phase transition occur as the rate of these control operations is increased: a measurement-induced entanglement transition, and a...
I will present the theory needed to apply a Gaussian-preserving operator to a fermionic Gaussian state. Then I will use this formalism to derive the equations of motions of a fermionic Kitaev chain following two different dynamic protocols, induced by the presence of the monitoring apparatus: a quantum-jump evolution with string operators and a quantum diffusion dynamics with long-range...
I will discuss some numerical results for the entanglement entropy dynamics
along the quantum trajectories of a fermionic Kitaev chain, in the presence of
measurements with a non-local character. The first part addresses a quantum-jump evolution with fixed-range string operators: a variety of behaviors emerge, ranging from volume-law, for extensive ranges of the string, to subvolume- and...
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,...
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...
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...
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.
Ergodicity in quantum systems is often defined through statistical properties of energy eigenstates, such as Berry's conjecture for single particle chaotic systems, and the eigenstate thermalization hypothesis (ETH) for many-body systems. In this talk, I would like to pose the question whether there are quantum systems which can exhibit a stronger form of ergodicity, namely whether dynamics is...
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...
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...
An alternative title could have been “How to characterise fluctuations in diffusive out-of-equilibrium many-body quantum systems?” In general, the difficulty to characterise non-equilibrium systems lies in the fact that there is no analog of the Boltzmann distribution to describe thermodynamic variables and their fluctuations. Over the last 20 years, however, it was observed that fluctuations...
Confining atoms to a single line (1D) results in a system, whose elementary excitations are quasi-particles with properties that may differ significantly from the atoms; in a Bose gas, correlation effects due to interactions in 1D prevent two quasi-particle excitations from occupying the same quantum state. This imposed Pauli exclusion leads to effective fermionization of the quantum Bose gas...
Due to its probabilistic nature, a measurement process produces a distribution of possible outcomes. This distribution — or its Fourier transform known as full counting statistics (FCS) — contains much more information than say the mean value of the measured ob- servable and accessing it is sometimes the only way to obtain relevant information about the system.
In fact, the FCS is the limit...
I will discuss several examples of non-ergodic dynamical systems with anomalous charge fluctuations.
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...
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...
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...
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...
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...
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...
