Mesoscopic cold atom systems in and out of equilibrium

From quark and nuclear matter down to solid-state systems and ultracold gases, mass asymmetry is known to profoundly influence the behavior of strongly interacting fermionic particles, both at the few- and many-body level. In my talk I will discuss the prospects offered by a novel ultracold 53Cr - 6Li Fermi mixture, currently under development within the PoLiChroM experiment in Florence. I will first show how the unique few-body properties enabled by the peculiar chromium-lithium mass ratio render such a mixture a qualitatively new framework for the investigation of elusive phenomena of highly-correlated fermionic matter. I will then provide an overview of our new setup, discussing the current status and near future perspectives of the PoLiChroM machine.

In our quest to obtain a deeper understanding of strongly interacting many body quantum systems, we turn to finite systems that can be prepared at very low entropies. Single-atom sensitive detection allow for the determination of high-order correlation functions. In our presentation we first show work with noninteracting spin polarized fermions and determine the correlations present solely due to indistinguishability, observing for the first time so-called Pauli crystals, geometric structures that form in the correlations between particles because of the antisymmetry of the wave function. We will also report on a significant breakthrough observing a precursor of many-body physics in a finite 2D system that due to degeneracies in the harmonic potential exhibits a shell structure: Increasing the attraction between particles, we observe a non-monotonous behavior of the excitation spectrum when the attraction becomes larger than the spacing between shells. In the many-body limit this can be interpreted as a quantum phase transition from normal to superfluid and associated with a long-lived Higgs mode. In the future we would like to use our newly established methods to measure correlations in (strongly) interacting systems. A first step could be to directly observe Cooper pairs by increasing the particle number.

Non-equilibrium quantum many-body systems can display fascinating phenomena relevant for various fields in science ranging from physics, to chemistry, and ultimately, for the broadest possible scope, life itself. The challenge with these systems, however, is that the powerful formalism of statistical physics, which have allowed a classification of quantum phases of matter at equilibrium does not apply. Therefore, using controllable cold atomic systems to shed light on the organizing principles and universal behaviors of dynamical quantum matter is highly appealing. One emerging paradigm is the dynamical phase transition (DPT) characterized by the existence of a long-time-average order parameter that distinguishes two non-equilibrium phases. I will report the observation of a DPT in two different but complementary systems: a trapped quantum degenerate Fermi gas and long lived arrays of atoms in an optical cavity. I will show how these systems can be used to simulate iconic models of quantum magnetism with tunable parameters and to probe the dependence of their associated dynamical phases on a broad parameter space. Besides advancing quantum simulation our studies pave the ground for the generation of metrologically useful entangled states which can enable real metrological gains via quantum enhancement.

Degenerate quantum gases in modulated optical lattices are a flexible testbed for the experimental study of quantum matter driven far from equilibrium. I will discuss results from a sequence of recent experiments in this area, starting with the tuning of band structure and transport properties in both rapidly-driven and slowly-driven lattices, moving on to experimental mapping of the properties of interacting prethermal Floquet matter, and concluding with probes of localization in driven quasicrystals and interacting quantum kicked rotors.

Collective effects in atom-light interactions typically emerge when the scatterers are separated by much less than an optical wavelength. However, in structured materials they can dominate even for comparably low densities. Here we report on the observation of collective backscattering by ordered atoms in a 2d optical lattice. With a single atomic monolayer this realizes the thinnest possible mirror. We observe a significant narrowing of the spectral response compared to the lifetime-limited line width of individual atoms. This underlines the collective nature of the scattering process in our sample of about 200 atoms.

I will discuss the dynamics of two impurities in a Bose-Einstein condensate (BEC) [1] and one impurity in a two-component BEC [2], when the problem is approached from the open quantum system perspective. For both cases and untrapped impurities, I will describe the long-time diffusive behavior. Particularly, I will show that this behavior can be controlled in the two component case. For two trapped impurities, I will discuss the realistic conditions for the final equilibrium state to be squeezed or to present entanglement. I will briefly discuss an application for quantum non-demolition thermometry in a BEC [3]; I will introduce in more detail an application in phononics, to construct a system that permits a heat current between two BECs, and evaluate this system as a thermal diode [4]. I will also discuss the effect of dimensionality in the dynamics described [5]. [1] C. Charalambous, et al., Sci. Post 6, 10 (2019). [2] C. Charalambous, et al., Quantum 4, 232 (2020) [3] M. Mehboudi, et al., Phys. Rev. Lett. 122, 030403 (2019). [4] C. Charalambous, et al., New Journal of Physics 21, 083037 (2019). [5] M. Miskeen Khan et al. arXiv:2007.04925.

We will briefly honour the memory of Prof. Artur Polls, who passed away in a completely unexpected way on Aug 12th 2020, see an obituary here [1]. We will share some recollections and pictures. Special attention will be taken to his last works related to mesoscopic quantum systems. We will finalize explaining one of his most recent work on the existence of quantum droplets of binary mixtures in 1D optical lattices [2]. [1] In memory of Artur Polls Martí, http://icc.ub.edu/news/616 [2] Quantum droplets of bosonic mixtures in a one-dimensional optical lattice Ivan Morera, Grigori E. Astrakharchik, Artur Polls, Bruno Juliá-Díaz Phys. Rev. Research 2, 022008 (R) (2020).

Driven quantum phase transition (QPT) occurs when a many body system is slowly or adiabatically swept across a quantum critical point (QCP). A recent example involves near deterministic production of an atomic Bose-Einstein condensate (BEC) in Twin-Fock as well as three-mode balanced Dicke states. The efficacy of the sweeping protocol, however, is limited by a compromise between finite coherence time (due to loss or decoherence) versus adiabaticity (from a finite-sized gap). This work presents a faster sweeping protocol with improved performance guided by deep reinforcement learning (DRL). Implemented in a BEC of upto 10000 87Rb atoms, the prepared balanced Dicke state ensemble exhibits an improved number squeezing of 13.08+/-0.29 dB within 750 ms, or in about half of the previously reported near adiabatic sweeping time. Our protocol highlights the potential of DRL to quantum dynamic control and quantum state preparation. With suitable adjustment, it can be applied to driving QPT along avoided crossing ground state over QCP in the broad class Lipkin-Meshkov-Glick (LMG) model of many coupled spins.

In this talk, I will report on recent developments on how to use ultracold fermionic alkaline-earth atoms (AEAs) –currently the basis of the most precise atomic clock in the world– for the investigation of complex many-body phenomena and magnetism. Specifically, I will discuss protocols that take advantage of the ultra long lifetime of AEAs dressed by an ultra-stable clock laser, which couples spin and motional degrees of freedom, to engineer superexchange spin models with high degree of tunability. I will show how these spin models can be a powerful resource for the generation of useful entangled states including cat states, cluster states and spin squeezed states. The proposed schemes are readily implementable in current state-of-the-art atomic clocks, and open a window to enhance their sensitivity beyond the standard quantum limit and a path to realize proof-of-principle demonstrations of one-way quantum computing with ultra-cold atomic systems.

We investigate the rotational properties of a dipolar Bose-Einstein condensate trapped in a toroidal geometry. Studying the ground states in the rotating frame and at fixed angular momenta, we observe that the condensate acts in distinctly different ways depending on whether it is in the superfluid or in the supersolid phase. We find that intriguingly, the toroidal dipolar condensate can support a supersolid persistent current which occurs at a local minimum in the ground state energy as a function of angular momentum, where the state has a vortex solution in the superfluid component of the condensate. The decay of this state is prevented by a barrier that in part consists of states where a fraction of the condensate mimics solid-body rotation in a direction opposite to that of the vortex. Furthermore, the rotating toroidal supersolid shows hysteretic behavior that is qualitatively different depending on the superfluid fraction of the condensate.

In the first part of my talk, I will present our study of quantum information scrambling in spin models with both long-range all-to-all and short-range interactions. We argue that a simple global, spatially homogeneous interaction together with local chaotic dynamics is sufficient to give rise to fast scrambling, which describes the spread of quantum information over the entire system in a time that is logarithmic in the system size. In the second part, we propose a realization of mesonic and baryonic quasiparticle excitations in Rydberg atom chains with programmable interactions. By engineering a $\mathbb{Z}_3$-translational-symmetry breaking field on top of the Rydberg-blockaded Hamiltonian, we show that different types of defects experience confinement, and as a consequence form mesonic or baryonic quasiparticle excitations. We propose an experimental protocol involving out-of-equilibrium dynamics to directly probe the spectrum of the confined excitations.

I will describe experiments where we probe how a strongly-interacting gas of ultracold atoms reacts to dissipation. Specifically, I will discuss how the spatial coherence disappears when the gas is exposed to a near-resonant laser driving absorption-spontaneous emission cycles. Spontaneous emission introduces random momentum changes leading to diffusion in momentum space. This momentum diffusion is a well-known process is well-known in quantum optics which, for example, limits the temperature achievable in laser cooling. For strongly interacting bosons, we observed that the momentum space dynamics becomes anomalously slow, more precisely sub-diffusive. I will discuss the interpretation of this observation by the existence of slowly-relaxing states (somewhat analog of subradiant states in quantum optics) that dominate the long-times dynamics.

The discovery of topological materials has triggered a huge research effort to understand and control the link between symmetry properties and the emergence of novel physical properties. In particular, the existence of topological invariants, related to the band structure properties of the bulk material, is associated with the emergence of edge states, robust to local perturbation of the lattice. With the development of a variety of synthetic experimental platforms, it is now possible to engineer lattices and explore topology in a variety of contexts. Here we are interested in the physics of 1D topological lattices in the presence of both dissipation and non-linearity. I will describe recent experiments we have performed on photonic Su-Schrieffer-Heeger (SSH) lattices made of arrays of coupled polariton micropillars. The coupling between neighbor resonators is modulated so that a topological gap opens. Such polariton lattice is non-hermitian because of photon losses through the cavity mirrors, and highly non-linear because of polariton-polariton interactions. The physical properties of the system can be probed via optical spectroscopy using either incoherent pumping of the bands or resonant excitation of the lattice. I will show that it is possible to retrieve the value of the bulk topological invariants by monitoring the propagation of polaritons across the lattice in reciprocal space. This technics based on the experimental measure of the mean chiral displacement can be extended to 2D lattices. The second part of the presentation will be dedicated to the non-linear response of such a topological chain under resonant coherent drive. Gap solitons can be formed which properties are reminiscent of the underlying topological properties of the lattice. Their exponential tails present chirality symmetry properties, which are responsible for robustness of these solitons to defects when located on one sublattice. Moreover, engineering of the drive enables generating exotic topological solitons, which do not have any counterpart in conservative systems. Our results offer new insights to the rich phenomenology of topological systems subject to nonlinearities and non-hermicity. They open the door to the exploration of many-body topological effects in open systems.

This talk will present recent results on the post-quench time evolution of one-dimensional systems of relevance to atomic physics. A quick review will be given of various methods based on integrability, including the Quench Action, Generalized Hydrodynamics and Numerical Renormalization. Applications to interaction quenches and to spatially inhomogeneous quenches will be discussed.

To characterize the generic behavior of open quantum many-body systems, we consider random, purely dissipative Liouvillians with a notion of locality. We find that the positivity of the map implies a sharp separation of the relaxation timescales according to the locality of observables. Specifically, we analyze a spin-1/2 system of size ℓ with up to n-body Lindblad operators, which are n local in the complexity-theory sense. Without locality (n=ℓ), the complex Liouvillian spectrum densely covers a “lemon”-shaped support, in agreement with recent findings [S. Denisov et al., Phys. Rev. Lett. 123, 140403 (2019)]. However, for local Liouvillians (n<ℓ), we find that the spectrum is composed of several dense clusters with random matrix spacing statistics, each featuring a lemon-shaped support wherein all eigenvectors correspond to n-body decay modes. This implies a hierarchy of relaxation timescales of n-body observables, which we verify to be robust in the thermodynamic limit. Our findings for n locality generalize immediately to the case of spatial locality, introducing further splitting of timescales due to the additional structure. K. Wang, F. Piazza, and D. J. Luitz, Phys. Rev. Lett. 124, 100604 (2020)