Session F66: Non-Equilibrium Physics in AMO Systems I

Spin-charge separation in an atomic Luttinger liquid

Strong correlations that occur in many electronic materials are responsible for some of the most remarkable phenomena in condensed matter physics, such as novel forms of superconductivity and topological quasi-particles, but at the same time, they present vexing challenges for theoretical description. Strong correlations are generally found in low-dimensional materials where low energy excitations are most likely collective. A paradigm for these strongly correlated materials is the one-dimensional (1D) Luttinger liquid.  The low energy excitations are bosonic sound waves that correspond to either spin-density or charge-density waves that, remarkably, propagate at different speeds, thus realizing a spin-charge separation.  This phenomena has been observed in electronic materials, but a quantitative analysis of these data has proved challenging because of the complexity of the electronic structure and the unavoidable presence of impurities and defects.

We have studied an artificial one-dimensional (1D) material in which the electrons are replaced by ⁶Li, a composite fermion. A 2D optical lattice confines the atoms to an array of 1D tubes. Quantum simulation with ultracold atoms takes advantage of the capability to adhere to a theoretical model, and the tunability of the model parameters to enable quantitative comparison with theory. Our experiment uses Bragg spectroscopy to measure the momentum and energy resolved structure factor, S(q,ω), from which the two speeds of sound are determined. In collaboration with our theory colleagues, we made a direct theory/experiment comparison and found excellent agreement as a function of interaction strength [1]. We found that it was necessary to include nonlinear corrections to the spin-wave dispersion arising from back-scattering, thus going beyond the Luttinger model. More recently, we explored the disruption of spin correlations with increasing temperature [2], an effect that destroys spin-charge separation.

1.  R. Senaratne*, D. Cavazos-Cavazos* et al, “Spin-charge separation in a 1D Fermi gas with tunable interactions”, Science 376, 1305 (2022).

2.  D. Cavazos-Cavazos, R. Senaratne, A. Kafle, and R.G. Hulet, “Realization of a spin-incoherent Luttinger liquid”, arXiv:2210.06306 (2022).   

A Bose-Einstein condensate thermal engine

The study of out-of-equilibrium thermodynamics of quantum systems has received increasing attention in recent years thanks to tremendous theoretical and experimental progress. While most of the studies in quantum thermodynamics bear a close resemblance to their classical counterparts, especially close to equilibrium, there are only a few examples of genuine quantum features, e.g. coherence and indistinguishability, that provide an advantage over classical thermodynamic devices. In this contribution, I will show the functioning and performance of an endoreversible Otto cycle operating with a harmonically confined Bose gas as the working medium. I analyze the engine operation in three regimes, with the working medium in the BEC phase, in the gas phase, and driven across the BEC transition during each cycle. The unique properties of the BEC phase allow for enhanced engine performance, including increased power output and higher efficiency at maximum power.

    

Capturing thermalization with Gaussian states in a virtual environment

While many-body systems in equilibrium are well understood by statistical mechanics, a general understanding of thermalization dynamics is still out of reach. One crucial obstacle is the formulation of suitable variational descriptions. The variational class of Gaussian states excellently describes early time dynamics by accounting for both mean field dynamics and spontaneous quasiparticle excitations. At late times, however, these linearized fluctuations inadequately capture the growth of entanglement.

We extend the Gaussian theory by coupling the system to an artificial environment, generalizing the evolution of observables to stochastic trajectories. At the expense of generating an ensemble of states, the environment provides a decoherence which suppresses entanglement within each trajectory, allowing to capture the evolution with Gaussian states beyond early times. The approach is applied to the dynamics of a quenched spinor condensate [Phys. Rev. A 105, 013305 (2022)] and the thermalization of cold atoms in an optical lattice. In the latter case, the trajectory approach proves an elegant solution to the UV catastrophe present in semiclassical field theories.   

Ultrafast dynamics of cold Fermi gas after a local quench

We consider energy dynamics of two initially independent reservoirs A and B filled with a cold Fermi gas coupled and decoupled by two quantum quenches following one another. The energy change in the system adds up the heat transferred between A and B and the work done by the quench to uncouple the reservoirs. In case when A and B interact for a short time, we find an energy increase in both reservoirs upon decoupling. This energy gain results from the quenches' work and does not depend on the initial temperature imbalance between the reservoirs. We relate the quenches' work to the mutual correlations of A and B expressed through their von Neumann entropies. Utilizing this relation, we show that once A and B become coupled, their von Neumann entropies grow (on a timescale of the Fermi time) faster than thermal transport within the system. For a metallic setup, this implies the characteristic timescale of correlations' growth τ to be in the femtosecond range, while for the ultracold atoms, we expect τ ~ 0.1 ms.   

Insulating BECs, fractured Bose glasses, and other surprises in strongly tilted optical lattices

We consider versions of Bose- and Fermi-Hubbard models whose dynamics conserves both total charge and total dipole moment, a situation which can be engineered in strongly tilted optical lattices. Related models have received significant attention recently for their interesting out-of-equilibrium dynamics, but analytic and numeric studies reveal that they also possess rather unusual ground states. As an example, the dipole-conserving Bose-Hubbard model realizes a phase of matter which contains a Bose-Einstein condensate, but which is not a superfluid. This model also exhibits an interesting dynamic instability towards an exotic type of glassy state, with implications for experiments on spinning BECs. This talk will present a survey of recent results on these models, as well as discuss connections to current and future AMO experiments.   

Inhomogeneous Kibble-Zurek Mechanism in 87Rb

Inhomogeneous Kibble-Zurek Mechanism (IKZM) extends the framework for understanding the non-equilibrium defect formation dynamics in a non-homogeneous system as it undergoes a continuous, second-order, thermal phase transition. Although the theory correctly predicts the qualitative power-law scaling of the defect number density with the thermal quench rate, recent studies in ⁸⁷Rb Bose gas show significant deviation from the predicted scaling exponent as well as the observation of defect density saturation at rapid quench rates that can be attributed to an early coarsening effect. The study presented here extends our investigations into the IKZM in ⁸⁷Rb Bose gas and examines the effect of the optical trapping potential geometry on certain key parameters such as the KZ power-law scaling exponent, the saturated defect number density, and the early coarsening effect. Our observations indicate a strong dependence of the KZ scaling exponent on the underlying trap geometry, with a trend currently unexplained by the IKZM theory, while the early coarsening effect is insensitive to the trap geometry. We present the relevant results and scope for future studies to better understand such non-equilibrium phase transition dynamics.

    

Quantum point contact transport in the presence of a particle loss

Ultracold atomic gases allow us to simulate interesting quantum phenomena.

Recently, atomtronics being the cold-atom analog of electronics has attracted attention due to experimental realizations of mesoscopic and circuit systems with ultracold atomic gases.

In this talk, I will discuss two-terminal mesoscopic transport phenomena in the presence of a particle loss, which can be realized in ultracold atomic gases. By analyzing a couple of systems related to the dissipative quantum point contact, it is shown that formal expressions of the particle and heat currents and the particle loss obey universal forms. If time permits, I will explain the recent result on the system with superfluid reservoirs.   

Doppler sensitivity and optimization of Rydberg antennas

Radio frequency antennas based on Rydberg atoms should in principle be able to achieve sensitivities beyond those of any conventional wire antenna, especially at lower frequencies where very long wires are needed to accommodate the growing wavelength. Given their low electromagnetic profile, they have already demonstrated improvement over conventional wire antennas as calibration standards for electric field measurements. This paper presents a detailed theoretical investigation of Rydberg antenna sensitivity, elucidating parameter regimes that could cumulatively lead to 2--3 orders of magnitude sensitivity increase over currently explored setups. Of special interest are three-laser setups proposed to compensate for atom motion-induced Doppler spreading. Such setups are in indeed shown to be advantageous, but only because they restore sensitivity to expected Doppler-limited value, removing significant additional off-resonance reductions.   

Measuring correlations in an ensemble of lattice-trapped dipolar atoms

Understanding the buildup of correlations is an important aspect of studying quantum dynamics.  In a recent theory-experiment collaboration, we perform collective spin measurements to investigate the two-body correlations between a large ensemble of  ultracold chromium atoms frozen in a three dimensional optical lattice. The chromium atoms have spin S=3 and interact via long range and anisotropic dipolar interactions. From measuring the fluctuations of the total magnetization, we estimate the dynamical growth of the connected pairwise correlations associated with magnetization. We theoretically analyze the short- and long-time behaviors of the correlations as well as perform numerical simulations for the full time evolution, which are compared with experimental observations to access the quantum nature of the correlations. Our work shows that measuring fluctuations of spin populations provides new ways to characterize correlations in quantum many- body systems, for S> 1/2 spins.

    

Diverging current fluctuations in critical Kerr resonators

The parametrically pumped Kerr model describes a driven-dissipative nonlinear cavity, whose nonequilibrium phase diagram features both continuous and discontinuous quantum phase transitions. We consider the consequences of these critical phenomena for the fluctuations of the photocurrent obtained via continuous weak measurements on the cavity. Considering both direct photodetection and homodyne detection schemes, we find that the current fluctuations diverge exponentially at the discontinuous phase transition. However, we find strikingly different current fluctuations for these two detection schemes near the continuous transition, a behaviour which is explained by the complementary information revealed by measurements in different bases. To obtain these results, we develop formulas to efficiently compute the diffusion coefficient -- which characterises the long-time current fluctuations -- directly from the quantum master equation, thus connecting the formalisms of full counting statistics and stochastic quantum trajectories. Our findings highlight the rich features of current fluctuations near nonequilibrium phase transitions in quantum-optical systems.   

Long-time correlations in nonequilibrium atom-surface interactions

The dynamics of atoms immersed in a complex electromagnetic environment is often fundamentally different from the respective isolated dynamics. Not only the fields used to control the atoms, but also thermal and quantum fluctuations in the vicinity of macroscopic bodies lead to changes in the decay rate, induce decoherence, modify the atomic resonances and cause fluctuation-induced forces.

We explore the quantum-optical dispersion interaction between a microscopic particle and a complex electromagnetic environment in nonequilibrium. We argue that long-time correlations, i.e. contributions to the autocorrelation function of quantum operators which scale as an inverse power law in the time delay, can be essential for understanding the dynamics of the particle.

    

Quasi-Floquet prethermalization in a disordered spin ensemble in dimond

Floquet (periodic) driving has recently emerged as a powerful technique to control quantum systems via the so-called Floquet engineering (i.e. pulsed periodic driving). Such technique can help to prevent environment-induced decoherence and more recently, has enabled the study of novel quantum dynamical phenomena. However, a central challenge to stabilizing Floquet systems and observing such phenomena is the inevitable energy absorption from the driving fields. One potential solution--prethermalization--arises when the driving frequency is sufficiently larger than the system's local energy scale; the heating process is significantly suppressed and there exists a long-lived regime described by an effective static Hamiltonian. Despite this promise, the presence of long-range interactions and multiple driving frequencies may significantly alter the existence of prethermal behaviors. Crucially, there are two possible culprits for the breakdown of prethermalization: (1) The existence of long-range interactions may lead to a divergent local energy scale; (2) In the quasi-Floquet scenario with multiple driving frequencies, multi-photon processes may allow resonant absorption of energy from the drive. Here, we report the observation of prethermalization in a strongly interacting dipolar spin ensemble in diamond, where the interplay between the angular dependence and the long-range interaction helps to stabilize the system. More intriguingly, we extend our experimental observation to quasi-Floquet drivings with multiple incommensurate frequencies. In contrast to single-frequency drive, we observe that the existence of prethermalization is extremely sensitive to the smoothness of the applied fields. To complement our experimental observation, we also present a theoretical understanding of the energy absorption under quasi-periodic drive. Our results open the door to robust Floquet engineering and experimental investigation of non-equilibrium phenomena with multi-frequency drive.   

Prethermalization in open quantum systems: an application of the fluctuation-regulated quantum master equation

Quantum master equations (QME) describe the nonequilibrium dynamics of open quantum systems. For systems that are weakly coupled to a nearly memoryless bath, Markovian QMEs are routinely used. Markovian QMEs include the drive terms in the first order and the system-bath coupling in the second order. Here we show that such QMEs can be extended to incorporate the dissipative higher-order contributions from the drive, provided one takes into account the effects of thermal fluctuations in the local environment. Such an extension to the higher-order terms can also be done for other coupling terms, such as dipolar couplings. The resulting master equation is named a fluctuation-regulated quantum master equation (FRQME). For a driven-dipolar open quantum system for which the system-bath coupling is very small compared to the drive and the dipolar interaction, the system exhibits multiple timescales of evolution, with the system-bath relaxation timescale being the slowest one. We observe, in such a system, the dissipators from FRQME give rise to a set of quasi-conserved quantities. In a timescale much shorter than the relaxation timescale, these quasi-conserved quantities dominate the dynamics. The presence of these quasi-conserved quantities ensures constrained dynamics and results in the emergence of a prethermal state followed by unconstrained evolution under system-bath coupling. In the talk, we shall discuss the dynamics of this driven dipolar system with the principal focus on the emergence of the prethermal state.