Session N33: Non-Equilibrium Physics with Cold Atoms and Molecules, Rydberg Gases, and Trapped Ions II

Observing emergent hydrodynamics in a long-range quantum magnet

Identifying universal properties of non-equilibrium quantum states is a major challenge in modern physics. A fascinating prediction is that classical hydrodynamics emerges universally in the evolution of any interacting quantum system. We study the dynamics of a long-range interacting spin system with non-equilibrium quantum field theory, predicting the emergence of a whole family of hydrodynamic universality classes, ranging from normal diffusion to anomalous superdiffusion. We experimentally test these predictions in the quantum dynamics of 51 individually controlled ions. By measuring space-time resolved correlation functions in an infinite temperature state, we observe the emergence of hydrodynamics at late time. We extract the transport coefficients of the hydrodynamic theory, reflecting the microscopic properties of the system. Our observations demonstrate the potential for engineered quantum systems to provide key insights into universal properties of non-equilibrium states of quantum matter and uncover the transport processes governing long-range interacting systems.   

Probing the edge between integrability and quantum chaos in interacting few-atom systems

Interacting quantum systems in the chaotic domain are at the core of various ongoing studies of many-body physics, ranging from the scrambling of quantum information to the onset of thermalization. We propose a minimum model for chaos that can be experimentally realized with cold atoms trapped in one-dimensional multi-well potentials. We explore the emergence of chaos as the number of particles is increased, starting with as few as two, and as the number of wells is increased, ranging from a double well to a multi-well Kronig-Penney-like system. In this way, we illuminate the narrow boundary between integrability and chaos in a highly tunable few-body system. We show that the competition between the particle interactions and the periodic structure of the confining potential reveals subtle indications of quantum chaos for 3 particles, while for 4 particles stronger signatures are seen. The analysis is performed for bosonic particles and could also be extended to distinguishable fermions.   

Optimizing Rydberg Antennas

Given their low electromagnetic profile, specially prepared atomic Rydberg vapors have already demonstrated improvement over conventional wire antennas as calibration standards for electric field measurements. Major efforts are now under way to develop practical room temperature Rydberg atom RF receivers with greater sensitivity, bandwidth, and dynamic range than any classical receiver. In this presentation I will summarize theoretical analysis of various laser and RF local oscillator setups, with the goal of optimizing the underlying electromagnetic transparency (EIT) sensitivity against environmental noise, atomic motion-induced Doppler, RF communication signal waveform, and other important effects. At the heart of the analysis is a carefully controlled resonant coupling of the low energy core and highly excited Rydberg atomic level subspaces. The incident RF signal perturbation of the Rydberg state amplitudes then induces a corresponding large perturbation of the core state amplitudes. This nonequilibrium multistate entanglement is enabled by an effective Hamiltonian eigenvalue near-degeneracy whose avoided crossing produces the required extreme sensitivity. This formulation of the problem enables a very efficient method for optimizing various proposed experimental setup designs.   

Strongly interacting two photon random walk in an array of single atom beamsplitters

Few photon quantum walk is a powerful tool for quantum simulation. However, most theoretical and experimental works related to this topic have been focused on linear beam splitter arrays, which lack photon-photon interactions. Here we propose a novel way of implementing a strongly interacting two photon discrete time quantum walk. We use single atom beam splitters to induce strong photon-photon interaction. Photonic quantum walks in such a nonlinear beam splitter array are still poorly understood. In this work, we theoretically investigated the two photon correlation functions at the output and observed strong photon-photon interaction which yield various types of photon statistics. Finally, we propose a practical realization of our random walk based on time-multiplexed synthetic dimensions. Our proposal has opened the door for a novel approach of quantum simulation using photons.   

Anisotropic sound propagation in dilute dipolar gases.

Ultracold dipolar gases have garnered extensive interest in recent years, attributed to their capacity for a rich variety of dynamical phenomena. One such instance is anisotropic scattering, dominant in thermal dipolar gases in the quantum collision regime. When taken out of equilibrium, this anisotropy can lead to axially differentiated spreading of energy through the gas. To this end, we study how dipolar collisions lead to an anisotropic viscosity that distorts acoustic wave propagation. The implications to diffusive transport are addressed, and extensions to this work are proposed. Analysis is performed with a semi-analytic theory based on the Enskog formalism, verified by numerical Monte Carlo simulations. This work is funded by the NSF.   

Thermalization dynamics of a U(1) lattice gauge theory on a Bose-Hubbard quantum simulator

Gauge theories form the foundation of modern physics, with applications ranging from elementary particle physics and early-universe cosmology to condensed matter systems. We demonstrate the emergence of irreversible thermal equilibrium behavior for far-from-equilibrium gauge field, by quantum simulating the fundamental unitary dynamics of a U(1) symmetric gauge field theory. While this is in general beyond the capabilities of classical computers, it is made possible through the experimental implementation of a 71-site cold atomic system in an optical superlattice. The highly constrained gauge theory dynamics is encoded in a one-dimensional Bose--Hubbard simulator, which couples fermionic matter fields through dynamical gauge fields. We investigate the far-from-equilibrium evolution and the equilibration to a steady state well approximated by a thermal ensemble. Our work establishes a new realm for the investigation of elusive phenomena, such as Schwinger pair production and string-breaking, and paves the way for more complex higher-dimensional gauge theories on quantum synthetic matter devices.   

Ferromagnetism in tilted fermionic Mott insulators

We investigate the magnetism of tilted fermionic Mott insulators. With a small tilt, the fermions are almost localized and still form a Mott-insulating state, where the localized spins interact with each other via antiferromagnetic exchange coupling modified by the tilt [1,2]. A large tilt, rather than destroying localization, induces the Wannier-Stark localization and the fermions can still be regarded as a localized spin system. The sign of the exchange coupling can be changed to realize ferromagnetic interaction. We show these behaviors via both perturbation theory and real-time numerical simulation for the fermionic Hubbard chain. Our simulation shows that it is possible to effectively control the speed and time-direction of the real-time dynamics with the tilt.

[1] K. Takasan and M. Sato, Phys. Rev. B 100, 060408 (2019).

[2] S. C. Furuya, K. Takasan, and M. Sato, Phys. Rev. Research 3, 033066 (2021).   

Valley Prize (2022): TBD

I will describe new universality classes of hydrodynamic phenomena and non-equilibrium fixed points which arise in constrained many-body systems.  These new theories are inspired by exotic physical systems with "fracton" excitations.  I will highlight how modern methods from effective field theory are leading us to a systematic understanding and classification of new universality classes of constrained hydrodynamics and how -- in turn -- these new fluids might lead us to a better understanding of hydrodynamics more generally. 

As an interesting example of a new universality class, I will describe a "fractonic" generalization of the non-equilibrium Kardar-Parisi-Zhang fixed point that can exist below four spatial dimensions. This non-equilibrium fixed point, and others, arise out of a fundamental instability of the hydrodynamics of a (constrained) fluid at rest.

One of these universality classes -- the subdiffusion of charge in the presence of dipole conservation -- has already been experimentally observed in ultracold atoms in a tilted optical lattice.  Time permitting, I will suggest ideas for how to discover further universality classes in experiments.   

Continuous time crystal from a spontaneous many-body Floquet state

We propose the concept of a spontaneous many-body Floquet state. This is a state that, in the absence of external periodic driving, still self-oscillates like in the presence of a time-periodic Hamiltonian, this behavior being spontaneously induced by many-body interactions. Furthermore, we prove that it is also a time crystal, presenting long-range time-periodic order. However, its time crystalline behavior is very different to that of conventional Floquet discrete time crystals: here, there is no external periodic driving, and the nature of the spontaneous symmetry breaking is continuous instead of discrete. We also demonstrate that a spontaneous many-body Floquet state can be implemented in a one-dimensional flowing atom condensate, resulting from a dynamical phase transition and stable against quantum fluctuations, and propose realistic experimental scenarios for its observation. The realization of a spontaneous many-body Floquet state would then not only provide a novel form of ordered quantum matter, but also a continuous time crystal.   

Emergence of Hilbert Space Fragmentation in the Ising Model with a Weak Transverse Field

The last two decades have witnessed a substantial advance in revealing conditions for quantum many-body systems to thermalize following the experimental progress in quantum simulators. The transverse-field Ising model is one of the fundamental models in quantum many-body systems, yet full understanding of its dynamics remains elusive for higher than in one dimension. Here, we show the emergence of non-ergodicity for the Ising model in a weak transverse field on a square lattice in arbitrary dimension d. Specifically we investigate the effective non-integrable model in the weak-transverse field limit and demonstrate that novel Hilbert-space fragmentation occurs for d>1 as a consequence of only one emergent U(1) conservation law, i.e., domain-wall-number conservation. This conservation law leads to a kinetic constraint in the model and the appearance of frozen regions, which give rise to exponentially many fragmented subspaces in the Hilbert space. Our results indicate nontrivial initial-state dependence for long-lived prethermal dynamics of the Ising models in a weak transverse field.   

Quench dynamics of 2D Bose gases across the BKT critical point

We report on the observation of non-equilibrium dynamics in 2D quantum systems across a critical point. We quench the system by coherently splitting a single 2D Bose gas into two, resulting in the dynamical crossing of the Berezinskii-Kosterlitz-Thouless phase transition. We monitor the relaxation dynamics towards the vortex-proliferated disordered phase using a matter-wave interferometry technique and find the time evolution of the phase correlation function and vortex density. We identify that the temporal decay of the correlation is related to the energy scale associated with vortex excitations. We further compare measured vortex unbinding dynamics with the real-time renormalisation group theory.

We acknowledge close collaboration with Professor Ludwig Mathey, Dr Vijay Singh (University of Hamburg, Germany) and Dr Junichi Okamoto (University of Freiburg, Germany) on the theoretical interpretation of this experimental work including extensive numerical simulation of specific cases.   

Finite speed of quantum information in models of interacting bosons at finite density

We prove that quantum information propagates with a finite velocity in any model of interacting bosons whose (possibly time-dependent) Hamiltonian contains spatially local single-boson hopping terms along with arbitrary local density-dependent interactions.  More precisely, with density matrix ρ∝exp[-μN] (with N the total boson number), ensemble averaged correlators of the form 〈[A₀ ,B_r (t)]〉, along with out-of-time-ordered correlators, must vanish as the distance r between two local operators grows, unless t≥r/v for some finite speed v.  In one dimensional models, we give a useful extension of this result that demonstrates the smallness of all matrix elements of the commutator [A₀ ,B_r (t)] between finite density states if t/r is sufficiently small.    Our bounds are relevant for physically realistic initial conditions in experimentally realized models of interacting bosons. In particular, we prove that v can scale no faster than linear in number density in the Bose-Hubbard model: this scaling matches previous results in the high density limit.  The quantum walk formalism underlying our proof provides an alternative method for bounding quantum dynamics in models with unbounded operators and infinite-dimensional Hilbert spaces, where Lieb-Robinson bounds have been notoriously challenging to prove.   

Nonequilibrium Hanbury-Brown-Twiss experiment: Theory and application to binary stars

In this work we consider the Hanbury-Brown and Twiss experiment for a configuration of two extended objects at different temperature. We show that intensity interference for these scenarios appears as an alternative method for measuring the features of the binary system. Unlike the case of equilibrium scenarios or the two-photon states, both temperatures and radii of each object take a role on the photon correlations. In addition to the interference oscillations on the second-order coherence, a long-baseline asymptotic value shows to depend on the observation frequency, temperatures and radii of both objects. We discuss the advantages of measuring each magnitude and also its combination according to the experimental possibilities. By finally including some aspects of the orbital motion to our approach, we apply it to the case of binary stars (in particular, Luhman 16 and Spica α Vir systems), showing that the method might be suitable for estimating not only the radii but the temperatures of the constituents. For the case of the Luhman 16 we show that measuring both magnitudes should be possible in the visible range with baselines of no more than hundreds of meters, while for the case of the Spica the same is possible in the ultraviolet regime but with baselines of tenths of meters. We believe that our work contributes to improve both fundamental and practical aspects of intensity interferometry as a tool for characterizing binary systems.