Session P04: Driven Quantum Matter and Non-Equilibrium Dynamics

Interferometry of non-Abelian band singularities and Euler class topology

In systems with a real Bloch Hamiltonian band nodes can be characterised by a non-Abelian frame-rotation charge. The ability of these band nodes to annihilate pairwise is path dependent, since by braiding nodes in adjacent gaps the sign of their charges can be changed. Here, we theoretically construct and numerically confirm two concrete methods to experimentally probe these non-Abelian braiding processes and charges in ultracold atomic systems. We consider a coherent superposition of two bands that can be created by moving atoms through the band singularities at some angle in momentum space. Analyzing the dependency on the frame charges, we demonstrate an interferometry scheme passing through two band nodes, which reveals the relative frame charges and allows for measuring the multi-gap topological invariant. The second method relies on a single wavepacket probing two nodes sequentially, where the frame charges can be determined from the band populations. Our results present a feasible avenue for measuring non-Abelian charges of band nodes and the experimental verification of braiding procedures directly, which can be applied in a variety of settings including the recently discovered anomalous non-Abelian phases arising under periodic driving.   

Engineering Dynamical Phase Diagrams with Driven Lattices in Spinor Gases

We experimentally demonstrate that well-designed driven optical lattices can enable the exploration of highly-controllable non-equilibrium dynamics in spinor gases in previously inaccessible parameter regimes.  Specifically, experimental signatures in our observations demonstrate that driven lattices can simultaneously independently determine the effective spin-dependent interactions, spinor phase, and quadradic Zeeman energy of spinor gases with negligible heating and atom losses compared to conventional techniques.  Modulation-induced higher harmonics additionally generate progressively narrower separatrices at driving-frequency determined higher quadradic Zeeman energies. We understand these observations with calculations based on a dynamical single spatial mode approximation and find qualitative agreement with our data. Our findings expand the working range of cold atoms for applications in quantum sensing and engineered Hamiltonians.   

Interacting dynamical Anderson Metal-Insulator transition in kicked Bose-Einstein condensates: the effect of trapping potentials

Recent experimental progress on the observation of interaction effect on the dynamical localization-delocalization transition in kicked BEC has opened a new avenue for studying the interplay of interaction and disorder physics. In the experiments, a Gaussian trapping potential is used, which is different from the ideal quantum kicked rotor on a ring that has no spatially inhomogeneous potential. On the other hand, the experimental progress on realizing box-type of hard wall potentials for cold atoms offers another attractive platform for studying interacting Anderson localization physics. In this talk, we present our mean field numerical study of the kicked BEC with three types of potentials: Gaussian, ring, and box. We find the phase diagram of interacting Anderson Metal-insulator transition for a box potential is very close to that for the ideal ring potential, indicating kicked atomic gases in a box potential is a better system for studying many-body dynamical localization physics.   

Floquet engineering XXZ spin models and two-axis twisting with ultracold molecules

Due to their strong, long-range, and tunable dipolar interactions, ultracold molecules in optical lattices are a versatile platform for studying quantum many-body physics. In addition to control with d.c. electric and magnetic fields, molecules are also amenable to Floquet Hamiltonian engineering with microwave pulse sequences. Using a spin-1/2 system encoded in rotational states of ultracold KRb molecules, we investigated two applications of Floquet engineering. First, we validated our method by benchmarking Ramsey contrast decay of Floquet engineered XXZ spin models theoretically against MACE simulations and experimentally against spin models tuned by d.c. electric fields. Second, we explored two-axis twisting mean-field dynamics in itinerant molecules using an XYZ Hamiltonian, which cannot be generated by d.c. fields.   

Collective oscillations and damping of spin-driven turbulent Bose-Einstein condensates

We present observations of altered hydrodynamic properties in spin-driven turbulent spinor Bose-Einstein condensates (BECs). Turbulence was sustained by spatially mixing spin with alternating magnetic fields, using a driving method that provided a unique timeframe for investigating coherent collective modes of the turbulent BEC confined in a harmonic potential. To excite the quadrupole mode in our anisotropic, pancake-shaped BEC, we periodically modulated optical dipole traps. Our findings show that for turbulent BECs, the mode's frequency experiences a slight shift, and the temperature-dependent damping rate is enhanced compared to single-component, non-driven BECs. These results suggest intricate turbulence dynamics reminiscent of classical fluid dynamics. Numerical simulations based on the Gross-Piteavskii equation reveal residual damping enhancement and propose a turbulence-induced viscosity model.   

Supersolidity in a driven quantum gas

Driven systems are of fundamental scientific interest, as they can display properties that are radically different from similar systems at equilibrium. However, systems out of equilibrium are difficult to describe theoretically, as they are inherently time-dependent and deeply nonlinear. This makes the study of such systems an ideal task for quantum field simulators, in which complex dynamics emerge naturally and can be probed experimentally. Here, we demonstrate the emergence of supersolidity in a driven, two-dimensional superfluid that only has contact interactions. The self-stabilized system emerges as a result of large occupations of phononic modes due to driving [1] and can be described theoretically using an out-of-equilibrium fixed point of amplitude equations [2]. To demonstrate the hallmarks of supersolidity, we induce collective modes of the lattice, and show that the system supports lattice phonon propagation. We also show that the state maintains phase rigidity, a key property of superfluidity. This work introduces a novel type of supersolid that is readily experimentally accessible, and establishes a conceptual framework for describing elementary excitations of driven systems.

[1] N. Liebster, et al., arXiv:cond-mat/2309.03792 (2023)

[2] K. Fujii, et al., arXiv:cond-mat/2309.03829 (2023)   

Dissipation induced coherence in partial condensate

Dissipation is unavoidable in quantum systems. It usually induces decoherences and changes quantum correlations. However in this work, we find that dissipation can be a tool to probe the coherence in the quantum gas. By introducing dissipation into a partial condensate of Rb87 through blue detuned light, we observe an oscillation of atom number between the condensate part and the non-condensate part before equilibrium state is reached. In addition, since these two parts have different loss rates under dissipation, total atom number after the dissipation also oscillates which indicates the coherence in the quantum gas. We also propose a model based on Bogoliubov theory and it shows qualitative consistence with our experimental result.   

Engineering Perfect Transport in Spin Chains

Simulating transport phenomena in quantum materials is crucial for uncovering new approaches to information processing and for developing practical quantum devices. Rydberg atom arrays have emerged as a promising platform for the quantum simulation of out-of-equilibrium quantum dynamics due to their ability to realize quantum spin models on arbitrary geometries with controllable disorder, defects, and interactions. However, the realization of spin models driving spin transport typically relies on Hamiltonian engineering methods with reduced accuracy when accounting for experimental imperfections and realistic noise parameters. Here, we characterize the performance of two complementary protocols to realize ballistic transport of single-spin excitations in chains of Rb-87 atoms. We find the set of parameters for which a perfect transport condition is achieved in a one-dimensional spin chain. We then use the probability of transporting a single-spin excitation as a metric to quantify the breakdown of perturbative methods and the detrimental effects of long-range interactions, noisy control parameters, and spin-motion entanglement. We optimize over the set of control parameters, highlighting a competition between stronger driving rates, shorter lifetimes, and breakdown of perturbative approximations. We further propose utilizing perfect spin transport fidelity as a tool for benchmarking the realization of effective spin models, as well as the engineering of information transport. This work paves the way for near-term study of spin transport in chains, rings, lattices, and arbitrary graphs. Our results pave the way for engineering control operations and distributing entanglement among distant spins, as well as offering an approach to certify the engineering of effective spin models.   

Wave turbulence in driven dipolar gases across the superfluid to supersolid transition

Turbulence in a physical system involves the transport of energy among different length

scales. Ultracold gases provide fertile platforms to explore this phenomenon, due to the

involvement of a wide range of length scales ranging from short ones e.g. the healing

length determined by the s-wave interactions to macroscopic scales set by external traps.

Gases with long-range interactions, such as magnetic dipoles, may further aid in

unveiling the universal characteristics of quantum turbulence due to the existence of

competing interactions. We unravel the emergent energy cascade in quasi-two-

dimensional dipolar gases within the extended Gross-Pitaevskii framework. To initiate

such a cascade, the system is periodically driven across the superfluid to supersolid

transition and vice versa. A power-law decay is observed in the long-wavelength limit of

the energy density spectrum and the momentum distribution. The exponent of such a

decay is investigated for different initial states and characteristics of the driving

protocol, revealing signatures of wave quantum turbulence.   

Bose polaron interactions in a cavity-coupled monolayer semiconductor

The interaction between a mobile quantum impurity and a bosonic bath leads to the formation of quasiparticles, termed Bose polarons. The elementary properties of Bose polarons, such as their mutual interactions, can differ drastically from those of the bare impurities. Here, we explore Bose polaron physics in a two-dimensional nonequilibrium setting by injecting σ− polarised exciton-polariton impurities into a bath of coherent σ+ polarised polaritons generated by resonant laser excitation of monolayer MoSe2 embedded in an optical cavity. By exploiting a biexciton Feshbach resonance between the impurity and the bath polaritons, we tune the interacting system to the strong-coupling regime and demonstrate the coexistence of two new quasiparticle branches. Using time-resolved pump-probe measurements we observe how polaron dressing modifies the interaction between impurity polaritons. Remarkably, we find that the interactions between high-energy polaron quasiparticles, that are repulsive for small bath occupancy, can become attractive in the strong impurity-bath coupling regime. Our experiments provide the first direct measurement of Bose polaron-polaron interaction strength in any physical system and pave the way for exploration and control of many-body correlations in driven-dissipative settings.