Session X09: Many-Body Dynamics in Trapped Gases

Interaction-Driven Spin Rotations in a Two-component BEC Reflecting from a Barrier

Reflection from a barrier perturbed by weak Raman coupling beams generates spin waves in a BEC of ⁸⁷Rb. Due to the coincidence of scattering lengths in ⁸⁷Rb, a BEC in a mixture of two hyperfine states behaves as a phase-coherent yet distinguishable two-component fluid. Reflection from the barrier creates a counter-propagating matter wave with spin partly transverse to the spin of the forward-going wave, initiating interaction-driven rotations. The observed spin rotations are well-described by mean-field simulations with equal inter and intra-spin interaction strengths, demonstrating that the spin dynamics do not arise from nonequilibrium dynamics caused by spin-dependent interactions or immiscibility of the two components. Rather, the driving mechanism for spin rotations is the different interaction energy experienced by parallel versus anti-parallel spins in different spatial modes, much in the same way the identical spin rotation effect is known to generate spin waves in non-condensed gases. We observe one oscillation of a spin wave for low Rabi frequencies and study the transition where spin rotations become independent of the external coupling and instead are dominated by the interaction-driven effects.   

Vortex-core phase transition iin the polar phase of spin-1 Bose-Einstein condensates

Vortex core structure in the polar phase of spin-1 Bose--Einstein condensates is investigated theoretically. Singly quantized vortices are categorized by the local ordered state in the vortex core and three types of vortices are found as lowest-energy vortices, elliptic AF-core vortices, axisymmetric F-core vortices, and N-core vortices. These vortices are named after the local ordered state, ferromagnetic (F), antiferromagnetic (AF), broken-axisymmetry (BA), and normal (N) states apart from the bulk polar (P) state. The N-core vortex is a conventional vortex, in the core of which the superfluid order parameter vanishes. The other two types of vortices are stabilized when the quadratic Zeeman shift is below a critical value. The axisymmetric F-core vortex is the lowest-energy vortex for ferromagnetic interaction, and it has an F core surrounded by a BA skin that forms a ferromagnetic-spin texture, as exemplified by the localized Mermin--Ho texture. The elliptic AF-core vortex is stabilized for antiferromagnetic interaction; the vortex core has both nematic-spin and ferromagnetic orders locally and is composed of the AF-core soliton spanned between two BA edges. The phase transition from the N-core vortex to the other two vortices is continuous, whereas that between the AF-core and F-core vortices is discontinuous. The critical point of the continuous transition is computed by the perturbation analysis of the Bogoliubov theory and the Ginzburg--Landau formalism describes the critical behavior. The influence of trapping potential on the core structure is also investigated.   

Squeezed Ground States in a Spin-1 Bose-Einstein Condensate

In this work, we generate spin squeezed ground states in an atomic spin-1 Bose-Einstein condensate tuned near the quantum critical point between the polar and ferromagnetic quantum phases of the interacting spin ensemble. In contrast to typical non-equilibrium methods for preparing atomic squeezed states by quenching through a quantum phase transition, squeezed ground states are time-stationary and remain squeezed for the lifetime of the condensate.

We use a nonadiabatic shortcut protocol consisting of a pair of controlled quenches of an external magnetic field to approach the quantum phase transition, significantly shortening the state preparation time compared to adiabatic methods. A squeezed ground state with a metrological improvement of up to 6-8 dB and a constant squeezing angle maintained over 2s is both simulated and experimentally demonstrated.    

Effects of the transverse direction on the many-body tunneling dynamics

Tunneling in a many-particle system appears as one of the novel implications of quantum physics.  Here, we theoretically investigate the  tunneling dynamics of a few intricate bosonic clouds in a two-dimensional symmetric double-well potential. We unravel how the inclusion of the transverse direction, orthogonal to the junction of the double-well, can intervene in the tunneling dynamics of bosonic clouds by employing a well-known many-body numerical method,  the multiconfigurational time-dependent Hartree for bosons (MCTDHB), which incorporates quantum correlations exhaustively.   We analyze the tunneling dynamics by preparing the initial state of the bosonic clouds in the left well of the double-well either as the ground, longitudinally or transversely excited, or a vortex state. We examine the detailed mechanism of the tunneling process by analyzing the evolution in time of the survival probability, depletion and fragmentation, and the many-particle position, momentum, and angular-momentum expectation values and their variances. As a general rule, all objects lose coherence while tunneling through the barrier and the states which include transverse excitations do so faster. In particular for the later states, we show that even when the transverse direction is practically frozen, prominent many-body dynamics in a two-dimensional bosonic Josephson junction occurs.   

Short time dynamics after a wavefunction quench in 1D Bose gases

Generalized hydrodynamics (GHD) has recently been shown to describe the dynamics in nearly-integrable 1D Bose gases very well, even after strong quenches of the trap [1]. We now study these gases shortly after a wavefunction quench, in which the atoms are put into a superposition of multiple momentum states by an axial Bragg pulse. After the quench, the gases are out of local equilibrium with the local generalized Gibbs ensemble (GGE), so GHD dose not apply. We measure how the momentum distribution of the center peak evolves up to the point that the local GGE is satisfied. We perform numerical calculations and GHD in the infinite coupling limit, to identify when the GGE is satisfied in that case [2,1]. We then apply this understanding to extracting the local equilibration time constants for different densities and coupling strengths. These time constants are at the frontier of what can be theoretically calculated about non-equilibrium nearly-integrable systems.

[1] N. Malvania, et al. Generalized hydrodynamics in strongly interacting 1D Bose gases. Science, 373.6559 (2021)

[2] R. Van den Berg, et al. Separation of time scales in a quantum Newton’s cradle. PRL, 116(22), 225302 (2016)   

Anomalous localization in spin chains coupled to a non-local degree of freedom

It has recently been predicted that many-body localization survives the presence of coupling to a non-local degree of freedom, such as a cavity mode [PRL 122, 240402 (2019)]. Such a cavity-QED system can host anomalous phases unique to non-equilibrium systems. Here we present recent results on anomalous localization in such setups. First, we show that for the right choice of non-local degree of freedom, an inverted mobility edge occurs, meaning that infinite temperature states are localized while low energy states are delocalized. Second, we show a similar model can be used for realizing time crystals in cavity-QED systems and in the absence of drive, i.e., a time-crystalline phase in a static Hamiltonian. Finally, we study the stability of localization in the presence of non-zero but small photon loss. All the models presented are realizable on experimental AMO platforms.

    

Rotating Bose gas dynamically enters the lowest Landau level

Motivated by recent experiments, we model the dynamics of a condensed Bose gas in a rotating anisotropic trap. The equations of motion of neutral particles in a rotating frame are analogous to those of charged particles in a magnetic field. As the rotation rate is ramped from zero to the trapping frequency, the condensate stretches along one direction and is squeezed along another, becoming long and thin. When the trap anisotropy is slowly switched off on a particular timescale, the condensate is left in the lowest Landau level. We use a time dependent variational approach to quantify these dynamics and give intuitive arguments about the structure of the condensate wavefunction. This preparation of a lowest Landau level condensate can be an important first step in realizing bosonic analogs of quantum Hall states.   

Interface properties and phase transitions in atomic Boson – Fermion mixtures

We study the density profiles of atomic boson – fermion mixtures confined in one dimensional box potentials by modelling the system with many-body density-density interactions. A variety of configurations were found in the different parameter regime. Atomic mixtures can remain mixed, and phase separate in 3-chunk or 2-chunk structure depending on the boson-boson and the boson-fermion interaction strengths. Phase diagram for all the structures as a function of the interactions is mapped out. For the 2-chunk structure, interface properties which describes the interaction between the separated bosons and fermions were analyzed. The width of the interface reveals information about interaction and other parameters. At the hard walls, the density profiles of bosons or fermions show the healing length of the corresponding system. And at the boson - fermion interface, the width depends on both kinetic and interaction energy which can be tuned by the atomic mass, boson-fermion interaction and their densities.   

Rainbow Scars: From Area to Volume Law

Quantum many-body scars (QMBS) constitute a new quantum dynamical regime in which rare “scarred” eigenstates mediate weak ergodicity breaking. One open question is to understand the most general setting in which these states arise. In this work, we develop a generic construction that embeds a new class of QMBS, rainbow scars, into the spectrum of an arbitrary Hamiltonian. Unlike other examples of QMBS, rainbow scars display extensive bipartite entanglement entropy while retaining a simple entanglement structure. Specifically, the entanglement scaling is volume-law for a random bipartition, while scaling for a fine-tuned bipartition is sub-extensive. When internal symmetries are present, the construction leads to multiple, and even towers of rainbow scars revealed through distinctive non-thermal dynamics. Remarkably, certain symmetries can lead rainbow scars to arise in translation-invariant models. To this end, we provide an experimental road map for realizing rainbow scar states in a Rydberg-atom quantum simulator, leading to coherent oscillations distinct from the strictly sub-volume-law QMBS previously realized in the same system.   

Eigenstate thermalization approaching for an atom interacting with fixed scatterers

Eigenstate thermalization is generally studied in many-body systems. It can be approached even if all but one particles have the infinite mass, as demonstrated here. Since n zero-range scatterers along the axis of a circular, transversely harmonic waveguide form a separable potential, millions of eigenstates are calculated using modest computational resources. On increase of n, the common characteristic of quantum chaos — the level spacing statistics — diverges from the Seba one [1] and at n=64 approaches the Wigner-Dyson one for the Gaussian orthogonal ensemble (GOE), expected for complete chaos. When the energy spectrum degeneracy with no scatterers is eliminated by an axial vector potential, at n=16 the level statistics approaches the one for the Gaussian unitary ensemble, expected for complete quantum chaos with no T-invariance. In this case, GOE takes place only for P-invariant distribution of the scatterers, when the Hamiltonian is TP-invariant. Chaotic behavior is revealed also in the inverse participation ratio [1], which drops from 0.4 to 0.028 on increase of n from 4 to 32. Simultaneous 4-time reduction of the eigenstate expectation value dispersion demonstrates the eigenstate thermalization approaching. 1.V. A. Yurovsky and M. Olshanii, PRL 106, 025303 (2011).