玻色凝聚原子气体跨越自发对称性破缺的普适非平衡动力学研究进展

Universal non-equilibrium dynamics in Bose condensed atomic gases undergoing spontaneous symmetry breaking

自发对称性破缺是自然界中普遍存在的物理现象,从宇宙星系的形成到超流涡旋的产生,它广泛存在于宇宙学、凝聚态物理以及原子分子光物理等各个领域之中.自发对称性破缺过程中的普适临界动力学可用KibbleZurek机制描述,该机制描述了体系非绝热激发和淬火速率之间的普适标度关系.研究普适临界动力学既可以加深人们对早期宇宙中复杂结构形成和演化的理解,又可以帮助人们找出更快的绝热路径,缩短量子态制备和量子调控的时间.超冷原子体系具有洁净的环境、较长的相干时间以及高度可控等优点,是研究普适临界动力学的理想平台之一.本文从自发对称性破缺相变出发,介绍了玻色凝聚原子气体跨越自发对称性破缺的两类普适非平衡动力学:(1)由无能隙Higgs模诱导的普适非平衡动力学;(2)由软化声子模诱导的普适非平衡动力学.

Spontaneous symmetry breaking(SSB)is a ubiquitous physical phenomenon in nature,ranging from the formation of cosmic galaxies to the generation of the vortices in superfluid,which widely occurs in cosmology,condensed matter physics,and atomic and molecular optical physics.Universal critical dynamics associated with the SSB can be described by the Kibble-Zurek mechanism(KZM),which describes the scaling of the non-adiabatic excitations when driving the system crossing the phase transition.The KZM was first described in the context of finite temperature transitions in the region of cosmology,and then extended to the condensed matter physics by Zurek.KZM was confirmed by experiments in superfluid helium,liquid crystals,both high-T;and low-T;superconductors and ultracold atomic gases.Due to the advantages of clean environment,robust coherence and highly controllable parameters,ultracold atomic gases become one of ideal simulation platforms for researching the universal non-equilibrium critical dynamics.In this review,we investigate the critical dynamics associated with the SSB phase transition,introducing two typical universal non-equilibrium dynamics in Bose condensed atomic gases undergoing spontaneous symmetry breaking:(1)Universal non-equilibrium dynamics induced by the gapless Higgs modes;(2)universal non-equilibrium dynamics induced by the soft phonon modes.In the first part,we investigate the universal non-equilibrium dynamics induced by the gapless Higgs modes.Considering the coupled two-component Bose-Einstein condensation(BEC),we explore its static symmetry breaking transition and its mean-field quenching dynamics.In the equilibrium transitions,the maximum probability of the full quantum states corresponds to the mean-field ground state.In the non-equilibrium dynamical transitions,as the Higgs mode vanishes at the critical point,the mean-field dynamics display critical phenomena obeying the KZM.Both the mean-field and the full quantum defect modes show damped oscillation as driving the coupling strength crossing the critical point.Furthermore,we consider the coupled two-component BEC confined in a one-dimensional optical lattice potential,and then explore its ground-state properties and real-time quenching dynamics.Through driving the system crossing from the symmetric Rabi oscillation to the broken symmetry self-trapping,the universal non-equilibrium dynamics,which is induced by the gapless Higgs mode at the critical point,will result in the generation of the domains and appearance of the phase transition delay.On the other hand,we investigate the universal non-equilibrium dynamics induced by the soft phonon modes.Here,by considering an atomic BEC ladder subjected to a synthetic magnetic field,we reveal that the spontaneous superfluidity breakdown induced by the soft phonon mode obeys the KZM.The critical exponents are extracted from the Landau critical velocity,which determines the systematic correlation length.Meanwhile,the numerical critical exponents are extracted from the universal non-equilibrium dynamics of the phase transition delay and the spontaneous vortex creation.Furthermore,we consider the universal non-equilibrium dynamics of the two-component BEC.By driving the atom-atom interaction strength crossing the critical point,the system occurs spontaneous superfluidity breakdown induced by the soft phonon mode,and the numerical simulation shows the spontaneous domain generation and its temporal delay.The studies of the universal non-equilibrium critical dynamics can not only deepen our understanding of the formation and evolution of complex structures in early universe,but also help us find out the suitable adiabatic pathway,and shorten the time for quantum state preparation and quantum control.