Excited-state quantum phase transitions and Loschmidt-echo spectra in a spinor Bose-Einstein condensate

Abstract

Phase transitions in nonequilibrium dynamics of many body quantum systems, the so-called dynamical phases transition (DPTs), play an important role for understanding various dynamical phenomena observed in different branches of physics. In general, there have two types of DPTs, the first one refers to the phase transition that is characterized by distinct evolution behaviors of a physical observable, while the second one is marked by the nonanalyticities in the rate function of the initial state survival probability. Here, we focus on such DPTs from both quantum and semiclassical perspectives in a spinor Bose-Einstein condensate (BEC), an ideal platform to investigate nonequilibrium dynamics. By using the sudden quench process, we demonstrate that the system exhibits both types of DPTs present as the control parameter quenches through the critical one, referring to as the critical quench. We show analytically how to determine the critical quenches by means of the semiclassical approach and carry out a detailed examination on both semiclassical and quantum signatures of two types of DPTs. Moreover, we further reveal that the occurrence of DPTs is closely connected to the separatrix in the underlying classical system. Our findings provide more insights into the properties of DPTs and verify the usefulness of semiclassical analysis for understanding DPTs in quantum systems with well-defined semiclassical limit.

Understanding dynamical phase transitions in a spinor Bose–Einstein condensate via quantum and semiclassical analyses

“…As an order parameter, M shows a crossover from M ≠ 0 in BA phase to M = 0 in polar phase when ξ straddles the critical point. Besides the ground state phase transition, in the BA phase, the Hamiltonian (1) also exhibits an excited state quantum phase transition [68,[81][82][83], which is characterized by the divergence in the density of states at certain excited energy in the thermodynamic limit [84][85][86][87].…”