Nonequilibrium Josephson Oscillations in Bose-Einstein Condensates without Dissipation

Abstract

We perform a detailed quantum dynamical study of nonequilibrium Josephson oscillations between interacting Bose-Einstein condensates confined in a finite-size double-well trap. We find that the Josephson junction can sustain multiple undamped Josephson oscillations up to a characteristic time scale tau(c) without quasipartcles being excited in the system. This may explain recent related experiments. Beyond a characteristic time scale tau(c) the dynamics of the junction is governed by fast, quasiparticle-assisted Josephson tunneling as well as Rabi oscillations between the discrete quasiparticle levels. We predict that an initially self-trapped state of the Bose-Einstein condensates will be destroyed by these fast dynamics.

Thermalization of Isolated Bose‐Einstein Condensates by Dynamical Heat Bath Generation

“…At this time, QPs get excited, and the dynamics becomes dominated by fast QP Rabi oscillations between the discrete trap levels, which in turn drive the BEC oscillations. [17] We note that after the time τ c inelastic QP collisions will become important and will be taken into account in section 4.2. The collisionless regime that exists up to τ c and, therefore, the time scale τ c itself can be described by the BHF approximation.…”

“…[45]. This issue, however, does not play an important role for the physics we discuss in this work, and we therefore proceed with the expansion (17).…”

“…We suggest the quasiparticle damping mechanism to play a crucial role in such a behaviour. Below we present a formalism [17][18][19] which, when applied to specific systems, will shed light on this issue and other problems related to thermalization of isolated closed systems.…”

“…First, Josephson oscillations without damping can occur up to a time t = τ c after the quench. [17,18] During a time interval τ c < t < τ f the condensate (BEC) and the quasiparticle (QP) subsystems are strongly coupled via a dynamically generated, parametric resonance, indicated by the BEC and the QP spectra being strongly correlated with each other. [19] In this regime incoherent excitations are thus created out of the condensate in an avalanche-like manner.…”

“…Most importantly, it is even valid in cases where either the bath or the subsystem Hilbert space is initially not populated, i.e., the bath is dynamically generated by the total system's time evolution, possibly involving multiple time scales. [17][18][19] We thus term this thermalization dynamics "dynamical bath generation" (DBG). The DBG mechanism can be understood also as a setup where the subsystem-bath coupling evolves in time.…”