# Thermalization of isolated Bose-Einstein condensates by dynamical heat bath generation

## TL;DR

This paper demonstrates how a Bose-Einstein condensate in a double-well potential can dynamically generate a heat bath through quasiparticle interactions, leading to thermalization despite initial conditions violating traditional thermalization criteria.

## Contribution

It introduces the concept of dynamical bath generation (DBG), showing how a BEC can act as a heat reservoir for quasiparticles, enabling thermalization without ETH compliance.

## Key findings

- Identification of three dynamical regimes in BEC thermalization.
- Demonstration of a dynamically generated heat bath via quasiparticle interactions.
- Observation of thermalization in conditions violating ETH requirements.

## Abstract

If and how an isolated quantum system thermalizes despite its unitary time evolution is a long-standing, open problem of many-body physics. The eigenstate thermalization hypothesis (ETH) postulates that thermalization happens at the level of individual eigenstates of a system's Hamiltonian. However, the ETH requires stringent conditions to be validated, and it does not address how the thermal state is reached dynamically from an inital non-equilibrium state. We consider a Bose-Einstein condensate (BEC) trapped in a double-well potential with an initial population imbalance. We find that the system thermalizes although the initial conditions violate the ETH requirements. We identify three dynamical regimes. After an initial regime of undamped Josephson oscillations, the subsystem of incoherent excitations or quasiparticles (QP) becomes strongly coupled to the BEC subsystem by means of a dynamically generated, parametric resonance. When the energy stored in the QP system reaches its maximum, the number of QPs becomes effectively constant, and the system enters a quasi-hydrodynamic regime where the two subsystems are weakly coupled. In this final regime the BEC acts as a grand-canonical heat reservoir for the QP system (and vice versa), resulting in thermalization. We term this mechanism dynamical bath generation (DBG).