Metastable Bose-Einstein Condensation in a Strongly Correlated Optical Lattice

TL;DR

This study investigates the metastability of Bose-Einstein condensates in optical lattices, revealing that non-adiabatic lattice loading causes condensate persistence above predicted critical temperatures, highlighting the importance of relaxation timescales.

Contribution

It demonstrates experimentally and theoretically that lattice turn-on is non-adiabatic, leading to metastable condensates at higher temperatures than predicted by equilibrium models.

Findings

Condensate persists above critical temperature due to non-adiabatic loading.

Relaxation times are comparable to heating rates, hindering adiabaticity.

Highlights need for better understanding of relaxation and thermometry in quantum gases.

Abstract

We experimentally and theoretically study the peak fraction of a Bose-Einstein condensate loaded into a cubic optical lattice as the lattice potential depth and entropy per particle are varied. This system is well-described by the superfluid regime of the Bose-Hubbard model, which allows for comparison with mean-field theories and exact quantum Monte Carlo (QMC) simulations. Despite correcting for systematic discrepancies between condensate fraction and peak fraction, we discover that the experiment consistently shows the presence of a condensate at temperatures higher than the critical temperature predicted by QMC simulations. This metastability suggests that turning on the lattice potential is non-adiabatic. To confirm this behavior, we compute the timescales for relaxation in this system, and find that equilibration times are comparable with the known heating rates. The similarity of these timescales implies that turning on the lattice potential adiabatically may be impossible. Our results point to the urgent need for a better theoretical and experimental understanding of the timescales for relaxation and adiabaticity in strongly interacting quantum gases, and the importance of model-independent probes of thermometry in optical lattices.