Coherent Quantum Tunneling of Bose-Einstein Condensates in Optical Lattices

TL;DR

This paper investigates the coherent quantum tunneling phenomena of Bose-Einstein condensates in optical lattices, highlighting the conditions for Josephson tunneling and free boson tunneling depending on superfluidity within the wells.

Contribution

It provides a detailed analysis of inter-well tunneling mechanisms in BECs, distinguishing between superfluid and non-superfluid regimes based on lattice parameters.

Findings

Josephson tunneling rate can be enhanced beyond naive estimates

Tunneling is always non-zero regardless of superfluidity within wells

Coherent tunneling depends on boson number difference in non-superfluid regime

Abstract

We study inter-well coherent quantum tunneling of Bose-Einstein condensates (BEC's) in optical lattices. Irrespective of whether the BEC is a superfluid in the whole lattice, it may or may not be a superfluid within each well of the lattice depending on whether the ratio of the well kinetic energy to the interaction energy is small or large. When the bosons in each well are superfluid, we have Josephson tunneling among the wells driven by the phase difference between the superfluids in the different wells. For typical lattice parameters the Josephson tunneling rate will be somewhat enhanced from the naive estimate of (tunneling energy scale)$/\hbar$. Whether it is enhanced or not will depend on lattice parameters, but it can never be zero. When the bosons in each well are not superfluid, there is coherent quantum tunneling of essentially free bosons between the wells. This tunneling is proportional to the boson number difference between the wells.