Bose-Einstein condensation by polarization gradient laser cooling

TL;DR

This study demonstrates that polarization gradient cooling alone can produce a Bose-Einstein condensate in a microscopic optical trap, with machine learning optimizing experimental parameters to significantly improve phase space density.

Contribution

It shows for the first time that simple polarization gradient cooling can generate a BEC inside a micrometer-sized optical trap, aided by machine learning optimization.

Findings

Machine learning increased atom number by 5 times.

Temperature decreased by 2.5 times, boosting phase space density.

A BEC of ~250 atoms was formed within 40 ms of cooling.

Abstract

Attempts to create quantum degenerate gases without evaporative cooling have been pursued since the early days of laser cooling, with the consensus that polarization gradient cooling (PGC, also known as "optical molasses") alone cannot reach condensation. In the present work, we report that simple PGC can generate a small Bose-Einstein condensate (BEC) inside a corrugated micrometer-sized optical dipole trap. The experimental parameters enabling BEC creation were found by machine learning, which increased the atom number by a factor of 5 and decreased the temperature by a factor of 2.5, corresponding to almost two orders of magnitude gain in phase space density. When the trapping light is slightly misaligned through a microscopic objective lens, a BEC of $\sim 250$ $^{87}$Rb atoms is formed inside a local dimple within 40 ms of PGC.