Dynamics of Cold Atoms Crossing a One-Way Barrier

TL;DR

This paper demonstrates an optical one-way barrier for ultracold atoms, enabling asymmetric transmission and reflection, with potential applications in cooling and phase-space manipulation of atoms and molecules.

Contribution

It presents a novel implementation of a one-way optical potential barrier for ultracold atoms, illustrating its robustness and analyzing its implications for Maxwell's demon and entropy.

Findings

Barrier effectively transmits atoms from one side and reflects from the other.

Simulations show initial momentum distribution influences atomic dynamics.

Light-assisted collisions are identified as a primary loss mechanism.

Abstract

We implemented an optical one-way potential barrier that allows ultracold $^{87}$Rb atoms to transmit through when incident on one side of the barrier but reflect from the other. This asymmetric barrier is a realization of Maxwell's demon, which can be employed to produce phase-space compression and has implications for cooling atoms and molecules not amenable to standard laser-cooling techniques. The barrier comprises two focused, Gaussian laser beams that intersect the focus of a far-off-resonant, single-beam optical dipole trap that holds the atoms. The main barrier beam presents a state-dependent potential to incident atoms, while the repumping barrier beam optically pumps atoms to a trapped state. We investigated the robustness of the barrier asymmetry to changes in the barrier beam separation, the initial atomic potential energy, the intensity of the second beam, and the detuning of the first beam. We performed simulations of the atomic dynamics in the presence of the barrier, showing that the initial three dimensional momentum distribution plays a significant role, and that light-assisted collisions are likely the dominant loss mechanism. We also carefully examined the relationship to Maxwell's demon and explicitly accounted for the apparent decrease in entropy for our particular system.