  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.
