  We demonstrate an asymmetric optical potential barrier for ultracold 87 Rb
atoms using laser light tuned near the D_2 optical transition. Such a one-way
barrier, where atoms impinging on one side are transmitted but reflected from
the other, is a realization of Maxwell's demon and has important implications
for cooling atoms and molecules not amenable to standard laser-cooling
techniques. In our experiment, atoms are confined to a far-detuned dipole trap
consisting of a single focused Gaussian beam, which is divided near the focus
by the barrier. The one-way barrier consists of two focused laser beams
oriented almost normal to the dipole-trap axis. The first beam is tuned to have
a red (blue) detuning from the F=1 -> F' (F=2 -> F') hyperfine transitions, and
thus presents a barrier only for atoms in the F=2 ground state, while letting
F=1 atoms pass. The second beam pumps the atoms to F=2 on the reflecting side
of the barrier, thus producing the asymmetry.
