Optical One-Way Barrier for Atoms

We demonstrate an asymmetric optical potential barrier for ultracold $^{87}$Rb atoms using laser light tuned near the D$_2$ transition. Such a one-way barrier, where atoms impinging on one side are transmitted but reflected from the other, is a literal realization of Maxwell's demon and has important implications for cooling atomic species 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 normal to the dipole trap. The first barrier beam is tuned between the $F=1\longrightarrow F'$ and the $F=2\longrightarrow F'$ families of hyperfine transitions, and 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. We study experimentally the reflection and transmission dynamics of atoms in the presence of the one-way barrier.