Experimental Realization of an Optical One-Way Barrier for Neutral Atoms

We demonstrate an asymmetric optical potential barrier for ultracold ${}^{87}\mathrm{Rb}$ atoms using laser light tuned near the $D_2$ optical transition. Such a one-way barrier, where atoms incident 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 present either a potential well or barrier, depending on the state of the incident atoms. On the reflecting side of the barrier, the second beam optically pumps the atoms to the reflecting (barrier) state, thus producing the asymmetry.

Figure 1

Schematic diagram of the optical setup, showing the dipole trap, barrier beams, and imaging system.

Figure 2

Relevant atomic levels of ${}^{87}\mathrm{Rb}$ on the $D_2$ cooling transition (not to scale), shown with the off-resonant coupling of the main barrier laser. Atoms with $F=1$ see this field as red detuned, while $F=2$ atoms see a blue detuning. The second barrier beam is resonant with the usual $F=1\to F^{\prime }=2$ repumping transition.

Figure 3

Atom spatial distributions along the dipole trap responding to the one-way barrier. The dipole-trap focus and barrier beams are located at the plot centers. Each image is an average of 78 repetitions of the experiment. (a) Atoms initially in $F=1$ are dropped from the left with no barrier present. (b) Atoms initially in $F=1$ are dropped onto the barrier from the left (transmitting side). (c) Atoms initially in $F=2$ are dropped onto the barrier from the left. (d) Atoms initially in $F=1$ are dropped onto the barrier from the right (reflecting side).

Figure 4

Populations on left- and right-hand sides of the one-way barrier, where atoms were initially loaded throughout the dipole trap, along with the total population. Error bars indicate statistical error from 38 repetitions.