A. V. Ponomarev et al., "ac-Driven Atomic Quantum Motor", Phys. Rev. Lett. v.102, p.230601 (2009) .
Abstract: We propose an ac-driven quantum motor consisting of two different, interacting ultracold atoms placed into a ring-shaped optical lattice and submerged in a pulsating magnetic field. While the first atom carries a current, the second one serves as a quantum starter. For fixed zero-momentum initial conditions the asymptotic carrier velocity converges to a unique nonzero value. We also demonstrate that this quantum motor performs work against a constant load.
ScienceNow has a review of this paper as well.
Now, Alexey Ponomarev, Peter Hänggi, and colleagues at the University of Augsburg, Germany, have devised the quantum-mechanical equivalent of such a motor. Their motor consists of bright spots of laser light that form a circle like so many pearls in a bracelet (see picture). The spots of laser light can trap two ultracold atoms: The first, called the carrier, is missing an electron and so is electrically charged; the second, called the starter, is uncharged. Instead of applying an oscillating current in a coil, the researchers envision applying an oscillating electric field perpendicular to the plane of the ring.
This will set the carrier into motion--but not in a simple way. Because it's a quantum particle, the carrier atom must be described by quantum waves that give the probability for finding it at one position or another. Applying an oscillating field alone will send waves of equal strength rippling around the ring in both directions, with the net result that, on average, the particle doesn't budge. To get it going, the physicists have to include the starter atom, which gives the carrier a shove whenever the two atoms happen to hop into the same light spot.
Even that is not quite enough. To make the motor turn over, the researchers find, the electric field has to oscillate in a pattern that would appear different if it were suddenly reversed--much as a song sounds different if you play it backward. Such oscillations set off quantum waves that flow predominantly either to the left or to the right, setting the motor in motion, the researchers report in a paper published online this week in Physical Review Letters.
Now let's see which experimental group will be the first to produce something similar to this! The race is on! :)