A quantum ratchet sounds like a handy device. The idea is to push a quantum particle in a specific direction by periodically kicking it with an unbiased force. But why would an unbiased force push it in a specific direction? It sounds as if you’re more likely to end up with a random walk/brownian-type motion.
The trick is to set up some kind of asymmetry in the system so that the particle “wants” to go in a particular direction. That’s how macroscopic ratchets work.
Now Itzhack Dana from Bar-Ilan University in Israel and some pals have set up the first quantum ratchet with a Bose Einstein Condensate (BEC).
They started with a symmetric pulsed optical standing wave. This periodically nudges the BEC, which has a momentum distribution that is symmetric when the particle is stationary.
Now this system looks symmetric because both the standing wave and the momentum are symmetric. The asymmetry arises when these two systems have different centers of symmetry. The BEC then migrates along the line between them.
Neat, huh! And a world first, according to the team (although that claim requires a coupla extra caveats).
Ref: arxiv.org/abs/0706.0871: Experimental Realization of Quantum-Resonance Ratchets at Arbitrary Quasimomenta