Experimental observation of Loschmidt time reversal of a quantum chaotic system.

We have performed an experiment to demonstrate the approximate time reversal of a "chaotic" time evolution of atomic de Broglie waves. We use ultracold atoms from a Bose-Einstein condensate in a quantum δ-kicked rotor experiment, and show that an initial state can be approximately re-created even after a period of "chaotic" evolution (a number of kicks). As this mechanism only works for a very narrow range of momenta, the net effect is a narrowing of the momentum distribution after the kick sequence.