One thing that has always confused people is the flow of time: almost every natural (microscopic) law is time-reversible. The only exception so far is a weak effect in beta-decay. On the other hand, it is obvious that the macroscopic world is not time reversable. Again, how do we build an everyday, irreversible world from reversible laws?

Chaos is the phenomenon that the outcome of events can depend very sensitively on the initial conditions of these events. One example is the weather, where the proverbial butterfly beats its wings in Beijing to cause a thunderstorm in New York. Now chaos is limited to the classical world: in quantum mechanics chaos cannot exist. But the world is made out of atoms, which are ruled by quantum mechanics. So where does this go wrong?

Our BEC forms a system where quantum effects are hugely magnified, so that it becomes easy to do experiments where quantum effects show up. We are studying a quantum system that is classically chaotic.

What we are working on currently is to first establish a bit of chaos, and then to carefully undo that chaos. Classically, this is impossible, as the outcome is extremely sensitive to the initial conditions and the experiment. But according to quantum mechanics, this is possible. Over the past few weeks we have found good evidence that quantum mechanics is right (again) and that we can undo the chaos.

This results are accepted for publication in Physical Review E, a preprint can be found at the preprint archive.