quantum revival

A quantum revival is the surprising moment when a wave packet that had spread out and lost its shape suddenly reassembles itself into nearly the form it started with. Launch a localized lump of probability in a suitable trap and watch it: at first it smears and disperses, seemingly for good, but if you keep waiting it can gather itself back together, briefly recovering its original sharp profile before dispersing once more.

The trick lies in the discrete energy levels of a bound system. The wave packet is a sum of many such levels, each carrying its own steady rhythm, and these rhythms quickly fall out of step, which is why the packet appears to dissolve. But because the levels are spaced in a regular, related way, after a special revival time all the rhythms come back into alignment at once, and the packet snaps back into shape. It is interference, run in reverse.

Along the way you often see partial, or fractional, revivals, where the packet temporarily splits into several smaller copies of itself before the full revival. These delicate effects have been observed in real systems — in Rydberg atoms, in molecular vibrations, and in cold atoms held in optical lattices — and they are a vivid reminder that the smooth spreading of a wave packet is, underneath, perfectly reversible unitary evolution, not true forgetting.