Nondispersing Wave Packets

We have formed nondispersing wave packets from atomic Li Rydberg eigenstates by adding a small microwave field approximately resonant with the $\Delta n=1$ transition. The orbital motion of the Rydberg electron in states of $70\le n\le 78$ becomes phase locked to a 17.5 GHz microwave field and remains so for 900 ns, roughly 15 000 orbits of the Rydberg electron. A Floquet approach provides a reasonable quantum-mechanical description.