Electric Dipole Echoes in Rydberg Atoms

We report the first observation of echoes in the electric dipole moment of an ensemble of Rydberg atoms precessing in an external electric field $F$. Quasi-one-dimensional Rydberg atoms oriented along the $x$ axis are first produced and then subjected to a dc field $F$ = 20mVcm$^{-1 }$that is suddenly applied in the $z$ direction to create a Stark wavepacket whose evolution is monitored using a half-cycle probe pulse. The wavepacket contains states of different n that precess at different rates leading to dephasing. Rapid reversal of the field $F \to -F$ at t = $\tau $ is shown to play the role akin to that of a $\pi $-pulse in NMR in rephasing the dephased ensemble resulting in the build-up of an echo at t $\sim $ 2$\tau $. The appearance of the echo is explained with the aid of classical trajectory Monte Carlo simulations.