  When ground state atoms are excited to a Rydberg state, van der Waals
interactions among them can lead to a strong suppression of the excitation.
Despite the strong interactions the evolution can still be reversed by a simple
phase shift in the excitation laser field. We experimentally prove the
coherence of the excitation in the strong blockade regime by applying an
`optical rotary echo' technique to a sample of magnetically trapped ultracold
atoms, analogous to a method known from nuclear magnetic resonance. We
additionally measured the dephasing time due to the interaction between the
Rydberg atoms.
