  Following a recent proposal by S. B. Zheng and G. C. Guo (Phys. Rev. Lett.
85, 2392 (2000)), we report an experiment in which two Rydberg atoms crossing a
non-resonant cavity are entangled by coherent energy exchange. The process,
mediated by the virtual emission and absorption of a microwave photon, is
characterized by a collision mixing angle four orders of magnitude larger than
for atoms colliding in free space with the same impact parameter. The final
entangled state is controlled by adjusting the atom-cavity detuning. This
procedure, essentially insensitive to thermal fields and to photon decay, opens
promising perspectives for complex entanglement manipulations.
