Single-photon entanglement between two modes of the electromagnetic field in a cavity

Summary form only given. We have prepared a single-photon entangled state involving two field modes of a high finesse microwave superconducting cavity, and we have probed its coherence after storing it for a variable time in the cavity. We use the resonant interaction between circular Rydberg atoms and the field. This coupling is stronger than any dissipation rate so that the energy exchange between the atoms and the cavity is periodic (Rabi oscillation). A first atom prepares the entangled field state (writer atom) and a second one probes its coherence (reader atom).