Abstract : Usual measurements in optics destroy the incoming photons and
convert their energy into a measurable signal. However, this
destruction is not recquired by fundamental quantum laws and
quantum nondemolition strategies have been proposed. They allow
for repeated measurements of electromagnetic fields. In this work,
we observed, for the first time, a single photon stored in a
superconducting cavity without destroying it. In this experiment,
we use circular Rydberg atoms which are strongly coupled to the
field. During its interaction with the cavity mode, a single atom
is able to absorb and reemit a single photon. This phenomenon is
the well-known quantum Rabi oscillation. At the end this
absorption--emission cycle, the photon is still in the cavity.
However, the atom--field system keeps a track of its past
evolution in the phase of its wavefunction, which changed by 180°.
This phase-shift can be detected by atomic interferometry. We
performed experiments wich show that we can correlate the atomic
state to the field state, and that this process is
non-destructive. The performances of our setup are analyzed and
its possible applications to quantum optics are discussed.