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Une mémoire quantique
We demonstrate the coherent quantum information transfer from an atom to the cavity and back to a second atom. The cavity is here a quantum memory, storing a qubit for a time in the second range. The write and read operations involve flying atomic qubits.
This experiment is based on the 
quantum Rabi pulse, perfoming a qubit-copy operation from atom to cavity or back. The state of a first atom is copied onto the initially empty cavity mode and stored in this mode for a while. It is finally read out by a second atom, initially in 
.
Energy transfer
In this simple experiment [1], the first atom is prepared in 
. A quantum Rabi pulse deposits a single photon in the cavity. In an ideal experiment, the second atom, undergoing also a pulse, should be found certainly in at the exit of the cavity.
Due to cavity relaxation, there is only a finite probability that the photon remains in the cavity before being read out by the second atom. The curve above presents the probability for finding the second atom in as a function of the storage time in the cavity. This experiment has been recently perfomed with the longest available cavity damping time, above 100 ms. We observe, as expected, an exponential decay of the transfer probability, which allows us to measure precisely the lifetime of the single photon state. Not surprisingly, it fits quite well with the lifetime of the classical energy in the cavity. Note that, at short times, the transfer probability is close to 80%, corresponding to a reasonable fidelity for the individual copy operations.
Coherence transfer
In a more complex experiment, we have stored a qubit coherence in the cavity. The first atom is prepared in an equal weight superposition of and in the Ramsey zone 
. This state is copied onto the cavity mode and read later by the second atom. The state of this atom is finally probed by a second Ramsey pulse in 
, followed by the atomic energy detection.
In fact, we are here performing a simple Ramsey fringes experiment. But the two pulses act on different atoms and the quantum coherence has been transferred between them through the cavity quantum memory.
This figure presents the observed fringes when we scan the relative phase of the two Ramsey pulses, for three cavity storage times (301, 436 and 581 
s from top to bottom). The phase is scanned by tuning the frequency of the source inducing the Ramsey pulses around the atomic resonance. Note that this data correspond to an early experiment, performed with a 112 s cavity damping time.
As the storage time increases, the fringes get narrower, corresponding to the increased resolution of the Rasmey spectroscopic method for an increasing time interval between pulses. Simultaneously, the contrast of the fringes decreases, due to cavity relaxation.
The measured decay time of the contrast is exactly twice the cavity damping time. This can be easily understood, since the intermediate state is a superposition of 
, which decays with the cavity damping time, and 
which does not decay. In other words, the coherence decays twice more slowly than the populations.
[1] X. Maître, E. Hagley, G. Nogues, C. Wunderlich, P. Goy, M. Brune, J.M. Raimond, S. Haroche, Phys. Rev. Lett. 79, 769 (1997) : "Quantum memory with a single photon in a cavity".
