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Intrication de deux modes du champ
A single atom is used to entangle the two non-degenrate modes of a cavity. In the entangled state, a single photon is shared by the two modes. The coherence of this state is probed by another atom.
In this experiment [1], we let an atom interact coherently with the two non-degenerate modes 
and 
( has the lowest frequency) of the cavity with orthogonal linear polarizations. This experiment has been performed with an early generation of cavity. The modes lifetime is of the order of 1 ms and the frequency difference between them is 
kHz. The atoms can be tuned at resonance with either mode via the Stark effect. When resonant with one mode, the atom does not appreciably interact with the other, since 
.
The above scheme presents the sequence for the atom preparing the entanglement (left) and the later probe atom (right). The atomic frequency is plotted as a function of time.
The preparation atom is initially in 
and both cavity modes are empty. The atom interacts with the upper mode for a 
quantum Rabi pulse and with the lowest mode for a 
pulse. This timing performs the sequence of transformations :
and the two cavity modes share a single photon in a state which is one of the Bell states of the two photonic qubits.
This state, once prepared, evolves at the frequency difference 
between the two modes, since the two superposed states have slightly different energies.
The second probe atom, initially in performs a pulse in the upper frequency mode and a in the second. A simple exercise shows then that the probability for getting finally the probe atom in oscillates with the relative phase of the two 
and 
states at the read-out time. These oscillations demonstrate the coherence of the whole process.
The figure above presents the probability for getting the probe in as a function of the delay time between the two atoms. We observe, as expected, oscillations at the detuning , revealing the intermediate coherent state superposition. The contrast of the oscillations eventually decays due to cavity relaxation, in good agreement with the theoretical models (solid line).
[1] A. Rauschenbeutel, P. Bertet, S. Osnaghi, G. Nogues, M. Brune, J.M. Raimond, S. Haroche, Phys. Rev. A 64, 050301 (2001) „Controlled entanglement of two field modes in a cavity quantum electrodynamics experiment“
