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Abstract. We use a quantum non-demolition measurement to generate a spin squeezed state and to create entanglement in a cloud of 10 5 cold cesium atoms. For the first time we operate an atomic clock improved by spin squeezing beyond the projection noise limit in a proof-of-principle experiment. For a clockinterrogation time of 10 µs, the experiments show an improvement of 1.1 dB in the signal-to-noise ratio, compared to the atomic projection noise limit.

We propose an original quantum memory protocol. It belongs to the class of rephasing processes and is closely related to two-pulse photon echo. It is known that the strong population inversion produced by the rephasing pulse prevents the plain two-pulse photon echo from serving as a quantum memory scheme. Indeed gain and spontaneous emission generate prohibitive noise. A second π-pulse can be used to simultaneously reverse the atomic phase and bring the atoms back into the ground state. Then a secondary echo is radiated from a non-inverted medium, avoiding contamination by gain and spontaneous emission noise. However, one must kill the primary echo, in order to preserve all the information for the secondary signal. In the present work, spatial phase mismatching is used to silence the standard two-pulse echo. An experimental demonstration is presented.