Experimental realization of a quantum optical tap

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Abstract
We have realized a system which is able to read out information encoded on the amplitude of a laser beam without degrading the signal-to-noise ratio of the carrier. This has been implemented using the two-photon cross-phase-modulation effect of a three-level system in a sodium atomic beam located in a doubly resonant optical cavity. We have achieved an improvement by a factor of 1.35 of the overall information transer compared to any classical optical tap, where a perfect quantum duplicator would reach the ideal value of 2.