Observation of backaction-evading measurement of an optical intensity in a three-level atomic nonlinear system

Backaction-evading measurement of an optical intensity has been observed, using a three-level system in a sodium atomic beam as a nonlinear medium. The principle of this experiment is to use the cross-phase-modulation effect, which in principle allows one to read nondestructively the quantum intensity fluctuations of the ‘‘signal’’ beam when detecting the phase fluctuations of the ‘‘meter’’ beam. Using a cavity-enhancement scheme, the quantum noise measured when recombining the signal and meter photocurrents is 0.8 dB below the shot-noise level.