Experimental realization of a quantum measurement for intensity difference fluctuation using a beam splitter

A new quantum measurement scheme of intensity difference fluctuation between two light beams of equal mean intensity is presented. In this system a beam splitter is used as the coupling device and the twin beams with high quantum correlation are injected into its dark port as the input meter wave instead of the usual vacuum field. A measurement satisfying all the quantum nondemolition criteria is experimentally achieved. The measured sum of the transfer coefficients and the conditional variance are, respectively, T-s + T-m = 1.31 and V-s/m = -2.1 dB.