Experimental demonstration of phase measurement precision beating standard quantum limit by projection measurement

We propose and demonstrate experimentally a projection scheme to measure the quantum phase with a precision beating the standard quantum limit. The initial input state is a twin Fock state vertical bar N, N > proposed by Holland and Burnett (Phys. Rev. Lett., 71 ( 1993) 1355) but the phase information is extracted by a quantum state projection measurement. The phase precision is about 1.4/N for large photon number N, which approaches the Heisenberg limit of 1/N. Experimentally, we employ a four-photon state from type-II parametric down-conversion and achieve a phase uncertainty of 0.291 +/- 0.001 beating the standard quantum limit of 1/root N = 1/ 2 for four photons. Copyright (c) EPLA, 2008.