Experimental demonstration of phase measurement precision beating standard quantum limit by projection measurement
F. W. Sun, B. H. Liu, Y. X. Gong, Y. F. Huang, Z. Y. Ou, G. C. Guo · Europhysics Letters (EPL) · 2008
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 | 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 for four photons.