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. {\bf 71}, 1355 (1993)] 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/N=1/2 for four photons.
@article{arxiv.0710.2922,
title = {Experimental demonstration of phase measurement precision beating standard quantum limit by projection measurement},
author = {F. W. Sun and B. H. Liu and Y. X. Gong and Y. F. Huang and Z. Y. Ou and G. C. Guo},
journal= {arXiv preprint arXiv:0710.2922},
year = {2008}
}