Volume 82, Number 2, April 2008
Article Number 24001
Number of page(s) 5
Section Electromagnetism, Optics, Acoustics, Heat Transfer, Classical Mechanics, and Fluid Dynamics
Published online 26 March 2008
EPL, 82 (2008) 24001
DOI: 10.1209/0295-5075/82/24001

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

F. W. Sun1, B. H. Liu1, Y. X. Gong1, Y. F. Huang1, Z. Y. Ou1, 2 and G. C. Guo1

1  Key Laboratory of Quantum Information, University of Science and Technology of China (CAS) - Hefei 230026, PRC
2  Department of Physics, Indiana University-Purdue University Indianapolis - 402 N. Blackford Street, Indianapolis, IN 46202, USA

received 20 January 2008; accepted in final form 18 February 2008; published April 2008
published online 26 March 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$\rangle$ 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$\pm$0.001 beating the standard quantum limit of $1/\sqrt{N}$ = 1/2 for four photons.

42.50.Dv - Quantum state engineering and measurements.
03.65.Ta - Foundations of quantum mechanics; measurement theory.
07.60.Ly - Interferometers.

© EPLA 2008