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

TL;DR

This paper experimentally demonstrates a quantum phase measurement method that surpasses the standard quantum limit by using a projection measurement on a twin Fock state, approaching the Heisenberg limit.

Contribution

The authors propose and experimentally validate a projection measurement scheme for quantum phase measurement that beats the standard quantum limit and approaches the Heisenberg limit.

Findings

Achieved phase uncertainty of 0.291 with four photons.

Demonstrated phase precision of about 1.4/N, approaching the Heisenberg limit.

Surpassed the standard quantum limit of 1/2 for four photons.

Abstract

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 $0.291\pm 0.001$ beating the standard quantum limit of $1/\sqrt{N} = 1/2$ for four photons.