Adaptive Measurements in the Optical Quantum Information Laboratory

TL;DR

This paper reviews adaptive measurement techniques in optical quantum information, demonstrating their ability to achieve fundamental quantum limits in state discrimination, phase measurement, and interferometry, with a focus on recent experimental implementations.

Contribution

It introduces adaptive measurement methods that enable quantum measurements at fundamental limits, highlighting recent experimental advances and their applications in quantum information processing.

Findings

Adaptive measurements reach the Helstrom bound for state discrimination

They achieve Heisenberg-limited phase estimation

Recent experiments demonstrate practical implementation of these techniques

Abstract

Adaptive techniques make practical many quantum measurements that would otherwise be beyond current laboratory capabilities. For example: they allow discrimination of nonorthogonal states with a probability of error equal to the Helstrom bound; they allow measurement of the phase of a quantum oscillator with accuracy approaching (or in some cases attaining) the Heisenberg limit; and they allow estimation of phase in interferometry with a variance scaling at the Heisenberg limit, using only single qubit measurement and control. Each of these examples has close links with quantum information, in particular experimental optical quantum information: the first is a basic quantum communication protocol; the second has potential application in linear optical quantum computing; the third uses an adaptive protocol inspired by the quantum phase estimation algorithm. We discuss each of these examples, and their implementation in the laboratory, but concentrate upon the last, which was published most recently [Higgins {\em et al.}, Nature vol. 450, p. 393, 2007].