Entanglement-enhanced measurement of a completely unknown optical phase

• DOI: 10.1038/nphoton.2010.268
• Publication Date: 2010-12-12T18:17:21Z
• Authors: G. Y. Xiang; B. L. Higgins; D. W. Berry; H. M. Wiseman; G. J. Pryde
• Keywords: Physical sciences; 0103 physical sciences; Mathematical sciences; Atomic; 530; 01 natural sciences; molecular and optical physics not elsewhere classified

Precise interferometric measurement is vital to many scientific and technological applications. Using quantum entanglement allows interferometric sensitivity that surpasses the shot-noise limit (SNL)1,2. To date, experiments demonstrating entanglement-enhanced sub-SNL interferometry3,4,5,6, and most theoretical treatments7,8,9,10,11,12,13, have addressed the goal of increasing signal-to-noise ratios. This is suitable for phase-sensing—detecting small variations about an already known phase. However, it is not sufficient for ab initio phase-estimation—making a self-contained determination of a phase that is initially completely unknown within the interval [0, 2π). Both tasks are important2, but not equivalent. To move from the sensing regime to the ab initio estimation regime requires a non-trivial phase-estimation algorithm14,15,16,17. Here, we implement a ‘bottom-up’ approach, optimally utilizing the available entangled photon states, obtained by post-selection5,6. This enables us to demonstrate sub-SNL ab initio estimation of an unknown phase by entanglement-enhanced optical interferometry. Entangled photon states, obtained by post selection, are used to perform interferometric phase measurement with a sensitivity beyond the shot-noise limit.