Adaptive Homodyne Measurement of Optical Phase

We present an experimental demonstration of the power of feedbackin quantum metrology, confirming the predicted [H. M. Wiseman, Phys. Rev. Lett. 75, 4587 (1995)] superior performanceof an adaptive homodyne technique for single-shotmeasurement of optical phase. For measurements performed on weak coherentstates with no prior knowledge of the signal phase, adaptive homodyneestimation approaches closer to the intrinsic quantum uncertaintythan any previous technique. Our results underscore the importanceof real-time feedback for reaching quantum limits in measurement andcontrol.