Using single-photon detection and an adaptive measurement strategy, we are able to determine the state of an optical pulse with errors below what is possible with even the best possible passive receiver, a limit often referred to as the "standard quantum limit".
We tested schemes to optimally measure unknown quantum states for the purpose of surpassing what are typically considered to be the classical limits to such measurements.
Experimental demonstration of a receiver beating the standard quantum limit for multiple nonorthogonal state discrimination, F. E. Becerra,J. Fan,G. Baumgartner, J. Goldhar,J. T. Kosloski & A. Migdall, Nature Photonics 7, 147–152 (2013)
Implementation of generalized quantum measurements for unambiguous discrimination of multiple non-orthogonal coherent states, F. E. Becerra, J. Fan & A. Migdall, Nature Communications 4, Article number: 2028 (2013)