Superiority of Entangled Measurements over All Local Strategies for the Estimation of Product Coherent States
Julien Niset, Antonio Acín, Ulrik L. Andersen, Nicolas J. Cerf, Raúl García−Patrón, Miguel Navascués, Metin Sabuncu · Physical Review Letters · 2007
It is shown that the ensemble {P(alpha),|alpha|alpha;{*}}, where P(alpha) is a Gaussian distribution of finite variance and |alpha is a coherent state, can be better discriminated with an entangled measurement than with any local strategy supplemented by classical communication. Although this ensemble consists of products of quasiclassical states without any squeezing, it thus exhibits a purely quantum feature. This remarkable effect is demonstrated experimentally by implementing the optimal local strategy on coherent states of light together with a global strategy that yields a higher fidelity.