Superiority of Entangled Measurements over All Local Strategies for the Estimation of Product Coherent States

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.

Figure 1

Schematic of the experimental setup. The states are measured using (a) a local strategy and (b) a global strategy.

Figure 2

Spectral power densities (normalized to the quantum noise level) of the local and global strategies. The resolution bandwidth is 100 kHz and the video bandwidth is 30 Hz.