Résumé : It is shown that the ensemble {P(alpha), lpha lpha;{*}}, where P(alpha) is a Gaussian distribution of finite variance and lpha 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.