Comment on ``Hidden quantum nonlocality revealed by local filters''

Abstract

In Section 3 of his paper (N. Gisin, Phys. Lett. A 210 (1996) 151), Gisin argues that a ``careless application of generalized quantum measurements can violate Bell's inequality even for mixtures of product states.'' However, the observed violation of the CHSH inequality is not in fact due to the application of generalized quantum measurements, but rather to a misapplication of the inequality itself -- to conditional expectations in which the conditioning depends upon the measurements under consideration.