A general scheme for ensemble purification

We exhibit a general procedure to purify any given ensemble by identifying an appropriate interaction between the physical system S of the ensemble and the reference system K. We show that the interaction can be chosen in such a way to lead to a spatial separation of the pair S-K. As a consequence, one can use it to prepare at a distance different equivalent ensembles. The argument associates a physically precise procedure to the purely formal and fictitious process usually considered in the literature. We conclude with an illuminating example taken from quantum computational theory. (C) 2003 Elsevier Science B.V. All rights reserved.