Exchangeable stochastic processes and symmetric states in quantum probability

Abstract

We analyze general aspects of exchangeable quantum stochastic processes, as well as some concrete cases relevant for several applications to Quantum Physics and Probability. We establish that there is a one-to-one correspondence between quantum stochastic processes, either preserving or not the identity, and states on free product C*-algebras, unital or not unital respectively, where the exchangeable ones correspond precisely to the symmetric states. We also connect some algebraic properties of exchangeable processes, that is the fact that they satisfy the product state or the block-singleton conditions, to some natural ergodic ones. We then specialize the investigation for the q-deformed Commutation Relations, q in(-1,1) (the case q=0 corresponding to the reduced group C*-algebra of the free group on infinitely many generators), and the Boolean ones. We also provide a generalization of De Finetti Theorem to the Fermi CAR algebra (corresponding to the q-deformed Commutation Relations with q=-1), by showing that any state is symmetric if and only if it is conditionally independent and identically distributed with respect to the tail algebra. The Boolean stochastic processes provide examples for which the condition to be independent and identically distributed w.r.t. the tail algebra, without mentioning the {\it a-priori} existence of a preserving conditional expectation, is in general meaningless in the quantum setting. Finally, we study the ergodic properties of a class of remarkable states on the full group C*-algebra of the free group on infinitely many generators, that is the so-called Haagerup states.