 0612 W. De Roeck, C. Maes, K. Netocny
 Quantum Macrostates, Equivalence of Ensembles and an HTheorem
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Jan 13, 06

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Abstract. Before the thermodynamic limit, macroscopic averages need not commute for a quantum system. As a consequence, aspects of macroscopic fluctuations or of constrained equilibrium require a careful analysis, when dealing with several observables. We propose an implementation of ideas that go back to John von Neumann's writing about the macroscopic measurement. We apply our scheme to the relation between macroscopic autonomy and an Htheorem, and to the problem of equivalence of ensembles. In particular, we prove a quantum version of the asymptotic equipartition theorem. The main point of departure is an expression of a law of large numbers for a sequence of states that start to concentrate, as the size of the system gets larger, on the macroscopic values for the different macroscopic observables. Deviations from that law are governed by the entropy.
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