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Time evolution of typical pure states from a macroscopic Hilbert subspace. (English) Zbl 1520.81055

Summary: We consider a macroscopic quantum system with unitarily evolving pure state \(\psi_t\in \mathcal{H}\) and take it for granted that different macro states correspond to mutually orthogonal, high-dimensional subspaces \(\mathcal{H}_{\nu}\) (macro spaces) of \(\mathcal{H}\). Let \(P_{\nu}\) denote the projection to \(\mathcal{H}_{\nu}\). We prove two facts about the evolution of the superposition weights \(\Vert P_{\nu} \psi_t\Vert^2\): First, given any \(T>0\), for most initial states \(\psi_0\) from any particular macro space \(\mathcal{H}_{\mu}\) (possibly far from thermal equilibrium), the curve \(t\mapsto \Vert P_{\nu} \psi_t\Vert^2\) is approximately the same (i.e., nearly independent of \(\psi_0)\) on the time interval \([0, T]\). And second, for most \(\psi_0\) from \(\mathcal{H}_{\mu}\) and most \(t\in [0,\infty), \Vert P_{\nu} \psi_t\Vert^2\) is close to a value \(M_{\mu\nu}\) that is independent of both \(t\) and \(\psi_0\). The first is an instance of the phenomenon of dynamical typicality observed by Bartsch, Gemmer, and Reimann, and the second modifies, extends, and in a way simplifies the concept, introduced by von Neumann, now known as normal typicality.

MSC:

81Q05 Closed and approximate solutions to the Schrödinger, Dirac, Klein-Gordon and other equations of quantum mechanics
58J51 Relations between spectral theory and ergodic theory, e.g., quantum unique ergodicity
80A10 Classical and relativistic thermodynamics
81P17 Quantum entropies
37A20 Algebraic ergodic theory, cocycles, orbit equivalence, ergodic equivalence relations

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