Thermalization in a closed quantum system from randomized dynamics

The emergence of statistical mechanics from quantum dynamics is a central problem in quantum many-body physics. Deriving observables aligned with the prediction of the canonical ensemble for a quantum system relies on the presence of a bath provided either as an external environment or as a larger part of a closed system. We demonstrate that thermal (canonical) observables for a whole closed quantum system of finite size can arise in the absence of a bath. These thermal observables stem from classical averaging over randomized unitary evolutions for a few-body system. The temperature in the canonical ensemble appears as a global constraint on the total energy of the system, determined by the choice of the initial state. From averaging randomized evolutions, we derive spin-spin correlation functions for a finite spin chain and show that they exhibit a temperature-dependent finite correlation length, in agreement with the prediction of the canonical ensemble. This establishes a method for computing thermal observables in a closed, finite-size system from real-time propagation without a bath. An implementation of this thermalization approach on a quantum computer can be utilized for thermal state preparation.

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