Środowiskowe Seminarium z Informacji i Technologii Kwantowych

The second law of thermodynamics states that the entropy of an isolated system can only increase over time, thereby distinguishing the past from the future. This seems to conflict with the reversible evolution of isolated quantum systems, which preserves the von Neumann entropy. However, counterintuitively, many observables in large isolated systems do reach equilibrium, despite the unitary evolution of the system's state. We characterise the extent to which any observable exhibits this emergent irreversibility, as determined by the relationship between the microstates associated with the reversible evolution and the macrostates associated with the observable. We demonstrate how a version of the second law of thermodynamics can be recovered in isolated quantum systems, and analyse the fluctuations from equilibrium that reveal the underlying reversible dynamics, finding that these fluctuations diminish as the system size increases. We then explore the hypothesis that the apparent irreversibility of the quantum measurement process is a manifestation of the second law of thermodynamics, resulting in possible criteria for when a physical system constitutes an observer.