Quantum Thermalisation

Statistical mechanics has been one of the most successful branches of physics. One fundamental question in modern quantum mechanics is how concepts in equilibrium statistical mechanics, such as ergodicity and thermalisation, can dynamically arise in isolated quantum systems. For classical systems, the notion of thermalisation is well understood as chaotic dynamics is intimately tied to ergodicity. At first glance, chaoticity appears to be incompatible with the time-reversible nature of unitary dynamics in closed quantum systems. 

Nevertheless, theoretical and experimental studies have successfully demonstrated the dynamical emergence of thermalisation at the level of local observables or subsystems in isolated quantum systems. We study quantum relaxation, or the lack thereof, using familiar concepts from statistical and classical physics by using semiclassical techniques to approximate quantum dynamics. From a fundamental perspective, our approach opens the possibility of connecting the quantum and classical worlds. Moreover, understanding quantum thermalisation is crucial in designing platforms for quantum simulation.