Our everyday experience suggests that all physical systems eventually reach equilibrium (hot coffee mixes with cold milk to instantly give a lukewarm cappuccino, etc.). It may then come as a surprise that quantum mechanical equations of motion respect a time-reversal symmetry that effectively precludes the possibility of an emergence of arrow of time associated with increase in entropy and thermalization. Despite this, numerical evidence overwhelmingly suggests that quantum mechanical systems also equilibrate in time, and only recently, in the past decade, have we started discovering quantum systems that break this dogma. In this talk, I will give an introductory discussion into the study of how thermalization can occur in quantum systems, and how it can be broken by giving an example of two particular systems—many-body localized systems, and quantum many-body scars.
