Work extraction from a single energy eigenstate.

Work extraction from the Gibbs ensemble by a cyclic operation is impossible, as represented by the second law of thermodynamics. On the other hand, the eigenstate thermalization hypothesis (ETH) states that just a single energy eigenstate can describe a thermal equilibrium state. Here, we attempt to unify these two perspectives and investigate the second law at the level of individual energy eigenstates, by examining the possibility of extracting work from a single energy eigenstate. Specifically, we performed numerical exact diagonalization of a quench protocol of local Hamiltonians and evaluated the number of work-extractable energy eigenstates. We found that it becomes exactly zero in a finite system size, implying that a positive amount of work cannot be extracted from any energy eigenstate, if one or both of the prequench and the postquench Hamiltonians are nonintegrable. We argue that the mechanism behind this numerical observation is based on the ETH for a nonlocal observable. Our result implies that quantum chaos, characterized by nonintegrability, leads to a stronger version of the second law than the conventional formulation based on the statistical ensembles.