  Many-body-localized (MBL) systems do not thermalize under their intrinsic
dynamics. The athermality of MBL, we propose, can be harnessed for
thermodynamic tasks. We illustrate this ability by formulating an Otto engine
cycle for a quantum many-body system. The system is ramped between a strongly
localized MBL regime and a thermal (or weakly localized) regime. The difference
between the energy-level correlations of MBL systems and of thermal systems
enables mesoscale engines to run in parallel in the thermodynamic limit,
enhances the engine's reliability, and suppresses worst-case trials. We
estimate analytically and calculate numerically the engine's efficiency and
per-cycle power. The efficiency mirrors the efficiency of the conventional
thermodynamic Otto engine. The per-cycle power scales linearly with the system
size and inverse-exponentially with a localization length. This work introduces
a thermodynamic lens onto MBL, which, having been studied much recently, can
now be considered for use in thermodynamic tasks.
