  A profound quest of statistical mechanics is the origin of irreversibility -
the arrow of time. New stimulants have been provided, thanks to unprecedented
degree of control reached in experiments with isolated quantum systems and
rapid theoretical developments of manybody localization in disordered
interacting systems. The proposal of (many-body) eigenstate thermalization (ET)
for these systems reinforces the common belief that either interaction or
extrinsic randomness is required for thermalization. Here, we unveil a quantum
thermalization mechanism challenging this belief. We find that, provided
one-body quantum chaos is present, as a pure many-body state evolves the arrow
of time can emerge, even without interaction or randomness. In times much
larger than the Ehrenfest time that signals the breakdown of quantum-classical
correspondence, quantum chaotic motion leads to thermal [Fermi-Dirac (FD) or
Bose-Einstein (BE)] distributions and thermodynamics in individual eigenstates.
Our findings lay dynamical foundation of statistical mechanics and
thermodynamics of isolated quantum systems.
