Quantum scattering as a work source.

We show that the state of two quantum systems $A$ and $B$ after a short time-dependent interaction equals the state after a collision between $A$ and $B$ provided that the wave packet associated to their relative motion is fast and broad compared to the energies of $A$ and $B$. The entropy change in the joint system $A$ and $B$ vanishes and the change in kinetic energy due to their collision is equivalent to (minus) the work generated by the time-dependent interaction.