  We consider the task of extracting work from quantum systems in the resource
theory perspective of thermodynamics, where free states are arbitrary thermal
states, and allowed operations are energy conserving unitary transformations.
Taking as our work storage system a 'weight' we prove the second law and then
present simple protocols which extract average work equal to the free energy
change of the system - the same amount as in classical thermodynamics.
Crucially, for systems in 'classical' states (mixtures of energy eigenstates)
our protocol works on a single copy of the system. This is in sharp contrast to
previous results, which showed that in case of almost-deterministic work
extraction, collective actions on multiple copies are necessary to extract the
free energy. This establishes the fact that free energy is a meaningful notion
even for individual systems in classical states. However, for non-classical
states, where coherences between energy levels exist, we prove that collective
actions are necessary, so long as no external sources of coherence are used.
