  Active states, from which work can be extracted by time-dependent
perturbations, are an important resource for quantum thermodynamics in the
absence of heat baths. Here we characterize this resource, establishing a
resource theory that captures the operational scenario where an experimenter
manipulates a quantum system by means of energy-preserving operations and
resets to non-active states. Our resource theory comes with simple conditions
for state convertibility and an experimentally accessible resource quantifier
that determines the maximum advantage of active states in the task of producing
approximations of the maximally coherent state by means of energy-preserving
quantum operations.
