  Thermodynamics imposes restrictions on what state transformations are
possible. In the macroscopic limit of asymptotically many independent copies of
a state---as for instance in the case of an ideal gas---the possible
transformations become reversible and are fully characterized by the free
energy. In this Letter, we present a thermodynamic resource theory for quantum
processes that also becomes reversible in the macroscopic limit, a property
that is especially rare for a resource theory of quantum channels. We identify
a unique single-letter and additive quantity, the thermodynamic capacity, that
characterizes the "thermodynamic value" of a quantum channel, in the sense that
the work required to simulate many repetitions of a quantum process employing
many repetitions of another quantum process becomes equal to the difference of
the respective thermodynamic capacities. On a technical level, we provide
asymptotically optimal constructions of universal implementations of quantum
processes. A challenging aspect of this construction is the apparent necessity
to coherently combine thermal engines that would run in different thermodynamic
regimes depending on the input state. Our results have applications in quantum
Shannon theory by providing a generalized notion of quantum typical subspaces
and by giving an operational interpretation to the entropy difference of a
channel.
