  Universality of classical thermodynamics rests on the central limit theorem,
due to which, measurements of thermal fluctuations are unable to reveal
detailed information regarding the microscopic structure of a macroscopic body.
When small systems are considered and fluctuations become important,
thermodynamic quantities can be understood in the context of classical
stochastic mechanics. A fundamental assumption behind thermodynamics is
therefore that of coarse-graning, which stems from a substantial lack of
control over all degrees of freedom. However, when quantum systems are
concerned, one claims a high level of control. As a consequence, information
theory plays a major role in the identification of thermodynamic functions.
Here, drawing from the concept of gauge symmetry, essential in all modern
physical theories, we put forward a new possible, intermediate route. Working
within the realm of quantum thermodynamics we explicitly construct physically
motivated gauge transformations which encode a gentle variant of
coarse-graining behind thermodynamics. As a consequence, we reinterpret quantum
work and heat, as well as the role of quantum coherence.
