  The act of describing how a physical process changes a system is the basis
for understanding observed phenomena. For quantum-mechanical processes in
particular, the affect of processes on quantum states profoundly advances our
knowledge of the natural world, from understanding counter-intuitive concepts
to the development of wholly quantum-mechanical technology. Here, we show that
quantum-mechanical processes can be quantified using a generic
classical-process model through which any classical strategies of mimicry can
be ruled out. We demonstrate the success of this formalism using fundamental
processes postulated in quantum mechanics, the dynamics of open quantum
systems, quantum-information processing, the fusion of entangled photon pairs,
and the energy transfer in a photosynthetic pigment-protein complex. Since our
framework does not depend on any specifics of the states being processed, it
reveals a new class of correlations in the hierarchy between entanglement and
Einstein-Podolsky-Rosen steering and paves the way for the elaboration of a
generic method for quantifying physical processes.
