Thermodynamics set limits on what machines can do: how much work they can produce, how much heat they release, and how much energy is wasted. But for microscopic and quantum devices, there is another equally important question: what state is fed into the machine in the first place? A quantum device generally works optimally for only one input - the input it anticipates from the environment. Finding this input state is therefore central to understanding the efficiency of a machine when in actual operation.
Here, we develop a general framework for identifying the thermodynamically ideal inputs to any finite-time process. The key insight is that important quantities such as heat, work, and entropy flow can be captured by special operators acting on the initial state. This means that, instead of testing every possible input, one can learn the full thermodynamic behaviour of a device from a limited set of test inputs. The work also shows that “ideal” depends on the task: the state that extracts the most work need not be the same as the one that minimizes wasted entropy or maximizes free-energy gain. This provides a practical roadmap for tailoring quantum and classical devices to different thermodynamic goals.
Thermodynamically Ideal Quantum State Inputs to Any Device
Paul M. Riechers, Chaitanya Gupta, Artemy Kolchinsky, and Mile Gu
PRX Quantum 5, 030318
