Entropy Flow at the Quantum Limit

Abstract: Thermal management is a key challenge, both globally and microscopically in integrated circuits and quantum technologies. The associated heat flow $I_Q$ has been understood since the advent of thermodynamics by a process of elimination, $I_Q{=}I_E{-}\mu I_N$, subtracting from the energy flow $I_E$ its convective contribution. However, in the quantum limit, this formula implies the paradoxical result that the entropy entrained by heat flow is unbounded even though the entropy itself tends to zero. We resolve this conundrum by recognizing that the traditional formula for heat is missing a quantum term. The correct quantum formula predicts that the heat produced in quantum processes is vastly smaller than previously believed, with correspondingly beneficial consequences for the efficiency of quantum machines.