Experimental and theoretical demonstration of quantum degeneracy enhancement in a thermodynamic engine*

Thermodynamic engines underpin much of modern technology. Relatively unexplored until recently is the question of whether a quantum mechanical thermodynamic engine can have inherent advantages in efficiency and power over its classical counterpart. As a first step towards answering this question, we experimentally and theoretically characterize an isentropic thermodynamic engine using an interacting quantum-degenerate gas of bosonic $^7$Li as the working fluid. In loose analogy to an Otto cycle, strokes of harmonic trap compression and relaxation are interleaved with strokes of strengthening and weakening contact interactions via a magnetic Feshbach resonance. By subjecting a thermal gas to the same cycle, we observe a quantitative and significant enhancement in both the efficiency and power transfer using the quantum-degenerate working fluid, as well as quantitative agreement with approximation-free interacting simulations. By running this cycle in reverse, we show that the process is isentropic and fully reversible. We characterize the power transfer and cycle efficiencies as a function of trap and interaction compression and cycle time, and show that we achieve high-efficiency, high-power energy transfer between optical and magnetic fields, quantitatively demonstrating quantum degeneracy enhancement of a thermodynamic engine.