Bending the rules: Quantum effects in the operation of a microscopic heat engine in diamond

A classical heat engine that extracts work from thermal sources and which does not include coherence amongst its microscopic degrees of freedom is a fundamental concept of classical thermodynamics. In contrast, the internal states of a quantum heat engine (QHE) can exist in a coherent superposition of energy levels and a question of interest for such a QHE is whether it can exhibit thermodynamic behavior fundamentally different to that allowed in a classical engine. QHEs have recently been implemented using for example trapped ions [1]. However, experiments so far have not shown any non-classical features in their thermodynamic quantities. While the efficiency of a QHE is still bound by the Carnot limit, recent theoretical predictions show that coherence can boost its power output above the classically allowed limit for an engine using the same thermal resources [2]. Moreover, the presence of coherence was predicted to result in the equivalence of different QHE types in the limit of weak driving and short cycle duration.