Thermodynamics was built for steam engines; quantum mechanics for atoms. The two meet again in a driven cavity, where an atom absorbs and emits photons while a laser continuously supplies energy and light leaks through the mirrors. Patrick Potts and colleagues at the University of Basel treat this textbook open system as a miniature light engine. Their key step is not to count every escaping photon as waste heat. Part of that outgoing energy can still do work on another quantum system. That b...