a Image of the ion trap set-up with a single ion acting as both an engine and a load. b Schematic representation of the working fluid of a quantum engine (two-level system) and c the load (the vibrational modes of a quantum harmonic oscillator). The two-level system undergoes a cyclic evolution. The sizes of the filled circles in the harmonic oscillator pictorially represent the occupation probabilities of different modes. Initially, when the ion is cooled near the ground state, the spread in occupation probabilities is small (smaller blue circle). After operating the energy-conversion device for a number of forward (engine type, as opposed to refrigerator type) cycles, the spread in occupation probabilities increases (larger, fuzzier, red circle).