A single-atom quantum heat engine driven by atomic collisions*

Recent advances in controlling nanoscopic objects suggest the realization of machines exploiting quantum properties. However, the increasing importance of fluctuations in quantum systems calls into question whether such devices can combine high efficiency, high output power, and small power fluctuations. 

We report on the experimental realization of a stable quantum-Otto engine by immersing single Cs atoms as single-atom impurities into an ultracold Rb bath. The machine is realized in the seven quasi-spin states formed by the Cs ground-state Zeeman levels. We employ controlled inelastic spin-exchange interactions to exchange heat between the engine and the bath while work is performed changing the external magnetic field.  The heat exchange is directed, allowing to prepare spin states beyond the paradigm of thermal states, maximizing the output power. At the same time, the finite quantum-spin space forming the machine minimizes power fluctuations. We find that maximum output power can be combined with high efficiency and sub-Poissonian power fluctuations. Our realization paves the way to experimentally explore new strategies for optimizing quantum machines using, for example, nonthermal or nonequilibrium baths.