A German team has fulfilled a prediction made decades ago by physicist Richard Feynman: a heat engine composed of a single atom (Science 2016, DOI: 10.1126/science.aad6320). Heat engines, which convert thermal energy to mechanical work, have been used in various forms for several hundred years. Over the past decade, scientists have designed ever-smaller heat engines, with the smallest being composed of a single molecule. Now, the German team, led by Kilian Singer of the University of Kassel and Johannes Rossnagel of the University of Mainz, has trapped a calcium ion (40Ca+) and alternately cooled and heated it with lasers and electric fields. The temperature differences produced by heating and cooling caused the atom to oscillate harmonically in an axial direction, “similar to the flywheel of a mechanical engine,” the authors say. They envision a wide variety of future applications, such as single-atom refrigerators and pumps.