Single impurities in a quantum gas form a paradigm of quantum physics. Controlling the exchange of heat at the level of single quanta between impurity and bath paves the way for
quantum technological applications, such as quantum probes or heat engines.
Quantum probes are atomic-sized devices mapping information of their environment to quantum mechanical states. By improving measurements and at the same time minimizing
perturbation of the environment, they form a central asset for quantum technologies. We have realized single-atom quantum probes for local thermometry based on the spin dynamic of individual neutral Cesium (probe) atoms in an ultracold gas (bath) of Rubidium atoms.
The competition of inelastic endo- and exoergic spin-exchange processes map the temperature onto the quasi-spin population of the probe. This can be used as temperature probe, where sensitivity is enhanced in a nonequilibrium state. Vice versa controlling the endo- and exoergic spin-exchange processes to have a directed heat exchange with the bath is a central building block of a quantum engine. We have realized a quantum Otto cycle in the quasi-spin states of single Cs impurities coupled to a Rb bath. Our engine combines high efficiency, high output power power, and small power fluctuations, as measured from full counting statistics of the quantum states.

Daniel Adam1, Quentin Bouton1, Sabrina Burgardt1, Jens Nettersheim1, Tobias Lausch1,
Daniel Mayer1, Felix Schmidt1, Eberhard Tiemann2, and Artur Widera1
1Department of Physics and Research Center OPTIMAS TU Kaiserslautern, Germany
2Institut für Quantenoptik, Leibniz Universität Hannover, 30167 Hannover, Germany