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Heat engines usually operate by exchanging heat with thermal baths at different (positive) temperatures. Nonthermal baths may, however, lead to a significant performance boost. Here, we experimentally analyze the power output of a single-atom quantum Otto engine realized in the quasispin states of individual cesium atoms interacting with an atomic rubidium bath. From measured time-resolved populations of the quasispin state, we determine the dynamics during the cycle of both the effective spin temperature and of the quantum fluctuations of the engine, which we quantify with the help of the Shannon entropy. We find that power is enhanced in the negative-temperature regime and that it reaches its maximum value at half the maximum entropy. Quantitatively, operating our engine at negative effective temperatures increases the power by up to 30% compared to operation at positive temperatures, including the case of infinite temperature. At the same time, entering the negative-temperature regime allows for reducing the entropy to values close to zero, offering highly stable operation at high power output. We further numerically investigate the influence of the size of the Hilbert space on the performance of the quantum engine by varying the number of levels of the working medium. Our work thereby demonstrates control of a multilevel single-atom quantum engine interacting with a realistic atomic bath having many degrees of freedom.

We study the sensitivity of a collisional single-atom probe for ultracold gases. Inelastic spin-exchange collisions map information about the gas temperature $T$ or external magnetic field $B$ onto the quantum spin-population of single-atom probes, and previous work showed enhanced sensitivity for short-time nonequilibrium spin dynamics [Phys. Rev. X 10, 011018 (2020)]. Here, we numerically investigate the steady-state sensitivity of such single-atom probes to various observables. We find that the probe shows distinct sensitivity maxima in the ($B,T$) parameter diagram, although the underlying spin-exchange rates scale monotonically with temperature and magnetic field. In parameter space, the probe generally has the largest sensitivity when sensing the energy ratio between thermal energy and Zeeman energy in an externally applied magnetic field, while the sensitivity to the absolute energy, i.e., the sum of kinetic and Zeeman energy, is low. We identify the parameters yielding sensitivity maxima for a given absolute energy, which we can relate to a direct comparison of the thermal Maxwell-Boltzmann distribution with the Zeeman-energy splitting. We compare our equilibrium results to nonequilibrium experimental results from a single-atom quantum probe, showing that the sensitivity maxima in parameter space qualitatively prevail also in the nonequilibrium dynamics, while a quantitative difference remains. Our work thereby offers a microscopic explanation for the properties and performance of this single-atom quantum probe, connecting thermodynamic properties to microscopic interaction mechanisms. Our results pave the way for optimization of quantum-probe applications in ($B,T$) parameter space beyond the previously shown boost by nonequilibrium dynamics.