Spin dynamics of open quantum systems driven by spin-exchange collisions

This thesis investigates the spin dynamics of single neutral cesium-133 (Cs) atoms while interacting with an ultracold rubidium-87 (Rb) bath. Cs is used as a single-atom probe to extract environmental parameters, such as the Rb bath temperature with boosted sensitivity, and as a working fluid for realizing a performant single-atom Otto engine driven by the Rb bath. The immersion of Cs into the Rb bath realizes an open nonequilibrium quantum system driven by Rb-Cs collisions. Elastic collisions thermalize the impurity's motional degree of freedom to the bath's temperature, whereas inelastic spin-exchange (SE) collisions change the internal spin states of the colliding Rb and Cs atoms. A Cs state-resolved readout procedure allows tracking of the Cs spin dynamics and the resulting interatomic energy (heat) exchange. Depending on the initial state conditions, spin- and heat exchange can occur in both directions, from Cs to Rb or vice versa. This thesis encompasses three different experimental investigations that are based on spin-changing collisions. First, the dependency of SE collisions on the bath temperature and the externally applied magnetic field is used to determine the temperature and magnetic field. Tracking of the SE dynamics allows extracting this information and determining the Rb bath temperature or the external magnetic field, employing individual Cs atoms as a quantum probe for the environment. Investigations of the probing sensitivity show that the singleatom thermometer and magnetometer both have a sensitivity in the nonequilibrium spin dynamics of up to an order of magnitude larger than for the steady state. The analysis of steady-state probing allows a deeper insight into the collision mechanisms and gives a fundamental understanding of the maximum sensitivity origin. Second, controlling the SE direction allows transferring energy from Cs to the bath (Cs cooling) or vice versa (Cs heating). The cyclic realization of these processes in combination with magnetic field control represents a thermodynamic cycle, resulting in an Otto engine. While Cs takes the role of the working fluid, the single-atom engine is fueled by the Rb bath's spin states. Optimizing the performance leads to a stable quantum heat engine with high efficiency, up to 91% at maximum power output. From time-resolved Cs spin populations, an effective spin temperature and the spin entropy are calculated, showing the engine working at a negative effective spin temperature while maximizing the power output and increasing the stability. Finally, the Cs entropy evolution is investigated in greater detail. Starting with Cs atoms occupying few to one highly excited state(s), the entropy is small and grows while SE occurs, broadening the Cs spin distribution. A uniform spin distribution corresponds to the highest entropy. Eventually, the spin distribution narrows by occupying few to one low excited state(s), and the entropy decreases. Calculating the effective temperature for the Cs spin shows that the population of the limited Cs spin-system of only seven levels goes from a negative to a positive effective spin temperature. Finite-size scaling of the spin dynamics that approach the theoretical maximum entropy is a precursor of a dynamical phase transition. This behavior is investigated for different SE-directed systems, initial states, and system sizes.