Title: Single atomic impurities coupled to an ultracold gas as a model system for quantum devices

Abstract:
The ability to prepare, manipulate, and detect individual quantum systems allows for studying nonequilibrium dynamics at an unprecedented level.

In my talk, I will report on our recent approaches to controlling single atomic Cs impurities in an ultracold Rb gas. In addition to elastic collisions of the impurity with the gas atoms, we employ inelastic spin exchange to realize proof-of-principle examples for single-atom quantum devices.

First, we show that spin-exchange collisions map information on the bath temperature or surrounding magnetic field onto the quantum-spin population of the impurity, where the sensitivity is found to be enhanced in the nonequilibrium phase before reaching the steady state. Preparing the impurity in a coherent superposition of the Cs clock states, we find that similar information can be obtained from the dephasing of the quantum phase when the system is prepared at an inter-species Feshbach resonance.

Second, we use directed spin-exchange for efficient energy exchange between impurity and bath, realizing a single-atom quantum engine driven by spin-fuel rather than thermal states. We show that the machine can be operated in a highly desirable mode with high efficiency, maximum power output, and sub-Poissonian fluctuations when using states with population inversion, corresponding to an effective negative spin temperature.

Finally, a closer study of the spin dynamics of the impurity shows that a large range of initial states shows a non-monotonous evolution of spin entropy. A significant fraction of states comes close to the state of maximum entropy. Subsequently, spin evolution shows quasi-universal dynamics. Numerical simulations extending the system's size show that when the maximum entropy is reached, the spin population also delocalizes in the thermodynamic limit, and finite-size scaling reveals critical scaling exponents independent of the details of the initial state. We interpret this observation as indicating an open-system phase transition in time.
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For more information on the Quantum Science Colloquia, please see the website https://phys.au.dk/ccq/events/quantum-science-colloquium