Abstract
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 realize spin-based quantum probes by immersing individual Cs atoms into an ultracold Rb bath. Controlling inelastic spin-exchange processes between probe and bath allows mapping motional and thermal information onto quantum-spin states. We show that the steady-state spin-population is well suited for absolute thermometry, reducing temperature measurements to detection of quantum spin distributions. Moreover, we find that the information gain per inelastic collision can be maximized by accessing the nonequilibrium spin dynamic. The sensitivity of nonequilibrium quantum probing effectively beats the steady-state Cram\'er Rao limit of quantum probing by almost an order of magnitude, while reducing the perturbation of the bath to only three quanta of angular momentum. Our work paves the way for local probing of quantum systems at the Heisenberg limit, and moreover for optimizing measurement strategies via control of nonequilibrium dynamics.
Highlights
Miniaturizing measurement probes is a strong technological driving force and yields fascinating new insights into various fields including biology [1], solid-state physics [2], and metrology [3]
We show that the steady-state spin population is well suited for absolute thermometry, reducing temperature measurements to detection of quantum-spin distributions
The different values of the scattering cross sections σ and their dependence on temperature T and magnetic field B are based on a precise model of the Rb-Cs molecular potential [22,26], which is in detail discussed in Appendix D
Summary
Miniaturizing measurement probes is a strong technological driving force and yields fascinating new insights into various fields including biology [1], solid-state physics [2], and metrology [3]. A paradigm for quantum probing is a single atom with discrete energy quantum levels coupled to an atomic environment. Thermometry of quantum systems is important for ultracold gases, and various probes including magnons [7], confined Bose-Einstein condensate [8], Fermi sea [9], or single atoms [10] have been reported. All these probes rely on the standard method of time-of-flight velocimetry [11] and are classical. Having access to the dynamics of this microscopic process of quantum probing allows us to optimize
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