Single-Atom Quantum Probes for Ultracold Gases Boosted by Nonequilibrium Spin Dynamics

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 the probe and bath allows us to map 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. Keeping the motional degree of freedom thermalized, individual spin-exchange collisions yield information about the gas quantum by quantum. We find that the sensitivity of this nonequilibrium quantum probing effectively beats the steady-state Cramér-Rao limit 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.

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Plain Language Summary

Quantum properties of materials promise many improved technological applications, among them probing of microscopic processes. The most elementary quantum probe is a single atom, which employs quantum properties to store information about its environment. While individual ions or ultracold atomic clouds have been used to measure surrounding fields, an atomic probe storing information in its quantum states about a surrounding atomic many-body system remains elusive. Here, we report on not only the realization of such a probe, but one that outperforms a fundamental quantum limit.

We realize individual atomic quantum probes by controlling microscopic inelastic collisions that couple the motion of cesium atoms to their spin. We find that the width of the spin distribution, upon reaching a steady state, provides an unambiguous measure of the temperature of a surrounding cold bath of rubidium atoms. This enables local absolute quantum thermometry, reducing temperature measurements to simply measurements of the spin distribution of the probe.

We also find that, surprisingly, we can outperform the so-called Cramer-Rao bound, a fundamental quantum limit on the precision of atomic probes. Since our coupling to spin is quantized, we obtain information about the system quantum by quantum, which optimizes the information flow. For only three spin-exchanging collisions, that is, perturbing the many-body system by three quanta of angular momentum, we obtain the maximum amount of information in the probe.

Our work opens the door to experimental quantum probing and paves the way for new optimization strategies using nonequilibrium dynamics.