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.

中文翻译：

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非平衡自旋动力学增强的超冷气体单原子量子探针

量子探针是原子大小的设备，可将其环境信息映射到量子力学状态。通过改进测量并同时最小化对环境的干扰，它们成为量子技术的核心资产。我们通过将单个Cs原子浸入超冷Rb浴中来实现基于自旋的量子探针。控制探针和熔池之间的非弹性自旋交换过程使我们能够将运动和热信息映射到量子自旋状态。我们表明稳态自旋种群非常适合绝对测温，减少了温度测量以检测量子自旋分布。此外，我们发现通过访问非平衡自旋动力学可以使每次非弹性碰撞的信息增益最大化。保持运动自由度热化，各个自旋交换碰撞会逐个量子地产生有关气体量子的信息。我们发现，这种非平衡量子探测的灵敏度有效地超过了稳态Cramér-Rao极限一个数量级，同时将镀液的扰动降低到了角动量的仅三个量子点。我们的工作为在Heisenberg极限处对量子系统进行局部探测，以及通过控制非平衡动力学优化测量策略铺平了道路。