# Single-atom quantum probes for ultracold gases using nonequilibrium spin dynamics

## TL;DR

This paper demonstrates a novel quantum probing technique using single-atom spins immersed in ultracold gases, achieving high sensitivity and minimal disturbance by exploiting nonequilibrium spin dynamics for thermometry and information extraction.

## Contribution

It introduces a method to enhance quantum sensing by controlling nonequilibrium spin dynamics of single-atom probes in ultracold gases, surpassing steady-state limits.

## Key findings

- Sensitivity exceeds steady-state quantum limits by nearly an order of magnitude.
- Steady-state spin populations enable absolute thermometry.
- Minimal perturbation involves only three quanta of angular momentum.

## 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.