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 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 Cramér 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.

Quantum sensing of open systems: Estimation of damping constants and temperature

“…Quantum sensing [15,16] involves the exploration of subtle quantum effects to increase the precision of parameter estimation. Quantum sensors have become one of the most promising applications of quantum technologies [17][18][19], involving single-or multiparameter estimation [20,21].…”

In Situ Thermometry of a Cold Fermi Gas via Dephasing Impurities

“…This method has proved useful in several recent experiments [28][29][30][31][32][33][34] but becomes challenging at low temperatures where equilibration is slow and the probe's energy levels must be finely tuned [26,[35][36][37][38][39][40]. These limitations can be overcome by harnessing the probe's nonequilibrium dynamics for thermometry [25,[41][42][43][44][45][46]. Perhaps the most extreme example is pure dephasing, where the energy of the probe is conserved and thus normal thermalization is completely suppressed.…”

A quantum heat engine driven by atomic collisions

“…We further detect the Zeeman populations of single Cs atoms at arbitrary times by position resolved fluorescence measurements combined with Zeeman-state-selective operations (Fig. 1 a inset) 21 . From each individual measurement, we can determine quantized spin transitions for each single atom.…”

“…1 d). Indeed, due to the different atomic Landé factors for Rb and Cs , only half ( ) of the energy change of a bath atom is effectively exchanged with the heat engine during an inelastic spin-exchange collision 21 . As a result, the heat emitted (absorbed) by the bath differs from the energy portions absorbed 〈 Q H 〉 (emitted 〈 Q C 〉) by the machine.…”