Coherent and dephasing spectroscopy for single-impurity probing of an ultracold bath

TL;DR

This paper demonstrates how Ramsey spectroscopy on single Cs impurities in an ultracold Rb bath can nondestructively measure the bath's density and temperature by analyzing phase evolution and fringe contrast decay.

Contribution

It introduces a method to use single-impurity Ramsey spectroscopy for nondestructive probing of ultracold atomic environments, highlighting the impact of interspecies interactions.

Findings

Measured bath density and temperature via Ramsey phase shifts.

Enhanced temperature sensitivity near a Feshbach resonance.

Showed impurity atoms as effective nondestructive quantum probes.

Abstract

We report Ramsey spectroscopy on the clock states of individual Cs impurities immersed in an ultracold Rb bath. We record both the interaction-driven phase evolution and the decay of fringe contrast of the Ramsey interference signal to obtain information about bath density or temperature nondestructively. The Ramsey fringe is modified by a differential shift of the collisional energy when the two Cs states superposed interact with the Rb bath. This differential shift is directly affected by the mean gas density and the details of the Rb-Cs interspecies scattering length, affecting the phase evolution and the contrast of the Ramsey signal. Additionally, we enhance the temperature dependence of the phase shift preparing the system close to a low-magnetic-field Feshbach resonance where the $s$-wave scattering length is significantly affected by the collisional (kinetic) energy. Analyzing coherent phase evolution and decay of the Ramsey fringe contrast, we probe the Rb cloud's density and temperature. Our results point at using individual impurity atoms as nondestructive quantum probes in complex quantum systems.