CBQS-QUANTOP seminar by Georgios Vasilakis

Towards Back-Action Evasion via Hyperfine Manifold Correlations in a Single Atomic Spin Ensemble

Quantum back-action noise, arising from the unavoidable disturbance of a system by the measurement process, is a key obstacle to achieving optimal sensitivity in atomic magnetometers. Back-action evasion can be realized by engineering a so-called negative-mass reference frame, in which the back-action noise of two subsystems cancels in the measured observable. Previous demonstrations of this approach have relied on two physically separate spin ensembles. Here we present a scheme that realizes such a reference frame within the two hyperfine manifolds of a single alkali-metal spin ensemble. Under appropriate conditions on the probe detuning and atomic polarization, the light-shift noise imparted on the two manifolds acquires opposite signs, and by exploiting the resulting anticorrelations in the measured observable, back-action is cancelled while preserving sensitivity to the magnetic field. We derive the conditions for back-action cancellation, analyze the noise spectrum in both stationary and non-stationary probing regimes, and discuss the effect of the differential Larmor precession between the two hyperfine manifolds. Preliminary experimental results using cesium atoms are presented. The proposed scheme offers a practical and compact route to back-action-evading RF magnetometry using continuous probe light.