Feedback control of coherent spin states using weak nondestructive measurements

TL;DR

This paper demonstrates how weak nondestructive measurements combined with real-time feedback can effectively protect coherent spin states of cold atoms from decoherence, enhancing quantum control in atom interferometry.

Contribution

It provides a theoretical and experimental analysis of feedback control using weak measurements to preserve quantum coherence in spin ensembles.

Findings

Maximum feedback efficiency occurs in the weak measurement regime.

Experimental stabilization of coherent spin states in cold atoms.

Analysis of parameters affecting feedback performance.

Abstract

We consider the decoherence of a pseudo-spin ensemble under collective random rotations, and study, both theoretically and experimentally, how a nondestructive measurement combined with real-time feedback correction can protect the state against such a decoherence process. We theoretically characterize the feedback efficiency with different parameters --- coherence, entropy, fidelity --- and show that a maximum efficiency is reached in the weak measurement regime, when the projection of the state induced by the measurement is negligible. This article presents in detail the experimental results published in [Phys. Rev. Lett. \textbf{110}, 210503 (2013)], where the feedback scheme stabilizes coherent spin states of trapped ultra-cold atoms, and nondestructively probed with a dispersive optical detection. In addition, we study the influence of several parameters, such as atom number and rotation angle, on the performance of the method. We analyze the various decoherence sources limiting the feedback efficiency and propose how to mitigate their effect. The results demonstrate the potential of the method for the real-time coherent control of atom interferometers.