Quantum Control of Two Critically Dressed Spin 1/2 Species in Magnetic Fluctuations

TL;DR

This paper analyzes noise effects in critically dressed spin systems used in neutron EDM experiments, employing perturbation theory and numerical methods to develop strategies that enhance coherence times.

Contribution

It introduces a theoretical framework for understanding noise-induced relaxation in dressed spins and proposes modulation techniques to improve coherence times.

Findings

Calculated relaxation and frequency shifts due to magnetic field fluctuations.

Validated theoretical predictions with numerical solutions of Bloch equations.

Demonstrated modulation methods that significantly extend coherence times.

Abstract

The neutron electric dipole moment experiment at the Spallation Neutron Source (nEDM@SNS experiment) proposes to measure the nEDM using the spin-dependent capture cross section of neutrons on $^3$He. The critical dressing mode of this experiment uses an oscillating magnetic field to dress the gyromagnetic ratios of neutrons and $^3$He to the same value. While this technique grants increased sensitivity to the nEDM by improving the signal-to-noise ratio, this mode of measurement also introduces additional noise from the power supply used to drive the dressing field. This can lead to randomly fluctuating magnetic fields which cause the spins of neutrons and $^3$He to drift apart over time. Here we use second-order time-dependent perturbation theory to compute relaxation and frequency shifts due to fluctuations in the dressing field in terms of the magnetic field noise power spectrum and compare these calculations to numerical solutions obtained by integrating the Bloch equations. We then use these results to develop mitigation strategies for this type of noise. Furthermore, we report on spin dressing modulation techniques that significantly amplify coherence times for the critically dressed system, and attempt to quantify the coherence time achievable.