Johnson-Nyquist Noise Effects in Neutron Electric-Dipole-Moment Experiments

TL;DR

This study analyzes how magnetic Johnson-Nyquist noise from metal electrodes affects neutron EDM experiments, showing that with proper design, its impact is negligible at high sensitivities, enabling cost-effective electrode choices.

Contribution

First dedicated modeling of JNN effects in large-scale neutron EDM experiments with co-magnetometry, including noise simulation methods and impact assessment.

Findings

JNN impact is negligible for sensitivities down to 4×10⁻²⁸ e·cm.

Optically pumped Cs and Hg magnetometers are not limited by JNN.

Solid aluminum electrodes are suitable due to minimal JNN effects.

Abstract

Magnetic Johnson-Nyquist noise (JNN) originating from metal electrodes, used to create a static electric field in neutron electric-dipole-moment (nEDM) experiments, may limit the sensitivity of measurements. We present here the first dedicated study on JNN applied to a large-scale long-measurement-time experiment with the implementation of a co-magnetometry. In this study, we derive surface- and volume-averaged root-mean-square normal noise amplitudes at a certain frequency bandwidth for a cylindrical geometry. In addition, we model the source of noise as a finite number of current dipoles and demonstrate a method to simulate temporal and three-dimensional spatial dependencies of JNN. The calculations are applied to estimate the impact of JNN on measurements with the new apparatus, n2EDM, at the Paul Scherrer Institute. We demonstrate that the performances of the optically pumped $^{133}$Cs magnetometers and $^{199}$Hg co-magnetometers, which will be used in the apparatus, are not limited by JNN. Further, we find that in measurements deploying a co-magnetometer system, the impact of JNN is negligible for nEDM searches down to a sensitivity of $4\,\times\,10^{-28}\,e\cdot{\rm cm}$ in a single measurement; therefore, the use of economically and mechanically favored solid aluminum electrodes is possible.