Tests of a SQUID-Based $^{3}\hbox{He}$ Co-Magnetometer Readout for a Neutron EDM Experiment

In a new experimental search for an electric dipole moment (EDM) of the neutron, polarized $^{3}\hbox{He}$ will occupy the same volume as the neutrons under study to serve as a co-magnetometer, enabling precise corrections for ambient magnetic field drifts that would otherwise severely limit the reach of the experiment. One of the two methods that will be built into the apparatus is to directly detect the $^{3}\hbox{He}$ magnetization signal using superconducting quantum interference device (SQUID)-based gradiometers. In a previous work in this Transactions, we proposed a candidate design for a SQUID system consistent with experimental requirements and the planned neutron EDM apparatus. Because the $^{3}\hbox{He}$ precession signal is at approximately 100 Hz, signal contamination from low-frequency magnetic noise could adversely affect the co-magnetometer readout precision; the addition of reference magnetometer channels to the SQUID system could mitigate this risk. In this paper, we present noise studies of the candidate SQUID system in a test apparatus and demonstrate effective ambient magnetic field noise cancelation with the implementation of reference channels. In addition, we report a demonstration of low-noise SQUID operation while a nearby photomultiplier tube and its high-voltage power supply are operating.