Comagnetometry using mirror-symmetric ions in a crystal

Searches for physics beyond the Standard Model using spin sensors are susceptible to spurious frequency shifts and noise due to magnetic fields. Therefore a comagnetometer – an auxiliary sensor that allows mundane magnetic field effects to be differentiated from new physics – is an essential feature of many precision searches. Here we demonstrate the operation of a novel type of comagnetometer using nuclear spins of dopant ions in a crystal, comparing four different sub-ensembles of ions. We demonstrate rejection of magnetic-field-induced shifts to better than 1 part in 10${}^5$ using this system, laying the groundwork for improved searches of time-reversal symmetry violation using solid-state systems.