Precise measurements of the dipole moment of the neutron test the standard model of particle physics. Typical experiments detect the evolution of neutrons in magnetic and electric fields. Achieving high sensitivity requires stable and homogeneous fields. We are investigating nitrogen-vacancy diamonds for sensing electric fields. As a first step we have measured electric fields by optically-detected magnetic resonance. Near avoided crossings a first-order Stark effect is observed. Line positions can be measured to about 5 kHz, allowing electric fields to be measured to about 2 kV/cm. Extending the technique to use electromagnetically-induced transparency will allow for an all-optical probe, but may introduce issues of systematic errors in the electric field measurement.
We report measurements of optically detected magnetic resonance spectra of ensembles of nitrogen-vacancy (NV-) centers in diamonds in the presence of electromagnetic fields. To reduce inhomogeneous broadening, the spectra are acquired from a region of 20 cubic microns in a CVD(Chemical Vapor Deposition) diamond through a confocal microscope. The Stark shift from transverse electric fields is enhanced at avoided crossings between the hyperfine levels that arise from interaction with <sup>14</sup>N(I = 1) nuclei in the diamond lattice. As expected from previous reports, the Stark shift of the spectral lines is stronger when there is no magnetic field along the NV axis. The shift is also strong, but for different transitions, at a field of about 100 uT.
Ensembles of negatively charged nitrogen vacancy centers in diamonds are investigated as optical sensors for electric and magnetic fields in the interaction region of a neutron electric dipole moment experiment. As a first step towards measuring electric fields, the Stark shift is investigated in the ground electronic state, using optically detected magnetic resonance (ODMR) to measure hyperfine-resolved fine structure transitions. One detection approach is to modulate the electric field and demodulate the ODMR signal at the modulation frequency or its harmonic. Models indicate that the ratio of the amplitudes of these signals provides information about the magnitude of the electric field. Experiments show line shapes consistent with the models. Methods are considered for extending this technique to all-optical measurement of fields. Additionally, progress is reported towards an all-optical, fiberized sensor based on electromagnetically-induced transparency (EIT), which may be suitable for measuring magnetic fields. The design uses total internal reflection to provide a long optical path through the diamond for both the 637 nm EIT laser and a green repump laser.