Frequency-tunable Three-dimensional Microwave Resonator for Coherent Control of Large NV Ensembles

Nitrogen-vacancy color centers in diamond show promise as high-sensitivity vector magnetometers, due to their excellent room temperature coherence time, and the ability to address them using optical and microwave pulses (Optically Detected Magnetic Resonance). One way to increase the sensitivity of measurements is to use large ensembles of NV^- centers to gain a statistical advantage; however, this requires the ability to apply an uniform microwave field to the whole ensemble. Combining optical excitation and readout with uniform microwave excitation in the same device has been shown to be challenging in past experimental realizations. Here, we report the development of a three-dimensional, re-entrant microwave resonator geometry which allows uniform (0.3% RMS error in field amplitude) and frequency-tunable (300 MHz tuning range) microwave excitation on macroscopic (~3mm³) volumes, as well as laser excitation and photoluminescence measurement. We also show results of ODMR measurements of a large (>10¹² spins) NV^- ensemble. This work will be used in an enhanced-sensitivity room-temperature magnetometer, but could also be applied to any experiments measuring spin resonance on large samples.