Versatile Quadrature Antenna for Precise Control of Large Electron Spin Ensembles in Diamond

An easily reproducible inexpensive microwave antenna that can generate a strong and homogeneous magnetic field of arbitrary polarization is presented, which enables fast and coherent control of electron spins over a large volume. Unlike preceding works, a resonant antenna with ample optical access and which maintains its resonant behavior regardless of the proximity of other experimental hardware components is presented. This robustness is crucial as it enables using microscope objectives with short working distances to perform wide-field imaging/sensing with bulk diamonds. The antenna generates a magnetic field strength of 22.3 A $m^{-1}$ for 1 W total driving power, which doubles the power efficiency compared with previously reported patch antenna designs. The magnetic field homogeneity in a volume of $0.3{\text{mm}}^3$, $0.6{\text{mm}}^3$ and $1{\text{mm}}^3$ is within 3%, 8% and 14%, respectively. The antenna can be driven off-resonance without affecting the ellipticity and inhomogeneity of the field, and has a full-width-at-half-maximum bandwidth of $\sim$200 MHz. Its resonant frequency can be tuned over a 400 MHz range via varactors. The PCB files are provided open-source. This work facilitates a robust and versatile piece of instrumentation, being particularly appealing for applications such as high sensitivity magnetometry and wide-field imaging/sensing with nitrogen-vacancy centers.