Modified Split-Ring Resonators for Efficient and Homogeneous Microwave Control of Large Volume Spin Ensembles

Quantum sensing using local defects in solid-state systems has gained significant attention over the past several years, with impressive results demonstrated both in academia and in industry. Specifically, using large volume and high-density ensembles for beyond state-of-the-art sensitives is of clear interest. A major obstacle for achieving such record sensitivities is associated with the need to realize strong, homogeneous driving of the sensor defects. Here, we focus on high-frequency microwave sensing using nitrogen-vacancy (NV) centers in diamond and develop a modified split-ring resonator design to address this issue. We demonstrate enhanced drive strengths and homogeneities over large volumes compared with previous results, with prospects for enabling the desired sensitivities. We reach Rabi frequencies of up to 18-MHz with an efficiency ratio of
$2~\text {Gauss}/\sqrt {\text {Watt}}$
, along with an inhomogeneity of < 0.7% in a volume of 0.1 mm3. This structure also has a narrow form factor that allows for efficient optical coupling and fluorescence collection.