Superconducting Diamond Microwave Resonators

Superconducting microwave resonators form the foundation of an increasing number of superconducting technologies for quantum information processing¹ and radiation detection.² A specific example is the Kinetic Inductance Detector (KID), which uses a high-Q resonator as a photon detector for purposes ranging from astronomy to security.³<br/><br/>In a KID, radiation is detected when an incoming photon breaks a Cooper pair which decreases the frequency of the microwave resonator. The relative size of this shift increases with the penetration depth of the material. Noise in the form of random fluctuations in the resonant frequency can occur due to two-level systems (TLSs), generally ascribed to dangling bonds in amorphous oxides that form at the surface of metallic superconductors. For practical applications, optimising for both sensitivity and signal-to-noise ratio, the superconducting material used for a microwave resonator should ideally have a high penetration depth and a low population of TLSs.<br/><br/>Here, I will present our results on a detailed characterisation of a λ/2 microwave resonator with a fundamental frequency of 409 MHz at 300 mK patterned from boron-doped nanocrystalline diamond (BNCD).⁴ We compare the observed resonant frequency to simulations based on the device geometry alone and show that the frequency is reduced by more than half, indicating a very large penetration depth enabled by the granularity of the material. I will also present early measurements of the effect of noise on this resonator and make a comparison to a single crystal equivalent device.<br/><br/>Our data show that BNCD is a good candidate for fabricating microwave resonators requiring a high kinetic inductance fraction. Knowledge of the penetration depth of a material is also vital for the design of low-frequency superconducting devices, including SQUIDs. Our results, therefore, pave the way for more complex superconducting circuits based on BNCD.<br/><br/><b>References</b><br/>[1] L. Grünhaupt, <i>et al</i>. <i>Nat. Mater.</i> <b>18</b> (2019): 816-819.<br/>[2] S. Doyle, <i>et al.</i> <i>J. Low Temp. Phys</i>. <b>151 </b>(2008): 530-536.<br/>[3] S. Rowe, <i>et al. Rev. Sci. Instrum.</i> <b>87</b> (2016): 033105.<br/>[4] J. A. Cuenca, <i>et al.</i> <i>Carbon</i> <b>201</b> (2023): 251-259.