Millikelvin Nb nanoSQUID-embedded tuneable resonator fabricated with a neon focused-ion-beam

Abstract

SQUID-embedded superconducting resonators are of great interest due to their potential for coupling highly scalable superconducting circuits with quantum memories based on solid-state spin ensembles. Such an application requires a high-$Q$, frequency-tuneable resonator which is both resilient to magnetic field, and able to operate at millikelvin temperatures. These requirements motivate the use of a higher $H_c$ metal such as niobium, however the challenge then becomes to sufficiently reduce the operating temperature. We address this by presenting a monolithic Nb nanoSQUID-embedded resonator, where neon focused-ion-beam fabrication of the nanoSQUID results in a device displaying frequency tuneability at $T=16$ mK. In order to assess the applicability of the device for coupling to small spin clusters, we characterise the flux sensitivity as a function of microwave drive power and externally applied magnetic field, and find that the noise is dominated by dielectric noise in the resonator. Finally, we discuss improvements to the device design which can dramatically improve the flux sensitivity, which highlights the promise of Nb SQUID-embedded resonators for hybrid superconductor-spin applications.

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