Yttrium orthosilicate (Y2SiO5, or YSO) has proved to be a convenient host for rare-earth ions used in demonstrations of microwave quantum memories and optical memories with microwave interfaces, and shows promise for coherent microwave--optical conversion owing to its favourable optical and spin properties. The strong coupling required by such microwave applications could be achieved using superconducting resonators patterned directly on Y2SiO5, and hence we investigate here the use of Y2SiO5 as an alternative to sapphire or silicon substrates for superconducting hybrid device fabrication. A NbN resonator with frequency 6.008 GHz and low power quality factor Q≈400000 was fabricated on a Y2SiO5 substrate doped with isotopically enriched Nd145. Measurements of dielectric loss yield a loss-tangent tanδ=4×10−6, comparable to sapphire. Electron spin resonance (ESR) measurements performed using the resonator show the characteristic angular dependence expected from the anisotropic Nd145 spin, and the coupling strength between resonator and electron spins is in the high cooperativity regime (C=30). These results demonstrate Y2SiO5 as an excellent substrate for low-loss, high-Q microwave resonators, especially in applications for coupling to optically-accessible rare earth spins.
@article{arxiv.1901.03262,
title = {High-cooperativity coupling of a rare-earth spin ensemble to a superconducting resonator using yttrium orthosilicate as a substrate},
author = {Gavin Dold and Christoph W. Zollitsch and James O'Sullivan and Sacha Welinski and Alban Ferrier and Philippe Goldner and S. E. de Graaf and Tobias Lindström and John J. L. Morton},
journal= {arXiv preprint arXiv:1901.03262},
year = {2019}
}