YBCO microwave resonators for strong collective coupling with spin ensembles

TL;DR

This paper reports the development and characterization of YBCO superconducting microwave resonators capable of achieving strong collective coupling with spin ensembles, demonstrating high quality factors and tunable anticrossings at various temperatures and magnetic fields.

Contribution

It introduces YBCO resonators that maintain high quality factors under magnetic fields and temperatures, enabling coherent spin-photon coupling at practical conditions.

Findings

Quality factor exceeds 10^4 below 55 K

Achieved clear anticrossings with 82 MHz splitting at 2 K

Spin-cavity coupling scales with the square root of the number of spins

Abstract

Coplanar microwave resonators made of 330 nm-thick superconducting YBCO have been realized and characterized in a wide temperature ($T$, 2-100 K) and magnetic field ($B$, 0-7 T) range. The quality factor $Q_L$ exceeds 10$^4$ below 55 K and it slightly decreases for increasing fields, remaining 90$\%$ of $Q_L(B=0)$ for $B=7$ T and $T=2$ K. These features allow the coherent coupling of resonant photons with a spin ensemble at finite temperature and magnetic field. To demonstrate this, collective strong coupling was achieved by using DPPH organic radical placed at the magnetic antinode of the fundamental mode: the in-plane magnetic field is used to tune the spin frequency gap splitting across the single-mode cavity resonance at 7.75 GHz, where clear anticrossings are observed with a splitting as large as $\sim 82$ MHz at $T=2$ K. The spin-cavity collective coupling rate is shown to scale as the square root of the number of active spins in the ensemble.