Low-temperature microwave properties of biaxial YAlO3

Low-loss crystals with defects due to paramagnetic or rare earth impurity ions are a major area of investigation for quantum hybrid systems at both optical and microwave frequencies. In this work we examine the single-crystal yttrium aluminium perovskite YAlO3 using the whispering gallery mode technique. Multiple resonant microwave modes were measured from room temperature to 20 mK allowing precise characterization of the permittivity tensor at microwave frequencies. We show that it is biaxial and characterize the tensor as a function of temperature with estimated uncertainties below 0.26%. Electron spin resonance spectroscopy was also performed at 20 mK, with transitions identified with zero-field splittings of 16.72 and 9.92 GHz. Spin-photon couplings of order 4.2 and 8.4 MHz were observed for residual levels of concentration, which are stronger than the photon cavity linewidths of 116 kHz but of the same order as the linewidths of the discovered spin transitions.