Potential sources of losses in superconducting quantum devices attributed to magnetism in oxides.*

Muon spectroscopy is a powerful tool to detect even weak magnetism presence. The low-energy range (up to 30 keV) gives a unique opportunity to study thin films and disordered systems. We have used a low-energy highly polarized muon beam and have detected magnetic fluctuations in niobium pentoxide in a wide temperature range with the rate of about 100 MHz below 35 K and down to 2.7 K. These fluctuations are most likely caused by oxygen deficiency in niobium pentoxide. Magnetic fluctuations are directly associated with magnetic flux noise and might lead to dephasing and decoherence in quantum devices. Since surface oxides are the limiting factor for all devices and tantalum-based devices demonstrated better performance, we made a comparison of niobium pentoxide with tantalum pentoxide. No magnetic fluctuations were detected in tantalum pentoxide in this range.