A Josephson junction defect spectrometer for measuring two-level systems

M. J. A. Stoutimore, M. S. Khalil, C. J. Lobb, K. D. Osborn
2012 Applied Physics Letters  
We have fabricated and measured Josephson junction defect spectrometers (JJDSs), which are frequency-tunable, nearly-harmonic oscillators that probe strongly-coupled two-level systems (TLSs) in the barrier of a Josephson junction (JJ). The JJDSs accommodate a wide range of junction inductances, L_J, while maintaining a resonance frequency, f_0, in the range of 4-8 GHz. By applying a magnetic flux bias to tune f_0, we detect strongly-coupled TLSs in the junction barrier as splittings in the
more » ... e spectrum. JJDSs fabricated with a via-style Al/thermal AlOx/Al junction and measured at 30 mK with single-photon excitation levels show a density of TLSs in the range σ_TLSh = 0.4-0.5 /GHz μm^2, and a junction loss tangent of tanδ_J = 2.9x10^-3.
doi:10.1063/1.4744901 fatcat:5gujemoyqzea7asgvatlzsxx6q