A Josephson junction defect spectrometer for measuring two-level systems
  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 device 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 $\sigma_{TLS}h = 0.4-0.5 /GHz {\mu}m^2$, and a junction loss tangent of $\tan \delta_{J} = 2.9x10^{-3}$. 
M. J. A. Stoutimore, M. S. Khalil, C. J. Lobb, K. D. Osborn
cond-mat.supr-con