A Josephson junction defect spectrometer for measuring two-level systems

Abstract

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 $\tan {\delta }_J=2.9x10^{-3}$.

4 pages, 3 figures