  The high-Q harmonic oscillator mode of a Josephson junction can be used as a
novel probe of spurious two-level systems (TLSs) inside the amorphous oxide
tunnel barriers of the junction. In particular, we show that spectroscopic
transmission measurements of the junction resonator mode can reveal how the
coupling magnitude between the junction and the TLSs varies with an external
magnetic field applied in the plane of the tunnel barrier. The proposed
experiments offer the possibility of clearly resolving the underlying coupling
mechanism for these spurious TLSs, an important decoherence source limiting the
quality of superconducting quantum devices.
