A Josephson junction defect spectrometer for measuring two-level systems

TL;DR

This paper introduces a Josephson junction defect spectrometer (JJDS) that detects and characterizes two-level systems in Josephson junction barriers by tuning resonance frequencies and observing spectral splittings at cryogenic temperatures.

Contribution

The study presents the design, fabrication, and measurement of a frequency-tunable JJDS capable of probing TLSs in junction barriers with high sensitivity and quantifies TLS density and junction loss tangent.

Findings

TLS density in junction barriers is approximately 0.4-0.5 per GHz per μm^2.

The junction loss tangent is measured to be about 2.9x10^{-3}.

JJDS operates effectively at 30 mK with single-photon excitation levels.

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 $\tan \delta_{J} = 2.9x10^{-3}$.