Design and Implementation of a Josephson Junction Spectrometer*

A Josephson tunnel junction can be used as an on-chip absorption spectrometer at frequencies up to several hundred gigahertz. As a result of the AC Josephson effect, a voltage biased junction acts as a microwave source. When emitted photons are absorbed in the junction's electromagnetic environment, a dc Cooper pair current flows (inelastic Cooper pair tunneling). By measuring this dc current as a function of applied voltage---the junction's current-voltage characteristic---one obtains a spectrum of the electromagnetic environment. We describe the design of a Josephson junction spectrometer which seeks to optimize bandwidth, sensitivity, coupling and linewidth. We present measurements of the spectra of miniature on-chip $LC$ circuits with resonant frequencies in the 25-100 GHz range. Our Josephson junction spectrometer will be used to study level transitions in mesoscopic systems.