A single conduction channel in a superconducting weak link provides two quantum “Andreev” levels whose energies are tunable by both magnetic flux and channel transmission. Exciting the transition between the ground and upper Andreev level requires photons spanning the range from 2GHz to 90GHz in aluminum and 2GHz to 600GHz in niobium.

A first generation Josephson tunnel junction spectrometer with the required bandwidth has been demonstrated and this transition has been measured [Bretheau, Girit et al]. The long sought-after goal of detecting the excited Andreev bound state has been achieved, validating the mesoscopic foundation for the Josephson effect. Now the Andreev level energy-phase dispersion is well established. But there are still fundamental questions concerning Andreev bound states (ABS), such as what is their lifetime and what limits it? These questions can be answered with a more sophisticated Josephson junction spectrometer with a uniform broadband coupling scheme and narrow emission linewidth.

A long quasiparticle or Andreev pair lifetime would have applications in quantum information, such as the Andreev qubit [Chelkatchev, ...]. This work may also help understand the observed non-equilibrium quasiparticle distribution in superconducting circuits and lead to applications in single photon detection.