A Mesoscopic Spectrometer Based on the Josephson Effect*

A key element of mesoscopic topological systems, such as hybrid semiconductor-superconductor circuits, are Andreev Bound States, single quasiparticles localized at superconducting weak links. The characteristic transition energy of these states is twice the superconducting gap (90 GHz in aluminum). Conventional microwave techniques allow probing these states but only in a limited bandwidth. We propose a new broadband spectrometer operating at frequencies up to 180 GHz based on the Josephson effect which converts a DC voltage to microwave oscillations at a frequency proportional to this voltage. Using a symmetrical SQUID biased at half a flux quantum allows decoupling the spectrometer from environmental modes. In addition, careful design of the biasing circuit reduces the number of remaining modes and damps them. The fabricated mesoscopic spectrometer has a linewidth of 2 MHz, a bandwidth of 180 GHz and a minimal theoretical sensitivity of 5 kHz.