Josephson Radiation from Gapless Andreev Bound States in HgTe-Based Topological Junctions

Frequency analysis of the rf emission of oscillating Josephson supercurrent is a powerful passive way of probing properties of topological Josephson junctions. In particular, measurements of the Josephson emission enable the detection of topological gapless Andreev bound states that give rise to emission at half the Josephson frequency f_{J} rather than conventional emission at f_{J}. Here, we report direct measurement of rf emission spectra on Josephson junctions made of HgTe-based gate-tunable topological weak links. The emission spectra exhibit a clear signal at half the Josephson frequency f_{J}/2. The linewidths of emission lines indicate a coherence time of 0.3–4 ns for the f_{J}/2 line, much shorter than for the f_{J} line (3–4 ns). These observations strongly point towards the presence of topological gapless Andreev bound states and pave the way for a future HgTe-based platform for topological quantum computation.