On-chip microwave spectroscopy of Andreev and Majorana bound states in semiconductor nanowires*

The microscopic picture of the Josephson effect, describing the flow of supercurrent through a weak link, is based on the formation of Andreev bound states, localized at the weak link. In narrow gap semiconductors, such as InAs and InSb, the interplay of induced superconductivity with spin-orbit coupling and magnetic field can give rise to 4Pi-periodic Andreev levels, signifying the presence of topological superconductivity and Majorana end modes.
In this talk, I will summarize our efforts to understand the Andreev level spectra in proximitized semiconductor nanowires by exploiting the AC Josephson effect and using on-chip microwave detection techniques based on superconducting tunnel junctions. With this technique, we investigate gate-tunable Andreev bound states in ballistic semiconductor nanowire channels [1] and the microwave spectrum of nanowire-based Cooper-pair transistors [2]. I will also showcase our measurements of the 4π-periodic Josephson effect above a threshold magnetic field [3], which is consistent with the topological phase transition and the emergence of Majorana bound states.
[1] D.J. van Woerkom et al, Nature Physics 13, 876 (2017)
[2] A. Proutski et al, Phys. Rev. B 99, 220504(R) (2019)
[3] D. Laroche et al, Nature Communications 10, 245 (2019)