Circuit quantum electrodynamics measurement of Andreev levels in a semiconducting nanowire weak link

Andreev levels are supercurrent-carrying fermionic modes responsible for the manifestation of the Josephson effect in superconducting weak links. Individual Andreev levels can be resolved in highly transparent, few-channel weak links by embedding them in superconducting microwave resonators. We apply such circuit quantum electrodynamics probing method to measure Andreev levels in a semiconducting nanowire proximitized by superconducting electrodes. We experimentally show that this mesoscopic system reproduces the hallmark phenomena of atomic physics: from radiation-induced transitions between the discrete levels to Coulomb repulsion between the quasiparticles. We also uncover phenomena with no direct analogues in atomic physics. They arise from the interplay between strong spin-orbit interaction and superconductivity. Among them are the “zero-field” spin splitting and the presence of supercurrent sensitive to the quasiparticle spin. The latter effects allow us to coherently control the spin of a superconducting quasiparticle [1].

[1] Hays et al., Science 373, 430 (2021)