The Josephson effect of topological superconductors

The AC Josephson effect defines a voltage-to-frequency ratio, h/2e, which only depends on the Planck contant, h, and the electron charge, e. This ratio, which is well established for various superconducting weak links, picks up a factor of two when the Josephson junction is formed between two topological superconductors hosting Majorana bound states. This quantitative signature of the topological phase transition can be observed in the microwave domain, where the charge parity lifetime does not play a role. We fabricated a Josephson weak link in a semiconductor InAs nanowire with epitaxial aluminum shell and coupled it to a superconducting tunnel junction acting as a on-chip frequency-sensitive microwave detector. We observed a dramatic change in the radiation frequency above a threshold magnetic field, in agreement with the expected topological phase transition. Find our manuscript here.

Andreev bound states in semiconductor nanowires

The modern description of superconductivity on the nanoscale is based on the Andreev bound states localized to the weak link between two bulk superconductors. These single electron states carry the dissipationless supercurrent via the junction. In semiconductor nanostructures, the Andreev level spectrum can be tuned by electrostatic gating, and by applying an external magnetic field. We presented direct microwave spectroscopy of the Andreev levels in a semiconductor InAs nanowire with epitaxial aluminum leads. Our results are published in Nature Physics. Building on these results, we collaborated with the QLAB group at Yale to create the first Andreev level qubit in a semiconductor nanowire. The results are reported here.

Microwave lab-on-a-chip

The detection of very weak microwave signals emitted by nanoscale devices requires special detection methods, especially when the microwave frequency has to be measured and the device is cooled down to millikelvin temperatures. Moreover, typical frequencies of interest range up to 100 GHz, determined for instance by the superconducting gap in the device. We utilize photon-assisted tunneling processes as a spectroscopic tool to measure the microwave signatures of various physical phenomena. In this work, we used inelastic Cooper-pair tunneling to measure the Andreev level spectrum in a nanowire Josephson junction. Another process, photon-assisted quasiparticle tunneling was utilized to assess the signatures of topological superconductivity. We have also measured the shot noise of a nanowire Josephson junction.

Charge parity lifetime in superconducting islands

At sufficiently low temperatures, a piece of superconductor should only contain paired electrons, known as Cooper pairs. However, stray unpaired quasiparticles can be a potential limiting factor for the coherence of superconducting quantum circuits and topological qubits. We used a nanoscale NbTiN island as its own charge parity detector, and characterized the charge parity lifetime. We demonstrated that this figure is very sensitive to the appropriate design of the superconducting circuit, and measures such as gap engineering and microwave shielding can increase it with four orders of magnitude! We published these results in Nature Physics.