PhD defense: Oscar Erlandsson

Title: Aspects of decoherence in superconductor-semiconductor qubits

Abstract: Hybrid superconductor-semiconductor devices offer unique advantages for solid-state quantum information processing. In particular, the gatemon qubit has proved to be a versatile experimental platform since its inception less than a decade ago. For all types of qubits, understanding and overcoming decoherence are important parts of the progression toward large-scale quantum computation. In this thesis, results from three different studies related to decoherence in gatemons are presented.

First, a gatemon formed in an InAs nanowire with a fully covering Al shell is studied in a finite magnetic field. Investigating this system in an applied field is motivated by the possible existence of Majorana zero modes, which could be used for protection against decoherence. A non-monotonic dependence of the qubit transition frequency on magnetic field is observed and interpreted as the destructive Little-Parks effect. No signature of finite Majorana coupling E_M is observed. By measuring the charge dispersion of the qubit, an upper bound is placed at E_M/h < 10 MHz.

Next, parity switching induced by quasiparticles in the nanowire gatemon is studied. Quasiparticle poisoning can lead to decoherence and is an important source of loss in superconducting qubits. At zero magnetic field, the switching is found to occur on a time scale of 100 ms. As either the temperature or magnetic field is increased, the switching rate is observed to be first constant and then increase exponentially, which is consistent with the conventional picture of coexisting non-equilibrium and thermal quasiparticles. Slow parity switching at zero magnetic field is promising for future development of gatemon coherence times.

Finally, early results from the study of a 2DEG-based gatemon with multiple gates close to the Josephson junction are presented. Precise gate control of the potential could allow for novel ways to limit decoherence. Operation of multiple gates is demonstrated in both transport and circuit-QED measurements. The devices are found to have short relaxation times on the order of 0.1 μs. Directions for future research are discussed.