PhD Defense: Deividas Sabonis

High frequency techniques for detection of Majorana zero modes in hybrid InAs/Al nanowires

In popular literature topological quantum computing is always presented as a much better, more stable approach towards the protection of quantum state compared to more conventional methods to quantum computing. However, even then a well defined quantum state could be for example destroyed by physical processes that change a number of particles on the superconductor. Protection against such and other errors requires qubit manipulation and readout on a time scale shorter than the typical time scale of such destructive process.

This contribution in a form of PhD thesis further investigates hybrid one dimensional structures as a potential future platform for topological quantum information processing. However, here low frequency transport makes only a small part of characterization methods. Instead, the work concerns with the development and implementation of high frequency measurement techniques for detection and readout of hybrid InAs/Al nanowires. The detection techniques presented span almost 10 orders of magnitude in frequency.

In particular, fast radio frequency nanowire-based charge sensors working in magnetic fields up to 1 T both for state readout and gate-space mapping are realized. Later on, dispersive sensing results in InAs/Al nanowires are presented that allow for the compact footprint state readout. Next, microwave spectroscopy is performed both at zero and finite magnetic field in superconducting double-island device resembling the theoretical proposed qubit design.

Finally, results from the full-shell nanowire-based offset charge sensitive transmon are presented. The long parity switching time in the nanowire system would imply a large number of quantum operations possible before the decoherence happens. As a result, we employ the nanowire transmon itself as a sensitive detector allowing to estimate the parity switching time both at zero and at finite magnetic field. The presented results will hopefully lay the foundation for the future high frequency experiments in Majorana-compatible, one-dimensional systems.