Public defence in Engineering Physics, M.Sc. (Tech.) András Gunyhó

Title of the thesis: Thermal detectors for superconducting quantum computers

Thesis defender: András Gunyhó
Opponent: Professor Andreas Wallraff, ETH Zürich, Switzerland
Custos: Professor Mikko Möttönen, Aalto University School of Science, Department of Applied Physics

This thesis introduces a novel method for measuring the state of quantum bits (qubits) in a superconducting quantum computer. The technique is based on superconductor–normal-metal–superconductor (SNS) microwave radiation sensors, and it has the possibility to avoid issues related to power consumption of conventionally used methods. It may therefore enable larger and more powerful quantum processors. In a proof of concept experiment, the technique was found to be feasible, and a straightforward path towards state of the art measurement accuracy was laid out.

The thesis builds up to the main result that combines qubits and SNS sensors by separately presenting advancements in both kinds of devices. These include introducing a novel kind of qubit, advanced qubit measurement techniques, and by investigating the sensitivity limits of SNS microwave sensors, which were found to be record-breaking.

The results of this thesis present steps forward in quantum computers based on superconducting circuits, as well as advances of SNS sensors towards applications such as cosmology and microwave quantum optics. Quantum processors utilizing SNS sensors may enable scalability that brings us closer to practically useful quantum computers.

Keywords: Josephson junction, radiation sensor, quantum computing

Contact information: andras.gunyho@aalto.fi 

Thesis available for public display 10 days prior to the defence at Aaltodoc.