Public defence, Engineering Physics, MSc Katja Kohopää

Title of the thesis: Optical control of superconducting quantum technology

Thesis defender: Katja Kohopää 
Opponent: Professor Johannes Fink, Institute of Science and Technology, Austria
Custos: Professor Jukka Pekola, Aalto University School of Science

This thesis studies optical control of superconducting quantum technology, aiming to support the scalability of quantum computing. Quantum computers are expected to be able to solve problems that classical supercomputers are not able to, but achieving this requires a significant increase in the number of qubits. This scale-up requires development in many parts of the quantum computer, including the information transfer between room temperature control electronics and qubits at extremely low temperatures. 

Currently, this information is typically transferred via metallic coaxial cables, which introduce a significant heat load to the cryostat. As cooling power is limited, alternatives need to be explored. This thesis focuses on optical control, where information is transferred between the temperature stages in optical form, via optical fibers that do not cause significant heat load to the cryostat. Then, at low temperature, the optical signal can be converted to electrical form to drive superconducting components. 

In this thesis, this topic is studied in two parts. Disordered superconducting thin films are studied as they are promising materials for photon detectors that could be used in optical-to-electrical conversion. Regarding disordered films, the effect of ion irradiation is studied on several materials and novel atomic layer deposition methods are described for two superconducting materials. 

In addition, the optical control of Josephson junction arrays is studied. When Josephson junction arrays are driven with pulse patterns, they can be used to produce quantized voltage waveforms at cryogenic temperatures. The main result of the thesis is that the maximum drive frequency of our optically driven junction array was significantly higher compared to typical methods that use electrical pulse pattern generators. The results can help pave the way for using optical pulses and fast Josephson junction arrays to enable an energy-efficient and scalable method to drive qubits that would decrease the heat load and aid in the required scale-up for large-scale quantum computing.

Thesis available for public display 7 days prior to the defence at Aalto University's public display page. 

Contact Information:
katja.kohopaa@vtt.fi