Public defence in Engineering Physics, M.Sc. (Tech) Arto Viitanen

Title of the doctoral thesis: Dissipation engineering of quantum-electric devices

Doctoral student: Arto Viitanen
Opponent: Assist.Prof. Simone Gasparinetti, Chalmers University of Technology, Sweden
Custos: Prof. Mikko Möttönen, Aalto University School of Science, Department of Applied Physics

Thesis available for public display 10 days prior to the defence at: https://aaltodoc.aalto.fi/doc_public/eonly/riiputus/

Superconducting quantum circuits are lossy due to their inevitable coupling to their environment. On one hand, minimizing these losses is vital for ensuring coherent circuit operation. On the other, momentarily increasing these losses can be advantageous for removing undesired energy from the circuit, potentially addressing a key challenge in quantum computing: qubit initialization. 

This thesis investigates the temporal control of dissipation in quantum circuits with a device known as a quantum-circuit refrigerator. The refrigerator is based on the phenomenon of electron tunneling, where the electron simultaneously absorbs microwave photons from the desired quantum circuit. While previous implementations of this refrigerator operated it with a direct voltage, a main contribution of this thesis is a method to refrigerate with a microwave signal, inducing a peculiar energy level shift in the quantum circuit. The study also introduces a streamlined version of the refrigerator and, using a qubit, measures at the level of individual quantum states its ability to cool a microwave resonator. The results deepen our understanding of operational properties of the quantum-circuit refrigerator and unlock advanced methods for its use in future quantum computers.

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Doctoral theses in the School of Science: https://aaltodoc.aalto.fi/handle/123456789/52