Defence of doctoral thesis in the field of applied physics, M.Sc. Elsa Mannila

Title of the doctoral thesis is "Counting nonequilibrium quasiparticles in a superconductor"

Superconducting electrical circuits have applications for instance as ultrasensitive radiation detectors and building blocks of quantum computers. In these devices, superconductivity is based on electrons pairing at low temperatures into Cooper pairs. The billions of electrons in a device can then be described with a single quantum-mechanical wavefunction. It is expected that at temperatures low enough, there should be no unpaired electrons left in the superconductor. However, in practice, unpaired electrons known as quasiparticle excitations are observed in experiments. Even if there may be as few as one nonequilibrium quasiparticle per billion Cooper pairs, they can still limit the performance of e.g. superconducting quantum bits.

In this dissertation, we study the nonequilibrium quasiparticles in superconductors using charge detectors based on single-electron transistors, which are sensitive enough to observe in real time the electrons tunneling on and off a mesoscopic superconducting aluminum island. We were able to count the number of quasiparticles on such an island in real time and demonstrate that the island is completely free of quasiparticles even for seconds, which is orders of magnitude longer than the operation times of superconducting qubits . We also present a superconducting detector of Cooper pair breaking radiation, in which one electron passes through the device per broken Cooper pair. The thesis also includes results on improving the performance of charge detectors and applications of such devices in studying stochastic thermodynamics. 

The work provides new information about processes that break Cooper pairs and create quasiparticles: When the backaction of the charge detector on the system studied was minimized, the Cooper pair breaking rate decreased over timescales of weeks, which rules out previously suggested origins of quasiparticles. The results presented in this dissertation can be used to improve the performance of superconducting devices. 

Opponent is Professor William D. Oliver, Massachusetts Institute of Technology, USA

Custos is Professor Jukka Pekola, Aalto University School of Science, Department of Applied Physics

The public defence will be organised via Zoom and on Campus. Link to the event

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The dissertation is publicly displayed 10 days before the defence in the publication archive Aaltodoc of Aalto University

Electronic thesis (

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