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Title of the thesis: Quantum Heat Engines in Superconducting Circuits

Thesis defender: Miika Rasola
Opponent: Professor John Goold, Trinity College Dublin, Ireland
Custos: Professor Mikko Möttönen, Aalto University School of Science

As superconducting quantized electric circuits continue to emerge as an
exceptionally versatile and powerful platform for quantum computing and other
quantum technologies, they also provide a rich foundation for exploring quantum
thermodynamics. The study of heat transport in these circuits offers invaluable
synergy as understanding the impact of thermal fluctuations becomes increasingly
important for their operation.

Although heat transport in superconducting circuits has been extensively studied, quantum heat engines
(QHE) operating at the limit of a few excitation quanta had not been experimentally
realized prior to this work. These engines offer valuable insight into the quantum
thermodynamics of superconducting circuits. Beyond fundamental research, the
beseeching goal is to develop integrated QHEs which generate useful work in
conjunction with other quantum technologies. In this context, autonomous QHEs,
capable of operating without coherent external control, are particularly compelling.

In this thesis, we explore thermodynamics and quantum heat engines, with a
particular focus on engineering quantum heat engines using superconducting
circuits as our experimental platform. The presented research includes both
experimental and theoretical advancements, focusing on driven and autonomous
QHEs, respectively.

First, we demonstrate that engineered environments in superconducting circuits can
serve as effective thermal reservoirs for externally driven quantum heat engines.
Experimentally, we show that a quantum-circuit refrigerator (QCR) can be used not
only for cooling but also for heating the coupled circuit. This discovery imminently
leads to the experimental realization of a quantum heat engine, where a quantum
Otto cycle is driven in a transmon qubit coupled to the QCR.

Second, regarding autonomous quantum heat engines, we theoretically show that
non-linear optomechanical coupling can eliminate the need for external driving while
providing a method for directly observing the work output. These promising
theoretical results suggest that the experimental realization of an autonomous QHE
is ultimately a matter of careful circuit engineering.

Keywords: Josephson junction, Quantum heat engine, Thermal noise

Thesis available for public display 7 days prior to the defence at .