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Title of the thesis: Superconducting flux qubit for quantum thermodynamics experiments

Thesis defender: Rishabh Upadhyay
Opponent:  Associate Professor Thilo Bauch, Chalmers University, Sweden
Custos: Prof. Jukka Pekola, Aalto University School of Science

In this thesis, a quantum thermal device is developed using a superconducting flux qubit, which forms the central element, integrated with superconducting resonators and metallic reservoirs on a silicon platform. The setup allows precise control and study of heat flow at the quantum level, with the flux qubit mediating energy transfer between the reservoirs through excitation and relaxation cycles.

Experiments harnessing strong and ultra-strong qubit-resonator coupling regimes are intriguing not only from a theoretical perspective but also in the experimental domain, where realization remains less explored, especially in the context of quantum thermodynamics. An initial focus of the work is designing a strong and stable coupling architecture between the qubit and resonators, achieving the ultra-strong coupling regime experimentally. The architecture is then extended to connect the qubit to two resonators on either side, each linked to a thermal reservoir, enabling controlled heat transport—a first demonstration of flux qubit-mediated thermal flow with high efficiency.

Additionally, the work introduces geometric asymmetry in the couplers and leverages the qubit nonlinearity to realize a non-reciprocal device, the Microwave Quantum Diode, paving the way for potential applications in thermal management at the quantum scale.

The thesis concludes with an outlook on future experiments and applications, using this flux qubit-based platform to explore novel quantum thermodynamic phenomena and develop advanced thermal devices.

Keywords: Quantum thermodynamics, Thermometry, Quantum electrodynamics, Superconducting flux qubit, Strong coupling regime, Superconducting circuits, Mesoscopic devices.

Contact information: Rishabh.Upadhyay@aalto.fi 

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