Public defence, Engineering Physics, M.Sc. Christoforus Dimas Satrya

Title of the thesis: Thermal sensing for quantum thermodynamics experiments on superconducting circuits

Thesis defender: Christoforus Dimas Satrya
Opponent: Professor Tero T. Heikkilä, University of Jyväskylä, Finland
Custos: Professor Jukka Pekola, Aalto University School of Science

Superconducting circuits provide a controllable and versatile platform for studying quantum phenomena and advancing quantum technologies. Josephson junctions and resonators are the central circuit elements, as they enable the realization of macroscopic quantum systems—such as qubits—that can be conveniently controlled, probed, and integrated with non-superconducting components. The implementation of a thermal reservoir through the integration of a dissipative element, such as a normal-metal resistor, offers an experimental platform for investigating quantum thermodynamics, heat management in circuits, on-chip thermal detectors, and the realization of quantum thermal machines. 

In this thesis, we integrate a mesoscopic normal metal and its associated thermometers as an on-chip bolometer within circuit quantum electrodynamics devices for thermal detection and quantum thermodynamics experiments. The on-chip bolometer is utilized as a sensitive and broadband thermal spectrometer to characterize the properties of a superconducting resonator and to investigate its dominant loss channels. We demonstrate the capability of our thermal detector to resolve the resonator modes over a wide frequency range extending above 20 GHz. When two thermal baths are connected through a qubit–resonator system, a quantum heat valve can be realized. This device has been demonstrated using both transmon and flux qubits. To model these experiments, we propose an approach based on electromagnetic simulation and microwave circuit theory, restricted to the linear-response regime. Despite this limitation, the method successfully captures the experimental observations and provides a practical tool for analysing and designing circuits for future experiments. 

A periodically driven qubit coupled to two unequal resonators, each terminated by a heat reservoir, enables the realization of a quantum Otto refrigerator. We explore alternative platforms for the experimental implementation of a quantum refrigerator based on charge and flux qubits, which exhibit strong anharmonicity. As an important step, we perform experiments on a driven flux qubit galvanically coupled to a resonator that is, in turn, weakly capacitively coupled to a heat reservoir. We observe interference patterns in the heat current arising from driving-induced coherence. These quantum features are governed by the relative phase accumulated by the qubit states under driving. 

Keywords: Quantum thermodynamics, thermal detector, bolometer, quantum heat transport, mesoscopic devices, superconducting circuits and qubits.

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

Contact information: 
christoforus.satrya@aalto.fi 
https://www.linkedin.com/in/christoforussatrya/ 
https://pico.aalto.fi/