Public defence in Engineering Physics, M.Sc. Timm Mörstedt

The Quantum-Circuit Refrigerator allows for dissipation control, reset of quantum bits, and thermodynamic control of superconducting quantum circuits.

Public defence from the Aalto University School of Science, Department of Applied Physics.
Doctoral hat floating above a speaker's podium with a microphone

Title of the thesis: Fast Qubit Control with a Quantum-Circuit Refrigerator

Doctoral student: Timm Mörstedt
Opponent: Professor Christopher Eichler, Friedrich-Alexander-Universität Erlangen-Nürnberg, Germany
Custos: Professor Mikko Möttönen, Aalto University School of Science, Department of Applied Physics

Superconducting circuits have emerged as powerful building blocks on the path toward a useful quantum computer. However, fast and accurate control over these circuits remains one of the key challenges. In particular, the fast initialization of superconducting qubits is a growing requirement in this era of constantly increasing qubit lifetimes.

We investigate different means of qubit control in the context of dissipation engineering. We use a quantum-circuit refrigerator (QCR), an on-chip microcooler based on one or two normal-metal–insulator–superconductor junctions, to create a tunable environment for superconducting circuits. We present and compare two different realizations of this device, the double-junction QCR directly coupled to a transmon qubit and the single-junction QCR coupled to the qubit via a superconducting resonator.

Beyond qubit reset, we explore other properties of the QCR, including the cooling and creation of exceptional points in superconducting resonators and the generation of thermal states in superconducting qubits. Through single-shot readout experiments, we gain insight into the quantum state of the qubit and its dynamics in response to different control signals. Combining the results of these experiments, we discuss the possible realization of a quantum heat engine using a QCR as a two-way tunable environment, extending the scope of applications toward the fundamental study of open quantum systems.

These results shed light on the versatile world of quantum-circuit refrigeration and present novel insights, experiments, and applications. At the intersection of circuit quantum electrodynamics and quantum thermodynamics, the QCR promises further possibilities for advancement and increased understanding of the behavior and control of superconducting quantum systems.

Keywords: superconducting circuits, qubits, quantum thermodynamics

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