Public defence in Engineering Physics, M.Sc. (Tech) Eric Hyyppä

Title of the thesis: High-fidelity elementary operations for superconducting quantum computers

Doctoral student: Eric Hyyppä
Opponent: Professor Andrew Houck, Princeton University, USA
Custos: Professor Mikko Möttönen, Aalto University School of Science, Department of Applied Physics

The thesis presents new methods to improve the speed and fidelity of operations in superconducting quantum computers. Some of the proposed techniques have already been taken into use by IQM Quantum Computers.

Quantum computers carry out computations by harnessing fundamental quantum mechanical phenomena, such as superposition and entanglement, which, in theory, enables an exponential speed-up in certain computational problems compared to classical computers. In the future, quantum computers may enhance, for example, material and molecular simulations and the solution of certain optimization problems. At low level, a quantum computation can be broken down to a series of elementary operations, which include the initialization of quantum bits, i.e., qubits, single- and two-qubit logic gates, and the measurement of the qubits. Even though there has been tremendous progress in the development of quantum computer hardware over the past decades, the elementary operations of current quantum computers are still too error-prone to outperform classical supercomputers in tasks of practical relevance.

In this thesis, we study novel methods for improving the speed and fidelity of elementary operations in superconducting quantum computers, in which the qubits are realized as superconducting circuits controlled with electric and magnetic signals. We focus especially on improving single- and two-qubit logic gates, and qubit initialization. To this end, we develop both new superconducting-circuit components and improved methods for qubit control. Most importantly, we demonstrate a new superconducting qubit, the unimon, and novel analytical control pulse shapes for implementing fast qubit control. The unimon qubit has several favorable properties for the implementation of high-fidelity quantum logic gates, including a high anharmonicity, protection against low-frequency charge noise, and a simple structure. The new analytical control pulse shapes significantly reduce leakage errors to non-computational states, which allows the implementation of faster high-fidelity single-qubit gates than previously possible. In addition, we study the application of a quantum-circuit refrigerator for the initialization of superconducting circuits and the implementation of high-fidelity two-qubit logic gates in a scalable quantum processing unit architecture. In conclusion, the presented methods help to build more accurate quantum computers on the journey towards quantum advantage.

Key words: Quantum computer, superconducting quantum circuit, qubit, quantum operation, single-qubit gate, two-qubit gate, leakage error, quantum-circuit refrigerator

Thesis available for public display 10 days prior to the defence at: https://aaltodoc.aalto.fi/doc_public/eonly/riiputus/

Contact information:

Doctoral theses at the School of Science: https://aaltodoc.aalto.fi/handle/123456789/52