BSc defense for Thorbjørn Clausen

Title: Single qubit gates beyond the rotating wave approximation

Abstract: The single-qubit gate is among the most fundamental building blocks for quantum computing and is used in a wide array of methods in high consecutive numbers. As such, the high fidelity of these is paramount, such as to reduce cumulative error.

Recently, the rotating wave approximation (RWA) (which is present in most traditional techniques for achieving high fidelities) has come under challenge by the adoption of low-frequency and high anharmonicity qubits like the fluxonium. Specifically, the assumption of a gate taking place over many Larmour periods has been challenged, and this has led to recent work redeveloping the quantum dynamics of gates outside this regime.

This work aims to review these methods for post-RWA pulses: commensurate, co-rotating, and Magnus-stroboscopic, and specifically learn more about how they apply to a range of qubit anharmonicities. Furthermore, pulse envelope shaping based on FAST is formulated to provide an alternative approach for these post-RWA pulses. All relevant methods are implemented and tested numerically, and results are commented upon.

Supervisor: Morten Kjaergaard