Masters Thesis Defense by Marcin Kepa

Title: Towards Spin Qubit Operations Without a Superconducting Magnet

Abstract: One of the typical elements of a spin qubit quantum computing experimental setup is a cryogenic superconducting magnet that is used for defining a quantization axis, splitting the two spin states of an electron or a hole.

In this work, we demonstrate a promising alternative to this, where the cryogenic magnet is replaced by a large-diameter external coil that is wrapped around the cryostat.

Taking advantage of a large out-of-plane g-factor of hole spin qubits in germanium, we observe that it is feasible to generate sufficient magnetic fields for qubit operations. We tune the spin qubit device into the double quantum dot regime, showcasing spin-related effects and establishing the necessary magnetic field strength of up to approximately 5 mT in the out-of-plane direction.

We calibrate the external coil by comparing its operation with a vector magnet, proving that this goal is achieved. We also demonstrate its practicality by studying the magnetization of a superconducting transmon chip mounted directly on the mixing chamber plate, taking advantage of the large volume of significant magnetic field offered by the large-diameter coil.

Together, these findings provide a clear path for future experiments and prove that it is a simple yet practical tool for spin hole qubits. It also provides a path towards scaling up towards devices with more qubits, as it circumvents the limitation of lines available on a cold finger, which is unavoidable bottleneck with small-diameter cryogenic vector magnets.

• Supervisor: Jens Paaske
• Co-Supervisor: William Lawrie
• Censor: Jørn Bindslev Hansen