Master's Defense: Lazar Lakic

Connecting the dots: Laying the foundation for coupling Ge/SiGe quantum dots via PtSiGe superconductors

In this thesis, we work towards realizing an extended range link between two semiconductor quantum dot (QD) spin qubits, mediated by a superconductor, as proposed by M.Leijnse and K.Flensberg in 2013 [1]. This is achieved by fabricating and characterizing quantum dots in strained planar Germanium/SiliconGermanium (Ge/SiGe) heterostructures, alongside a novel approach to fabricating nanoscale Platino-Germano-Silicide (PtSiGe) superconducting structures [2]. We demonstrate single hole occupation of quantum dots and double quantum dots, making use of charge sensing techniques such as dynamic sensor compensation, and RF-reflectometry. Furthermore, we characterize nanoscale junctions between superconducting (S) and normal conducting (N) materials. Specifically, SNS- and NSN-junction measurements of PtSiGe superconducting nanostructures. In addition to this, we characterize a S-QD-S junction, constituting the first reported measurements of a proximitized hybrid quantum dot in the group-IV planar Ge/SiGe platform. By demonstrating these individual components, we provide scope for extended range interactions for quantum dot based spin qubits mediated by a superconductor.

[1] Leijnse, M., & Flensberg, K. (2013, August 6). Phys. Rev. Lett. 111, 060501 (2013) - Coupling Spin Qubits via Superconductors. Physical Review Link Manager. Retrieved August 17, 2023, from https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.111.060501

[2] Tosato, A., Levajac, V., Wang, J. Y., Boor, C. J., Borsoi, F., Botifoll, M., Borja, C. N., Martin-Sanchez, S., Arbiol, J., Sammak, A., Veldhorst, M., & Scappucci, G. (2023, 04 06). Hard superconducting gap in germanium. Communications Materials, 4(1), 9. doi.org/10.1038/s43246-023-00351-w