Zhaoen Su

  • Graduate Student
OFF CAMPUS

Research

PhD Defense - Abstract

Andreev bound states in superconductor-quantum dot chains can provide a platform for  quantum simulation and topologically protected quantum computation. This thesis focuses on quantum transport in  superconductor-semiconductor nanowire hybrid structures. With InSb nanowires, we study Andreev bound states in single, double and triple dot chains. We first implement highly tunable single quantum dots in nanowires coupled to superconductors facilitated by local gates and transparent contacts. We explore the tunneling resonance of Andreev bound states in a wide parameter regime: from co-tunneling regime to spinfull singlet Andreev bound states, and find simultaneous transitions of superconducting and normal transports as the dot is tuned to be strongly coupled to the superconductor. In the open dot regime we investigate the  zero bias feature that is strongly relevant to Majorana zero modes based on continuous nanowire sections. With two copies of this superconductor-quantum dot structure, we study the hybridization of Andreev bound states in a double dot. We observe tunneling spectra of the hybridized Andreev bound states and resolve their spin structure. Finally we implement a chain made of three superconductors and three quantum dots in series. Each dot is strongly coupled to a superconductor and has a single electron near the superconductor chemical potential. Spectroscopy measurement demonstrates resonances through Andreev bound states in the triple dot. A zero-bias peak is observed when a magnetic field is applied and it sustains in magnetic fields for a wide range, which can provide a signature of Majorana zero modes in this chain structure. We also evaluate the potential of Ge/Si core/shell nanowires for the realization of Majorana zero modes. To that end we  establish three of the necessary ingredients for realizing Majorana zero modes based in nanowires: we achieve induced superconductivity from NbTiN, we estimate spin-orbit coupling (lSO » 100-500 nm) based on spin blockade, and we measure g-factors (up to 8) in Ge/Si double dots.

Dissertation

Thesis Title

Andreev bound states in superconductor-quantum dot chains

Graduate Advisor

Sergey Frolov