Quantum Properties in Hybrid Nanowire Devices

Quantum Properties in Hybrid Nanowire Devices

Xu, D. (TU Delft QRD/Kouwenhoven Lab)

Kouwenhoven, Leo P. (promotor)
Wimmer, M.T. (promotor)

Delft University of Technology

2022-05-12

Quantum computing is a flourishing field of scientific and technological research. The development of quantum computing in the past decades is the so-called second quantum revolution, where various aspects of quantum physics, such as entanglement and superposition, are used to form the main building block of computation — the quantum bit, or qubit. With quantum technologies rapidly growing, people have been paying attention to a few promising implementation schemes for quantum computing, including schemes based on topological protection.
Majorana bound states (MBSs) are predicted to be non-Abelian anyons that enable topological quantum computing, which uses the topological phase of matter to protect quantum information against noise from the environment. The search for MBSs has drawn enormous interests in condensed matter physics community, where hybrid semiconductor-superconductor nanowire systems are currently the most promising candidates. Great advances have been achieved in this field over the last decade, due to efforts ranging from material growth, to transport experiments and to theoretical understanding.
When a hybrid nanowire undergoes a transition to a topologically nontrivial phase, two MBSs appear at the ends of the hybrid region. As a result, zero-bias peaks (ZBPs) should appear in tunneling spectroscopy performed on normal-conductor - semiconductor-nanowire - superconductor (N-nanowire-S) junctions. In this thesis, we firstly demonstrate large ZBPs in vapor-liquid-solid (VLS) InSb nanowires with epitaxial Al with heights on the order of 2e²/h. Besides the original Majorana interpretation of these ZBPs, we discuss alternative explanations such as quasi-Majoranas due to a smooth potential and random disorder. (Chapter 4)
In the same system, we then demonstrate that the induced superconducting gap, the effective Landé g-factor and the spin-orbit coupling strength can be tuned by the electrostatic environment, i.e. applied gate voltage. The change of these quantities is dominated by the coupling between the semiconductor and the superconductor. (Chapter 5)
The remaining part of the thesis focuses on selective area growth (SAG) InSb nanowires with epitaxial Al. Quantum transport results on these nanowires show high-quality phase-coherence, hard superconducting gap and 2e-Coulomb blockaded transport. We then study the properties on induced superconductivity as well as phase coherence in SAG nanowire networks. We establish a fitting model to extract the phase coherence length based on the temperature dependence of the Aharonov-Bohm (AB) effect.
The SAG platform will allow scalable experiments for more complicated quantum transport, paving the way towards Majorana braiding. (Chapters 6-8)

Majorana
hybrid device
semiconductor nanowire
InSb
superconductivity
Andreev bound state
selective area growth
Aharonov-Bohm effect

https://doi.org/10.4233/uuid:26c9e59f-5d64-40a4-adb9-cabe2c272bcb

978-90-8593-519-3

Institutional Repository

doctoral thesis

© 2022 D. Xu