Zero-energy states in Majorana nanowire devices

In the voyage towards solving increasingly challenging computations of physical systems, quantum computation has arisen as a contender for conventional computational approaches. To address the issue of keeping the required quantum mechanical states sufficiently stable against environmental disturbances, novel proposals suggested to employ topological quantum states, where information can be stored nonlocally, essentially by sharing the information over physically different locations. Because suitable topological states are elusive in existing materials, an approach of great interest is to engineer the required topological Majorana modes by combining a spin-orbit coupled semiconductor nanowire exposed to a magnetic field with a superconducting material: a Majorana nanowire. After the first experimental signs of Majorana modes were observed in 2012, it also became clear that the experiments showed deviations from the theoretical expectations and alternative interpretations were suggested. This dissertation explores the intricate physics that emerges in Majorana nanowires, with the aim to find improved Majorana signatures in transport experiments. By addressing disorder at the interface between the nanowire and the superconductor, we find Majorana signatures through the electrical transport through a ballistic tunnel junction, which allows us to exclude certain alternative explanations based on disorder. We also look into two key elements required to obtain Majorana modes: spin-orbit interaction and induced superconductivity. First, through measurements of the effect of a magnetic field and its direction on the size of the induced superconducting gap, we show that spin-orbit interaction counteracts the closing of the superconducting gap. This protection of the superconducting gap is ultimately responsible for the possibility of a topological nontrivial phase in nanowires. Second, we investigate the influence of an electric field in the nanowire on the coupling between electronic states in the nanowire and the superconductor and find that the electric field modifies the strength of the effective nanowire parameters essential to Majorana physics. Returning to the study of transport signatures of Majorana modes, we explore plateaus in the zero-bias conductance near the quantization value predicted for topological Majorana modes. Instabilities of the observed quantized plateaus on tunnel-barrier details indicate instead the presence of topologically trivial zero-energy states, which can be described as local Majorana modes and may offer an alternative route towards the demonstration of non-Abelian exchange statistics. Finally, we address the nonlocal distribution of Majorana nanowire zero-energy states through the modulation of the energy splitting due to a remote electrostatic gate decoupled from the tunneling barrier region. We identify states consistent with overlapping Majorana modes in a short nanowire. The dissertation is concluded by discussing interesting future avenues to solidify the understanding of Majorana nanowires and we indicating a possible alternative approach to demonstrate non-Abelian properties by deliberately stabilizing local Majorana modes.QRD/Kouwenhoven La

In-plane selective area InSb–Al nanowire quantum networks

Strong spin–orbit semiconductor nanowires coupled to a superconductor are predicted to host Majorana zero modes. Exchange (braiding) operations of Majorana modes form the logical gates of a topological quantum computer and require a network of nanowires. Here, we utilize an in-plane selective area growth technique for InSb–Al semiconductor–superconductor nanowire networks. Transport channels, free from extended defects, in InSb nanowire networks are realized on insulating, but heavily mismatched InP (111)B substrates by full relaxation of the lattice mismatch at the nanowire/substrate interface and nucleation of a complete network from a single nucleation site by optimizing the surface diffusion length of the adatoms. Essential quantum transport phenomena for topological quantum computing are demonstrated in these structures including phase-coherence lengths exceeding several micrometers with Aharonov–Bohm oscillations up to five harmonics and a hard superconducting gap accompanied by 2e-periodic Coulomb oscillations with an Al-based Cooper pair island integrated in the nanowire network.QRD/Kouwenhoven LabQuTechQRD/Goswami LabBUS/Quantum DelftQN/Kouwenhoven La

Author Correction: In-plane selective area InSb–Al nanowire quantum networks (Communications Physics, (2020), 3, 1, (59), 10.1038/s42005-020-0324-4)

The Data availability statement of this article has been modified to add the accession link to the raw data. The old Data availability statement read “Materials and data that support the findings of this research are available within the paper. All data are available from the corresponding author upon request”. This has been replaced by “Materials and data that support the findings of this research are available within the paper. The raw data have been deposited at https://zenodo.org/record/4589484#.YEoEOy1Y7Sd”. This has been corrected in both the HTML and PDF version of the article.Correction include: The Data availability statement of this article has been modified to add the accession link to the raw data.QRD/Kouwenhoven LabQuTechQRD/Goswami LabBUS/Quantum DelftQN/Kouwenhoven LabElectronic Components, Technology and Material

Electric field tunable superconductor-semiconductor coupling in Majorana nanowires

We study the effect of external electric fields on superconductor-semiconductor coupling by measuring the electron transport in InSb semiconductor nanowires coupled to an epitaxially grown Al superconductor. We find that the gate voltage induced electric fields can greatly modify the coupling strength, which has consequences for the proximity induced superconducting gap, effective g-factor, and spin-orbit coupling, which all play a key role in understanding Majorana physics. We further show that level repulsion due to spin-orbit coupling in a finite size system can lead to seemingly stable zero bias conductance peaks, which mimic the behavior of Majorana zero modes. Our results improve the understanding of realistic Majorana nanowire systems.QRD/Kouwenhoven LabQuTechApplied SciencesQN/Bakkers La

Ballistic superconductivity in semiconductor nanowires

Semiconductor nanowires have opened new research avenues in quantum transport owing to their confined geometry and electrostatic tunability. They have offered an exceptional testbed for superconductivity, leading to the realization of hybrid systems combining the macroscopic quantum properties of superconductors with the possibility to control charges down to a single electron. These advances brought semiconductor nanowires to the forefront of efforts to realize topological superconductivity and Majorana modes. A prime challenge to benefit from the topological properties of Majoranas is to reduce the disorder in hybrid nanowire devices. Here we show ballistic superconductivity in InSb semiconductor nanowires. Our structural and chemical analyses demonstrate a high-quality interface between the nanowire and a NbTiN superconductor that enables ballistic transport. This is manifested by a quantized conductance for normal carriers, a strongly enhanced conductance for Andreev-reflecting carriers, and an induced hard gap with a significantly reduced density of states. These results pave the way for disorder-free Majorana devices.QRD/Kouwenhoven LabQN/Conesa-Boj LabQRD/Wimmer LabQubit Research DivisionQN/Bakkers LabBUS/GeneralQRD/Goswami La

Quantized Majorana conductance

Majorana zero-modes - a type of localized quasiparticle - hold great promise for topological quantum computing. Tunnelling spectroscopy in electrical transport is the primary tool for identifying the presence of Majorana zero-modes, for instance as a zero-bias peak in differential conductance. The height of the Majorana zero-bias peak is predicted to be quantized at the universal conductance value of 2e 2 /h at zero temperature (where e is the charge of an electron and h is the Planck constant), as a direct consequence of the famous Majorana symmetry in which a particle is its own antiparticle. The Majorana symmetry protects the quantization against disorder, interactions and variations in the tunnel coupling. Previous experiments, however, have mostly shown zero-bias peaks much smaller than 2e 2 /h, with a recent observation of a peak height close to 2e 2 /h. Here we report a quantized conductance plateau at 2e 2 /h in the zero-bias conductance measured in indium antimonide semiconductor nanowires covered with an aluminium superconducting shell. The height of our zero-bias peak remains constant despite changing parameters such as the magnetic field and tunnel coupling, indicating that it is a quantized conductance plateau. We distinguish this quantized Majorana peak from possible non-Majorana origins by investigating its robustness to electric and magnetic fields as well as its temperature dependence. The observation of a quantized conductance plateau strongly supports the existence of Majorana zero-modes in the system, consequently paving the way for future braiding experiments that could lead to topological quantum computing.Accepted Author Manuscript This title has a addendum: editorial expression of concern, see Relations belowQRD/Kouwenhoven LabApplied SciencesIntegral Design and ManagementQN/Bakkers La