Shadow-wall lithography as a novel approach to Majorana devices

Abstract

The development of quantum computers is perhaps one of the most exciting innovations of our time. The most investigated quantum computers, however, suffer from the fact that quantum information is lost due to interaction between the quantum bits and their environment. As a radically different approach, it has been proposed that one can instead use topological phases of matter to create quantum bits that are immune to environmental noise. The most prominent example of such a topological state of matter is the topological superconductor, which hosts Majorana zero modes. These quasiparticles can be used to store information non-locally, and their non-abelian exchange statistics allow for the implementation of protected quantum gates. Their postulated appearance at the edges of a one-dimensional semiconductor coupled to a superconductor has been a hot research topic over the last decade. Yet, their claimed observation in condensed-matter experiments has not been unequivocal. While the experiments produce some of the signatures of Majorana zero modes, they often exhibit significant deviations from the theory. The main obstacle here is that one of the fundamental properties of Majorana zero modes, namely their non-locality, has not yet been accessible due to the design of these experiments.

In this thesis, we have developed shadow-wall lithography as a novel approach to Majorana devices. One of the key concepts of this technique is to move the majority of the required nanofabrication steps prior to the formation of a semiconductor-superconductor hybrid, which significantly improves the performance of the device. Moreover, the shallow-angle deposition of a thin superconducting film allows the hybrid section to be grounded. This facilitates the simultaneous investigation of both ends of the device, enabling the search for the predicted end-to-end correlation of the Majorana zero modes. We extend the fabrication improvements by also considering the material used in these devices. For their operation, a magnetic field is required, which quenches the superconductivity in the superconducting film due to both orbital and paramagnetic effects. The paramagnetic effects are suppressed through the use of Pt impurities, which provide spin-orbit scattering centers in the film. For the thinnest films, we are able to extend the critical magnetic field up to 7 T. We further demonstrate that the inclusion of Pt does not prevent the quantum states in the semiconductor from obtaining a Zeeman splitting. We combine the improved nanofabrication technique and material developments with novel measurement schemes, such as the use of radio-frequency reflectometry and non-local conductance spectroscopy. The former allows us to map out large regions of the available experimental parameters while looking for the predicted end-to-end correlation of zero energy states. We demonstrate that such correlations are lacking in these devices, indicating that they do not exhibit an extended topological superconducting phase with Majorana zero modes at their ends. With non-local measurements, we instead focus on the induced superconducting gap in the bulk of such a hybrid. We demonstrate a significant tunability through electrostatic gating and show a closing and reopening of the induced gap, though the absence of zero-bias peaks also indicates that this is not due to an extended topological phase transition. These experiments strongly suggest that the realization of a topological superconductor in semiconductor-superconductor hybrids requires monumental efforts in the development of better materials.

While the bulk of this thesis is devoted to the creation of a topological superconductivity, the final chapters take an alternative approach. We demonstrate that these hybrids possess all the necessary ingredients to form a topological superconductor by using the shadow-wall lithography technique to realize an artificial Kitaev chain. By coupling two quantum dots via a gate-tunable proximitized quantum state in the hybrid segment, we show that the system can be brought to a sweet spot that hosts unpaired Majorana zero modes. To demonstrate the versatility of the developed platform, we finally move away from the study of Majorana zero modes and instead focus on the superconducting diode effect. We show that the tunability of the superconducting properties in a hybrid segment can be used to control the presence and magnitude of the superconducting diode effect in short nanowire Josephson junctions. These two chapters offer an inspiring perspective on the future of semiconductor-superconductor hybrid devices.