Topological materials constitute an exciting research field directed at the investigation of novel materials with potential for applications in electronics, spintronics and quantum technologies. These materials present a bulk with “conventional” behavior but exhibiting boundaries with robust states with exotic properties. Topological superconductors, for example, are associated with boundary states known as Majorana zero modes, which have attracted great attention due to the prospect of realizing qubits that are resilient against local sources of noise, differently than other existing technologies. If these expectations are confirmed, Majorana-based topological qubits could pave the way for obtaining a universal quantum computer.
While topological superconductors are not readily available in nature, theory predicts that they can be realized by combining different existing materials. In this context, one of the most explored approaches makes use of a hybrid combination of conventional superconductors, such as Al, and nanoscale semiconductors, typically InAs or InSb. Interestingly, these hybrid superconductor-semiconductor systems also hold promise for other non-topological applications in quantum technologies. Despite the experimental advances, a fully conclusive demonstration of Majorana modes is still missing. This can be mainly attributed to disorder in the above materials and related devices. TOPOQDot proposed to experimentally explore an alternative route for the realization of a topological superconductor that is expected to be more robust against disorder. This approach relies on assembling a topological superconductor from the bottom up using simpler building blocks known as quantum dots, which potentially allows the effects of disorder to be minimized. The objectives of TOPOQDot were to develop hybrid superconductor-semiconductor devices for the alternative route above, to study how to tune quantum dots for assembling a topological superconductor, and finally to observe signatures of Majorana modes. We have concluded from this project that disorder indeed plays a key role in hybrid superconductor-semiconductor systems, and it can be a limiting factor even for quantum dot-based approaches towards topology. At the same time, the project has led to the investigation of new phenomena related to the physics of hybrid superconductor-semiconductor nanowires, and to uncovering important heating effects in hybrid devices. Overall, the results contribute to the development of quantum devices based on superconductor-semiconductor hybrids.
