Periodic Reporting for period 4 - TOPOQDot (A bottom-up topological superconductor based on quantum dot arrays)
Reporting period: 2021-12-01 to 2023-05-31
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.
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.
The first main achievements of the project were to set up a new quantum transport laboratory and to develop most of the fabrication processes completely from scratch. The HI delivered a new facility for the development of the action in June 2019 and the main experimental instrumentation was delivered and installed in the following months. While this was taking place, the research group worked towards the fabrication of the devices envisioned in the project (despite the basic fabrication infrastructure at the HI, most required fabrication processes had not been implemented/optimized due to the absence of other experimental groups with similar fabrication needs).
The above developments allowed the research team to explore two distinct material platforms for the devices: nanowires and two-dimensional electron gases. Each of these systems displays their own set of advantages/disadvantages and, as such, we aimed to find the most suitable for implementing the project. The goal was to develop devices with a robust superconducting proximity effect and minimal disorder. We have found the most promising platform to be InAs nanowires with epitaxial superconducting Al shells. From this point on, the research team worked in two parallel directions. On the one hand, we have carefully investigated the nanowires and related devices to build an understanding of the physics of this material system. At the same time, we have studied devices with geometries targeting the original objectives of the project. Concerning the latter activities, we concluded that the disorder level in our devices was not sufficiently low for implementing our original proposal. By contrast, we have uncovered a rich physics concerning epitaxial InAs-Al nanowires, including some effects that could have important implications for hybrid superconductor-semiconductor devices in general. Specifically, we have identified severe heat bottlenecks that could lead to increased temperatures in such devices and ultimately affect their operation. To this end, we have developed a new technique that we named Joule spectroscopy, capable of unraveling heat dissipation mechanisms and with potential as a powerful characterization tool for this type of hybrid device. We note that this work opens several interesting research possibilities for the future, considering that these effects had not been previously explored. In addition, we have also obtained interesting results regarding the behavior of the epitaxial Al shell of the wires, and of devices based on full-shell InAs-Al nanowires.
While some of the results of this projet have already been published, other manuscripts are currently in preparation and the remaining experimental data is being analyzed for future publication. Furthermore, our results have been disseminated in national and international scientific conferences and workshops, with a growing number of contributions following the improvement of the pandemic and the increased scientific output of the project. Dissemination to non-academic audiences or to undergraduate-level students has been carried out in the form of invited talks and participations in events. The PI has also participated in the organization of a Summer School on Quantum Transport in Topological Materials. The school was a remarkable success, having been attended by around 100 students and researchers from around the world.
The above developments allowed the research team to explore two distinct material platforms for the devices: nanowires and two-dimensional electron gases. Each of these systems displays their own set of advantages/disadvantages and, as such, we aimed to find the most suitable for implementing the project. The goal was to develop devices with a robust superconducting proximity effect and minimal disorder. We have found the most promising platform to be InAs nanowires with epitaxial superconducting Al shells. From this point on, the research team worked in two parallel directions. On the one hand, we have carefully investigated the nanowires and related devices to build an understanding of the physics of this material system. At the same time, we have studied devices with geometries targeting the original objectives of the project. Concerning the latter activities, we concluded that the disorder level in our devices was not sufficiently low for implementing our original proposal. By contrast, we have uncovered a rich physics concerning epitaxial InAs-Al nanowires, including some effects that could have important implications for hybrid superconductor-semiconductor devices in general. Specifically, we have identified severe heat bottlenecks that could lead to increased temperatures in such devices and ultimately affect their operation. To this end, we have developed a new technique that we named Joule spectroscopy, capable of unraveling heat dissipation mechanisms and with potential as a powerful characterization tool for this type of hybrid device. We note that this work opens several interesting research possibilities for the future, considering that these effects had not been previously explored. In addition, we have also obtained interesting results regarding the behavior of the epitaxial Al shell of the wires, and of devices based on full-shell InAs-Al nanowires.
While some of the results of this projet have already been published, other manuscripts are currently in preparation and the remaining experimental data is being analyzed for future publication. Furthermore, our results have been disseminated in national and international scientific conferences and workshops, with a growing number of contributions following the improvement of the pandemic and the increased scientific output of the project. Dissemination to non-academic audiences or to undergraduate-level students has been carried out in the form of invited talks and participations in events. The PI has also participated in the organization of a Summer School on Quantum Transport in Topological Materials. The school was a remarkable success, having been attended by around 100 students and researchers from around the world.
Hybrid superconductor-semiconductor devices have attracted great interest owing to their potential for applications in quantum technologies, both in the trivial and topological regimes. Despite the intensive activity in this research direction, many aspects of the underlying physics are still not fully understood. While exploring an alternative approach for the realization of a topological superconductor, TOPOQDot has led to interesting discoveries that contribute to the development of these hybrid devices. Most notably, we observed that heating effects are considerable in these systems owing to the poor thermal conductivity of superconductors and the inefficiency of other cooling mechanisms (e.g. electron phonon coupling). Consequently, hybrid superconductor-semiconductor devices are particularly susceptible to heating even when low powers are applied, an aspect that is not typically considered in experiments. By studying these effects, we have uncovered the dominant dissipation mechanisms in different device geometries and have developed a new technique (Joule spectroscopy) for the characterization of hybrid devices.