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A bottom-up topological superconductor based on quantum dot arrays

Periodic Reporting for period 3 - TOPOQDot (A bottom-up topological superconductor based on quantum dot arrays)

Reporting period: 2020-06-01 to 2021-11-30

Topological materials constitute an exciting and very active field of research in condensed matter physics. It relies on the fact that topology, a concept borrowed from mathematics and related to the properties of objects that are conserved upon continuous deformation, can have important implications in the electronic properties of materials. As a consequence, topological materials present bulk properties that are similar to those of ordinary materials while displaying, at the same time, exotic, robust boundary states. 1D topological superconductors, for instance, are associated with fascinating states known as Majorana modes. In the past decade, this type of material has attracted a great deal of attention, largely due to expectations that Majorana modes could lead to qubits that are protected against local sources of noise. If these expectations are confirmed, Majorana-based topological qubits would pave the way to quantum computers that are robust against decoherence.

While topological superconductors are not readily available in nature, theory predicts that they can be engineered by combining different materials and physical effects. Perhaps the most explored approach for realizing a 1D topological superconductor relates to the hybrid combination of superconductors and semiconductor nanowires with strong spin-orbit interaction in the presence of an external magnetic field. In spite of the great experimental advances reported in the past years, a fully conclusive demonstration of a topological superconductor and Majorana modes is still lacking. TOPOQDot investigates an alternative route towards the realization of a 1D topological superconductor: to controllably assemble it from an array of quantum dots with induced superconductivity. Indeed, theory suggests that the subgap states of a quantum dot coupled to a superconductor, also known as Shiba states, are natural precursors towards this goal. The approach offers the advantage of minimizing the effects of disorder, as the tuning of the quantum dot array can be corrected by means of electrostatic gating and applied magnetic fields. In addition, the entire evolution of the trivial subgap states into Majorana modes can be followed during the tuning, thus providing an unambiguous demonstration of their realization. The main objectives of the project are: to study the hybridization of Shiba states into molecular levels, to obtain robust signatures of Majorana modes in a triple quantum dot geometry, to study the properties of the detected subgap states, and to scale to longer and non-linear arrays.
A first important achievement during this period was the successful set up of new laboratory facilities fully dedicated to the development of the project. Indeed, when the project first started, I did not have access to an exclusive workspace for my research group. Supported by my host institution, I have invested time in designing a new lab space with the appropriate infrastructure for carrying out the low-noise, low temperature transport experiments that are a focus of the project. Since the laboratory was finally delivered, my research group and I have installed our main experimental equipment, including a state-of-the-art dilution refrigerator, whose base temperature is below 7 mK and that is equipped with a triaxial vector magnet. The latter equipment is essential for performing the experiments in the project. Importantly, the new laboratory allowed me to set up a new and independent research line within the host institution.

Another important achievement concerns the sample preparation, which is a crucial part of the project. In order to prepare the envisioned hybrid devices, wherein low-dimensional semiconductors are coupled to superconductors, we employ state-of-the-art nanofabrication techniques, such as e-beam lithography and metal deposition techniques. These are needed to achieve the nanometer-scale features required in our devices. My research group and I have established a full process flow for the fabrication of hybrid devices based on semiconductor nanowires, as well as many of the processes required for the fabrication of devices based on two-dimensional electron gases. These developments put us in a good position to perform the experiments planned in the project.

While setting up the new lab space and the nanofabrication in the host institution, our scientific output benefited from internal and external local collaborations. This includes a published article reporting on a joint theoretical and experimental study of replicas of bound states in nanowire devices, and a review article discussing trivial and topological sub-gap states in hybrid nanowires. The results have been disseminated in international scientific conferences and workshops. In addition, I have participated in the organization of a Summer School on Quantum Transport in Topological Materials. The school was a great success, having been attended by around 100 students and researchers from around the world.
Hybrid devices based on superconductors and low-dimensional semiconductors have attracted a growing interest in the past decade owing to their potential for quantum technology applications. Indeed, they are studied as a platform for realizing sources of entangled electrons, superconducting and topological qubits, etc. A common characteristic shared among these different devices is the fact that their operation strongly depends on the corresponding spectrum of subgap (Andreev) states. The main scientific progress expected from this project is to obtain a better understanding of Andreev bound states in the trivial and topological regimes of hybrid superconductor-semiconductor nanostructures. By studying the hybridization of these subgap states in the trivial regime, we aim to shed light on the fundamental mechanisms involved in the formation of a 1D topological superconductor and of Majorana modes, which are special Andreev states with potential application in topological quantum computing. The main expected results are: (i) demonstration of the hybridization of Shiba states in a double dot geometry, from which we will learn the basic mechanisms involved in the tuning of quantum dot arrays, (ii) demonstration of robust signatures of Majorana modes in a triple quantum dot geometry, as predicted by theory, and (iii) investigation of longer and non-linear quantum dot arrays.
Hybrid superconductor-semiconductor nanowire device for realizing quantum dot arrays