Majorana zero modes (MZMs) are a topic of intense research as they constitute the main building block of topological qubits - a qubit type with potentially enhanced coherence time. A promising way to create these quantum states is to couple a one-dimensional (1D) semiconducting segment with spin-orbit interaction to a superconductor, in the presence of an external magnetic field. Growing the active semiconductor as a 2D layer and creating 1Dstructures by top-downprocessing might allowto realize complex multiqubit devices in the future. This thesis explores antimony-based two-dimensional electron gases (2DEGs), known for their favorable material properties, as platforms for topological superconductivity....@en

Quantum computers can solve some problems exponentially faster than classical computers. Unfortunately, the computational power of quantum computers is currently limited by the number of working qubits. It is difficult to scale up these systems, because qubits are easily affected by noise in their environment. This noise leads to decoherence: loss of the qubit’s encoded information. A possible solution to diminish decoherence is using Majorana box qubits, as these qubits are predicted to be insensitive to local noise. However, this promising type of qubit does not exist yet. With the research described in this thesis, we aim to contribute to the development of Majorana box qubits (MBQs). In these qubits, Majorana zero modes, the basic elements of MBQs, are contained within a superconducting island to suppress Majorana parity fluctuations caused by quasiparticle poisoning. To enable parity readout of the MBQ, these modes are coupled to quantum dots within a nanowire network. To help realize MBQs, we need a better understanding of quasiparticles in superconducting islands, parity-readout techniques, and ways to fabricate nanowire networks. These three aspects are the focus of the experiments presented in this thesis. To study superconducting islands and readout techniques, we used InAs semiconductor nanowires with an epitaxially grown Al shell. Majorana signatures have already been observed in such nanowires. We addressed quasiparticle dynamics in superconducting islands by measuring the gate-charge modulation of the switching current. We found a consistent 2e-periodic modulation at zero magnetic field, and an exponential decrease of parity lifetime with increasing magnetic field. We explored MBQ readout, using a quantum dot level as a proxy for a Majorana zero mode, and measured its charge hybridization with another dot using gate-based readout. We showed that we can rapidly discriminate between two settings with different tunnel couplings, demonstrating the potential of gate-based readout to measure MBQs. And, using gate-based readout, we could study charge-transfer processes occurring in hybrid structures of superconducting islands coupled to quantum dots. Finally, to find a good material platform for nanowire networks, we characterized two two-dimensional systems. We realized quantum point contacts in InSb, which we used to measure the $g$-factor anisotropy, and effective electron mass in this system. And, we studied the spin-orbit interaction in InAs/GaSb by extracting the difference in density between electrons with different spin orientations. This thesis finishes with a proposal for a series experiments to realize MBQs. These experiments make use of superconducting islands and the reflectometry setup we developed for gate-based readout. @en

The search for Majoranas bound states has witnessed heated efforts in the past decade. This field of research lies at the intersection of both scientific and commercial interests. The Majorana quasiparticle, being its own antiparticle and exhibiting non-abelian exchange statistics, is a unique member of the family of condensed-matter quasiparticles, distinct from most fermions or bosons. These properties are predicted to be instrumental in the building of a new type of qubits, having no energy splitting between qubit states and intrinsically protected from decoherence. In addition, the theory describing Majorana modes has a rich connection to the mathematical language of topology, making its study also of theoretical value. Thus, the prediction of the existence of Majorana zero modes in hybrid semiconducting-superconducting nanowires has been a strong driving force behind the recent technological progress in the making of these materials and devices. In this thesis, the most recent advance in materials, specifically the making of clean interfaces between semiconductors and superconductors, are applied to the study of the physical properties of superconducting-proximitized electronic states in semiconductors. This technology is combined with quantum dot techniques to investigate electron transport between individual quantum states in proximitized nanowires. The findings include better understanding of electron transport in these systems as well as presenting new potential applications to the field of Majoranas and beyond. Following the introductory chapters, this thesis first demonstrates a high-efficiency Cooper-pair splitter, enabled by quantum dots with narrow linewidth and a superconductor with a hard gap. The techniques behind the improved efficiency can be used to make a generator of entangled pairs of electrons. We also demonstrate the use of quantum dots as spin detectors capable of revealing the spin structure of individual Cooper pairs. Next, we report the effect of a Cooper-pair splitter's peculiar response to the tuning of electrical gates in both experiment and theory. This includes the discovery of a new interference effect in electron co-tunneling processes through a superconductor. The key to observing this response is to ensure the hybrid nanowire is also a discrete quantum state instead of a superconducting bulk. The discovery above forms the foundation of fine-tuning the types of electron couplings between two quantum dots coupled via a superconductor. The power of this tunability can been seen via the successful making of a minimal artificial Kitaev chain, opening up new possibilities in the search for Majorana zero modes. This approach is less prone to difficulties encountered in other platforms such as material disorder and the interpretability of data. Moving from studying quantum dots under the influence of a superconducting hybrid, later chapters of this thesis focus on investigating electron properties in the hybrid nanowire using quantum dots as spin-, charge- and energy-selective probes. We first use them to detect and quantify the spin polarization of Andreev bound states in the hybrid nanowire. Using quantum dots as charge and energy detectors instead, we observe how electrons traverse through the bulk of a hybrid nanowire and reveal a thermoelectric conversion process in the conductance measurements of these devices. Finally, we report on the selective-area growth of InSb, the semiconductor used throughout this thesis, that can form the basis of future developments.@en

A topological superconductor is a new state of matter that attract a lot of interest for its potential application in quantum computers. However, there is no single material known to host this state of matter. In this thesis, combinations of superconductors and semiconductors are investigated experimentally with the goal to engineer such a topological superconductor. The materials chosen combine spin-orbit interaction, superconductivity and onedimensionality. Then, under influence of a magnetic field, the hybrid superconductor semiconductor system is predicted to become topological. First, the theoretical background of the experiments is presented, with special attention to the superconducting quantum interference in semiconducting Josephson junctions. In addition, a description of the different materials used and the fabrication of the devices, is provided. In the first experiment we explore hole transport through GeSi core-shell nanowires. Electronic measurements reveal two transport channels only, which underlines the onedimensionality of the nanowire. On top of that, high-quality induced superconductivity is observed in both the tunneling and open regime, and evidence for strong spin-orbit interaction is presented. Then, we switch materials to a two-dimensional electron and hole gas in an InAs/GaSb double quantum well. The spin-orbit interaction is studied by measuring the difference between the densities of electrons with opposite spin orientation. Two types of spin-orbit interaction are identified by tuning the magnitude of one of them, with an applied electric field. InAs quantum wells are known to exhibit enhanced conduction at their edges. We find supercurrent through these edges in Josephson junction devices using superconducting quantum interference measurements. The interference pattern reveals a flux periodicity of h/e. Interestingly, while this periodicity is observed in the trivial regime, it was considered a signature of topological superconductivity before. We argue and show that nonlocal processes lead to the h/e effect in our devices. The correlated occurence of enhanced edge conduction and the h/e periodicity is confirmed in Josephson junctions made of InSb flakes. The final experimental chapter considers a superconducting quantum interference device, fabricated in an InAs quantum well. This geometry allows for control of the superconducting phase difference of the Josephson junction, potentially reducing the magnetic field needed for the device to become topological. Unfortunately, in the measurements we do not observe signatures of topological superconductivity. At last, we describe what device geometry and material combination could be used to do reach the topological regime. In addition, we discuss ideas for future research of the othermaterial systems used in this thesis.@en