After a century from the quantum description of nature, the scientific community has laid the basis for using nature's properties to our advantage. The quantum technology vision stems from the idea of capitalizing these principles in various sectors, such as computation and communication. However, in contrast to classical processors, encoding and processing quantum information suffer from the quantum states' fragility to environmental disturbances. To mitigate their susceptibility, disruptive proposals suggested encoding information in non-local degrees of freedom such as in pairs of delocalized Majorana modes in topological superconductors. Although these materials remain elusive in nature, it is possible to engineer solid-state devices with the same properties such as semiconducting-superconducting nanowires. Starting from this idea, experimental signatures of zero-energy Majorana modes have been accompanied in recent years by continuous theoretical validations and rejections. The refinement in the theoretical understanding aligns with the swift advances on the experimental side, and this thesis finds its place in this phase of advancement, focusing on the intricate physics of the building blocks of Majorana qubits and proposing solutions to various nanofabrication challenges. In particular, we consider with attention the challenge of reading out the Majoranas information by detecting changes in their transmission phase. To this purpose, the minimal circuit requires a phase-coherent interferometer embedding a semiconducting-superconducting segment. Despite the apparent simplicity of this experiment, the Majoranas fingerprint in the transmission phase remains mostly unexplored due to the complexity of the circuit building blocks. Motivated by this challenge, our quest begins by considering each piece of the puzzle separately. We start by exploiting recent breakthroughs in the growth of nanowire-based interferometers to study the transmission phase of a large quantum dot, a setup similar to the one required for the Majoranas read-out. The conductance of this Aharonov-Bohm loop manifests gate- and magnetic field-tunable Fano resonances, that arise from the interference between electrons that travel through the reference arm and undergo resonant tunnelling in the dot. This experiment serves to point out the limitations of the currently available nanowire networks and provide critical insights for future topological interferometers' design. Thereafter, we explore the intricate physics of Coulomb semiconducting-superconducting wires, commonly known as hybrid island devices. Here, we demonstrate for the first time that InSb nanowires coupled to superconducting Al films manifest charging mediated by Cooper pairs of electrons. This observation implies that the low-energy spectrum of the semiconductor is fully proximitized by the superconductor, a fundamental requirement for achieving parity control in topological circuits. Starting from a Cooper pair condensate with an even electron parity, we can tune the nature of the island ground state with experimental knobs such as magnetic field and gate voltages. In particular, when a spin-resolved subgap state moves from the edge of the induced gap down to zero energy, single electrons can charge the island leading to conductance oscillations with a gate-voltage periodicity halved than for Cooper pairs. By mapping out such a 2e-to-1e transition in large ranges of gate voltage and magnetic field, we identify potential topological regions where the 1e oscillations are caused by discrete subgap states oscillating around zero energy. Part of the challenges concerning the realization of scalable hybrid devices lies in the complexity of their nanofabrication and the open questions in the material science involved. Stimulated by these interrogatives, the second part of this thesis introduces significant advances in the arena of hybrid nanowire devices. Having so far dealt with InSb nanowires with a maximum length of 3 µm, we turn our attention to the synthesis and the characterization of much longer InSb nanowires with a higher chemical purity than their predecessors and electron mobility exceeding 40000 cm2/Vs. Having quantified their pronounced spin-orbit interaction, adding a superconductor in the game is the logical next step. At the time of these experiments, hybrid nanowire devices were obtained by interfacing the two materials in situ, directly after the growth of the semiconductor. Despite ensuring a barrier-free semiconducting-superconducting interface, this approach has significant drawbacks in creating gate-tunable junctions due to the challenges in controlling the selectivity and the accuracy of the superconductor etching step. Considering that the semiconducting-superconducting interface is unstable even at room temperature, the devices quality, turnaround, and reproducibility become severely affected by extensive and low-yield fabrication processes. To circumvent these roadblocks, we have established a new fabrication paradigm based on on-chip shadow walls and shadow evaporations that offers substantial advances in device quality and reproducibility. Our approach results in devices with a hard induced superconducting gap and ballistic hybrid junctions. In Josephson junctions, we observe large gate-tunable supercurrents and high-order multiple Andreev reflections indicating the resulting junctions' exceptional coherence. Crucially, our approach enables the realization of three-terminal devices, where zero-bias conductance peaks emerge in a magnetic ﬁeld concurrently at both boundaries of the one-dimensional hybrids. In the near future, correlating such Majoranas' signatures with the measurement of the induced gap in the bulk will enable a better classification of the observed subgap states. In conclusion, once this technology is applied to nanowire networks, it will allow verifying topological parity read-out schemes, which is a milestone toward verifying the Majorana states' exotic exchange statistics.
|Qualification||Doctor of Philosophy|
|Award date||14 Apr 2021|
|Publication status||Published - 2021|
- Hybrid devices
- Semiconducting nanowires
- Josephson junctions
- Aharonov-Bohm interferometers