Advances in semiconducting-superconducting nanowire devices

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.QRD/Kouwenhoven La

Transmission phase read-out of a large quantum dot in a nanowire interferometer

Detecting the transmission phase of a quantum dot via interferometry can reveal the symmetry of the orbitals and details of electron transport. Crucially, interferometry will enable the read-out of topological qubits based on one-dimensional nanowires. However, measuring the transmission phase of a quantum dot in a nanowire has not yet been established. Here, we exploit recent breakthroughs in the growth of one-dimensional networks and demonstrate interferometric read-out in a nanowire-based architecture. In our two-path interferometer, we define a quantum dot in one branch and use the other path as a reference arm. We observe Fano resonances stemming from the interference between electrons that travel through the reference arm and undergo resonant tunnelling in the quantum dot. Between consecutive Fano peaks, the transmission phase exhibits phase lapses that are affected by the presence of multiple trajectories in the interferometer. These results provide critical insights for the design of future topological qubits.QRD/Kouwenhoven LabQuTechQN/Kouwenhoven La

Shared control of a 16 semiconductor quantum dot crossbar array

The efficient control of a large number of qubits is one of the most challenging aspects for practical quantum computing. Current approaches in solid-state quantum technology are based on brute-force methods, where each and every qubit requires at least one unique control line—an approach that will become unsustainable when scaling to the required millions of qubits. Here, inspired by random-access architectures in classical electronics, we introduce the shared control of semiconductor quantum dots to efficiently operate a two-dimensional crossbar array in planar germanium. We tune the entire array, comprising 16 quantum dots, to the few-hole regime. We then confine an odd number of holes in each site to isolate an unpaired spin per dot. Moving forward, we demonstrate on a vertical and a horizontal double quantum dot a method for the selective control of the interdot coupling and achieve a tunnel coupling tunability over more than 10 GHz. The operation of a quantum electronic device with fewer control terminals than tunable experimental parameters represents a compelling step forward in the construction of scalable quantum technology.QCD/Veldhorst LabBUS/TNO STAFFQCD/Vandersypen LabQCD/Scappucci LabQN/Veldhorst La

Selectivity Map for Molecular Beam Epitaxy of Advanced III-V Quantum Nanowire Networks

Selective-area growth is a promising technique for enabling of the fabrication of the scalable III-V nanowire networks required to test proposals for Majorana-based quantum computing devices. However, the contours of the growth parameter window resulting in selective growth remain undefined. Herein, we present a set of experimental techniques that unambiguously establish the parameter space window resulting in selective III-V nanowire networks growth by molecular beam epitaxy. Selectivity maps are constructed for both GaAs and InAs compounds based on in situ characterization of growth kinetics on GaAs(001) substrates, where the difference in group III adatom desorption rates between the III-V surface and the amorphous mask area is identified as the primary mechanism governing selectivity. The broad applicability of this method is demonstrated by the successful realization of high-quality InAs and GaAs nanowire networks on GaAs, InP, and InAs substrates of both (001) and (111)B orientations as well as homoepitaxial InSb nanowire networks. Finally, phase coherence in Aharonov-Bohm ring experiments validates the potential of these crystals for nanoelectronics and quantum transport applications. This work should enable faster and better nanoscale crystal engineering over a range of compound semiconductors for improved device performance.QRD/Kouwenhoven LabQuTechSafety and SecurityBUS/Genera

A 2D quantum dot array in planar ²⁸Si/SiGe

Semiconductor spin qubits have gained increasing attention as a possible platform to host a fault-tolerant quantum computer. First demonstrations of spin qubit arrays have been shown in a wide variety of semiconductor materials. The highest performance for spin qubit logic has been realized in silicon, but scaling silicon quantum dot arrays in two dimensions has proven to be challenging. By taking advantage of high-quality heterostructures and carefully designed gate patterns, we are able to form a tunnel coupled 2 × 2 quantum dot array in a 28Si/SiGe heterostructure. We are able to load a single electron in all four quantum dots, thus reaching the (1,1,1,1) charge state. Furthermore, we characterize and control the tunnel coupling between all pairs of dots by measuring polarization lines over a wide range of barrier gate voltages. Tunnel couplings can be tuned from about 30 μ eV up to approximately 400 μ eV . These experiments provide insightful information on how to design 2D quantum dot arrays and constitute a first step toward the operation of spin qubits in 28Si/SiGe quantum dots in two dimensions.QCD/Vandersypen LabQCD/Veldhorst LabBUS/TNO STAFFQCD/Scappucci LabQN/Veldhorst LabQN/Vandersypen La

Impact of Junction Length on Supercurrent Resilience against Magnetic Field in InSb-Al Nanowire Josephson Junctions

Semiconducting nanowire Josephson junctions represent an attractive platform to investigate the anomalous Josephson effect and detect topological superconductivity. However, an external magnetic field generally suppresses the supercurrent through hybrid nanowire junctions and significantly limits the field range in which the supercurrent phenomena can be studied. In this work, we investigate the impact of the length of InSb-Al nanowire Josephson junctions on the supercurrent resilience against magnetic fields. We find that the critical parallel field of the supercurrent can be considerably enhanced by reducing the junction length. Particularly, in 30 nm long junctions supercurrent can persist up to 1.3 T parallel field─approaching the critical field of the superconducting film. Furthermore, we embed such short junctions into a superconducting loop and obtain the supercurrent interference at a parallel field of 1 T. Our findings are highly relevant for multiple experiments on hybrid nanowires requiring a magnetic-field-resilient supercurrent.QRD/Kouwenhoven LabQRD/Wimmer GroupQCD/Veldhorst LabBUS/Quantum DelftQN/Kouwenhoven La

Full parity phase diagram of a proximitized nanowire island

We measure the charge periodicity of Coulomb blockade conductance oscillations of a hybrid InSb-Al island as a function of gate voltage and parallel magnetic field. The periodicity changes from to at a gate-dependent value of the magnetic field, , decreasing from a high to a low limit upon increasing the gate voltage. In the gate voltage region between the two limits, which our numerical simulations indicate to be the most promising for locating Majorana zero modes, we observe correlated oscillations of peak spacings and heights. For positive gate voltages, the transition with low is due to the presence of nontopological states whose energy quickly disperses below the charging energy due to the orbital effect of the magnetic field. Our measurements highlight the importance of a careful exploration of the entire available phase space of a proximitized nanowire as a prerequisite to define future topological qubits.Architecture and the Built EnvironmentQuTechQCD/Veldhorst LabBUS/Quantum DelftQRD/Kouwenhoven LabOptical and Laser Remote SensingQN/Kouwenhoven La

Single-Shot Fabrication of Semiconducting–Superconducting Nanowire Devices

Semiconducting–superconducting hybrids are vital components for the realization of high-performance nanoscale devices. In particular, semiconducting–superconducting nanowires attract widespread interest owing to the possible presence of non-abelian Majorana zero modes, which are quasiparticles that hold promise for topological quantum computing. However, systematic search for Majoranas signatures is challenging because it requires reproducible hybrid devices and reliable fabrication methods. This work introduces a fabrication concept based on shadow walls that enables the in situ, selective, and consecutive depositions of superconductors and normal metals to form normal-superconducting junctions. Crucially, this method allows to realize devices in a single shot, eliminating fabrication steps after the synthesis of the fragile semiconductor/superconductor interface. At the atomic level, all investigated devices reveal a sharp and defect-free semiconducting–superconducting interface and, correspondingly, a hard induced superconducting gap resilient up to 2 T is measured electrically. While the cleanliness of the technique enables systematic studies of topological superconductivity in nanowires, it also allows for the synthesis of advanced nano-devices based on a wide range of material combinations and geometries while maintaining an exceptionally high interface quality.QCD/Veldhorst LabQRD/Kouwenhoven LabBUS/Quantum DelftQN/Kouwenhoven La

Shadow-wall lithography of ballistic superconductor–semiconductor quantum devices

The realization of hybrid superconductor–semiconductor quantum devices, in particular a topological qubit, calls for advanced techniques to readily and reproducibly engineer induced superconductivity in semiconductor nanowires. Here, we introduce an on-chip fabrication paradigm based on shadow walls that offers substantial advances in device quality and reproducibility. It allows for the implementation of hybrid quantum devices and ultimately topological qubits while eliminating fabrication steps such as lithography and etching. This is critical to preserve the integrity and homogeneity of the fragile hybrid interfaces. The approach simplifies the reproducible fabrication of devices with a hard induced superconducting gap and ballistic normal-/superconductor junctions. Large gate-tunable supercurrents and high-order multiple Andreev reflections manifest the exceptional coherence of the resulting nanowire Josephson junctions. Our approach enables the realization of 3-terminal devices, where zero-bias conductance peaks emerge in a magnetic field concurrently at both boundaries of the one-dimensional hybrids.BUS/Quantum DelftQuTechQCD/Veldhorst LabQRD/Kouwenhoven LabGeneralQCD/Vandersypen LabBUS/TNO STAFFQN/Kouwenhoven La

Magnetic-Field-Resilient Superconducting Coplanar-Waveguide Resonators for Hybrid Circuit Quantum Electrodynamics Experiments

Superconducting coplanar-waveguide resonators that can operate in strong magnetic fields are important tools for a variety of high-frequency superconducting devices. Magnetic fields degrade resonator performance by creating Abrikosov vortices that cause resistive losses and frequency fluctuations or suppress the superconductivity entirely. To mitigate these effects, we investigate lithographically defined artificial defects in resonators fabricated from Nb-Ti-N superconducting films. We show that by controlling the vortex dynamics, the quality factor of resonators in perpendicular magnetic fields can be greatly enhanced. Coupled with the restriction of the device geometry to enhance the superconductors critical field, we demonstrate stable resonances that retain quality factors ≃105 at the single-photon power level in perpendicular magnetic fields up to B⊥ ≃20mT and parallel magnetic fields up to B⥠≃6T. We demonstrate the effectiveness of this technique for hybrid systems by integrating an In-Sb nanowire into a field-resilient superconducting resonator and use it to perform fast charge readout of a gate-defined double quantum dot at B=1T.QRD/Kouwenhoven LabQuTechApplied SciencesBUS/GeneralQCD/DiCarlo LabQN/Kouwenhoven La