This thesis project aims to construct optimal, tunable superconductor-semiconductor junctions for electronic quantum devices. Such junctions will be useful for solid state quantum computers where the tunability of the semiconducting element allows for new types of superconducting quantum bits (qubits). The archetypical device is the gate tunable Josephson junction that has recently been shown to work in different qubit architectures.Growth of semiconductor nanowires will be carried out in a III-V molecular beam epitaxy reactor and coat these with superconducting shells such as Al, Ta, and MoGe, several unexploited materials combinations will be tested. During the project I will also take part in device fabrication using these nanowires and low temperature measurement campaigns to probe superconductivity and quantum transport phenomena.
This thesis project aims to construct optimal, tunable superconductor-semiconductor junctions for electronic quantum devices. Such junctions will be useful for solid state quantum computers where the tunability of the semiconducting element allows for new types of superconducting quantum bits (qubits). The archetypical device is the gate tunable Josephson junction that has recently been shown to work in different qubit architectures.Growth of semiconductor nanowires will be carried out in a III-V molecular beam epitaxy reactor and coat these with superconducting shells such as Al, Ta, and MoGe, several unexploited materials combinations will be tested. During the project I will also take part in device fabrication using these nanowires and low temperature measurement campaigns to probe superconductivity and quantum transport phenomena.
Superconductor-semiconductor hybrid materials have been extensively used for experiments on electrically tunable quantum devices. Notably, Josephson junctions utilizing nanowire weak links have enabled a number of new gate-tunable qubits, including gatemons, Andreev level qubits and spin qubits. Conversely, superconducting parametric amplifiers based on Josephson junctions have not yet been implemented using nanowires, even though such nearly quantum limited amplifiers are key elements in experiments on quantum circuits. Here we present Josephson parametric amplifiers based on arrays of parallel InAs nanowires that feature a large critical current as required for linear amplification. The resonance frequency of the devices is gate-tunable by almost 1 GHz, with a gain exceeding 20 dB in multiple frequencies and noise approaching the quantum-limit. This new platform enables on-chip integration of gate-tunable qubits with quantum limited amplifiers using the same hybrid materials and on any substrate.
Développement et caractérisation d'amplificateur paramétrique Josephson basé sur des nanofils inas/al Ce projet de thèse vise à construire des jonctions supraconducteur-semiconducteur optimisées et ajustables pour des dispositifs électroniques quantiques. De telles jonctions seront utiles pour l’informatique quantique où le réglage de l'élément semi-conducteur permet de nouveaux types de bits quantiques supraconducteurs (qubits). L'archétype de ce dispositif est la jonction Josephson réglable par un potentiel de grille, dont le fonctionnement a été récemment démontré avec différentes architectures de qubits.La croissance de nano-fils semi-conducteurs sera réalisée dans un réacteur d'épitaxie par jets moléculaires III-V et ces derniers seront recouverts de coquilles supraconductrices telles que Al, Ta et MoGe, plusieurs combinaisons de matériaux encore non-exploitées seront testées. Au cours du projet, je participerai également à la fabrication de dispositifs utilisant ces nano-fils et à des campagnes de mesures à basse température pour sonder la supraconductivité et ces phénomènes de transport quantique.
Quantum computers promise to solve certain problems such as quantum chemistry simulations much more efficiently than their classical counterparts. Although it is still unclear what material system will ultimately host large-scale quantum computers, solid-state systems are promising candidates due to their inherent scalability and advanced fabrication techniques that can be adapted from comparable technologies. Crucially, a future quantum computer will depend on the quality of its most fundamental building block, the quantum bit, or qubit. Qubits, although ideally insensitive to potential noise, are very susceptible to slight changes in their environment. Therefore, they do not only make the building block for quantum computers, but are also precise sensors. One of the most studied solid-state implementations of a qubit is the transmon, a weakly anharmonic oscillator based on superconducting capacitive and nonlinear inductive elements. Typically, Al-AlOx-Al superconductor-insulator-superconductor Josephson junctions are used for the latter. The interaction of the transmon with the control circuitry, typically superconducting resonators, is described by circuit quantum electrodynamics. In this PhD thesis, a more recently demonstrated type of qubit is further developed and studied in detail using circuit quantum electrodynamics. In these qubits, the Josephson element of the transmon is replaced with indium arsenide nanowires, forming a superconductor-normal metalsuperconductor junction. In addition to the standard flux tunability, these qubits can also be voltage tuned. Due to the compatibility of all the materials used with an applied magnetic field, this type of qubit is a good candidate to be used as a precise and accurate sensor in a magnetic field. The goal of this work is to introduce the in-plane magnetic field as a new tuning knob to the toolbox of circuit quantum electrodynamics. Advances in material science, especially the epitaxial growth of an aluminum shell directly on the indium arsenide nanowire, have enabled the fabrication of nanowire transmons with state-of-the-art coherence. An understanding of their workings in a zero-field environment is important before applying a magnetic field. Thus, we characterize the noise these qubits are subject to (Chapter 4) and find a strong coupling of charge two-level systems to their Josephson energy next to the expected weakly coupled flux and voltage noise. Applying a magnetic field reveals that coherence in these qubits can be observed up to 70 mT, substantially above the superconducting gap of bulk aluminum (Chapter 5). Effects limiting the performance include the thick and fully covering aluminum shell, and the alignment and stability of the magnetic field. The use of different nanowires, the installation of a persistent-current vector solenoid and additional magnetic shielding then enables the operation of voltage- and flux-tunable devices in a magnetic field (Chapter 6). This constitutes a good starting point for circuit quantum electrodynamics experiments in a magnetic field, such as the investigation of the microscopic origin of flux-noise.
A practical quantum computer demands a physical quantum bit (qubit) that is scalable, has a well-defined initial state, and brings together coherence with reliable control and read-out. A compelling idea is to put together a locally protected Majorana qubit with a superconducting qubit. The superconducting qubit is used to read out the Majorana states while the Majorana qubit may act as a quantum memory. This thesis presents a series of studies on quantum transport in semiconducting nanowires coupled to superconductors to investigate various platforms for realizing Majorana. These studies are followed by microwave measurements on superconducting circuits compatible with semiconducting-superconducting heterostructures. We first evaluate the potential of Ge/Si core/shell nanowires by achieving induced superconductivity as well as estimating spin-orbit coupling. Next we explore the transport mediated by Andreev bound states formed in InSb nanowire quantum. A subgap negative differential conductance is investigated together with the coalescing Andreev resonances at zero bias relevant for the correct interpretation of Majorana experiments done on the same structures. We conclude our studies of semiconducting nanowires by exploring tunnel junctions in Sn-InSb nanowires that are prepared by in-situ shadowing using nearby nanowires and flakes. Tin shells are found to induce a hard superconducting gap persisting up to high magnetic fields. We observe the two-electron charging effect from a small superconducting island of Sn-InSb. This effect is attributed to charge parity stability, which makes this nanowire system an intriguing candidate for superconducting and topological quantum circuits. These Sn-InSb junctions are then used as the nonlinear element in a transmon qubit design where we observe a dispersive coupling between this nanowire Josephson junction and a superconducting resonator. We also present our progress towards building a magnetic field resilient superconducting circuit that allows integration with semiconducting and topological structures. In the last chapter, InSb semiconducting nanowires are utilized as shadow masks prior to superconductor deposition on an InAs quantum well. We study Josephson current properties in Josephson junctions made from these nanowire shadows. Our results point to highly transparent junctions that can be developed further for hybrid superconductor-semiconductor qubit systems.
The performance and coherence of superconducting quantum systems are fundamentally tied to the quality of the materials from which they are built. In superconducting transmon qubits, many loss mechanisms and sources of decoherence originate from material imperfections or unwanted oxide formation. Likewise, semiconductor–superconductor hybrid systems depend on high-quality interfaces to ensure strong coupling between electronic states in the two materials. In such hybrid devices, the accessible operating regime is often dictated by the intrinsic material properties of either the semiconductor or the superconductor. Advancing quantum device performance therefore requires exploring new material systems and optimizing their physical characteristics. In this thesis, we present the development of several superconducting material platforms for quantum technologies, including superconducting qubits, single-photon detectors, and semiconductor superconductor hybrid devices. We focus on the growth and characterization of superconducting films and heterostructures, and we establish clear connections between their material properties and the resulting device performance. First, we will present a novel semiconductor-superconductor hybrid material based on in-situ deposited epitaxial layers of Nb and Al on a shallow InAs quantum well. In this structure, the Al interlayer provides an epitaxial connection between an in-situ magnetron sputtered Nb or NbTi thin film and a shallow InAs two dimensional electron gas. This metal-to-metal epitaxy is achieved by optimization of the material stack and results in an induced superconducting gap of approximately 1 meV, determined from transport measurements of superconductor semiconductor Josephson junctions. This induced gap is approximately five times larger than the values reported for Al-based hybrid materials and indicates the formation of highly-transparent interfaces that are required in high-quality hybrid material platforms. Then, we discuss a different approach in the next chapter where we develop a hybrid material based on a molecular beam epitaxy grown InAsSb surface quantum well with an in-situ deposited Nb layer directly on top of the quantum well. Relative to conventional Al-InAs based systems, the InAsSb surface quantum well offers a lower effective mass and stronger spin-orbit interaction, while the Nb layer has a higher critical temperature and a larger critical magnetic field. The in-situ deposition of the Nb layer directly on top of the quantum well results in a high-quality interface that enables strong coupling to the InAsSb quantum well. Transport measurements on Josephson junctions reveal an induced superconducting gap of 1.3 meV and a critical current density of 3.75 × 10³A/cm². Furthermore, a planar asymmetric SQUID is realized, exhibiting gate-tunable superimposed oscillations originating from both the individual Josephson junction and the full SQUID loop. The large induced superconducting gap combined with strong spin-orbit interaction position this material as an attractive platform for experiments exploring gate-tunable superconductivity and topological superconducting devices. In the next chapter, we will focus on establishing a link between Nb film crystal properties and their performance in superconducting circuits. In this study, we investigate the microwave properties of niobium resonators with different crystalline properties and related surface topographies. We analyze a series of magnetron sputtered films in which the Nb crystal orientation and surface topography are changed by varying the substrate temperatures between room temperature and 975 K. The lowest-loss resonators that we measure have quality factors of over 10⁶ at single-photon powers, among the best ever recorded using the Nb on sapphire platform. We observe the highest quality factors in films grown at an intermediate temperature regime of the growth series (550 K) where the films display both preferential ordering of the crystal domains and low surface roughness. Furthermore, we analyze the temperature-dependent behavior of our resonators to learn about how the quasi-particle density in the Nb film is affected by the niobium crystal structure and the presence of grain boundaries. Then, we explore yet a different material platform by attempting to realize onchip NbTiN single photon detectors on LiNbO₃, an emerging photonic platform with high potential for scalable quantum information processing. We use DC reactive magnetron sputtering to grow high-quality NbTiN thin films and investigate the impact of substrate temperature during NbTiN growth. We achieve four nm thick superconducting films with a critical temperature (𝑇𝑐) of 12.3 K grown at a substrate temperature of 825 K. We demonstrate that the increased substrate temperature reduces the oxygen concentration in our films and improves the overall stoichiometry. In addition, we integrate these films with the LiNbO3 platform and investigate the obtained interface quality. Finally, we fabricate superconducting nanowire single photon detectors from the NbTiN film on LiNbO₃ and characterize the detector performance. Lastly, we report a gate-tunable Josephson diode effect in hybrid nanowire junctions consisting of a spin-orbit-coupled semiconductor core coated with epitaxial ferromagnetic insulator and superconductor shells. The wires display a hysteretic superconducting window as a function of axial magnetic field. In the superconducting regime, the devices exhibit nonreciprocal current transport, with the diode efficiency showing a strong dependence on back-gate voltage. The effect persists in a remanent magnetization state following a controlled demagnetization procedure, establishing zero-field operation. The combination of results presented in this thesis stress the importance of a material-focused approach in advancing the performance of quantum hardware. The novel hybrid material platforms developed in this thesis expand the operating regime of devices, which could enable future experiments involving gate-tunable superconductivity or topological properties. Similarly, we demonstrate that optimizing superconductor crystal structure benefits quantum devices, such as single-photon detectors and superconducting resonators.
With a large portfolio of elemental quantum components, superconducting quantum circuits have contributed to dramatic advances in microwave quantum optics. Of these elements, quantum-limited parametric amplifiers have proven to be essential for low noise readout of quantum systems whose energy range is intrinsically low (tens of µeV). They are also used to generate non classical states of light that can be a resource for quantum enhanced detection. Superconducting parametric amplifiers, like quantum bits, typically utilize a Josephson junction as a source of magnetically tunable and dissipation-free nonlinearity. The magnetic control is not an industry standard for devices and starts already to be an issue in large scale circuits. In recent years, efforts have been made to introduce semiconductor weak links as electrically tunable nonlinear elements, with demonstrations of microwave resonators and quantum bits using semiconductor nanowires, a two dimensional electron gas, carbon nanotubes and graphene. However, given the challenge of balancing nonlinearity, dissipation, participation, and energy scale, parametric amplifiers have not yet been implemented with a semiconductor weak link. The work presented in this PhD thesis demonstrates the design, fabrication and performances of a parametric amplifier leveraging a graphene Josephson junction.The graphene is encapsulated in between hBN flakes in order to improve its quality. Using such high quality junctions, we demonstrate amplification with 20 dB gain which is the milestone for overcoming noise of the classical amplifiers in the rest of the amplification chain. The Josephson parametric amplifier being based on a resonant structure (around 6 GHz) suffers from the gain bandwidth product limiting the amplification to a few MegaHertz frequency range. The use of a side gate to electrically tune the graphene gives control of the Josephson junction inductance and thus the resonant frequency of the amplifier. We demonstrate a near 1 GHz tunability of the amplification frequency with the use of a side gate which is as good as the tunability offered by using a magnetic flux on a SQUID for similar amplifier designs. Moreover, graphene has shown to exhibit nonlinear loss under a strong microwave irradiation arising because of dynamics in the Andreev states. Dissipation is known to add intrinsic noise on quantum amplifiers and could be a problem for quantum limited parametric amplification. We demonstrate that despite the presence of nonlinear loss, graphene based Josephson parametric amplifiers can reach near quantum limited amplification. A two photon loss model was used to describe the behaviour of the device but we show that a more complex model is required in order to take into account the nonlinear dissipation and the non-sinusoidal current phase relation. We also studied the dynamic range and showed that the graphene based Josephson parametric amplifier can reach a 1 dB compression point of -123 dBm which is as good as single tunnel junctions based parametric amplifiers. Our results expand the toolset for electrically tunable superconducting quantum circuits and offer new opportunities for the development of quantum technologies such as quantum computing, quantum sensing and fundamental science.