This PhD thesis explores a hybrid semiconductor-superconductor circuit named the FerBo qubit. By replacing the conventional Josephson tunnel junction of a fluxonium with a mesoscopic weak link hosting Andreev bound states, the circuit couples a fermionic degree of freedom to a bosonic electromagnetic mode. This architecture enables a parity-based protection mechanism at zero external flux: the qubit states share the same parity under phase inversion, strongly suppressing the matrix elements of charge and phase operators. Combined with a high-impedance environment provided by the bosonic mode, this offers simultaneous protection against both relaxation and dephasing.This work encompasses both the theoretical modeling and a first experimental realization of this hybrid qubit architecture. Reaching this goal required an incremental approach, starting with the characterization of a standalone Josephson junction array and a conventional fluxonium, and culminating in the implementation of a prototype utilizing a semiconductor nanowire as the weak link. We found that this device operated outside of the protected FerBo regime, and could be described as a tunable nanowire fluxonium. To reach this conclusion, we developed a novel theoretical framework that allowed interpreting the rich spectra of the circuit. This method was designed to efficiently treat the complex hybridization between a few non-linear modes---such as those of transmons or fluxoniums---and the numerous linear modes present in the extended circuit. Validated on a conventional fluxonium, this framework proved essential for identifying transitions and extracting Hamiltonian parameters in both the conventional and the nanowire fluxonium circuits. Together with a theoretical description of the FerBo qubit, this work identifies the multimode design constraints---such as managing readout hybridization and linear-mode interactions---required to achieve hardware-level protection in future hybrid superconducting qubits.
This PhD thesis explores a hybrid semiconductor-superconductor circuit named the FerBo qubit. By replacing the conventional Josephson tunnel junction of a fluxonium with a mesoscopic weak link hosting Andreev bound states, the circuit couples a fermionic degree of freedom to a bosonic electromagnetic mode. This architecture enables a parity-based protection mechanism at zero external flux: the qubit states share the same parity under phase inversion, strongly suppressing the matrix elements of charge and phase operators. Combined with a high-impedance environment provided by the bosonic mode, this offers simultaneous protection against both relaxation and dephasing.This work encompasses both the theoretical modeling and a first experimental realization of this hybrid qubit architecture. Reaching this goal required an incremental approach, starting with the characterization of a standalone Josephson junction array and a conventional fluxonium, and culminating in the implementation of a prototype utilizing a semiconductor nanowire as the weak link. We found that this device operated outside of the protected FerBo regime, and could be described as a tunable nanowire fluxonium. To reach this conclusion, we developed a novel theoretical framework that allowed interpreting the rich spectra of the circuit. This method was designed to efficiently treat the complex hybridization between a few non-linear modes---such as those of transmons or fluxoniums---and the numerous linear modes present in the extended circuit. Validated on a conventional fluxonium, this framework proved essential for identifying transitions and extracting Hamiltonian parameters in both the conventional and the nanowire fluxonium circuits. Together with a theoretical description of the FerBo qubit, this work identifies the multimode design constraints---such as managing readout hybridization and linear-mode interactions---required to achieve hardware-level protection in future hybrid superconducting qubits.
Hybridizing Andreev states with electromagnetic modes in a superconducting circuit : the FerBo qubit Cette thèse de doctorat explore les propriétés d'un circuit hybride semiconducteur-supraconducteur qu'on a appelé le qubit FerBo. En remplaçant la jonction tunnel Josephson conventionnelle d'un fluxonium par un lien faible mésoscopique hébergeant des états liés d'Andreev, le circuit couple un degré de liberté fermionique à un mode électromagnétique bosonique. Cette architecture permet un mécanisme de protection basé sur la parité: à flux externe nul, les états du qubit ont la même parité en phase, ce qui supprime fortement les éléments de matrice des opérateurs de charge et de phase. Combinée à un environnement haute impédance fourni par le mode bosonique, cette propriété apporte une protection simultanée contre la relaxation et le déphasage.Ce travail comprend la modélisation théorique et une première réalisation expérimentale de cette architecture de qubit hybride. Nous avons suivi une approche incrémentale, en commençant par la caractérisation d'un réseau de jonctions Josephson et d'un fluxonium conventionnel, pour aboutir à l'implémentation d'un prototype utilisant un nanofil semiconducteur comme lien faible. Il s'est avéré que le circuit que nous avons mesuré n'a pas atteint le régime protégé, et qu'il pouvait être décrit comme un fluxonium accordable. Pour arriver à cette conclusion, nous avons dû développer une approche théorique novatrice qui permet de comprendre les détails des spectres complexes observés dans l'expérience. Cette méthode a été conçue pour traiter efficacement l'hybridation complexe entre quelques modes non-linéaires --- tels que ceux de transmons ou de fluxoniums --- et les nombreux modes linéaires présents dans le circuit étendu. Validé sur le fluxonium conventionnel, ce cadre s'est avéré essentiel pour identifier les transitions et extraire les paramètres du hamiltonien à la fois pour le fluxonium conventionnel et le fluxonium à nanofil. Associé à une description théorique du qubit FerBo, ce travail identifie les contraintes de conception associées à la présence de nombreux modes --- telles que la gestion de l'hybridation avec le mode de lecture et les interactions avec les modes linéaires --- qu'il est nécessaire de prendre en compte pour atteindre la protection matérielle dans ce type de qubit supraconducteur hybride.
Cette thèse de doctorat explore les propriétés d'un circuit hybride semiconducteur-supraconducteur qu'on a appelé le qubit FerBo. En remplaçant la jonction tunnel Josephson conventionnelle d'un fluxonium par un lien faible mésoscopique hébergeant des états liés d'Andreev, le circuit couple un degré de liberté fermionique à un mode électromagnétique bosonique. Cette architecture permet un mécanisme de protection basé sur la parité: à flux externe nul, les états du qubit ont la même parité en phase, ce qui supprime fortement les éléments de matrice des opérateurs de charge et de phase. Combinée à un environnement haute impédance fourni par le mode bosonique, cette propriété apporte une protection simultanée contre la relaxation et le déphasage.Ce travail comprend la modélisation théorique et une première réalisation expérimentale de cette architecture de qubit hybride. Nous avons suivi une approche incrémentale, en commençant par la caractérisation d'un réseau de jonctions Josephson et d'un fluxonium conventionnel, pour aboutir à l'implémentation d'un prototype utilisant un nanofil semiconducteur comme lien faible. Il s'est avéré que le circuit que nous avons mesuré n'a pas atteint le régime protégé, et qu'il pouvait être décrit comme un fluxonium accordable. Pour arriver à cette conclusion, nous avons dû développer une approche théorique novatrice qui permet de comprendre les détails des spectres complexes observés dans l'expérience. Cette méthode a été conçue pour traiter efficacement l'hybridation complexe entre quelques modes non-linéaires --- tels que ceux de transmons ou de fluxoniums --- et les nombreux modes linéaires présents dans le circuit étendu. Validé sur le fluxonium conventionnel, ce cadre s'est avéré essentiel pour identifier les transitions et extraire les paramètres du hamiltonien à la fois pour le fluxonium conventionnel et le fluxonium à nanofil. Associé à une description théorique du qubit FerBo, ce travail identifie les contraintes de conception associées à la présence de nombreux modes --- telles que la gestion de l'hybridation avec le mode de lecture et les interactions avec les modes linéaires --- qu'il est nécessaire de prendre en compte pour atteindre la protection matérielle dans ce type de qubit supraconducteur hybride.
We propose a novel superconducting quantum circuit that should be robust against both relaxation and dephasing over a wide and experimentally accessible parameter range. The circuit consists of a parallel arrangement of a large inductance, a small capacitor, and a well-transmitting Josephson weak link. Protection against relaxation arises from the hybridization between the fermionic degree of freedom associated with Andreev levels in the weak link and the bosonic electromagnetic mode of the LC circuit, hence its name: FerBo. Furthermore, as in the fluxonium qubit, delocalization of the wavefunctions in phase space provides resilience against dephasing.
Hybrid semiconductor-superconductor qubits have recently emerged as a promising alternative to traditional platforms, combining material advantages with device-level tunability. A defining feature is their gate-tunable Josephson coupling, enabling superconducting qubit architectures with full electric-field control and offering a path toward scalable, low-crosstalk quantum processors. This approach seeks to merge benefits of superconducting and semiconductor qubits, for instance by encoding quantum information in the spin of a quasiparticle occupying an Andreev bound state, thus combining long coherence times with fast, flexible control. Progress has accelerated through bottom-up engineering of Andreev states in coupled quantum dot arrays, leading to architectures such as minimal Kitaev chains hosting Majorana zero modes. In parallel, Hamiltonian-protected designs aim to enhance resilience against local noise and decoherence by exploiting superconducting phase dynamics and discrete charge or flux degrees of freedom. This article reviews recent theoretical and experimental advances in hybrid qubits, providing an overview of physical mechanisms, device implementations, and emerging architectures, with emphasis on their potential for (topologically) protected quantum information processing. While many designs remain at proof-of-concept stage, rapid progress suggests practical demonstrations may soon be achievable.
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.
Opportunités de manipulation directe d'un qubit de spin d'Andreev piloté en phase The research presented in this thesis delves into a theoretical exploration of Andreev bound states (ABS) hosted in Josephson junctions with spin-orbit coupling (SOC). ABS are fermionic bound states that appear at the junction between two superconducting electrodes forming a Josephson junction. These bound states are at the core of the microscopic description of the Josephson effect and provide an explanation for the transfer of Cooper pairs from one superconducting electrode to the other. Crucially, when the superconducting phase difference between the two electrodes is non-zero, a supercurrent can flow in the junction, depending on the occupancy of the ABS. The presence of this supercurrent facilitates the coupling between a microwave resonator and a junction due to its sensibility to the electromagnetic field. This coupling then allows for the detection and manipulation of ABS.Furthermore, in the presence of SOC along with a superconducting phase bias, the spin degeneracy of ABS can be lifted. Therefore such a junction provides a unique opportunity to realize a special kind of spin qubit known as Andreev spin qubit (ASQ). The qubit operation can be performed via an AC modulation of an electrostatic gate or magnetic flux, thanks to the sensitivity of the ABS to the electric potential or the phase difference, respectively. So far, the coherent manipulation of such qubits has been realized in gate-driven experiments as well as in flux-driven experiments with the use of additional Andreev levels. However, the direct transition between the two ABS forming the ASQ has remained out of reach in flux-driven experiments. In this thesis, we investigate two distinct types of Josephson junctions with SOC with the aim of estimating the amplitude of the matrix elements of the current operator between two ABS forming an ASQ. These elements characterize the coupling strength between the qubit and an external flux drive, indicating which transitions between ABS are within reach. Our first project focuses on a superconductor-normal-superconductor junction where the normal region consists of a nanowire with Rashba SOC. As a minimal model, we consider a generic scatterer located along the nanowire. Unless this scatterer possesses additional spatial (mirror) symmetries, it is responsible for inducing spin-flip transmission probability. Using the scattering formalism, we derive the energy spectrum of ABS and assess how SOC influences the spin-splitting between opposite spin states. We then obtain analytical expressions for the matrix elements of the current operator. Notably, our study reveals that SOC allows one to have finite elements between opposite spin states. In a second project, the system we study consists of a superconductor-double quantum dots-superconductor junction. A quantum dot consists of a tiny region defined by electrostatic gates or impurities. Due to its small size, Coulomb repulsion can be significant. However, in the simplest case of a quantum dot with a single level, even in the presence of SOC in the coupling between the dot and the leads, spin degeneracy of ABS is still preserved. However, recent experiments showed that additional levels allow to lift the spin degeneracy. Here, we introduce a second quantum dot which provides an additional level and offers more control on each level. By using an effective model, we identify the minimal ingredients that are necessary to lift the spin degeneracy. Our results reveal that this splitting is achievable through spin-dependent couplings with the leads and is reduced by the Coulomb repulsion. Additionally, the presence of a finite coupling between the two dots, combined with SOC in the couplings with the leads, allows one to have finite matrix elements of the current operator between opposite spin states, which we obtain numerically. In short, we present in this thesis two platforms for the realization of phase-driven ASQ.