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The defence will be in English.
Abstract: The tunnel Josephson junction consists in a thin insulating barrier inserted between two super-conducting electrodes, where Cooper pairs can tunnel from one electrode to the other, with a very low transmission probability. This element serves as the common and required source of non-linearity in superconducting quantum circuits, enabling the implementation of superconducting quantum bit (qubit) and superconducting amplifier circuits: two building blocks of quantum computing. The fabrication of such junctions does not allows for any in-situ control, in which their behaviour is fixed by their geometry. It is common to embbed at least two junctions in a superconducting loop to form a superconducting quantum interference device (SQUID), whose superconducting macroscopic wave function phase can be modified by applying an external magnetic field. Doing so allows one to recover some control on a quantum degree of freedom, by tuning the phase-dependent energy of the SQUID. Integrating a SQUID in a superconducting qubit eventually makes its transition frequency tunable. In the last decade, new architectures of superconducting quantum circuits have emerged, where the standard tunnel Josephson junction is replaced by a superconductor-normal-superconductor (SNS) junction. This junction benefits from the normal material properties, which can be a semiconductor or a semimetal, and whose density of states can be controlled by applying an external electric field. Consequently, the energy of a single junction can be tuned without relying on a magnetic field, a solution which may face scalability issues. Currently, superconducting qubits with an InAs nanowire, a 2D electron-gas (2DEG), and graphene (with low coherence times) have been demonstrated. In this PhD work, we investigate the use of graphene, a 2D van der Waals semimetal encapsulated between h-BN (to protect it from its environment), as a gate-tunable SNS junction. The graphene Josephson junction is integrated in a standard superconducting qubit design, where the Fermi energy of graphene is controlled by the use of a gate electrode, on which a voltage is applied. The gate has two effects: it changes the number of conduction channels within the graphene sheet, as well as their transmission probabilities (or transparencies). The relaxation time is measured between 300 and 800 ns, one order of magnitude above the current graphene based-qubit state-of-the-art. We measure a large tunability of the qubit transition frequency (several GHz). Surprinsingly, the anharmonicity higly depends on the gate voltage with large oscillations in the hole-doped regime of graphene. Based on a superconducting quantum point contact analytical model, which we solved numerically, our analysis suggests that large mesoscopic fluctuations of transparency occur in our devices, with values ranging from 0.2 to 1, an indication of a high quality graphene Josephson junction. The model, valid for any regime in a short junction limit, actually demonstrates the important effect of the transparency on the qubit spectrum. The charge dispersion (i.e. the qubit sensitivity to charge noise) is similarly reduced with the Josephson energy but also with the transparency. In particular, at perfect transparency, the charge dispersion is expected to vanish, which is highly desirable for qubits more resilient against charge noise. We demonstrate that the charge dispersion is indeed smaller than the one expected for a tunnel junction-based superconducting qubit operating in the same regime, by up to a factor 2, with an inferred transparency ranging from 0.6 to 0.8.
