Hybridizing Andreev states with electromagnetic modes in a superconducting circuit: the FerBo qubit

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.