Geometric Superinductance Qubits: Controlling Phase Delocalization across a Single Josephson Junction

We present the characterization of a large variety of loop-based qubits [1] made by the parallel connection of a single Josephson junction with a geometric superinductor [2], an uninterrupted coiled aluminum wire. They are all stemming from the same rf-SQUID circuit but with drastically different characteristic energy scales. Just changing the coil's number of turns it is possible to delocalize the phase variable from the flux-qubit and fluxonium regimes up to the recently introduced quasicharge regime with strongly enhanced zero-point phase fluctuations and a heavily suppressed flux dispersion. The use of geometric superinductors allows a direct implementation of the rf-SQUID Hamiltonian without approximations, and a precise control of the inductive and capacitive energy as guaranteed by top-down lithography - a key ingredient for intrinsically protected superconducting qubits.