Fourier engineering of the energy-phase relation for parity protected superconducting qubits

One promising candidate for a protected superconducting qubit is the so-called cos2φ qubit, which employs nonlinear elements that enable coherent tunneling of Cooper pairs. This non-linear element results in a Josephson potential containing only even harmonic terms, providing intrinsic protection against dielectric loss. In this work, we explore the realization of such a qubit using probably the simplest source of a non-sinusoidal current–phase relation (CPR): two Josephson tunnel junctions connected in series as described in Ref.[1]. In this talk we demonstrate that this basic SIS–SIS configuration can exhibit an effective SNS-like CPR, supported by measurements of the qubit spectrum, and we discuss possible noise contributions within this system.