Fig. 1: Principle of the protected qubit.
From: Superconducting circuit protected by two-Cooper-pair tunneling
a Electrical circuit for the transmon qubit. b Potential energy of the transmon with energy levels and wavefunctions for the first few eigenstates. c Electrical circuit for the idealized protected qubit. The cross-hatched circuit element comprises a capacitance in parallel with an inductive element that exclusively permits the tunneling of pairs of Cooper pairs. The superconducting island is indicated by color. d Potential energy of the ideal charge-protected qubit with the lowest-energy levels and wavefunctions.
