Before revision: Because of the non-linearity of the Josephson junction, operations can be performed on the two lowest eigenvalue states without exciting the higher energy states; thus Josephson junctions are very important element of flux qubits and superconducting circuits in general.
After revision: Because of the non-linearity of the Josephson junction, operations can be performed on the two lowest eigenvalue states without exciting the higher energy states; thus Josephson junctions are an integral element of flux qubits and superconducting circuits in general.