  We propose a qubit implementation based on exciton condensation in
capacitively coupled Josephson junction chains. The qubit is protected in the
sense that all unwanted terms in its effective Hamiltonian are exponentially
suppressed as the chain length increases. We also describe an implementation of
a universal set of quantum gates. Most gates also offer exponential error
suppression. The only gate that is not intrinsically fault-tolerant needs to be
realized with about 50% precision, provided the other gates are exact.

Mesoscale and Nanoscale Physics;Superconductivity;Quantum Physics