Protected qubit based on a superconducting current mirror

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.