We propose and investigate a novel method for the controlled coupling of two Josephson charge qubits by means of a variable electrostatic transformer. The value of the coupling capacitance is given by the discretized curvature of the lowest energy band of a Josephson junction, which can be positive, negative, or zero. We calculate the charging diagram of the two-qubit system that reflects the transition from positive to negative through vanishing coupling. We also discuss how to construct a phase gate making use of the controllable coupling.
@article{arxiv.cond-mat/0304166,
title = {Variable electrostatic transformer: controllable coupling of two charge qubits},
author = {D. V. Averin and C. Bruder},
journal= {arXiv preprint arXiv:cond-mat/0304166},
year = {2009}
}