Long-range coupling and scalable architecture for superconducting flux qubits

Constructing a fault-tolerant quantum computer is a daunting task. Given any design, it is possible to determine the maximum error rate of each type of component that can be tolerated while still permitting arbitrarily large-scale quantum computation. It is an underappreciated fact that including an appropriately designed mechanism enabling long-range qubit coupling or transport substantially increases the maximum tolerable error rates of all components. With this thought in mind, we take the superconducting flux qubit coupling mechanism described by Plourde et al. [Phys. Rev. B 70, 140501(R) (2004)] and extend it to allow approximately $500\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$ coupling of square flux qubits, $50\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$ a side, at a distance of up to several millimeters. This mechanism is then used as the basis of two scalable architectures for flux qubits taking into account cross-talk and fault-tolerant considerations such as permitting a universal set of logical gates, parallelism, measurement and initialization, and data mobility.