Information transfer fidelity in spin networks and ring-based quantum routers

Spin networks are endowed with an information transfer fidelity (ITF), which defines an absolute upper bound on the probability of transmission of an excitation from one spin to another. The ITF is easily computable, but the bound can be reached asymptotically in time only under certain conditions. General conditions for attainability of the bound are established, and the process of achieving the maximum transfer probability is given a dynamical model, the translation on the torus. The time to reach the maximum probability is estimated using the simultaneous Diophantine approximation, implemented using a variant of the Lenstra–Lenstra–Lovász (LLL) algorithm. For a ring with uniform couplings, the network can be made into a metric space by defining a distance (satisfying the triangle inequality) that quantifies the lack of transmission fidelity between two nodes. It is shown that transfer fidelities and transfer times can be improved significantly by means of simple controls taking the form of nondynamic, spatially localized bias fields, opening up the possibility for intelligent design of spin networks and dynamic routing of information encoded in them, while being more flexible than engineering fixed couplings to favor some transfers, and less demanding than control schemes requiring fast dynamic controls. B E. Jonckheere [email protected] F. C. Langbein [email protected] S. G. Schirmer [email protected] 1 Center for Quantum Information Science and Technology, University of Southern California, Los Angeles, CA 90089, USA 2 School of Computer Science and Informatics, Cardiff University, Cardiff CF24 3AA, UK 3 College of Science (Physics), Swansea University, Singleton Park, Swansea SA2 8PP, UK