Quantum network reduced-state synchronization part I-convergence under directed interactions

We consider reduced-state synchronization of qubit networks with the aim of driving the qubits´ reduced states to a common trajectory. The evolution of the quantum network´s state is described by a master equation, where the network Hamiltonian is either a direct sum or a tensor product of identical qubit Hamiltonians, and the coupling terms are given by a set of permutation operators over the network. The permutations introduce naturally quantum directed interactions. This part of the paper focuses on convergence conditions. We show that reduced-state synchronization is achieved if and only if the quantum permutations form a strongly connected union graph. The proof is based on an algebraic analysis making use of the Perron-Frobenius theorem for non-negative matrices. The convergence rate and the limiting orbit are explicitly characterized. Numerical examples are provided illustrating the obtained results.