Coherence in a network of two-level systems coupled to a bosonic field

We study the quantum dynamics of an open network of two-level systems which is coupled to a correlated common environment represented by a bosonic field in a thermal equilibrium state. Extensive numerical simulations of the full second-order time-convolutionless quantum master equation for the density matrix of various types of networks are performed, in order to investigate dissipation and decoherence processes and, in particular, their dependence on the spatial separation of the network sites and on the speed of the bosonic field modes. In the limit of an infinite speed the influence of the environment disappears due to the emergence of a decoherence-free subspace, while the limit of zero speed corresponds to the case in which the network sites are coupled to independent reservoirs, a case which is much easier to treat numerically. The main result of the paper is a general intuitive criterion which states that the simpler model of independent reservoirs can be used as long as the network relaxation time is smaller than the time it takes for the modes to travel the minimal distance between the network sites.