Long-lived quantum entanglement of multiple qubits: Excitons in strained graphene

We investigate the entanglement dynamics of systems consisting of up to five qubits coupled to an optical microcavity. This is depicted by a system of excitons in strained graphene. We numerically calculate the time evolution of multiple entanglement monotones in such systems when subjected to a coherent source of photons (e.g., a laser). We propose a different way of estimating the total amount of quantum entanglement contained within a multipartite system based on the negativity which we refer to as total negativity, and to which we give clear upper and lower bounds for this quantity. We compared the results for a system of excitons in strained graphene along with two other systems which can be described by the same Hamiltonian as that for superconducting qubits coupled to a waveguide, and for trapped Rb atoms. We have shown that, depending on the physical parameters of the system, a reasonable amount of long-living entanglement can be created between the qubits. This entanglement is exempt from decay as long as the systems are being continuously pumped with coherent photons. We have reached the interesting conclusion that freely adding new qubits to the system does not, in general, increase the overall amount of entanglement created by the dynamics: there exist an optimum number of system qubits which depend on the Rabi coupling as well as the cavity decay rate for maximal entanglement. We believe that such systems are good candidates for the generation of resilient sets of long-lived partially entangled states suitable for experiments in entanglement distillation.