Quantum Information Transfer In Simple Networks

Quantum state transfer is of central importance for scalable quantum informationprocessing and most feasible approaches that have been proposed for quantum computa-tionrelyupontheuseofquantumchannelswhichservetoconnectremoteregisters. Bothcoupled quantum spin systems and coupled quantum resonator systems have also beenextensively studied to act as such channels. Moreover, both one-dimensional structuresand hypercube structures are simple and popular models in quantum channels. For thesereasons, we investigate quantum state transfer in both one-dimensional and hypercubestructures, in both spin and resonator systems.First, we investigate a high-dimensional quantum state transfer protocol. An arbi-trary unknown high-dimensional state can be transferred with high fidelity between tworemote registers through an XX coupling spin chain of arbitrary length. The evolutionof the state transfer is determined by the natural dynamics of the chain without externalmodulation and coupling strength engineering. As a consequence, entanglement dis-tribution with a high efficiency can be achieved. Also the strong field and high spinquantum number can partly counteract the effect of finite temperature to ensure the highfidelity of the protocol when the quantum channel is in a thermal equilibrium state underan external magnetic field.Second, we study a high-dimensional quantum state transfer in hypercube spin net-works. A high-dimensional SWAP gate between two antipodes is implemented and gov-erned by the natural dynamic of such networks with neither external modulation norcoupling strength engineering. Moreover, the evolution time is independent of the dis-tance between the two antipodes. As a consequence, we extend this result to a coupledquantum spin chain with different intra-spin coupling strengths.Third, we propose and analyze a multi-photon state coherent transport protocol in acoupled-resonatorquantumnetwork. Amulti-photonSWAPgatebetweentwoantipodescan be achieved with neither external modulation nor coupling strength engineering.Moreover, we extend this scheme to a coupled-resonator chain of arbitrary length withdifferent coupling strengths. Effects of decoherence via quantum non-demolition inter-action are studied with sources including vacuum quantum fluctuation and bath thermalexcitations when the bath is in the thermal equilibrium state. These observations are helpful to understand the decoherence effects on quantum communication in quantumcoupled-resonator systems.Four, we propose a controllable quantum state transfer in a spin chain. A controlspin interacts with the spin chain to control quantum state transfer. When the couplingbetween the control spin and the spin chain vanishes, the quantum state switch turns onand a perfect quantum state transfer is allowable. When such coupling strength is muchstronger than that between the chain spins, the quantum state switch turns off and thequantum state becomes invariant.