Quantum Entanglement Swapping Between Multipartite Gaussian Entangled States

Quantum information science is a new subject developed on quantum mechanics and information science.It can improve the calculation speed,increase the information capacity,ensure the information security and improve the detection accuracy.Quantum information science can break through the limit of classical information systems.In recent years,quantum information science has attracted scientists' attention,and there are rich contents to be researched and good applications.With the development of quantum information,the construction of quantum information network has become one of the hot topics.Multipartite entangled state is an important resource for realizing quantum information network and quantum computing.It plays an important role in quantum computing and quantum network.Greenberger-Horne-Zeilinger(GHZ)entangled states and Cluster entangled states are two common multipartite entangled states.GHZ entangled states have been used to construct quantum teleportation networks and realize controlled quantum dense coding.Cluster entangled state is a basic quantum resource to realize one-way quantum computing.A larger scale cluster entanglement state can be used to realize the one-way quantum computation by measurements and feedforward of measurement results.If we have established some local quantum networks,we can construct a global quantum network by connecting these local quantum networks.So the question is how do we can connect local networks? And at the same time maintain the entanglement characteristics of the multipartite entangled state? One feasible method of merging two multipartite entangled states into a larger multipartite entangled state is quantum entanglement swapping.In 2014,it has been proposed to build a global quantum network of clocks that may allow the construction of a real-time single international time scale(world-clock)with unprecedented stability and accuracy.Quantum entanglement swapping is an important technical method to realize the construction of quantum communication network.Quantum entanglement swapping is originally proposed and demonstrated in discrete variable optical systems,and then it is extended to continuous variable systems.Recently,entanglement swapping between discrete and continuous variable systems has been demonstrated,which shows the power of hybrid quantum information processing.However,quantum entanglement swapping between two multipartite entangled states has not been verified.We design the corresponding joint measurement and feedforward scheme according to the types of multipartite entangled states,and complete the quantum entanglement between two multipartite entangled states.Our presented experiment can provide a feasible technical reference for constructing more complicated quantum networks.The main completed works are as following:1.We present an experimental demonstration of the entanglement swapping between two independent multipartite entangled states,each of which involves a tripartite continuous variable GHZ entangled state.The entanglement swapping is implemented deterministically by means of a joint measurement on two optical modes coming from the two multipartite entangled states respectively and the classical feedforward of the measurement results.After entanglement swapping the two independent multipartite entangled states are merged into a large entangled state in which all unmeasured quantum modes are entangled.2.We demonstrate the entanglement swapping between a tripartite continuous variable GHZ state and an continuous variable Einstein-PodolskyRosen(EPR)entangled state and the dependence of the resultant entanglement on transmission loss is investigated.3.We present a scheme for connecting two Gaussian continuous variable cluster states by entanglement swapping,and we analyze the connection schemes between different types of four-mode cluster states.Our project can be used to connect two local quantum networks composed by Gaussian continuous variable cluster states.The creative works are as follows:1.We experimentally realize the connection between two spaceseparated multipartite entangled states for the first time.And the dependence of the resultant entanglement on transmission loss is investigated.2.We present a scheme for connecting two Gaussian cluster states with different types by entanglement swapping,and we show that the structure of the output states after entanglement swapping may be not the same as that of the input states.