Quantum Network With Multiple Cold Atomic Ensembles

The construction of quantum network means realization of quantum entanglement among multiple remote quantum memories,which enables multi-party quantum com-munication.Such a network can also apply to distributed quantum computing,dis-tributed quantum precision measurement and fundamental tests of physics under large-scale space,etc.As photon can transfer quantum information,it suits well for connecting different quantum memories.Generating entanglement between photon and quantum memory with high quality would be important.There are many physical systems for quantum memory,while quantum memory with cold atomic ensemble is favored for its supe-riority in terms of retrieval efficiency and storage lifetime.In cold atomic ensembles,photon-atom entanglement can be generated directly,which suits very well for quantum network.In this paper,we mainly focused on research of quantum memory with cold atomic ensembles.By using ring cavity,interaction between atoms and photons can be enhanced,which helps increase the intrinsic retrieval efficiency,and the filter device can be also excluded by setting the control field off resonance from cavity while signal field in resonance with cavity.Thus we can acquire photon-atom entanglement with high brightness.After optimizing the influence of polarization,multi excitation events,etc,we acquire atom-photon entanglement with high fidelity.Combining with interference of identical photons,entanglement swapping can be realized with high fidelity.Three quantum network nodes are connected,GHZ-type entanglement of three atomic ensem-bles and hybrid entanglement of three atomic ensembles and three photons are generated for the first time.Furthermore,to extend the distance among quantum network nodes,quantum frequency conversion is used and non-classical correlation between photons at telecom band with atoms is observed.Then heralded entanglement between atomic ensembles separated by 50 km fiber is created.Besides,by making using of three cold atomic ensembles,we generate three photons with different color simultaneously,and experimentally demonstrate time-resolved interference among the three photons,which paves the way to improving compatibility of quantum networks in the future.Our work of entanglement of multiple cold atomic ensembles and interference of non-identical photons would greatly promote the construction of quantum network.In the future work,deterministic atom-photon entanglement from Rydberg system and long-lived quantum memory can be used to increase the success probability of nodes connection,which would promote the realization of a global quantum network.