Entanglement Of Remote Quantum Memories

A quantum network means a network connecting quantum processors located in different places and tranfering quantum states among them.It could provide some revo-lutionary functions,such as global secure communication,distributed quantum comput-ing,etc.It relies on the entanglement of remote quantum memories over long distances.In this thesis,we try to extend the distance by using quantum memories based on cold atoms ensembles.The development of remote entanglement is hindered by low brightness of atom-photon entanglement,and high transmission loss in fiber channel.We first use a ring cavity to enhance the atom-photon interaction,thence the retrieval efficiency is promoted.Besides,the cavity serves as a filter itself.Hence no extra filter loss is introduced.Compared with our previous results,the brightness of atom-photon entanglement is one order of magnitude higher.Then by using quantum frequency conversion,we shift the atomic wavelength from 795 nm to 1342 nm in the telecom-munications band.The transmission attenuation drops from 3.5 dB/km to 0.3 dB/km.Next,we create remote entanglement by photon interference.There are two main schemes in remote entanglement creation:two-photon interference scheme and single-photon interference scheme.Generally,the two-photon interference scheme requires less on phase stability.However,it offers a lower entanglement probability.The single-photon interference scheme offers a higher entanglement probability.Nevertheless,it requires a more stable phase environment.We realize entanglement first via two-photon interference over 22 kilometres of field-deployed fibres,and second via single-photon interference over 50 kilometres of coiled fibres.To achieve remote single-photon inter-ference,we design and perform two-stage phase stabilization.The phase uncertainty of 50 kilometres is suppressed to about 50 nm.Though two memories are located in the same laboratory now,our experiment could be extended to nodes physically separated by similar distances with some minor amendments.In addition,we make other two considerations about our quantum memory.One is about the verification of a quantum memory.We,for the first time,demonstrate the verification of a quantum memory via the measurement-device-independent scheme.Besides,we use a small atomic ensemble to generate high-quality single photons with the help of the Rydberg blockade effect,and a quantum random number generator to ensure a random polarization preparation.Our approach takes into account maneuver-ability while ensuring safety,thus has a strong practicality.The other one is that we propose some protocols to enhance the performance of current quantum memory.One part is about achieving deterministic atom-photon entanglement and deterministic en-tanglement swapping inside a quantum memory.The other part is about spin-wave ma-nipulation in a DLCZ quantum memory.We introduce how to achieve a long-lifetime and multimode storage of a qubit.Our work of 22 km and 50 km entanglement between quantum memories by two different schemes paves the way towards a quantum repeater-based quantum network.By combining with techniques such as Rydberg-based deterministic entanglement,long lifetime storage,etc.,it will greatly promote the experimental research of quantum re-peaters and full quantum networks.