Long-lived Quantum Memory And Entanglement With Cold Atoms

Quantum memory is important constituent part in quantum information process-ing.It can be used in various aspects of quantum communication,quantum network construction,quantum computation and so on.As quantum memory medium,atomic ensemble has relatively strong interaction with photons,so the atomic state can be re-trieved as photons with high retrieval efficiency.On the other hand,cold atomic en-semble is a very promising candidate for long-lived memory.Therefore these features make atomic ensemble advantageous to be used in quantum repeater.With the develop-ing of cold-atom technique,great advances have been achieved in cold atomic quantum repeater during the past two decades.However,the strengths of cold atomic quantum memory still can not meet the needs of reality,such as the short lifetime,low measured retrieval efficiency and low bright-ness of atom-photon entanglement source.Hence the research contents of this thesis are mainly about long lifetime,high retrieval efficiency and atom-photon entanglement of quantum memory.At first,we carefully studied various factors of quantum mem-ory in atomic ensemble,and solved many problems,like differential light shift induced memory dephasing,sudden decline of efficiency at early storage time(during a few mil-liseconds).Eventually we successfully demonstrate an efficient(76%)and long-lived(0.22 s)quantum memory.Based on analyses of our experimental system,we prolong the memory lifetime further by careful optimizing the bias magnetic gradient and bet-ter isolation of leaking laser light.The entanglement can violate Bell's inequality with 2.5 standard deviations after 1 second storage.We investigate different schemes about atom-photon entanglement,to combine the ring cavity technique we developed,we generate highly retrievable spin-wave-photon entanglement source.Based on this ex-periment,we make further efforts to optimize the experimental system,with techniques of filtering ring cavity and Raman pulse assisted basis measurement,we make a break-through on both brightness and fidelity of the atom-photon entanglement source.Then we successfully demonstrate hybrid entanglement of three photons and three atomic ensembles,and by projection measurement of three write-out photons,we also achieve three quantum memory entanglement.The fidelities of both six-partite and three-partite GHZ state are about 0.7.At last,we efficiently store and retrieve three color-different single photons via three atomic ensembles,and we accomplish a multi-photon interfer-ence experiment with time-resolved method.The experiment shows us rich features of multi-photon interference.The advances we achieved are extremely valuable for future application in quantum repeater based long-distance quantum communication,quantum network construction,distributed quantum computing and so on.