Generation Of Long Life Multimode Entanglement Between An Atomic Memory And A Photon In Atom Ensemble

Quantum repeaters(QRs)hold promise for distributing entanglement over long distances,thereby providing a feasible path to realize long-distance quantum communications and large-scale quantum networks.The entanglement between photonic and atomic(spin-wave storage)qubits is a basic building block for QRs.Great progress has been made in optical quantum storage based on cold atomic ensemble in the recent 10 years.However,the premise for quantum storage to acquire practical application in quantum information and other fields is that its storage performance is good enough.Large multimode capacity,long lifetime and high storage efficiency are three important indicators to achieve high quantum storage.In this dissertation,we have carried out a series of research work based on DLCZ(Duan–Lukin–Cirac–Zoller)quantum storage scheme.The details of the main work are followed:(1)The photon-atom entanglement is realized by spontaneous Raman scattering process in the ⁸⁷Rb cold atom ensemble.We study the influence of read power and optical depth(OD)on the readout efficiency,and optimize the readout efficiency.Based on this,we measure the variation of the entanglement quality of photon and atom(Bell parameter)with the retrieval efficiency.It shows that,the bell parameter increases with the increase of retrieval efficiency;when the retrieval efficiency increases to 5%,the bell parameter changes very small.By further analysis,we find out that the retrieval efficiency is related to the signal-to-noise ratio,and the higher the signal-to-noise ratio is,the higher the entanglement quality is.(2)In the experiment,we establish the millisecond quantum memory and demonstrate the deterministic generation of and partial retrieval of a spin wave excitation.The creation probability of the spin wave excitation within a given time(?730?s)is approximately 1(?99.6%).Such deterministic generation depends on a feedback scheme with a millisecond quantum memory.The spin wave is partially read out by two read pulses.In the experiment,the partial retrievals of spin wave is partially by using a first read pulse,and the remained spin wave excitation is completely retrieved by a second pulse.Meanwhile,we measure the anti-correlation function between the detections in the first and second readouts.The results show that the partial-retrieval operation on the spin wave excitation is in the quantum regime.(3)We establish spatial multimode and long lifetime entanglement between photonic and atoms.In the experiment,we encode the multimode optical qubits in the cold atom DLCZ process using a phase stable polarization interferometer combined with near collinear configuration and magnetic-field-insensitive spin waves,generating long-lived,spatial multiplexed photon-atom quantum entanglement interface.The quantum interface(QI)can store three long-lived spin-wave qubits simultaneously.By using feed-forward-controlled system,we demonstrate that multiplexed quantum interface(QI)gives rise to a 3-fold increase in the atom–photon(photon–photon)entanglement-generation probability compared with single-mode QIs.The measured bell parameter is 2.51±0.01 and the memory lifetime is 1 ms.This work is an effective way to realize fiber-based long-distance quantum communications.(4)In the experiment,we use optical cavity to enhance the interaction between light and atom.The results show that the generation rate of correlated pairs of photons and atomic spin waves is increased by 3.8 times through spontaneous Raman scattering process in the ⁸⁷Rb cold atom ensemble.Meanwhile,we study the influence of optical cavity on the read field efficiency of atomic spin wave.The results show that the quantum read efficiency using optical cavity is 1.5 times higher than that of without optical cavity,and its corresponding intrinsic read efficiency is 40.6%.In this paper,based on the entanglement of photon-atom quantum storage,we realize the long-lived storage of quantum memory in the cold atom ensemble,generate the deterministic spin wave,and establish the spatial multiplexed and long-lived entanglement source.These works provide experimental basis for long-distance quantum communication,distributed quantum computing and quantum repeater.