Preparation Of Atom-photon Entanglement Based On Magnetic-field-sensitive Spin Waves

The development of quantum communication is a prerequisite for the construction of large-scale quantum networks.Quantum communication can achieve the secure transmission of information through distant entanglement distribution.However,due to loss and decoherence of quantum information during the transmission channel,the communication rate exponentially decreases with transmission channel distance.So it is difficult to achieve direct long-distance quantum communication,with greatly restricts the development of quantum networks.To address this issue,Briegel et al.proposed a quantum repeater scheme in 1998,whose basic idea is to divide the long transmission channel into many segments and extend the entanglement to the whole transmission channel through entanglement swapping.Since the generation probability of entangled photon pairs is very low and the entanglement manipulation is probabilistic,it results in very low success rate of long-distance entanglement distribution.If quantum storage is introduced into quantum nodes to store the entanglement generated,the waiting mechanism can be provided for the subsequent entanglement swapping,and the success rate of long-distance entanglement swapping is greatly improved.In 2001,Duan et al.proposed a Duan-Lukin-Cirac-Zoller（DLCZ）quantum repeater scheme,making it possible to achieve successful long-distance entanglement distribution.The scheme directly uses the non-classical correlated pairs between the scattered photon and the spin wave in the Raman scattering process to construct the quantum repeater.This paper focuses on DLCZ quantum memory,and carries out the following two works:Firstly,by utilizing the process of spontaneous Raman scattering in an ensemble of cold ⁸⁷Rb atoms,correlated pairs between atomic spin waves and photons are generated.To generate entanglement between magnetic-field-sensitive spin waves and photons two spatial modes of Stokes photons are encoded as a polarization qubit by a phase stable polarization interferometer.The long-wavelength spin wave is obtained by adopting the nearly-collinear style between the read-writing optical path and the detecting optical path.In this system,a long-lived entanglement source between photon and atom is realized,with the storage lifetime of 900 μs and the Bell parameter S=2.58±0.03.This work enriches the research content of quantum interface between photon and atom,and provides a foundation for long-distance quantum communication.Secondly,based on photon-atom entanglement generated through the DLCZ scheme,we have achieved tunable pulse width of the Stokes photon-atom entangled with atomic spin waves by varying the interaction time between the light and atoms.The pulse width can range from 40ns to 50μs.When the pulse width is 40ns,the Bell parameter S=2.64±0.02,which violates the Bell inequality by 32 standard deviations.When the pulse width increases to 50μs,the Bell parameter S=2.26±0.05,which violates the Bell inequality by 5.2 standard deviations.This work provides a foundation for the interconnection between heterogeneous network nodes.