Generation Of Multimode Entanglement Between An Atomic Memory And A Photon And Cavity-enhanced Optical Storage In Atom Ensemble

Quantum repeater(QR)holds promise for the distribution of entanglement over long distances and then find critical application in long-distance quantum communications and large-size quantum networks.Optical quantum memories are basical blocks for building the QR.In the last decade,siginificants on quantum memories based on electromagnetically-induced-transparency(EIT)dynamic and Duan-Lukin-Cirac-Zoller(DLCZ)protocol have been made.The storage efficiency,lifetime and multimold capacity are three important performance indicators of the memory.High storage efficiency,long lifetime and large multimold capacity are riquied for practically realizng QR.Toword to goals,we do the following works:(1)Based on cavity-enhanced coupling between cold atoms and light mode,we demonstrated long-lived and high-efficiency optical memories for two orthogonal polarizations modes in cold atoms.(2)we have demonstrated temporal multimode memories and then build a multiplexed spin-wave–photon entanglement source.The main contents are the following aspects:(1)Highly-efficient optical storage of two orthogonal polarization modes in a cold atom ensemble.We use a moderate intensity magnetic field to split the Zeeman energy level of atom and store the two orthogonal polarization modes on the magnetic-field-insensitive spin waves to suppress decoherence induced by resident magnetic field.And then we make the signal and control beams collinearly pass through the atoms to increase spin-wave length and suppress decoherence caused by atomic thermal motion.We obtain cigar-type cold atomic ensemble loading in a magneto-optical trap(MOT)and then optical depth of the atomic ensemble is increased.The storage efficiency is 30% and the storage lifetime is 3ms.The presented work represents a key step toward the realization of an efficient and long-lived optical memory.(2)Based on cavity-enhanced EIT optical memory,we demonstrate a highly-efficient optical storage of two orthogonal polarization modes in a cold atom ensemble.In the experiment,we use EIT process to store the left and right polarization signal light modes in cold atom.We use a medium intensity magnetic field to split the Zeeman energy level of atom and store the two orthogonal polarization modes on the magnetic-field-insensitive spin waves.And then we make the signal and control beams collinearly pass through the atoms to realize the spin-wave long wavelength memory.By using a low-finesse optical cavity to enhance the coupling between the atoms and the signal light field,the storage efficiency and storage lifetime of this quantum memory system is 30% and 3.5ms,which corresponding to its intrinsic value of 45%.The presented work represents a key step toward the realization of an efficient and long-lived optical memory for polarization entangled photons and may find applications in scale-up quantum information networks.(3)Experimental study on the generation of spin-wave-photon entanglement based on temporal multimode memories.By applying a train of write pulses in time,with each pulse coming from a different direction,to a homogeneously broadened atomic ensemble to induce Duan-Lukin-Cirac-Zoller-like Raman processes,we prepare up to 19 pairs of modes,namely,one spin-wave mode and one photonic time bin.Spin-wave-photon(i.e.,atom-photon)entanglement is probabilistically produced in these mode pairs.We transfer 19 time bins to a detector,if a Stokes photon is detected by the detector in one of these time bins,the storage of one spin-wave excitation is heralded.Based on the feed-forward-controlled readout which is realized by FPGA(field-programmable gate array),the heralded spin-wave excitation will be translated into an anti-Stokes photon.We build a temporally multiplexed source and then demonstrate an 18.8-fold increase in the probability of thegeneration of spin-wave-photon entanglement compared to the sources that use individual modes.The measured Bell parameter for the temporally multiplexed source is 2.30?0.02,corresponding to the memory lifetime is 30?s.The increase of entanglement generation rate provides a method and experimental basis for long-distance quantum entanglement distribution.(4)Experimental study on the generation of spin-wave-photon entanglement based on spatial multimode memories.By applying a write pulse into the atoms,which will lead to spontaneous Raman scattering process.We establish six atom–photon entanglement source by collecting Stokes photons in six different directions.By using feedback control mechanism,we realize a ~6-fold increase in the probability of generating of spin-wave–photon entanglement state.The measured Bell parameter for the spatial multiplexed source is 2.49 ± 0.03 and lifetime is ~60 ?s.