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Heralded entanglement distribution between two absorptive quantum memories

Abstract

Owing to the inevitable loss in communication channels, the distance of entanglement distribution is limited to approximately 100 kilometres on the ground1. Quantum repeaters can circumvent this problem by using quantum memory and entanglement swapping2. As the elementary link of a quantum repeater, the heralded distribution of two-party entanglement between two remote nodes has only been realized with built-in-type quantum memories3,4,5,6,7,8,9. These schemes suffer from the trade-off between multiplexing capacity and deterministic properties and hence hinder the development of efficient quantum repeaters. Quantum repeaters based on absorptive quantum memories can overcome such limitations because they separate the quantum memories and the quantum light sources. Here we present an experimental demonstration of heralded entanglement between absorptive quantum memories. We build two nodes separated by 3.5 metres, each containing a polarization-entangled photon-pair source and a solid-state quantum memory with bandwidth up to 1 gigahertz. A joint Bell-state measurement in the middle station heralds the successful distribution of maximally entangled states between the two quantum memories with a fidelity of 80.4 ± 2.2 per cent (±1 standard deviation). The quantum nodes and channels demonstrated here can serve as an elementary link of a quantum repeater. Moreover, the wideband absorptive quantum memories used in the nodes are compatible with deterministic entanglement sources and can simultaneously support multiplexing, which paves the way for the construction of practical solid-state quantum repeaters and high-speed quantum networks.

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Fig. 1: Schematic diagram for a quantum repeater and an elementary link with absorptive quantum memories.
Fig. 2: Experimental setup.
Fig. 3: Quantum state tomography of two-photon polarization entanglement of two sources.
Fig. 4: The performance of the quantum memory in node A.
Fig. 5: Verification of heralded remote entanglement between two quantum memories by polarization analysis of photons retrieved from two memories.

Data availability

The data presented in the figures within this paper and other findings of this study are available from the corresponding authors upon reasonable request. Source data are provided with this paper.

Code availability

The custom codes used to produce the results presented in this paper are available from the corresponding authors upon reasonable request.

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Acknowledgements

This work is supported by the National Key R&D Program of China (no. 2017YFA0304100), the National Natural Science Foundation of China (nos 11774331, 11774335, 11504362, 11821404 and 11654002), Anhui Initiative in Quantum Information Technologies (no. AHY020100), the Key Research Program of Frontier Sciences, CAS (no. QYZDY-SSW-SLH003), the Science Foundation of the CAS (no. ZDRW-XH-2019-1), and the Fundamental Research Funds for the Central Universities (nos WK2470000026, WK2470000029 and WK2030000022). Z.-Q.Z. acknowledges the support from the Youth Innovation Promotion Association, CAS.

Author information

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Authors

Contributions

Z.-Q.Z. and C.-F.L. designed the experiment. X. Liu and J.H. carried out the experiment with assistance from Z.-F.L. and X. Li. P.-Y.L. and P.-J.L. helped collect the data. X. Liu, J.H. and Z.-Q.Z. analysed the data and wrote the paper with input from all other authors. The project was supervised by Z.-Q.Z., C.-F.L. and G.-C.G. All authors discussed the experimental procedures and results.

Corresponding authors

Correspondence to Zong-Quan Zhou or Chuan-Feng Li.

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The authors declare no competing interests.

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Peer review information Nature thanks Daniel Oblak and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data figures and tables

Extended Data Fig. 1 Experimental setup and characterizations of entangled photon-pair sources.

a, Detailed setup for entangled photon-pair sources. b, c, Power dependence of the coincidence counting rate and second-order-correlation function \({g}_{12}^{(2)}(2\,{\rm{ns}})\) for each entangled photon-pair source located at two nodes, respectively. Error bars in b and c represent one standard deviation. DM, dichroic mirror; IF, interference filter; QM, quantum memory.

Extended Data Fig. 2 Characterization of the heralded entangled photon pairs.

a, The fidelity of heralded entangled photon pairs as a function of average \({g}_{12}^{(2)}(2\,{\rm{ns}})\) of two sources. b, Measured S parameter of CHSH-type Bell’s inequality with respect to the average \({g}_{12}^{(2)}(2\,{\rm{ns}})\). Error bars are one standard deviation.

Extended Data Fig. 3 An example of an AFC structure with a bandwidth of 1 GHz.

The central 200-MHz AFC is generated by an AOM, and the four sidebands are generated by an EO-PM in parallel. The total AFC bandwidth is approximately 1 GHz. The AFC structure is determined by the transmission of weak probe light using single-photon detectors. The polarization of the probe light is chosen as H + V.

Extended Data Fig. 4 The AFC echo intensity as a function of storage time.

The echo intensity (blue point) is normalized to the value of 55.6-ns storage time. Error bars represent one standard deviation. The red line is double exponential fit (Aet/τ1 + Bet/τ2) of the experimental data, with A = 1.04, B = 0.29, τ1 = 134 ns and τ2 = 1,141 ns. The 1/e lifetime of storage efficiency is deduced to be 193 ns based on fitted data. a.u., arbitrary units.

Extended Data Fig. 5 Temporal multiplexed operations of a quantum memory.

a, A schematic diagram of temporally multiplexed operation. Compared to a single-mode scenario, four temporal modes are stored within the 55.6-ns storage time in the experiment. b, The estimated entanglement distribution rate (EDR) as a function of the mode number. The red star is achieved EDR with multiplexing of four time modes achieved in the experiment and the grey dots are estimated values based on experimental data.

Extended Data Fig. 6 Estimated fidelity of heralded remote entanglement between two quantum memories as a function of storage efficiency.

The blue dot is the data measured in the experiment and the black solid line is the simulation based on experimentally determined background noise. The red dashed line represents the fidelity of the classical bound. The error bar of the blue dot is one standard deviation.

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Liu, X., Hu, J., Li, ZF. et al. Heralded entanglement distribution between two absorptive quantum memories. Nature 594, 41–45 (2021). https://doi.org/10.1038/s41586-021-03505-3

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