Preparation and storage of frequency-uncorrelated entangled photons from cavity-enhanced spontaneous parametric downconversion

Journal name:
Nature Photonics
Year published:
Published online


The preparation and storage of photonic entanglement are central to the achievement of scalable linear optical quantum computation1, 2, 3 (LOQC). The most widely used photonic entanglement source (a spontaneous parametric downconversion (SPDC) source)4, 5 is not directly suitable for storage, because its working frequency bandwidth is significantly larger than any available quantum memory. To remedy this problem, cavity-enhanced narrow-band SPDC sources6, 7, 8, 9, 10, 11, 12 have been developed. However, the storage of cavity-enhanced narrow-band entangled photons has not yet been achieved. Also, the spectral correlations between the entangled photons can make them practically useless for scalable LOQC5, 13, 14. Here, we report the preparation and storage of frequency-uncorrelated narrowband (5 MHz) entangled photons from a cavity-enhanced SPDC source. The frequency correlation between the entangled photons is eliminated by changing the continuous UV pumping beam to short pulses. The storage of the polarization state of a single photon, and of a photon entangled with another flying in the fibre, is demonstrated. Our work demonstrates a quantum interface between narrow-band entangled photons from cavity SPDC and atomic quantum memory, and thus provides an important tool towards the achievement of all-optical quantum information processing.

At a glance


  1. Experimental set-up.
    Figure 1: Experimental set-up.

    In the source laboratory, a pair of photons is generated by applying UV pump light to a periodically poled KTiOPO4 crystal (PPKTP) inside a cavity. The flying photon is coupled into a 60-m-long single-mode fibre and guided to a polarization state analyser, which consists of a half wave plate (HWP), a quarter wave plate (QWP), a polarizing beamsplitter (PBS) and single photon detectors. The signal photon is directed through a 20-m-long fibre to the memory laboratory located 10 m away. In the memory laboratory, a cold atomic cloud trapped in an MOT serves as the quantum memory for the signal photon, with spatial modes U and D selected as two ensembles for storage of the vertical (V) and horizontal (H) polarization components, respectively. The path-length difference between the U and D modes is set to zero and actively stabilized. The two polarization modes are both transferred to be σ+ for storage. A strong control light is applied at an angle of 2° relative to the two spatial modes. The signal photon is stored and read out of the atomic ensemble by changing the strength of the control light. The retrieved signal photon is transmitted through a Fabry–Perot cavity and an atomic filter cell to absorb the leakage from the control light, and then subject to a polarization analyser for state analysis.

  2. Characterizations of storage medium.
    Figure 2: Characterizations of storage medium.

    a, Illustration of laser configuration. Inset: relevant atomic levels of 87Rb and associated light fields. b, Transmission spectra of coherent 50-μs-long probe light versus probe detuning from the |5S1/2,F = 1right fence right arrow |5P1/2,F = 2right fence transition in the presence (red circles) and absence (blue circles) of the control field with a Rabi frequency of 12.6 MHz. The optical depth (OD) derived from the absorption profiles is 55 and the EIT widow is ~5.5 MHz. Each data point represents an average of 20 experimental trials. c, Coincidence rates between D1 and D3. The reference coincidence (without storage) has been multiplied by factor of 0.2 for the purpose of illustration. Leakage efficiency is defined as the ratio of coincidence during storage and before storage, and overall storage efficiency is defined by the ratio of the coincidence after retrieval and before storage.

  3. Performance of the quantum memory.
    Figure 3: Performance of the quantum memory.

    a, Cross-correlation g13(2) versus storage time. bd, Fidelities after a storage time of 200 ns for six initial polarization states: 0.954 ± 0.026 (|Hright fence), 0.989 ± 0.010 (|Vright fence), 0.909 ± 0.027 (| + right fence), 0.889 ± 0.037 (| − right fence) 0.920 ± 0.031 (|Rright fence) and 0.881 ± 0.035 (|Lright fence). e, Normalized polarization correlation for the retrieved entanglement. The red (blue) curve represents that the polarization of the flying photon is projected onto | + right fence (|Hright fence). Error bars represent ±1 standard deviation.


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Author information

  1. These authors contributed equally to this work

    • Han Zhang,
    • Xian-Min Jin &
    • Jian Yang


  1. Hefei National Laboratory for Physical Sciences at Microscale and Department of Modern Physics, University of Science and Technology of China, Hefei, Anhui, 230026, PR China

    • Han Zhang,
    • Xian-Min Jin,
    • Jian Yang,
    • Han-Ning Dai,
    • Sheng-Jun Yang,
    • Tian-Ming Zhao,
    • Jun Rui,
    • Yu He,
    • Xiao Jiang,
    • Ge-Sheng Pan,
    • Zhen-Sheng Yuan,
    • Youjin Deng,
    • Zeng-Bing Chen,
    • Xiao-Hui Bao,
    • Shuai Chen &
    • Jian-Wei Pan
  2. Physikalisches Institut, Universität Heidelberg, Philosophenweg 12, D-69120 Heidelberg, Germany

    • Fan Yang
  3. Institute for Theoretical Physics, University of Innsbruck, A-6020 Innsbruck, Austria

    • Bo Zhao


X-M.J., J.Y., H.Z., Z.-B.C., Y-J.D., X-H.B, S.C., B.Z. and J-W.P. designed the experiment. H.Z., X-M.J., J.Y., H-N.D., S-J.Y., T-M.Z., J.R., Y.H., X.J., F.Y., G-S.P., Z-S.Y. and S.C. performed the experiment and analysed the data. X-M.J., J.Y., H.Z., Y-J.D., X-H.B, B.Z. and J-W.P. edited the manuscript.

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