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Satellite-based entanglement distribution over 1200 kilometers

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Science  16 Jun 2017:
Vol. 356, Issue 6343, pp. 1140-1144
DOI: 10.1126/science.aan3211
  • Fig. 1 Schematic of the spaceborne entangled-photon source and its in-orbit performance.

    (A) The thickness of the KTiOPO4 (PPKTP) crystal is 15 mm. A pair of off-axis concave mirrors focus the pump laser (PL) in the center of the PPKTP crystal. At the output of the Sagnac interferometer, two dichromatic mirrors (DMs) and long-pass filters are used to separate the signal photons from the pump laser. Two additional electrically driven piezo steering mirrors (PIs), remotely controllable on the ground, are used for fine adjustment of the beam-pointing for an optimal collection efficiency into the single-mode fibers. QWP, quarter-wave plate; HWP, half-wave plate; PBS, polarizing beam splitter. (B) The two-photon correlation curves measured on-satellite by sampling 1% of each path of the entangled photons. The count rate measured from the overall 0.01% sampling is about 590 Hz, from which we can estimate the source brightness of 5.9 MHz.

  • Fig. 2 The transmitters, receivers, and APT performance.

    (A) The entangled-photon beam (810 nm) is combined and co-aligned with a pulsed infrared laser (850 nm) for synchronization and a green laser (532 nm) for tracking by three DMs and sent out from an 8× telescope. For polarization compensation, two motorized QWPs and a HWP are remotely controlled. A fast steering mirror (FSM) and a two-axis turntable are used for closed-loop fine and coarse tracking, based on the 671-nm beacon laser images captured by cameras 1 and 2. BE, beam expander. (B) Schematic of the receiver at Delingha. The cooperating APT and polarization compensation systems are the same as those on the satellite. The tracking and synchronization lasers are separate from the signal photon and detected by single-photon detectors (SPDs). For polarization analysis along bases that are randomly switching quickly, two QWPs, a HWP, a Pockels cell (PC), and a PBS are used. BS, beam splitter; IF, interference filter. (C) The APT system starts tracking after the satellite reaches a 5° elevation angle. The left panel is a 50-s trace of the real-time image readout from the camera. Fine-tracking accuracy of ~0.41 μrad is achieved for both the x and y axes.

  • Fig. 3 Physical distances from the satellite to two ground stations and the measured channel attenuation.

    (A) A typical two-downlink transmission from the satellite to Delingha and Lijiang that lasted for about 275 s in one orbit. The distance from the satellite to Delingha varies from 545 to 1680 km. The distance from the satellite to Lijiang varies from 560 to 1700 km. The overall length of the two-downlink channel varies from 1600 to 2400 km. (B) The measured two-downlink channel attenuation in one orbit, using the high-intensity reference laser co-aligned with the entangled photons. The highest loss is ~82 dB at the summed distance of 2400 km, when the satellite has just reached a 10° elevation angle as seen from Lijiang station. Because the telescope has a diameter of 1.8 m (the largest) and thus has a higher receiving efficiency than other stations, when the satellite flies over Lijiang at an elevation angle of more than 15°, the channel loss remains relatively stable, from 64 to 68.5 dB.

  • Fig. 4 Measurement of the received entangled photons after transmission by the two-downlink channel.

    (A) Normalized two-photon coincidence counts in the measurement setting of the Embedded Image basis. (B) Normalized counts in the diagonal Embedded Image basis. Numbers in parentheses represent the raw coincidence counts of different measurement settings.

  • Fig. 5 Space-time diagram and Bell inequality violation.

    (A) The top panel illustrates the space-time relationship among the entanglement generation point (S), the quantum random-number generation points (R1 and R2), and the measurement results points (M1 and M2). The horizontal axis represents the distances between the ground stations and the satellite, which vary from 500 to 1700 km. In our experimental configuration, M1 and M2 are about 100 ns behind the light cone of S. The rate of quantum random-number generation is 5 kHz with an output delay below 200 ns. That is, the duration between R1 (R2) and M1 (M2) is in the range of 0.2 to 200.2 μs. Therefore, R1 (R2) and S are spacelike-separated, which implies that the freedom-of-choice loophole is distinctly closed, under the additional assumption that all the possible hidden variables must originate together with the entangled particles. The bottom panel illustrates the relationship between two ground stations, which are 1203 km apart. Taking into account the orbit height of 500 km, the length difference between the two free-space channels does not exceed 944 km. Thus, the spacelike criterion is satisfied between R1 and R2, R1 and M2, M1 and R2, and M1 and M2. As a result, the locality loophole is addressed. (B) Correlation functions of a CHSH-type Bell inequality for entanglement distribution. The measurement settings are the angles (ϕ1, ϕ2) used for the measurement of the polarization of photons by the Delingha and Lijiang stations, respectively. Error bars are one standard deviation, calculated from propagated Poissonian counting statistics of the raw photon detection events.

Supplementary Materials

  • Satellite-based entanglement distribution over 1200 kilometers

    Juan Yin, Yuan Cao, Yu-Huai Li, Sheng-Kai Liao, Liang Zhang, Ji-Gang Ren, Wen-Qi Cai Wei-Yue Liu, Bo Li Hui Dai, Guang-Bing Li, Qi-Ming Lu, Yun-Hong Gong, Yu Xu, Shuang-Lin Li, Feng-Zhi Li, Ya-Yun Yin, Zi-Qing Jiang, Ming Li, Jian-Jun Jia, Ge Ren, Dong He, Yi-Lin Zhou, Xiao-Xiang Zhang, Na Wang, Xiang Chang, Zhen-Cai Zhu, Nai-Le Liu, Yu-Ao Chen, Chao-Yang Lu, Rong Shu, Cheng-Zhi Peng, Jian-Yu Wang, Jian-Wei Pan

    Materials/Methods, Supplementary Text, Tables, Figures, and/or References

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    • Materials and Methods 
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