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Long-distance free-space measurement-device-independent quantum key distribution

Yuan Cao, Yu-Huai Li, Kui-Xing Yang, Yang-Fan Jiang, Shuang-Lin Li, Xiao-Long Hu, Maimaiti Abulizi, Cheng-Long Li, Weijun Zhang, Qi-Chao Sun, Wei-Yue Liu, Xiao Jiang, Sheng-Kai Liao, Ji-Gang Ren, Hao Li, Lixing You, Zhen Wang, Juan Yin, Chao-Yang Lu, Xiang-Bin Wang, Qiang Zhang, Cheng-Zhi Peng, Jian-Wei Pan

arXiv:2006.05088v1quant-ph

TL;DR

Atmospheric turbulence makes free-space MDI-QKD challenging by distorting spatial modes and causing intensity fluctuations in the channels. Adaptive optics is used to eliminate higher-order Zernike aberrations, including focus, astigmatism, and coma. The experiment between sites separated by approximately 20 km achieved a final key rate of 6.11 bps and generated over 295 kbit of secure key.

  • Problem

    Atmospheric turbulence makes free-space MDI-QKD challenging by distorting spatial modes and causing intensity fluctuations in the channels.

    Source: p. 3

  • Method

    Adaptive optics is used to eliminate higher-order Zernike aberrations, including focus, astigmatism, and coma.

    Source: S1. ADAPTIVE OPTICS · p. 13

  • Results

    The experiment between sites separated by approximately 20 km achieved a final key rate of 6.11 bps and generated over 295 kbit of secure key.

    Source: pp. 3, 7

  • Takeaways & Limitations

    The demonstration applies two-photon interference for detector-vulnerability removal to a noisy free-space channel between distant sites.

    Source: p. 3

    The reported analysis is parameterized by dark-count, basis-misalignment, and Alice/Bob channel-efficiency terms.

    Source: p. 7

Abstract

from arXiv · show

Measurement-device-independent quantum key distribution (MDI-QKD), based on two-photon interference, is immune to all attacks against the detection system and allows a QKD network with untrusted relays. Since the MDI-QKD protocol was proposed, fibre-based implementations have been rapidly developed towards longer distance, higher key rates, and network verification. However, owing to the effect of atmospheric turbulence, MDI-QKD over free-space channel remains experimentally challenging. Here, by developing the robust adaptive optics system, high precision time synchronization and frequency locking between independent photon sources located far apart, we realised the first free-space MDI-QKD over a 19.2-km urban atmospheric channel, which well exceeds the effective atmospheric thickness. Our experiment takes the first step towards satellite-based MDI-QKD. Moreover, the technology developed here opens the way to quantum experiments in free space involving long-distance interference of independent single photons.

S1. ADAPTIVE OPTICS

The adaptive-optics system represents wavefront aberrations with Zernike modes and corrects them to improve signal-photon coupling into a single-mode fibre. Acquiring, pointing, and tracking handles beam tilt, while a deformable mirror and SPGD algorithm target higher-order aberrations.

S1. ADAPTIVE OPTICS

  • The wavefront is represented as a modal expansion of Zernike polynomials, with each coefficient weighting its corresponding polynomial.Piston is ignored, tilt is handled by acquiring, pointing, and tracking, and higher-order modes such as focus and astigmatism are targeted by adaptive optics.Source: S1. ADAPTIVE OPTICS · p. 13
  • A tracking camera senses arrival direction and a fast steering mirror compensates tilt, while adaptive optics targets higher-order aberrations.The passage identifies focus, astigmatism, and coma among the higher-order modes.Source: S1. ADAPTIVE OPTICS · p. 13
  • A 40-unit deformable mirror and SPGD algorithm adjust wavefront compensation to maximize reference-beam power and, in turn, signal-photon coupling into the single-mode fibre.The reference beam shares the free-space channel and fibre with the signal beam; its measured power supplies the algorithm input.Source: S1. ADAPTIVE OPTICS · p. 13

S2. TIME SYNCHRONIZATION.

The timing system estimates and corrects clock time and frequency differences between the measurement station and each photon source, yielding a 32 ps standard deviation in arrival-time difference over the long term.

S2. TIME SYNCHRONIZATION.

  • 32 ps standard deviation in long-term arrival-time difference was achieved after estimating and correcting clock time and frequency differences.The measurement station directly measures a portion of arriving photons with superconducting nanowire single-photon detectors; the crystal oscillator's short-term Allan deviation is 8×10−14.Source: S2. TIME SYNCHRONIZATION · p. 14
  • The crystal oscillator has a guaranteed short-term Allan deviation of 8×10−14 under oven-controlled temperature.The clock differences between the measurement station and each source are estimated and corrected.Source: S2. TIME SYNCHRONIZATION · p. 14
  • A portion of arriving photons is measured at the station with superconducting nanowire single-photon detectors for clock-difference estimation and correction.The reported long-term arrival-time-difference standard deviation is 32 ps.Source: S2. TIME SYNCHRONIZATION · p. 14

S3. ASYMMETRIC FOUR-INTENSITY DECOY-STATE MDI-QKD PROTOCOL.

The protocol uses independently optimized source parameters for Alice and Bob and four intensity choices per source. Joint constraints on statistical fluctuations are used in the decoy-state key-rate analysis.

S3. ASYMMETRIC FOUR-INTENSITY DECOY-STATE MDI-QKD PROTOCOL.

  • Four intensity choices are used per source: three non-vacuum intensities and one vacuum state for each of Alice and Bob.The non-vacuum intensities are selected with source-specific probabilities.Source: S3. ASYMMETRIC FOUR-INTENSITY… · p. 14
  • Alice and Bob's Z-basis states use intensities µaz and µbz, while their other non-vacuum choices are prepared in the X basis.The vacuum states have intensities oa and ob.Source: S3. ASYMMETRIC FOUR-INTENSITY… · p. 14
  • The decoy-state key-rate analysis uses single-photon-pair counting and phase-flip bounds, Z-basis counting and bit-flip rates, binary entropy, and correction efficiency.The four-intensity protocol applies joint constraints to statistical fluctuations of different observables, which the passage says improves the key rate when statistical fluctuation is considered.Source: S3. ASYMMETRIC FOUR-INTENSITY… · p. 14
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