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Measurement device independent quantum key distribution over 404 km optical fibre
Hua-Lei Yin, Teng-Yun Chen, Zong-Wen Yu, Hui Liu, Li-Xing You, Yi-Heng Zhou, Si-Jing Chen, Yingqiu Mao, Ming-Qi Huang, Wei-Jun Zhang, Hao Chen, Ming Jun Li, Daniel Nolan, Fei Zhou, Xiao Jiang, Zhen Wang, Qiang Zhang, Xiang-Bin Wang, Jian-Wei Pan
TL;DR
Stable, enduring MDIQKD remains challenging. This work implements an optimized four-intensity decoy-state MDIQKD system over 404 km of ultralow-loss fibre and 311 km of standard fibre, achieving a 3.2 × 10−4 bps key rate and the longest reported distance for any QKD system.
Problem
Achieving a stable and enduring MDIQKD system remains a nontrivial research challenge.
Method
The experiment uses an optimized four-intensity decoy-state MDIQKD implementation over ultralow-loss and standard optical fibre.
Results
3.2 × 10−4 bps was achieved as the key rate, representing the longest distance reported for any QKD system.
Takeaways & Limitations
The implementation establishes a distance record for QKD systems.
Takeaways & Limitations
System performance could be further improved by increasing the system clock rate and single-photon detector efficiency.
Abstract
from arXiv · showhide
Quantum key distribution (QKD) can provide unconditional secure communication between two distant parties. Although the significance of QKD is undisputed, its feasibility has been questioned because of certain limitations in the practical application of real-life QKD systems. It is a common belief the lack of perfect single-photon source and the existence of detection loss will handicap the feasibility of QKD by creating security loopholes and distance limitations. The measurement device independent QKD (MDIQKD) with decoy-state method removes the security threats from both the imperfect single-photon source and the detection loss. Lengthening the distance and improving the key rate of QKD with such a superior method is thus the central issue in the practical application of QKD. Here, we report the results of MDIQKD over 404 km of ultralow-loss optical fibre and 311 km of standard optical fibre by employing an optimized four-intensity decoy-state method. This record-breaking implementation of MDIQKD method not only provides a new distance record for both MDIQKD and all types of QKD systems, more significantly, it achieves a distance that the traditional BB84 QKD would not be able to achieve with the same detection devices even with ideal single-phone sources. For the first time, our work demonstrates that with the MDIQKD method, imperfect devices can achieve better results than what ideal sources could have achieved. This work represents a significant step towards proving and developing a feasible long-distance QKD.
respectively. An beam splitter (BS) and two superconducting nanowire single-photon detectors
The optimized MDIQKD system combines a Bell-state measurement with calibrated, feedback-controlled operation and achieves record-distance key generation. Four-intensity optimization substantially improves key rates, including secure finite keys at 311 km and 404 km, where passive BB84 cannot generate a secure key with the same devices.
- Key-rate optimization: At 207 km, the key rate reached 9.55 bps, more than 500 times higher than the earlier experiment for the same accumulation time.About 50 times arose from the four-intensity method and device improvements, while the time-window efficiency increased the rate by about 10 times.
- Comparison with BB84: At 311 km, traditional passive BB84 generated no secure key even with an ideal single-photon source and without statistical fluctuations.The asymptotic BB84 key rate is bounded below zero because eX > 7.55% and eX + eZ > 26.25%.
Methods
The experiment improves synchronization, spectral matching, polarization control, and phase stabilization to support high-quality interference over long fibre links. Key implementations include 10 ps timing compensation, tightly matched laser spectra, real-time visibility feedback, and AMZI phase stabilization.
- Synchronization: Charlie records Alice’s and Bob’s pulse arrival times and compensates their difference using a programmable delay chip with 10 ps timing resolution.A synchronization signal delay method is also used to compensate timing differences.
- Synchronization: Chromatic dispersion is less than 30 ps in both 404 km ultralow-loss and 311 km standard optical fibre, versus a 2.5 ns signal pulse width.The dispersion is therefore much smaller than the signal pulse width.
- Spectral indistinguishability: Two semiconductor laser diodes are selected and controlled so their central wavelength difference remains less than 0.3 pm for each spectrum calibration.Their laser pulse spectrum FWHM is 3.2 pm, measured with a Fabry-Perot interferometer of 0.06 pm resolution.
- Polarization control: An extra SNSPD with 30 dark counts per second enables high-visibility real-time feedback during the 404 km ultralow-loss-fibre experiment.The effective SNSPD count remains about 3900 per second.
National Fundamental Research Program (under 2013CB336800), the National Natural Science
The work acknowledges support from multiple Chinese national, provincial, academic, and corporate funding sources, including specified grant programs and QuantumCTek Co., Ltd.
- Additional support came from the Chinese Academy of Science and the 10000-Plan of Shandong Province.
- The project also received support from the Science Fund of Anhui Province for Outstanding Youth and the National High-Tech Program of China.
- The National High-Tech Program of China provided grants 2011AA010800 and 2011AA010803, alongside support from QuantumCTek Co., Ltd.