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Integrating quantum key distribution with classical communications in backbone fiber network
Yingqiu Mao, Bi-Xiao Wang, Chunxu Zhao, Guangquan Wang, Ruichun Wang, Honghai Wang, Fei Zhou, Jimin Nie, Qing Chen, Yong Zhao, Qiang Zhang, Jun Zhang, Teng-Yun Chen, Jian-Wei Pan
TL;DR
Backbone QKD coexistence is difficult because intense Tbps classical traffic generates noise in weak quantum signals, while dark-fiber deployment limits scalability. The paper demonstrates QKD integrated with a commercial 3.6 Tbps backbone network over 66 km at 21 dBm, using narrow filtering and fiber characteristics to manage noise. Secure key rates reach 4.5 kbps for co-propagation and 5.1 kbps for counter-propagation at maximum launch power, supporting coexistence with existing backbone infrastructure within the demonstrated scope.
Problem
Backbone networks carry high-power Tbps classical traffic that generates strong Raman noise, while conventional QKD demonstrations often require dedicated dark fibers, increasing costs and limiting network scale.
Method
The paper integrates decoy-state BB84 QKD with commercial backbone traffic over deployed fiber and models filtering, attenuation, and effective core area as coexistence factors.
Results
4.5 kbps and 5.1 kbps secure key rates are achieved for co-propagation and counter-propagation, respectively, with 3.6 Tbps data at 21 dBm over 66 km.
Takeaways & Limitations
The demonstrated results validate the feasibility of quantum networks coexisting with current classical backbone fiber infrastructure.
Takeaways & Limitations
Transient problems from dropping or adding data channels are not included, and the classical-channel bandwidth does not cover all C+L-bands.
Abstract
from arXiv · showhide
Quantum key distribution (QKD) provides information-theoretic security based on the laws of quantum mechanics. The desire to reduce costs and increase robustness in real-world applications has motivated the study of coexistence between QKD and intense classical data traffic in a single fiber. Previous works on coexistence in metropolitan areas have used wavelength-division multiplexing, however, coexistence in backbone fiber networks remains a great experimental challenge, as Tbps data of up to 20 dBm optical power is transferred, and much more noise is generated for QKD. Here we present for the first time, to the best of our knowledge, the integration of QKD with a commercial backbone network of 3.6 Tbps classical data at 21 dBm launch power over 66 km fiber. With 20 GHz pass-band filtering and large effective core area fibers, real-time secure key rates can reach 4.5 kbps and 5.1 kbps for co-propagation and counter-propagation at the maximum launch power, respectively. This demonstrates feasibility and represents an important step towards building a quantum network that coexists with the current backbone fiber infrastructure of classical communications.
1. Introduction
QKD offers unconditional security, but conventional demonstrations often require dark fibers, increasing deployment costs and limiting network scale. This work addresses the challenge of coexistence with high-power backbone traffic, demonstrating QKD alongside commercial classical communications.
- QKD provides proven unconditional security for communication between remote users based on quantum physics.
- Dedicated dark fibers around 1550 nm impose fiber leasing and maintenance costs and limit the scale of quantum communication networks.
- Existing classical infrastructures use TDM and WDM to support data throughput of up to Tbps over ultra-long distances, motivating QKD integration with deployed fibers.
- Earlier coexistence work multiplexed a 1300 nm QKD channel with 1.2 Gbps conventional data over 28 km, but reported neither privacy amplification nor secret-key yield.
- Recent demonstrations used spectral and temporal controls, including 100 GHz filtering, to combine QKD with high-speed classical channels over fiber spans up to 150 km or 80 km.
- Backbone coexistence is harder because Tbps traffic can require approximately 20 dBm launch power, producing stronger Raman noise and exposing systems to field environmental factors.
- The paper demonstrates coexistence with 3.6 Tbps commercial backbone traffic over 66 km at 21 dBm, using filtering and fiber design to obtain secure keys.
2. Experiment
The field experiment integrates polarization-encoded decoy-state BB84 QKD with commercial backbone traffic across deployed fibers. WDM, narrow filtering, and large-core-area fibers are used to manage classical-channel noise and evaluate real-time operation.
- Field configuration: The experiment uses a 66 km installed fiber segment between Zhucheng and Huangshan within an 800 km backbone loop network spanning eight nodes.
- Fiber selection: G654 fibers with larger effective core areas reduce optical power density and can enhance QKD performance during coexistence experiments.
- Classical communications: The classical system provides an aggregate transmission rate of 3.6 Tbps using commercial line cards and super-channels carrying 200 Gbps and 100 Gbps channels.
- Classical communications: The classical spectrum contains 20 channels across approximately 1528–1538 nm, with launch power tunable from 8 to 21 dBm and optimal power around 18 dBm.
- QKD system: QKD uses polarization encoding and a decoy-state BB84 protocol, transmitting 1310 nm pulses at a 625 MHz repetition rate.
- QKD system: Real-time secure-key extraction applies authentication, key and basis sifting, error correction, and subsequent post-processing after detector registration.
- Coexistence design: WDM filters and a temperature-compensated fiber Bragg grating form a 20 GHz pass-band filter, while measured crosstalk is negligible relative to detector dark counts.
- Noise analysis: At 1310 nm, the wavelength gap makes four-wave mixing negligible and low-bandwidth Brillouin scattering ineffective; spontaneous Raman scattering remains the relevant scattering noise.
3. Results and discussion
Experiments and modeling show that QKD can coexist with high-power classical traffic, with performance governed by propagation direction, filtering, fiber attenuation, and effective core area. The measured system maintained secure key generation and stable classical-channel performance under backbone-network conditions.
- Measured QKD performance: At launch powers above 18 dBm, counter-propagation key rates significantly declined; at 19 dBm, QBER reached 4.0% and no secure keys were generated.The decline corresponds to more SRS photons and lower QSNR than in co-propagation.
- Measured QKD performance: 4.5 kbps and 5.1 kbps secure key rates were achieved for co-propagation and counter-propagation, respectively, in G654-110 fiber at maximum launch power.The higher counter-propagation rate is attributed to the relatively lower attenuation of G654-110-2 fiber.
- SRS modeling: 18, 10, and 8 cps/dBm·km were the calculated β20 GHz SRS coefficients for G652, G654-110, and G654-130 fibers, respectively.The coefficients were obtained by averaging measurements at different launch powers for both propagation directions.
- Factors affecting key rate: 20 GHz filtering was necessary in the simulations, because 100 GHz filtering produced QSNR values below 8 dB for counter-propagation and no secure keys in standard fiber.The modeled comparison evaluated filter bandwidth, fiber attenuation, and effective core area over 66 km at 21 dBm launch power.
- Factors affecting key rate: Low-loss fiber enhanced QSNR by 15.4 dB on average and increased key rate at least twofold, while large effective core area provided a moderate additional improvement.The combined modeled configuration reached 6.0 kbps for co-propagation and 5.2 kbps for counter-propagation.
- Stability and classical-channel impact: 6.2 kbps, 3.0 kbps, and 2.0 kbps were the average three-hour co-propagation key rates in G654-110, G652, and G654-130 fibers at approximately 18 dBm.The stability tests were conducted despite traffic, electricity, and construction activity.
- Stability and classical-channel impact: QKD inclusion caused negligible OSNR variation in the classical system, while WDM insertion losses slightly increased Q-factor above 16 dBm.The insertion losses lowered launch power and reduced optical effects between adjacent classical channels.
4. Conclusion
The study demonstrates coexistence of QKD with a commercial 3.6 Tbps backbone network over 66 km at maximum launch power, while identifying future work for broader operating conditions.
- 4. Conclusion: 3.6 Tbps classical data coexisted with QKD over 66 km fiber at the maximum launch power.The authors describe this as the first such demonstration to the best of their knowledge.
- 4. Conclusion: Filter bandwidth, fiber attenuation, and effective core area were modeled and analyzed as factors affecting coexistence performance.
- 4. Conclusion: The work validates the feasibility of building a quantum network that coexists with current classical backbone fiber infrastructure.
- 4. Conclusion: Transient problems from dropping or adding data channels were not included, and the classical-channel spectrum did not cover all C+L-bands.The authors identify both topics for future investigation.
Funding
The research was supported by Chinese national and provincial funding programs and a priority R&D plan project.
- Funding: The National Key R&D Program of China supported the research.Grant numbers 2017YFA0303900 and 2017YFA0304004 are listed.
- Funding: The Anhui Provincial Natural Science Foundation provided support through Grant No. 1508085J02.
- Funding: The Priority R&D Plan Project supported the work through Grant No. 2015GGX101035.