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Field and long-term demonstration of a wide area quantum key distribution network
Shuang Wang, Wei Chen, Zhen-Qiang Yin, Hong-Wei Li, De-Yong He, Yu-Hu Li, Zheng Zhou, Xiao-Tian Song, Fang-Yi Li, Dong Wang, Hua Chen, Yun-Guang Han, Jing-Zheng Huang, Jun-Fu Guo, Peng-Lei Hao, Mo Li, Chun-Mei Zhang, Dong Liu, Wen-Ye Liang, Chun-Hua Miao, Ping Wu, Guang-Can Guo, Zheng-Fu Han
TL;DR
Wide-area QKD requires network architectures and field-tested devices that support multiple users over conventional telecommunications infrastructure. This paper demonstrates the Hefei-Chaohu-Wuhu network, combining metropolitan and intercity links with standardized devices and networking techniques. The network covered more than 150 kilometers and operated for more than 5000 hours, while addressing device symmetry and dynamic link switching.
Problem
QKD must be extended from point-to-point links to multi-user, large-scale networks that can operate over metropolitan and intercity fiber infrastructure.
Method
The authors deployed a wide-area QKD network over China Mobile fiber, combining full-mesh and point-to-multipoint metropolitan schemes with standardized devices, symmetry resolution, and switching techniques.
Results
More than 5000 hours of operation and over 150 kilometers of coverage were demonstrated, with the network integrating full-mesh core and point-to-multipoint access configurations.
Takeaways & Limitations
The demonstrated architecture provided a field-deployed platform for secure and stable key distribution across cities using shared communication infrastructure.
Takeaways & Limitations
The authors note that device-independent or measurement-device-independent QKD could further enhance practical security in future work.
Abstract
from arXiv · showhide
A wide area quantum key distribution (QKD) network deployed on communication infrastructures provided by China Mobile Ltd. is demonstrated. Three cities and two metropolitan area QKD networks were linked up to form the Hefei-Chaohu-Wuhu wide area QKD network with over 150 kilometers coverage area, in which Hefei metropolitan area QKD network was a typical full-mesh core network to offer all-to-all interconnections, and Wuhu metropolitan area QKD network was a representative quantum access network with point-to-multipoint configuration. The whole wide area QKD network ran for more than 5000 hours, from 21 December 2011 to 19 July 2012, and part of the network stopped until last December. To adapt to the complex and volatile field environment, the Faraday-Michelson QKD system with several stability measures was adopted when we designed QKD devices. Through standardized design of QKD devices, resolution of symmetry problem of QKD devices, and seamless switching in dynamic QKD network, we realized the effective integration between point-to-point QKD techniques and networking schemes.
1. Introduction
QKD networks extend secure quantum key sharing from point-to-point links to multi-user, wide-area deployments over conventional fiber infrastructure. This paper demonstrates such a network across three cities, combining full-mesh and point-to-multipoint metropolitan configurations with improved networking techniques.
- Motivation: QKD networks extend quantum key distribution from point-to-point links to multi-user and large-scale scenarios using metropolitan and intercity network architectures.Metropolitan core networks typically use mesh configurations, while intercity backbones connect metropolitan networks through long-haul links.
- Contribution: The demonstrated network used China Mobile’s telecommunication fiber infrastructure to link three cities and two metropolitan QKD networks across more than 150 kilometers.The Hefei network provided full-mesh all-to-all interconnections, while the Wuhu network used point-to-multipoint access-network configuration.
- Contribution: The Hefei-Chaohu-Wuhu network was presented as the first wide area QKD network and introduced improved core and access networking schemes.The core scheme combined passive and active optical elements, while the access scheme used optical switching; the authors report greater reliability, flexibility, and reconfigurability.
- Contribution: The work addressed QKD networking challenges by resolving device symmetry and enabling seamless switching among different links.These techniques were combined with standardized device design for field deployment in complex network environments.
2. Overview of the Hefei-Chaohu-Wuhu wide area QKD network
The Hefei-Chaohu-Wuhu network connected nine nodes across three cities through eight fiber links and an intercity connection. Its architecture combined a full-mesh Hefei metropolitan core with a time-division-multiplexed Wuhu quantum access network.
- Network composition: Three cities—Hefei, Chaohu, and Wuhu—were connected through two metropolitan QKD networks and an HCW intercity link using a trusted intermediate node.The Hefei metropolitan network had five nodes, the Wuhu network had three, and Chaohu hosted the intermediate node.
- Network composition: Almost 200 km of installed fiber formed eight optical fiber links among nine network nodes, with the intercity fiber length exceeding 150 km.The link characteristics were catalogued in Table 1.
- Networking schemes: The Hefei metropolitan network used a full-mesh topology in which every node had a direct link to every other node.This configuration served as a metropolitan core network providing all-to-all interconnections.
- Networking schemes: The Wuhu metropolitan network used time-division multiplexing with one point-to-point QKD link active at each time.Its three-node arrangement simulated a quantum access network using shared fiber and additional optical components.
3. P2P QKD devices in the wide area QKD network
The network used standardized Faraday-Michelson phase-coding QKD devices in divided and integrated transceiver designs, incorporating countermeasures for practical attacks. Device design also addressed operational stability, synchronization, detection, random-number generation, and future security enhancements.
- Device architectures: All point-to-point devices adopted Faraday-Michelson interferometers for phase-coding BB84 with decoy states.The network included divided devices with separate transmitters and receivers, and integrated QKD-Transceivers combining both with an optical switch.
- The divided type: The divided device transmitted synchronization and quantum signals through the same fiber channel.The transmitter generated synchronized and phase-encoded quantum pulses, while the receiver demultiplexed, decoded, and detected them.
- The integral type: The QKD-Transceiver used a 2 × 2 optical switch to alternate between remote key sharing and internal self-calibration.Its cross state connected transmitter and receiver sections to remote devices; its bar state connected them for calibrating half-wave voltages and delay times.
- Security consideration: Practical-security measures combined decoy-state BB84, four-phase modulation, physical random-number generation, and bright-light monitoring.These measures addressed photon-number-splitting, fake-state, time-shift, and bright-light attacks while accommodating speed and cost constraints.
- Security consideration: Future security enhancement could use device-independent or measurement-device-independent QKD.The paper identifies these techniques as possible additions beyond the practical-security measures already considered.
4. Detailed layout of the HCW wide area QKD network
The HCW network integrated a full-mesh Hefei metropolitan network, a point-to-multipoint Wuhu access network, and intercity trusted-node links. Router, switch, multiplexing, device-symmetry, and seamless-switching techniques supported the resulting reconfigurable architecture.
- Network integration: The network combined Hefei full-mesh routing, Wuhu point-to-multipoint access, and HCW intercity trusted-repeater connectivity.The architecture used 6 QKD devices for Hefei, N+1 devices for Wuhu, and multiplexing to share optical-fiber channels and devices.
- Full-mesh Hefei metropolitan area QKD network: Hefei connected each of 4 QKD nodes physically to every other node while using only 4 fiber channels.A wavelength-saving real-time full-mesh router and time-division-multiplexing full-mesh optical switch dynamically routed and blocked connections.
- Full-mesh Hefei metropolitan area QKD network: The Hefei router and switch created three connection states, with 8 direct QKD links supporting the 4-node full-mesh network.The states corresponded to different full-mesh and duplex-link configurations controlled by the FMOS and optical switch.
- Point-to-multipoint Wuhu metropolitan area QKD network: Wuhu used a 1×2 optical switch for time-division point-to-multipoint access, allowing R7 to communicate with either T7 or T8 at each time.Replacing the switch with a 1×N switch would support N transmitters and one receiver in the upstream configuration.
- HCW-intercity QKD link: The Hefei–Wuhu intercity link exceeded 150 km and -32 dB channel loss and used Chaohu as a trusted intermediate node.Hop-by-hop key sharing connected Hefei and Wuhu, with different device and detector selections for the two intercity segments.
- Symmetry and seamless switching: Multiplexing required resolving device symmetry and seamless switching for dynamic QKD states.The device-symmetry procedure matched FMI properties and measured back-to-back QBER; switching retrieved pre-stored configuration parameters for known states.
5. Long-term performance with the field environment
The QKD network was tested across varied field environments and commercial fiber channels for long-term operation. Field conditions increased noise and performance fluctuations, while compensation and suppression measures supported stable operation.
- Stability measures: DWDM channel separation, an added time-delay module, and SPD time gating reduced reflective and leakage noise from neighboring high-power optical streams.DWDM and SPD time gating functioned as spectral and temporal filters, respectively.
- Operating environments: Four operating environments exposed QKD devices to differing temperature, humidity, dust, vibration, acoustic noise, and smoke conditions.The makeshift kitchen was the harshest setting, requiring a large operating temperature range, especially for the SPD.
- Environmental effects: Weather variations changed fiber birefringence, optical path length, and loss, so automatic compensation measures were adopted.These variations accumulate with distance and affect QKD-device stability.
- Field test scope: More than 5000 hours of field testing covered commercial metropolitan and intercity fiber links in varied operating environments.The network included telecom rooms, laboratories, normal rooms, and a makeshift kitchen.
- Measured performance: 0.77 kbps was the field secure key rate for the Hefei-Chaohu link, compared with 0.87 kbps in laboratory examination.Field QBERs increased from 0.94% to 1.16% for signal states and from 4.18% to 5.26% for decoy states.
- Measured performance: About 0.1% of the signal-state QBER increase was attributed to background noise, while field QBER and key-rate fluctuations also reflected fiber-loss variation.The background noise mainly came from leaked photons in parallel fibers sharing the same multi-core cable.
- Network-level limitation: The intercity QKD link limited wide-area performance, with a 0.77 kbps secure key rate versus 29.54 kbps for the NC-WC metropolitan link in state (III).The authors expected hardware and software improvements to increase performance, especially secure key rate.
6. Performance of the encryption applications
The network integrated QKD-generated keys with two encryption applications for telephone, fax, VPN, and public-network security. These demonstrations used either one-time-pad key transport or QKD-refreshed AES seed keys.
- One-time-pad application: A one-time-pad encryption medium was inserted between PSTN infrastructure and conventional telephone and fax terminals.The medium was transparent during normal operation and used QKD-server keys through real-time transport or secure SD-card storage.
- Application integration: The applications demonstrated QKD integration with both legacy communication terminals and conventional public-network security infrastructure.The two application paths used different key-handling modes: real-time or stored one-time-pad keys, and refreshed AES seed keys.
- VPN application: A symmetric-encryption VPN gateway integrated QKD with the existing classical IPsec protocol and remained compatible with conventional Internet hardware and software.The gateway used 256-bit AES, with secret seed keys derived from and refreshed by the QKD server.
7. Conclusions
The field-installed dynamic QKD network operated steadily over a wide area for long periods. Its metropolitan full-mesh and point-to-multipoint configurations integrated point-to-point QKD with networking components.
- Conclusions: More than 5000 hours of operation demonstrated steady field installation and long-term running of the dynamic QKD network.The QCN-USTC part operated for about two years.
- Conclusions: Over 150 kilometers separated subscribers in Hefei and Wuhu through shared China Mobile communication infrastructure.The network provided a secure and stable key-distribution platform across two cities.
- Conclusions: Four Hefei nodes formed a dynamic full-mesh metropolitan core with up to eight direct QKD links using four multiplexed fiber channels.Wuhu used three nodes and one 1×2 switch to simulate a point-to-multipoint quantum access network.
- Conclusions: The stable operation supported the reported reliability and robustness of QKD networks in field environments.The prototype integrated point-to-point QKD techniques with networking schemes.