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Metropolitan all-pass and inter-city quantum communication network
Teng-Yun Chen, Jian Wang, Hao Liang, Wei-Yue Liu, Yang Liu, Xiao Jiang, Yuan Wang, Xu Wan, Wei-Qi Cai, Lei Ju, Luo-Kan Chen, Liu-Jun Wang, Yuan Gao, Kai Chen, Cheng-Zhi Peng, Zeng-Bing Chen, Jian-Wei Pan
TL;DR
The paper tackles secure QKD networking beyond point-to-point links by demonstrating a metropolitan all-pass network integrated with an inter-city trusted relay. It combines arbitrary optical switching, multifunctional QKD terminals, decoy-state protocols, and automated control, achieving field operation with real-time one-time-pad voice communication and key rates reaching 4.5 kbps on a long link.
Problem
Secure QKD connections beyond point-to-point links are needed for practical network applications.
Method
The system combines arbitrary-port optical switching, trusted-relay integration, multifunctional QKD terminals, decoy-state protocols, and tailored hardware and software control.
Results
4.5 kbps key rates were obtained on the Feixi–USTC link, while the field network successfully supported automated QKD and real-time one-time-pad voice communication.
Takeaways & Limitations
The demonstration shows a metropolitan all-pass QKD network can be combined with trusted relaying to connect arbitrary nodes and extend network reach.
Abstract
from arXiv · showhide
We have demonstrated a metropolitan all-pass quantum communication network in field fiber for four nodes. Any two nodes of them can be connected in the network to perform quantum key distribution (QKD). An optical switching module is presented that enables arbitrary 2-connectivity among output ports. Integrated QKD terminals are worked out, which can operate either as a transmitter, a receiver, or even both at the same time. Furthermore, an additional link in another city of 60 km fiber (up to 130 km) is seamless integrated into this network based on a trusted relay architecture. On all the links, we have implemented protocol of decoy state scheme. All of necessary electrical hardware, synchronization, feedback control, network software, execution of QKD protocols are made by tailored designing, which allow a completely automatical and stable running. Our system has been put into operation in Hefei in August 2009, and publicly demonstrated during an evaluation conference on quantum network organized by the Chinese Academy of Sciences on August 29, 2009. Real-time voice telephone with one-time pad encoding between any two of the five nodes (four all-pass nodes plus one additional node through relay) is successfully established in the network within 60km.
1. Introduction
The paper addresses the need for secure QKD connections beyond point-to-point links by demonstrating a network that supports arbitrary node-to-node communication. It combines optical switching, integrated terminals, trusted relaying, and automated control toward larger-scale QKD networking.
- Secure QKD connections beyond point-to-point links are identified as important for network applications.
- Trusted relaying can extend QKD communication distance and accommodate different types of QKD links, but relay-site privacy must be ensured.
- An 8-port optical switching design enables arbitrary interconnection between input and output ports, supporting any two-node communication.
- The demonstrated metropolitan network uses star-type topology and polarization coding, with real-time one-time-pad voice encryption between nodes.
- Integrated terminals can operate as transmitters, receivers, or both, while tailored hardware and software automate quantum-channel distribution and QKD processing.
2. Optical switching and network architecture
The network architecture uses an 8-port mechanical optical switch to connect arbitrary node pairs in a metropolitan star network and combines this switching with trusted relaying for extended links.
- An 8-port optical switching equipment allows interconnection of any two ports, enabling arbitrary connections among network nodes.
- Connecting every node to the switch creates a star-type all-pass metropolitan network without special requirements beyond the optical switching arrangement.
- Mechanical switches provide high isolation, no additional noise when all ports operate simultaneously, and standard single-mode fiber channels.
- Trusted relay architecture is combined with the metropolitan network to extend communication distance and support interoperability among heterogeneous QKD devices.
3. QKD terminal devices for network applications
The network uses integrated QKD terminals and decoy-state processing to support flexible transmitter-receiver roles, synchronized field operation, and secure-key generation across metropolitan and inter-city links.
- Terminal architecture: Integrated terminals support arbitrary two-node connections by operating as either transmitters or receivers.
- Decoy-state protocol: Decoy-state processing is implemented on all links using randomly inserted states of different intensities to improve secure distance and key generation rate.
- Decoy-state protocol: Signal and decoy states are analyzed through detection rates and quantum bit error rates to estimate eavesdropper information before error correction and privacy amplification.
- Network links: Four metropolitan nodes are connected by an all-pass network, with an additional 60 km, extendable to 130 km, inter-city link through a trusted relay.
- Terminal implementation: The terminals generate 0.6, 0.2, and 0 photons per pulse for signal, decoy, and vacuum states, respectively, with a 6:1:1 occupancy ratio.
- Terminal implementation: Fiber circulators separate transmitted and received pulses, while narrowband filtering reduces synchronization-light disturbance and supports low QBER.
- Terminal implementation: The terminal schematic combines polarization-state preparation, intensity modulation, attenuation, circulators, CWDM multiplexing, and BB84 detection.
4. Performance in field fiber
The field-fiber network used integrated QKD terminals, decoy-state processing, and a trusted-relay connection to support automated metropolitan and inter-city operation. Measurements yielded key rates above 1.2 kbps below 2% QBER, with 4.5 kbps obtained on the Feixi–USTC link.
- Network configuration: Four metropolitan nodes connect through an optical switching module, while Feixi is linked through USTC as a trusted relay.The network uses field fibers and includes a 10 km circle link simulating a remote node.
- QKD terminals: The integrated terminals operate as both transmitters and receivers, with 4 MHz laser repetition and approximately 10% detector efficiency.Average photon numbers are 0.6 for signal states and 0.2 for decoy states.
- Decoy-state analysis: Decoy-state analysis estimates single-photon gain and error-rate bounds from measured signal, decoy, and vacuum-state parameters.The processing uses measured gains, QBER, pulse counts, and vacuum counting rates to calculate secure key rates.
- Measured performance: More than 1.2 kbps final key rate was achieved when QBER was below 2% during a typical 400 s run.The reported rate excludes one-fifth of the period used for adaptive feedback control.
- Measured performance: 4.5 kbps key rate was obtained on the Feixi–USTC link, with typical QBER below 1%.The system used a 320 MHz optical source, superconducting detectors, and approximately 10 kbps detection counting per arm.
5. Conclusions and perspectives
The demonstrated field network combines all-pass optical switching, trusted relays, decoy-state QKD, integrated terminals, and tailored control hardware and software. The authors present this hybrid architecture as a scalable route toward practical QKD networks over arbitrary distances.
- Conclusions and perspectives: The hybrid architecture integrates metropolitan all-pass networking with trusted relays to extend network reach over arbitrary distances.The system combines optical switching, relay operation, decoy-state protocols, integrated terminals, and automated control.
- Conclusions and perspectives: Integrated terminals can switch between transmitter and receiver operation and may support simultaneous operation to double key generation rate.The simultaneous-operation capability depends on suitable software and electrical control hardware.