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Large scale quantum key distribution: challenges and solutions
Qiang Zhang, Feihu Xu, Yu-Ao Chen, Cheng-Zhi Peng, Jian-Wei Pan
TL;DR
Global QKD seeks information-theoretically secure communication at worldwide scale, but practical devices and long-distance channels remain challenging. This letter reviews experimental efforts addressing device imperfections, fiber networks, and satellite links, including demonstrations reaching 1 Mbps key rate in MDI-QKD systems. The reviewed approaches show progress toward large-scale QKD while retaining important limitations in implementation and infrastructure.
Problem
Global QKD remains experimentally challenging because practical devices can introduce security loopholes, while fiber loss and decoherence hinder large-scale networks.
Method
The letter briefly reviews experimental efforts on practical QKD security, metropolitan and backbone fiber networks, and satellite-based QKD.
Results
1 Mbps key rate was demonstrated as feasible for MDI-QKD using a 1 GHz system, while MDI-QKD was characterized as practical for metropolitan networks.
Takeaways & Limitations
Experimental progress spans device-imperfection-resistant protocols, fiber networking, and satellite links as routes toward global QKD.
Takeaways & Limitations
Full implementation of CV MDI-QKD remains an experimental challenge, and trusted-relay networks require strictly secure relay nodes.
Abstract
from arXiv · showhide
Quantum key distribution (QKD) together with one time pad encoding can provide information-theoretical security for communication. Currently, though QKD has been widely deployed in many metropolitan fiber networks, its implementation in a large scale remains experimentally challenging. This letter provides a brief review on the experimental efforts towards the goal of global QKD, including the security of practical QKD with imperfect devices, QKD metropolitan and backbone networks over optical fiber and satellite-based QKD over free space.
1. Introduction
Global QKD aims to combine QKD and one-time-pad encoding for information-theoretically secure worldwide communication, but scaling beyond metropolitan deployments remains experimentally difficult. The central obstacles are securing imperfect devices and overcoming loss and decoherence over long distances.
- QKD combined with one-time-pad encoding provides information-theoretically secure communication based on quantum mechanics.
- Practical QKD devices deviate from ideal single-photon sources and detectors, creating security loopholes or side channels.Protocols such as decoy-state QKD and MDI-QKD address security against device imperfections.
- Large-scale QKD must contend with high channel loss and decoherence that limit long-distance fiber transmission.At 1000 km of fiber, even idealized systems would detect only 0.3 photons per century.
- Candidate approaches to long-distance QKD include quantum repeaters, trusted relays, and satellite-based quantum communication.Quantum memories constrain repeater deployment, trusted relays require protected nodes, and space links offer much less channel loss and negligible decoherence.
- The letter reviews 30 years of experimental progress on practical security, metropolitan and backbone fiber networks, and satellite-based QKD.Its final reviewed area is recent satellite-based QKD experiments, followed by an outlook for global quantum communication.
2. Secure QKD with imperfect devices
Practical QKD must address security loopholes caused by imperfect sources and detectors while maintaining useful distance and key rates. Decoy-state QKD mitigates source vulnerabilities, whereas MDI-QKD removes detector attacks from the trusted-device assumptions and has demonstrated metropolitan-scale performance.
- Security challenges: BB84 requires single-photon sources and detectors, but practical devices can introduce security loopholes or side channels.Security proofs must account for the actual implementation devices while leaving the channel under Eve’s control.
- Decoy-state QKD: Decoy states use pulses with varied expected photon numbers to detect photon-number-splitting attacks while reserving signal states for key generation.This makes weak coherent lasers suitable for secure BB84 over long distances.
- Decoy-state QKD: 100 km fiber and 144 km free-space experiments demonstrated decoy-state BB84 under real-world conditions, surpassing the earlier 30 km PNS-limited distance.The free-space experiment later reached 144 km, while the fiber demonstrations first exceeded 30 km.
- MDI-QKD: MDI-QKD counters detector attacks by assigning all single-photon detections to an untrusted measurement platform that performs Bell-state measurements.Alice and Bob send signals to the platform, and use post-selected entanglement to establish security without trusting the detectors.
- MDI-QKD: 404 km fiber distance and a 1 GHz clock-rate experiment show that MDI-QKD can combine long-distance operation with metropolitan-network practicality.At 100 km, the reported key rate was around 3 kbps; the 1 GHz system demonstrated feasibility for reaching 1 Mbps.
- Continuous-variable QKD: Continuous-variable QKD has reached 100 km under restricted collective attacks, but general-attack security limits distance and full CV MDI-QKD remains experimentally challenging.Locally generated local oscillators address one practical loophole, while detector-side-channel removal has only been demonstrated as a proof of principle.
3. QKD network over optical fiber: metropolitan and backbone
Metropolitan QKD networks have been deployed, while backbone scaling over optical fiber remains constrained by loss and decoherence. Experiments have combined trusted-relay backbones, metropolitan networks, WDM integration, and emerging untrusted-relay architectures.
- Metropolitan networks: The first reported metropolitan fiber QKD field test connected four users over several kilometers and achieved a secure key rate around 1 kbps.The setup used one Alice, three Bobs, and split weak coherent pulses.
- Metropolitan networks: Early metropolitan networks in Boston, Vienna, and Tokyo combined multiple QKD protocols with trusted relays to connect remote users.Trusted relays publicly announce XOR results from keys shared separately with Alice and Bob.
- Fiber integration: Wavelength division multiplexing supports optical routing and coexistence between QKD and conventional telecom data on the same fiber.Such integration can reduce cost and increase robustness in practical QKD applications.
- Fiber integration: 66 km of backbone field fiber carried QKD alongside 3.6 Tbps optical communication traffic, demonstrating integrated operation with high-rate classical data.The experiment demonstrated coexistence of QKD and conventional telecom traffic in a backbone setting.
- Backbone networks: Backbone scaling is difficult because optical-fiber loss and decoherence reduce quantum-signal intensity and fidelity exponentially, while classical repeaters cannot amplify unknown quantum states without errors.A practical quantum repeater is considered beyond current technology, leaving large-scale deployment reliant on alternatives such as trusted relays.
- Backbone networks: China’s Beijing–Shanghai backbone exceeded 2000 km, linked four metropolitan networks through 32 trusted nodes, and supported trials in banking, securities, and insurance.Each adjacent-node link used QKD, while trusted nodes performed XOR operations and forwarded keys without quantum repeaters or memories.
- Untrusted-relay networks: MDI-QKD enables a star network with an untrusted relay, allowing users to send quantum signals to a central Bell-state-measurement platform instead of trusting relay security.This architecture can also reduce system cost because most nodes use transmitters rather than single-photon detectors.
4. Satellite-based QKD
Satellite-based QKD addresses the severe distance limitations of fiber by exploiting lower atmospheric attenuation, but practical links must also handle substantial channel loss and moving-platform conditions. Experiments progressed from short free-space demonstrations to satellite downlinks and intercontinental key distribution.
- Motivation: 1000 km of standard telecom fiber has channel transmittance 10^-20, whereas satellite links can exploit lower atmospheric attenuation for long-distance QKD.Fiber attenuation is approximately 0.2 dB/km; atmospheric attenuation is less significant, especially above the atmosphere.
- Free-space experiments: 32 cm, 10 km, 23.4 km, 10.5 km, 100 km, and 143 km experiments mark the progression of free-space QKD, entanglement distribution, and teleportation.These demonstrations established that quantum states and entanglement can survive transmission through the atmosphere.
- Ground tests: 35 dB attenuation over a 144 km free-space link supported a secure decoy-state QKD rate of 12.8 bit/s.Ground tests also examined longer two-link free-space channels before satellite deployment.
- Satellite demonstrations: Micius demonstrated polarization-encoded decoy-state downlink QKD at 1 kbps over distances up to 1200 km.The experiment ran across 23 days, with performance varying according to distance and weather; daily coverage was approximately 273 seconds.
- Satellite demonstrations: Using Micius as a trusted relay, QKD connected ground stations in China and Europe separated by up to 7600 km.Sifted key rates of approximately 3 kbps at approximately 1000 km physical separation and approximately 9 kbps at approximately 600 km were routinely obtained under reasonably good weather.
5. Outlook of the future global QKD
Future global QKD will combine practical protocols with satellite and backbone fiber networks while improving satellite coverage and reducing trust assumptions. Longer-term approaches aim to remove the need to trust satellites or implementation devices.
- Practical security: Decoy-state QKD and MDI-QKD offer practical experimental solutions for imperfect sources and detectors.These protocols support practically secure QKD under device imperfections.
- Network development: A prototype global quantum communication network has been demonstrated using satellites and backbone fiber networks.Further metropolitan and backbone fiber QKD networks are expected in China and Europe.
- Network development: Higher-orbit satellites and daytime QKD using telecommunication-wavelength photons with tighter filtering could increase satellite coverage time and area.These measures are proposed to improve the efficiency of satellite-based QKD networks.
- Open limitations: Current global QKD implementations require the satellite itself to be trusted.Entanglement-based QKD or MDI-QKD fiber networks are identified as ways to overcome this limitation.
- Open limitations: Device-independent QKD could eventually remove the need to trust implementation devices, but remains impractical with current technology.The proposed route relies on a loophole-free Bell test.