#qkd

They're sisters?! >:D

A point-to-point long-distance quantum key distribution (QKD) over a distance of 1,002 km has been achieved by scientists from the University of Science and Technology of China (USTC) of the Chinese Academy of Sciences (CAS), and their collaborators from Tsinghua University, Jinan Institute of Quantum Technology, and Shanghai Institute of Microsystem and Information Technology (SIMIT), CAS. This milestone not only sets a new world record for non-relay QKD but also provides a solution for high-speed intercity quantum communication. The results were published in Physical Review Letters on May 25th.

QKD is based on the principles of quantum mechanics and enables secure key distribution between two remote parties. When combined with the "one-time pad" encryption method, it can achieve the highest level of security for confidential communication. However, the distance of QKD has been limited by factors such as the channel loss and system noise.

The twin-field QKD (TF-QKD) using sending-or-not-sending (SNS) protocol was demonstrated in the experiment, improving the relation between the key rate and channel transmittance from a linear η to its square root η. Therefore, it can achieve a much longer secure distance than traditional QKD protocols.

To achieve long-distance QKD, the research team collaborated with Yangtze Optical Fiber and Cable Joint Stock Limited Company (YOFC) and used ultra-low-loss fiber based on pure silica core technology, which achieved a maximum attenuation of 0.16 dB/km. SIMIT developed ultra-low-noise superconducting single-photon detectors.

By implementing multiple filters at temperatures of 40 K and 2.2 K to suppress dark counts caused by thermal radiation, the noise of the single-photon detectors was reduced to around 0.02 cps. Furthermore, the team also developed a dual-band phase estimation scheme to avoid the spontaneous Raman scattering noise, reducing the system noise to below 0.01 Hz.

Based on the aforementioned technological developments, the team achieved TF-QKD over a record distance of 1,002 km, with a key rate of 0.0034 bps. This work not only verifies the feasibility of the SNS-TF-QKD scheme at extremely long distances but also demonstrates that this protocol can achieve high key rates in many practical scenarios.

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An improvement in secure long-distance communication using twin-field quantum key distribution.

Overview of Twin-Field Quantum Key Distribution

For secure quantum communication, Twin-Field Quantum Key Distribution (TF-QKD) revolutionises range and key rates. It advances long-distance quantum networks by overcoming conventional QKD's distance limits.

TF-QKD uses an untrusted central node to secure communication between Alice and Bob, two distant participants, unlike typical Quantum Key Distribution (QKD) techniques. This technique succeeds because it can give secure key rates over longer distances than before.

Operating Principle and Security

Alice and Bob send weak coherent pulses to the untrusted central node in TF-QKD instead of conversing. Typical weak coherent pulses are phase-encoded. The centre node, which analyses Alice and Bob's pulses' single-photon interference, is crucial to improving performance.

Twin-Field Quantum Key Distribution has great security. The protocol is measurement-device-independent, therefore the system is safe even if the central node's measuring devices are compromised. By requiring Alice and Bob to send quantum states to the central measurement station, detector-side attacks are prohibited and measurement-device independence is achieved.

Scholars also explore security frameworks from theoretical asymptotic studies to finite-key regimes, ensuring system resilience to attacks and taking into account real-world defects and weaknesses. In fact, decoy-state approaches accurately characterise single-photon contributions in weak coherent pulses while keeping the cost benefits of traditional laser sources.

Overcoming Distance and Scaling Benefit

Twin-Field Quantum Key Distribution (TF-QKD) improves rate scaling with distance, its key innovation. Traditional QKD algorithms feature a linear relationship between key rate and channel distance. In contrast, TF-QKD scales securely with the square root of the channel length since the secret key rate is proportionate to the channel transmittance. This qualitative shift alters the crucial rate-communication distance relationship.

Improved scaling allows TF-QKD to break long-range repeaterless QKD's absolute key-rate limit. TF-QKD has consistently shown that it can achieve secure key rates higher than those of systems without trusted repeaters.

By eliminating the need for difficult-to-implement quantum repeaters, TF-QKD solves the major distance constraint of earlier quantum communication systems. This function increases secure communication range from hundreds to thousands of kilometres. TF-QKD can scale efficiently over long distances without repeaters, making it vital for future global quantum networks.

Experimental Results and Real-World Use

Recent investigations have proved that TF-QKD works in fiber-optic networks. In a historic experiment, sending-or-not-sending TF-QKD secured key distribution over a 509-kilometer ultralow loss optical fibre.

We found that the secure key rate at 509 km was nearly seven times the relative bound of repeaterless QKD for the same detection loss. Even with infinite pulses, the secure key rate was higher than with a traditional QKD protocol and flawless repeaterless QKD hardware.

Modern technology was employed to stabilise two laser sources over the 509-kilometer fibre distance utilising remote-frequency-locking. Additional examples show secure communication over 511 kilometres of optical fibre connecting far urban locations.

Recent advances have extended fiber-based secure communication distances to over 1000 km. Field tests of 546 kilometres and other verified realistic secure lines over 500 and 830 km have validated the technique. These impressive examples show that TF-QKD has moved from a lab curiosity to a technology suitable for inter-city and possibly continental-scale quantum networks.

Next steps and improvements

Twin-Field Quantum Key Distribution (TF-QKD) protocols and implementation methods are being studied. One study uses distinct optical frequency combs and open quantum channel stabilisation to improve the TF-QKD algorithm. Researchers are also improving experimental approaches like TF-QKD demonstrations without phase locking. Source monitoring, often with Hong-Ou-Mandel interference, improves system performance and security.

TF-QKD network topologies are also prioritised. Polarisation, wavelength, and time division multiplexing configurations like long-fiber Sagnac interferometers and 2xN networks are being researched. These projects aim to build scalable, high-rate TF-QKD networks for multi-party quantum key agreement protocols and conference key agreement applications.