Setting New Records for Data Rate and Distance in Optical Fiber Networks Advances Quantum Security.
CV QKD
In order to protect communication from adversaries with powerful quantum computers, quantum key distribution (QKD) is a fundamental approach for exchanging secret encryption keys with information-theoretic security. However, co-propagation with classical data across the current optical fiber infrastructure is necessary to integrate QKD into large-scale, economically viable networks. Historically, the possible secure distance of QKD transmissions has been severely limited to a few tens of kilometers due to the noise produced by these classical channels.
These restrictions have been broken by recent experimental advances in a variety of QKD protocols, which have shown record coexistence distances, set new records for data rates in urban settings, and improved security against real-world device vulnerabilities.
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Breaking the Distance Barrier: CV-QKD over 120 km
A new distance record has been set by researchers from the Czech Republic and Denmark for continuous-variable QKD (CV QKD) coexisting with conventional traffic. Their research showed that, in the asymptotic regime, secure quantum communication could coexist with fully occupied classical channels over an unprecedented 120 km of optical fiber.
In the finite-size regime, this experiment also generated keys at a distance of 100 kilometers. The group used ultra-low-loss fiber and Gaussian-modulated coherent states in a local-oscillator (LO) CV QKD system.
By effectively reducing the noise caused by classical channels, which is often the predominant impairment, the distance record was attained. This mitigation was achieved by taking advantage of a previously unnoticed built-in filter offered by the CV-QKD setup and by tailoring the modulation variance to decrease phase noise-induced surplus noise.
Importantly, the accomplishment was attained without the need for wavelength reallocation or additional optical filters, indicating that CV-QKD offers a “plug-and-play solution” for incorporation into standard 80–100 km long-haul optical lines. The viability of this CV QKD approach was validated by benchmarking, which revealed that it performed better than a commercial discrete-variable QKD (DV-QKD) system in comparable noise settings where the DV-QKD system failed.
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A High-Rate Solution for Metro Networks
High secret key rates are essential for applications in highly connected urban areas. A high-rate discrete-modulated CV-QKD system that is optimized for good performance and compatibility in urban networks was described by researchers in a separate breakthrough.
A probability-shaped 16-quadrature amplitude modulation (16QAM) protocol was used by this system. The system obtained an impressive composable secret key rate of 18.93 Mbps across a 25 km fiber line by optimizing this discrete-modulation technique. In comparison to earlier CV-QKD systems, this rate establishes a new performance advantage of more than an order of magnitude.
Because discrete-modulation protocols employ a smaller constellation space than conventional Gaussian-modulated protocols, they are preferred in high-speed applications due to their improved compatibility with high-speed wireline components and ability to reduce excess noise at high repetition rates.
Semidefinite programming (SDP) was used in the implementation to guarantee composable security, a strict type of security proof. By reducing surplus noise to incredibly low levels utilizing a fully digital and exact quantum signal processing technology, the system significantly improved security and performance. At a high system repetition frequency of 1 GHz, the experimental setup functioned. Additionally, studies revealed that the composable secret key rate was more than twice as high across a 25-kilometer distance when a decent post-selection technique was implemented.
Coexistence Beyond Single-Mode Fiber: Using Few-Mode Fiber
Investigating different fiber kinds also yields integration solutions. Coexistence with classical optical communication across 86 km of weakly-coupled few-mode fiber (FMF) was demonstrated by one QKD implementation.
This method makes use of the spatial degrees of freedom of the FMF using a new mode-wavelength dual multiplexing methodology. This technique takes advantage of the FMF’s huge effective core area and further modal isolation by assigning the classical data channel and QKD signal to various linear-polarized (LP) modes (in this experiment, LP 01 and LP 02 modes were used).
The main obstacle when QKD signals share fiber with strong classical signals is spontaneous Raman scattering (SRS) noise, which is much reduced by this approach. A secure key generation rate of 1.3 kbps in real time over 86 km was attained with the successful co-propagation of a 100 Gbps classical data link. Using wavelength-division multiplexing (WDM) at the same input power as standard single-mode fiber (SMF), the FMF scheme’s modal isolation decreased the SRS noise by an average of 86%.
Future optimization, according to the scientists, might increase the safe transmission distance to 185 km by reducing the FMF attenuation coefficient and enhancing single-photon detectors (e.g., 20% detection efficiency and 230 cps dark count rate).
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Securing Against Hidden Flaws Over 200 km
Separately, researchers developed a useful Side-Channel-Secure QKD (SCS-QKD) protocol to solve basic security vulnerabilities caused by defective equipment. Perfect sources and detectors are frequently assumed in QKD security proofs; however, actual devices differ, opening the door to possible attack through side channels like frequency spectrum or pulse form.
By eliminating the need for pristine vacuum state sources, the useful SCS-QKD technique protects emitted photons against side-channel attacks. The experiment used fiber spools to produce a 200 km long-distance distribution, which is consistent with recent theoretical work. With finite-key effects taken into account, this configuration produced a secure key rate of 1.29×10−7 bits per pulse. This study establishes a new distance record for SCS-QKD and shows that even with flawed and realistic sources, strong side-channel security can be preserved.
Future Outlook for Large-Scale Deployment
These diverse accomplishments, which include securing protocols over 200 km against source flaws, proving dependable coexistence using FMF for 86 km, achieving high 18.93 Mbps key rates in metro links, and demonstrating CV QKD coexistence over 120 km, collectively represent significant advancements toward the industrialization of QKD. QKD is getting closer to becoming a large-scale, reasonably priced solution for safeguarding international telecommunications by overcoming significant obstacles in terms of both distance and compatibility with traditional infrastructure.
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