Author: Denis Avetisyan
Researchers demonstrate that hollow-core fiber can maintain signal integrity for long-distance quantum key distribution, offering a pathway to more secure and robust networks.
Phase noise characterization of a 2-km hollow-core nested antiresonant nodeless fiber shows comparable performance to standard single-mode fiber for Twin-Field Quantum Key Distribution.
Achieving secure long-distance quantum communication remains a significant challenge due to signal degradation in optical fibres. This is addressed in ‘Phase noise characterisation of a 2-km Hollow-Core Nested Antiresonant Nodeless Fibre for Twin-Field Quantum Key Distribution’, which investigates the potential of hollow-core fibre to mitigate these limitations for advanced protocols like Twin-Field Quantum Key Distribution. Our results demonstrate that a 2-km hollow-core fibre exhibits comparable phase noise performance to standard single-mode fibre, indicating its viability for long-distance quantum communication. Could this technology pave the way for more robust and extended-range quantum networks?
The Fragility of Quantum Signals: A Fundamental Challenge
Quantum Key Distribution (QKD) offers the potential for unhackable communication by leveraging the laws of quantum mechanics to securely distribute encryption keys. However, a fundamental limitation to its practical implementation lies in signal loss during transmission. As photons, the carriers of quantum information, travel through optical fibres or the atmosphere, they are susceptible to attenuation – a gradual reduction in signal strength. This attenuation increases exponentially with distance, meaning that after a certain point, the quantum signal becomes indistinguishable from noise, rendering secure key exchange impossible. Consequently, the distance over which QKD can be reliably performed is severely restricted, currently posing a significant obstacle to the development of global, secure quantum networks. Researchers are actively pursuing various solutions, including quantum repeaters and advanced detection schemes, to overcome this distance barrier and extend the reach of quantum communication.
The practical implementation of quantum key distribution (QKD), while theoretically secure, faces a fundamental challenge: signal attenuation in transmission media. Specifically, standard single-mode fibre, the backbone of modern telecommunications, introduces a considerable loss of signal strength. At a wavelength of 1550 nanometers – a common choice for optical communication – this attenuation reaches approximately 0.18 decibels per kilometer. This seemingly small loss accumulates rapidly over distance, exponentially reducing the number of photons carrying quantum information. Consequently, direct transmission via conventional fibre is limited to around 100 kilometers, effectively creating a distance barrier that hinders the development of truly long-range, secure quantum networks. Overcoming this attenuation is therefore paramount to realizing the full potential of QKD and building a global quantum internet.
The realization of a truly interconnected and secure future hinges on overcoming the distance limitations currently plaguing quantum communication networks. While quantum key distribution (QKD) offers theoretically unbreakable encryption, the inherent fragility of quantum states means signals weaken considerably with distance – a phenomenon exacerbated by fibre optic cable attenuation. Establishing practical, widespread quantum networks necessitates innovative solutions to extend transmission range, such as quantum repeaters or trusted nodes, to counteract signal loss and maintain the integrity of quantum information. Without addressing this distance barrier, the potential benefits of quantum communication – secure financial transactions, protected government communications, and a fundamentally more private internet – will remain largely unrealized, restricting quantum technology to short-range applications and limiting its transformative impact on society.
Harnessing the Potential of Empty Space: Hollow Core Fibres
Hollow Core Fibre (HCF) utilizes a central core filled with air or vacuum to transmit optical signals, fundamentally differing from standard optical fibres which employ solid glass. This design drastically reduces signal attenuation; typical HCF exhibits attenuation levels of approximately 3.5 dB/km at 1550nm, compared to 20-60 dB/km in conventional single-mode fibres. The lower attenuation stems from the reduced interaction of light with the material medium; photons primarily propagate through air, minimizing absorption and scattering losses. This characteristic is particularly advantageous for long-distance communication and applications requiring high signal fidelity, such as quantum key distribution and sensor networks.
Chromatic dispersion and nonlinearity are critical limitations in standard optical fibres for quantum key distribution (QKD) and other quantum communication protocols. Chromatic dispersion, the spreading of optical pulses due to varying wavelengths, is significantly reduced in hollow core fibres (HCFs) due to the air or vacuum core, increasing the maximum transmission distance. Similarly, nonlinear optical effects, such as stimulated Brillouin scattering and four-wave mixing, which distort the signal and introduce errors, are minimized because the effective interaction length between photons and the fibre material is substantially decreased. This results in a higher signal fidelity and allows for the preservation of delicate quantum states over extended distances, essential for secure quantum communication and distributed quantum computing applications. The reduced nonlinearity and dispersion contribute to lower error rates and improved key rates in QKD systems utilizing HCFs.
Preservation of signal coherence in Hollow Core Fibre (HCF) necessitates mitigation of phase noise, which arises from environmental factors and inherent fibre properties. While HCF offers reduced latency, maintaining coherence requires careful implementation of stabilization techniques, including temperature control and active feedback loops. Recent experiments demonstrate high interference visibility – exceeding 99% – achievable in both HCF and conventional Single-Mode Fibre (SMF) configurations. This parity in visibility confirms that, while presenting unique challenges, comparable coherence levels are attainable in HCF through precise control of the optical path and minimization of disruptive noise sources. These results validate HCF as a viable medium for applications demanding high-fidelity signal transmission, such as quantum key distribution and coherent optical communication systems.
Precision Measurement: Unveiling the Secrets of Phase Noise
The Double Asymmetric Mach-Zehnder Interferometer (DAMZI) facilitates sensitive phase noise measurement in optical fibres by employing two imbalanced Mach-Zehnder interferometers configured asymmetrically. This configuration enhances sensitivity to phase fluctuations by creating interference patterns dependent on the accumulated phase difference between the two paths. Both Hollow Core Fibre (HCF) and Single-Mode Fibre (SMF) can be evaluated using the DAMZI, allowing for comparative analysis of phase noise characteristics. The interferometer’s output, typically observed via photodetectors, provides data that can be processed to quantify the magnitude and frequency of phase fluctuations, expressed in units such as rad/s or rad/ms. The asymmetry of the interferometer is crucial for distinguishing between common-mode and differential phase noise, enabling more precise characterization of the fibre’s impact on quantum key distribution (QKD) systems.
The Double Asymmetric Mach-Zehnder Interferometer facilitates the detailed characterization of phase fluctuations relevant to Quantum Key Distribution (QKD) systems. Specifically, the interferometer measures random phase variations that introduce errors in QKD bit decoding, impacting the system’s secure key rate and potentially compromising security. These fluctuations, expressed as phase noise, are quantified by analyzing the statistical distribution of phase differences observed at the interferometer’s output. By precisely measuring these fluctuations, researchers can identify and mitigate sources of noise, such as temperature variations, mechanical vibrations, and laser frequency drift, thereby optimizing QKD performance and ensuring robust secure communication. The interferometer’s sensitivity allows for analysis of both low-frequency phase drift and high-frequency phase noise components, providing a comprehensive assessment of phase stability crucial for practical QKD deployments.
During Time-Frequency Quantum Key Distribution (TF-QKD)-like experiments, phase drift rates of up to $10$ rad/ms were observed in both Hollow Core Fibre (HCF) and Single-Mode Fibre (SMF). However, implementation of phase stabilization techniques effectively mitigated these drifts. Analysis demonstrated that, following stabilization, the phase noise performance of HCF and SMF were comparable within the frequency ranges relevant to quantum key distribution. This finding indicates that the increased phase noise typically associated with HCF does not preclude its use in QKD systems when appropriate stabilization methods are employed.
Expanding the Horizon: Extending QKD Range with Advanced Fibre Design
The development of Nested Anti-Resonant Nodeless Hollow Core Fibre marks a substantial leap forward in minimizing signal degradation during transmission. Unlike traditional fibres which guide light through a solid core, this innovative design utilizes a hollow core surrounded by layers of precisely engineered cladding. This unique structure prevents light from interacting with the material of the fibre itself, drastically reducing intrinsic signal loss – a persistent challenge in long-distance quantum communication. By eliminating much of the attenuation caused by material absorption and scattering, this fibre allows for significantly extended transmission distances, particularly crucial for applications like Quantum Key Distribution (QKD). The nested anti-resonant design effectively confines light within the hollow core, enabling propagation over kilometers with remarkably low signal decay and paving the way for more practical and secure quantum networks.
The implementation of precise polarization control and strategic optical attenuation proved crucial in maximizing the performance of nested anti-resonant hollow core fibre for quantum key distribution (QKD). By carefully managing signal polarization and minimizing losses, researchers demonstrated a quantum bit error rate (QBER) of only 1.75% during a 2km transmission experiment mirroring a time-frequency-referenced QKD system. This significant reduction in error, achieved within a hollow core fibre environment, directly correlates with extended transmission distances and enhanced security for quantum communication protocols. The results suggest a pathway toward practical, long-range quantum networks capable of reliably distributing cryptographic keys with a markedly reduced risk of eavesdropping.
Despite the 66 dB attenuation incurred when connecting the hollow core fibre to standard single-mode fibre (SMF) patch cables, the overall system performance gains are substantial. This attenuation, while significant, is offset by the dramatically reduced signal loss within the hollow core fibre itself, resulting in more secure and reliable Quantum Key Distribution (QKD) systems. The ability to maintain a low Quantum Bit Error Rate (QBER) over extended distances – demonstrated with a 1.75% QBER over 2km – directly addresses a critical limitation in early QKD implementations. Consequently, this advancement moves practical, long-distance quantum networks closer to reality, enabling secure communication over distances previously unattainable with conventional fibre optic technologies and laying the groundwork for future quantum internet infrastructure.
The research highlights a system where localized interactions-the properties of the hollow-core fiber and its impact on phase noise-produce a global effect: the potential for extended quantum communication distances. This aligns with the notion that order doesn’t require central direction. The study doesn’t control quantum phenomena, but rather influences them by carefully managing the fiber’s characteristics to minimize phase noise, a critical parameter for Twin-Field Quantum Key Distribution. As Richard Feynman once stated, ‘The best way to learn is to try to explain something to someone else.’ This work embodies that principle, demonstrating a complex system through meticulous experimentation and clear articulation of its implications for secure communication.
Where Do We Go From Here?
The demonstration of viable phase noise characteristics in hollow-core fiber for Twin-Field Quantum Key Distribution (QKD) isn’t a destination, but rather a widening of possibilities. The pursuit of longer distances in quantum communication invariably encounters limitations imposed not by fundamental physics, but by the subtle, emergent properties of the transmission medium itself. This work rightly sidesteps the question of ‘can it be done?’ and begins to address ‘how will it actually manifest?’ Order doesn’t need architects; it emerges from local rules – in this case, the interaction of photons with a carefully engineered glass structure.
Future investigations will likely not center on achieving ever-lower noise floors – a diminishing returns proposition – but on understanding and mitigating the correlations within that noise. A perfectly quiet channel is an abstraction. Real systems exhibit fluctuations, and it is the statistical properties of those fluctuations, not their amplitude, that ultimately dictate the security and range of QKD. Furthermore, exploring alternative fiber designs, perhaps those that deliberately introduce controlled perturbations, might prove more fruitful than striving for an idealized, but unattainable, perfection.
Sometimes inaction is the best tool. A complete rejection of single-mode fiber is not the implication. Instead, the field should focus on a nuanced understanding of how different fiber technologies respond to the quantum signals, rather than attempting to force them into conformity. The goal is not control, but influence – a gentle nudge towards a more robust and secure communication paradigm.
Original article: https://arxiv.org/pdf/2512.04605.pdf
Contact the author: https://www.linkedin.com/in/avetisyan/
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2025-12-06 11:42