Researchers at Universidad de Concepción have achieved a quantum key distribution rate of 6.19 x $10^{- 3}$ bits per pulse, a new record for high-dimensional QKD operating over links with 10dB of channel loss. The team demonstrated this performance by directly using the full set of core modes within a four-core multicore fiber installed across the university campus.

This approach to encoding information offers a viable architecture for realistic, high-rate quantum-secure communications, moving beyond previous hybrid methods that limited efficiency. The work establishes core-mode encoding as a promising path for integrating quantum cryptography into existing telecommunications infrastructure.

Full Core-Mode Encoding for High-Dimensional QKD

A composable finite-key rate of 6. The Universidad de Concepción team’s implementation operates over an installed fiber network on its campus, maintaining performance under continuous environmental disturbances. This advance moves beyond hybrid qudit encoding strategies that combine core modes with time-bin photonic degrees of freedom, which incur efficiency penalties as dimensionality increases. Instead, the researchers exploited the full potential of spatial modes within the fiber, allowing for a more efficient transmission of quantum information.

The work demonstrates the feasibility of preparing, transmitting, and measuring coherent spatial qudits in deployed multicore fibers, a capability with implications beyond QKD itself. These include potential applications in metropolitan quantum networks using space-division multiplexing and quantum communication schemes reliant on high-dimensional spatial entanglement. The experimental setup used superconducting nanowire detectors to register single photons, a critical component for achieving high key rates even with signal attenuation. The team’s success in a real-world, deployed network environment highlights the growing maturity of QKD technology and its potential for securing future communications.

Four-Core Multicore Fibers Enable QuDit Superpositions

The stability of qudit superpositions, essential for high-dimensional quantum key distribution (QKD), benefits from the unique properties of multicore fibers. Unlike single-mode fiber systems, the common cladding shared by multiple cores within a multicore fiber significantly reduces differential phase drift, maintaining the coherence of qudits over longer distances. This slower drift, observed during experiments on the Universidad de Concepción campus, helps preserve the phase stability of the qudit superposition across the communication link, a critical factor for reliable key exchange.

The researchers designed the system to take advantage of this stability, enabling the preparation and transmission of coherent spatial qudits within the deployed fiber network. Fabrication of key components also contributes to the system’s efficiency. Four-by-four multiport beam splitters, vital for state preparation and measurement, can be directly integrated into multicore fibers with minimal insertion loss.

This direct fabrication contrasts with approaches requiring discrete component assembly, reducing signal degradation and improving overall performance. The use of these low-loss beam splitters, coupled with superconducting nanowire single-photon detectors, allowed the team to benchmark the scheme at 10dB of channel loss, a level of attenuation representative of long-distance fiber optic links. Achieving a composable finite-key rate of R = 6.19× 10^(-3)bits/pulse, the highest per-pulse rate reported to date for HD-QKD at comparable loss.

Deployed Network Tests Under Environmental Perturbations

The Universidad de Concepción’s installed multicore fiber network proved resilient to real-world disturbances during recent quantum key distribution tests, maintaining coherent qudit propagation. Positive asymptotic secret-key rates were consistently obtained across all tested network configurations, ranging from a baseline reference to the 1. 3-kilometer cross-campus link, and regardless of detector technology employed. Researchers benchmarked the system using both commercially available InGaAs avalanche photodiode detectors and high-efficiency superconducting nanowire single-photon detectors, demonstrating the protocol’s adaptability to different hardware options.

This flexibility is important for wider adoption, as it avoids reliance on specialized, expensive components and facilitates integration with existing telecommunications infrastructure. The ability to achieve viable key exchange rates with both detector types emphasises the robustness of the core-mode encoding approach. This work represents the first demonstration of a four-dimensional quantum key distribution protocol, using a d = 4 system, based on pure core-mode encoding over a deployed multicore fiber network.

19e-3 Bits/Pulse Rate Achieved with 10dB Loss

This per-pulse rate represents the highest reported to date for HD-QKD under comparable conditions, demonstrating a significant step toward viable, long-distance quantum-secure communication. The experiment used superconducting nanowire detectors to quantify key generation, a critical component for assessing real-world performance. The system’s success stems from a full exploitation of core-mode encoding within a four-core multicore fiber, a departure from previous demonstrations that combined core modes with time-bin encoding.

This approach avoids the efficiency penalties inherent in hybrid systems, allowing for increased information density and a more robust signal. The team benchmarked the scheme and established core-mode encoding as a viable architecture for realistic, high-rate quantum-secure communications.

HD-QKD Advantages: Noise Tolerance and Key Yield

Achieving a key exchange rate of 6. This performance, detailed in recent work from Universidad de Concepción, demonstrates improved resilience against noise compared to traditional qubit-based QKD systems. The increased tolerance stems from the use of high-dimensional encoding, where each photon carries more information, allowing for a more robust key exchange even with imperfect transmission. The team’s approach directly exploits the full capacity of a four-core multicore fiber, encoding information across all available spatial modes.

The researchers report that this full core-mode encoding allows for a higher secret-key yield, extracting more usable key material from each detected photon. This is a critical advantage as practical HD implementations have historically struggled to overcome efficiency losses that offset the theoretical gains of higher dimensionality. The authors state that this result establishes core-mode encoding as a viable architecture for realistic, high-rate quantum-secure communications.

These components facilitate the creation of even higher-dimensional alphabets, promising further increases in information density per photon and enhanced noise tolerance. “Notably, this d = 4 result approaches within a factor of 1.5 the current state of the art in qubit QKD,” the authors state, “suggesting that HD-QKD is entering a regime of genuine practical competitiveness.” The potential for scaling this technology is significant.

Superconducting Detectors Benchmark Performance

This performance benchmark was achieved using a system operating over an installed multicore fiber network at Universidad de Concepción, demonstrating a functional key exchange even with 10dB of signal attenuation, a significant hurdle for practical quantum communication. The system’s performance was initially characterized using InGaAs avalanche photodiode single-photon detectors, confirming viability with lower-cost hardware, before transitioning to superconducting nanowire detectors with 85% mean detection efficiency to define the performance bound.

Analysis of quantum bit error rate (QBER) distribution, integrated over 100ms blocks from over 8.18 x $10^{7}$ detections, revealed a stable spread primarily influenced by detector dark counts and timing jitter rather than phase drifts. This stability is important for maintaining secure key generation over extended periods and under real-world environmental conditions.

The observed QBER distribution remained largely within the 1.5%, 2.5% range, further supporting the claim that detector noise is the dominant factor limiting performance, rather than signal degradation within the fiber optic network. This finding suggests that further improvements in detector technology could yield even higher key rates and extended transmission distances. The authors state this work represents the first field deployment of a core-encoded four-dimensional HD-QKD system over a fully installed multicore fiber network under uncontrolled operational conditions, establishing an important benchmark for realistic high-dimensional quantum communication.

Core-Mode Encoding vs. Hybrid QKD Approaches

Recent demonstrations of quantum key distribution (QKD) over deployed multicore fibers have typically relied on hybrid encoding strategies, combining core modes with time-bin encoding to create higher-dimensional qudits. This approach, while practical, introduces efficiency penalties that scale with dimension, limiting potential key rates. Researchers recently achieved a composable finite-key rate of 51.5 kbps at 22dB of channel loss using a hybrid d = 4 QKD system over a 52km installed fiber in L’Aquila, demonstrating the feasibility of this method.

This work, however, presents a distinct path, establishing a four-dimensional QKD session based on pure core-mode encoding for the first time on an installed fiber network operating under real-world conditions. The team’s system, deployed across the Universidad de Concepción campus, avoids the inherent limitations of time-bin encoding, offering a quantitative benchmark for future MCF-based quantum networks.

Universidad de Concepción Campus Network Implementation

This full exploitation of core modes, tested across 200m and 1.3km links connecting campus laboratories, enabled operation under continuous environmental disturbances including temperature fluctuations and vibrations. The authors state positive asymptotic secret-key rates were obtained over all tested links and with both detector technologies, confirming the system’s viability across diverse operational scenarios. This robustness is attributed to the fiber’s design, which mitigates the effects of external disturbances on the quantum signals. The team’s approach circumvents the efficiency penalties inherent in hybrid encoding schemes that combine core modes with other degrees of freedom.

👉 More information
🗞 High-Dimensional Quantum Key Distribution via Full Core-Mode Encoding over Deployed Multicore Fibers
✍️ G. H. dos Santos et al.
🧠 DOI: http://link.aps.org/doi/10.1103/y779-t6ry

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