Quantum optical combs stabilise timing to 0.85 fs over 100km fibre

Researchers at Peking University, the Beijing Institute of Technology, and Beijing University of Posts and Telecommunications have achieved a timing stability of 0.85 femtoseconds over 100 kilometers of fiber. This level of precision, comparable to measuring time on the scale of attoseconds, helps maintain signal integrity in long-distance quantum communication.

The team unified precision timing, classical communication and quantum communication using a shared fiber link and optical-frequency-comb technology, streamlining the complex requirements for building practical quantum networks. This work establishes optical frequency combs as a common physical layer for integrated quantum-network services. Optical-Frequency-Comb Infrastructure Unifies Quantum Network Functions A timing jitter of just 0.85 femtoseconds was sustained over 100 kilometers of fiber optic cable, demonstrating a new level of precision in long-distance quantum communication infrastructure. The approach encodes classical data onto the intensity of an optical frequency comb, simultaneously transmitting information while preserving the comb’s inherent pulse timing. This infrastructure moves beyond the traditional role of optical frequency combs as mere measurement tools, instead positioning them as a foundational element for complete quantum networks. By fusing timing and communication functions onto a single optical carrier, the system reduces reliance on separate, parallel networks. This consolidation helps build scalable quantum systems by streamlining the complex requirements for coordinating distributed quantum operations. The researchers developed a spectral-engineering strategy to prevent interference between the data being transmitted and the timing signals, ensuring both functions operate reliably. Conventional absolute time synchronization typically relies on separate infrastructures for timing and communication, with security implemented through encryption. This separation limits the ability to provide trustworthy synchronization in quantum networks where both temporal accuracy and information security are paramount. The newly demonstrated system offers a different path, combining precision time transfer with quantum-secured information exchange within the same optical network. This integration allows for quantum-secured timestamp transmission, achieving a time deviation of 0.26 femtoseconds at one second, a performance consistent with standalone precision timing systems. “Precision timing, classical communication and quantum security can be jointly provided within a common optical network architecture,” the researchers state, highlighting the potential for a streamlined and secure quantum internet.

Precision Timing Achieved: 0.85 fs Stability Over 100km Fibre The demonstrated infrastructure achieves a timing jitter of only 0.26 femtoseconds at one second, a level of stability maintained even while simultaneously transmitting quantum-secured timestamps. This performance mirrors that of dedicated precision timing systems, indicating the integration of security features does not degrade temporal accuracy. This result is particularly notable given the 100-kilometer distance over which the timing signal is maintained, a significant step toward practical long-distance quantum networks. The system achieves simultaneous data transmission at 100 megabits per second alongside the 0. This approach allows for the co-propagation of timing, classical and quantum information over the same optical fiber, streamlining the infrastructure requirements for future quantum networks. The experimental setup used non-return-to-zero coding and a 50% duty cycle to maximize communication efficiency, achieving a bit error rate meeting the 10-6 criterion at a received optical power of -46 decibel milliwatts. Optical frequency combs, functioning as high signal-to-noise ratio light sources, proved effective in achieving this low bit error rate without relying on forward-error correction. The researchers highlight that this unified infrastructure provides a pathway toward scalable quantum network deployment, offering simultaneous support for precision timing, classical communication and quantum tasks.

Classical Data Transmission at 100 Mbit s⁻¹ via Comb Intensity The researchers developed a method to suppress interference between the data and timing signals, a critical step in achieving stable, unified transmission. A key element of the design is the use of dual-comb linear optical sampling to extract high-precision timing information at the receiving end. Local oscillation combs interfere with both the local clock comb and the received remote clock comb, generating interferograms used for timing recovery. Clock-referenced direct digital synthesizers and proportional-integral-derivative feedback loops maintain the stability of the comb repetition frequencies, ensuring accurate time-frequency transfer. Classical data encoding occurs at one site using an arbitrary waveform generator driving an intensity modulator, which imprints the data onto the transmitted output of Comb A. This modulation does not disrupt the comb’s pulse timing, preserving the integrity of the timing signal.

Spectral Engineering Suppresses Data-Timing Signal Coupling Spectral engineering techniques minimize interference between classical data and the precise timing signals essential for unified quantum networks. Manipulating coding formats, symbol distribution, and modulation depth allows for separation of spectral components, reducing instability introduced by data transmission. For instance, switching from non-return-to-zero coding to Manchester coding suppresses low-frequency components, redistributing spectral power at the expense of communication efficiency. Further control over energy levels is achieved by adjusting the fraction of unmodulated symbols and modulation depth. A higher fraction of unmodulated symbols increases the carrier-to-sideband power ratio, while a smaller modulation depth suppresses sideband power. These adjustments create a trade-off between minimizing timing-communication coupling and maintaining communication performance, specifically a low bit-error rate. Optimizing these parameters allows for a system that simultaneously transmits data and preserves timing integrity over extended distances. Wavelength allocation and propagation direction also play critical roles in noise reduction. Increasing the wavelength separation between classical and quantum channels minimizes spectral overlap and subsequent noise transfer. Proper selection of propagation direction further diminishes Raman-induced noise, a common source of signal degradation in fiber optic systems. “By jointly optimizing propagation direction, wavelength allocation and optical power, the performance of the fused architecture can be optimized,” according to the research.

The team demonstrated that a carefully controlled power range suppresses both phase distortion and photon contamination, maintaining low excess noise levels and high system performance. The analysis of timing error processes reveals that the variance of timing errors, σx2, is determined by a summation over the power of harmonic components, dependent on frequency and modulation width. This detailed understanding of noise sources allows for targeted mitigation strategies, furthering the development of robust and scalable quantum networks.

Excess Noise Model for Continuous-Variable QKD Channels A model detailing excess noise in continuous-variable quantum key distribution (CV-QKD) channels has been developed, identifying optical power, wavelength allocation and propagation direction as key control parameters for minimizing interference. The research establishes a power-dependent model for this excess noise, expressed as ξtot(Pc)≃ξ0 + c1Pc + c2Pc2 + c3Pc3, where Pc represents the mean optical-comb power and the coefficients account for Raman scattering, optical leakage, and nonlinear effects. This effect is demonstrated through fitted curves showing the dependence of excess noise on wavelength allocation and propagation direction, and is reflected in corresponding secret-key rate (SKR) results. The model quantitatively links classical-channel conditions to quantum excess noise, providing a means to predict and optimize performance. The research identifies specific noise mechanisms contributing to the overall excess noise, with dominant contributions to coefficient c1 arising from optical leakage and Raman scattering, while c2 and c3 are primarily influenced by cross-phase modulation and four-wave mixing. The time-domain equivalent of this excess noise is expressed as σ_x(τ;f_m,m,W) = 4/3sum_(k = 1)^∞Ak2(fm,m,W)[(| sin^3(π kf_mτ)|)/(π kf_mτ)]^21/2, allowing for precise characterization of timing jitter. After fusing communication and time-frequency layers, the system introduces only 0.0037 shot noise unit (SNU) of additional excess noise, limiting the secret-key-rate penalty to 6.7%, a result demonstrating the viability of co-propagating CV-QKD with classical data transmission.

Optimized System Limits Key Rate Penalty to 6.7% After optimizing system parameters, the integrated infrastructure limited the penalty to secret-key rates to 6.7%, a figure demonstrating the viability of shared infrastructure for quantum communication and precision timing.

The team established a power-dependent model to predict this excess noise, allowing for targeted mitigation strategies within the unified system. By jointly optimizing these three parameters, the fused architecture’s performance can be maximized. The recovered data undergoes parameter estimation, information reconciliation and privacy amplification to finalize the secure key exchange. Beyond secure communication, the unified infrastructure also enables absolute time synchronization, achieving a stability of 0.26 femtoseconds at one second. This level of precision is critical for coordinating distributed quantum operations, and the researchers demonstrate that it can be achieved alongside high-speed classical data transmission. The system introduces only 0.0037 shot noise units of additional excess noise, a minimal impact on quantum signal integrity. Quantum-Secured Timestamping Enables 0.26 fs Synchronization This synchronization surpasses conventional methods reliant on classical encryption, offering both enhanced security and temporal accuracy for distributed quantum networks. The system co-transmits weak quantum states and classical data using wavelength-division multiplexing on a shared optical fiber, a key element in streamlining network complexity. Optimization of these parameters resulted in the introduction of only 0.0037 shot noise units of additional excess noise, limiting the penalty to the secret-key rate to 6.7%, demonstrating a viable path toward shared infrastructure for both quantum communication and precision timing. Experimentally, the team demonstrated this secure synchronization, with time-difference increments remaining at the sub-femtosecond level while maintaining secure timestamp exchange. “Hence, the coexistence of these functions enables absolute time synchronization with both high stability and enhanced security with quantum-key protection,” compared with traditional schemes, according to the research. OFCs as Integrated Physical Layer for Quantum Network Services This unified approach, detailed in recent work, moves beyond separate infrastructures for precision timing and communication, streamlining the architecture of future quantum networks.

The team’s spectral-engineering strategy successfully suppresses coupling between data modulation and timing extraction, a previously challenging aspect of integrated systems. This suppression is essential for maintaining both the integrity of the classical data stream and the precision of the timing signal. The foundation of this integration lies in optical frequency combs, which carry precision timing through pulse timing and classical communication through intensity modulation. 👉 More information🗞 An optical-frequency-comb-enabled unified infrastructure for quantum networks✍️ Yiting Lin, Zishuo Ren, Zimeng Li, Yufei Zhang, Ziyang Chen and Hong Guo (Beijing Key Laboratory of Quantum Sensing and Precision Measurement, and Center for Quantum Information Technology, and Institute of Quantum Electronics, Peking University, Beijing, China); Yunkun Zhao and Kemi Xu (State Key Laboratory of Environment Characteristics and Effects for Near-space, Center for Photonic Quantum Precision Measurement, Advanced Research Institute of Multidisciplinary Science, Beijing Institute of Technology, Beijing, China); Jiayu Ma, Yanhao Sun and Xiangyu Wang (State Key Laboratory of Information Photonics and Optical Communications, Beijing University of Posts and Telecommunications, Beijing, China); Yang Sun and Lei Yang (Technology and Engineering Center for Space Utilization, Chinese Academy of Sciences, China)🧠 ArXiv: https://arxiv.org/html/2611.16349v1 More like thisQuantum CryptographyEntanglement-based QKD network links four cities in Slovakia with real-time keysQuantum Computing Business NewsLumen invests in quantum networking research at Ohio StateQuantum CryptographyBarcelona test shows 18.9km quantum key exchange over a cityQuantum NetworkingQunnect launches US open-access quantum network in New MexicoStay currentSee today’s quantum computing news on Quantum Zeitgeist for the latest breakthroughs in qubits, hardware, algorithms, and industry deals. Tags: