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

Precision Timing Meets Quantum Communication

A collaboration between Peking University, the Beijing Institute of Technology, and Beijing University of Posts and Telecommunications has shown that an optical‑frequency‑comb‑based link can keep timing jitter below one femtosecond (0.85 fs) across a 100‑km fiber span. The experiment merges three functions—ultra‑precise time transfer, classical data transmission, and quantum‑secured timestamping—onto a single optical carrier, pointing toward a more compact and scalable quantum‑network architecture.

Unified Physical Layer

Traditional long‑distance quantum networks rely on separate infrastructures for timing, classical traffic, and quantum key distribution (QKD). By encoding classical bits onto the intensity of the comb while preserving the pulse train’s timing, the team eliminates the need for parallel fibers. Dual‑comb linear optical sampling extracts the timing information at the receiver, while an arbitrary waveform generator drives an intensity modulator for 100 Mbit s⁻¹ data streams. The result is a bit‑error‑rate that meets the 10⁻⁶ threshold at –46 dBm without forward‑error‑correction.

Spectral Engineering for Interference Suppression

A key obstacle is the spectral overlap between data modulation sidebands and the comb’s timing tones. The researchers applied a suite of spectral‑engineering tricks: switching from non‑return‑to‑zero to Manchester coding, adjusting the fraction of unmodulated symbols, and fine‑tuning modulation depth. These measures shift energy away from low‑frequency components that would otherwise corrupt the timing signal. Wavelength allocation and propagation direction were also optimized to curb Raman‑induced noise, keeping excess noise to a mere 0.0037 shot‑noise units.

Timing Performance and Quantum Security

The integrated system delivers a time deviation of 0.26 fs at a one‑second averaging time—on par with dedicated precision‑timing setups. Simultaneously, continuous‑variable QKD runs over the same fiber, incurring only a 6.7 % penalty to the secret‑key rate. This demonstrates that quantum‑secured timestamping can coexist with high‑speed classical traffic without sacrificing either temporal accuracy or cryptographic strength.

Implications for Future Quantum Networks

By collapsing three layers into one, the approach reduces hardware complexity, fiber deployment costs, and synchronization latency. It also offers a pathway to quantum‑internet services where timing, data, and security are intrinsically linked. While the current demonstration is a laboratory proof‑of‑concept, scaling to metropolitan or intercity distances will require further work on amplification, dispersion management, and long‑term stability.

Bottom line: The experiment validates optical‑frequency combs as a versatile backbone for next‑generation quantum networks, delivering sub‑femtosecond timing stability alongside secure, high‑rate data transmission over 100 km of fiber.