Phase-stable Optical Fiber Links Achieve 0.998 Fidelity for Quantum Network Protocols

The development of secure quantum communication networks requires the reliable transmission of quantum information over significant distances, a challenge complicated by signal degradation and timing errors in optical fibres. Nicholas V. Nardelli, Dileep V. Reddy, Michael Grayson, and colleagues at the National Institute of Standards and Technology and the University of Colorado now demonstrate a crucial advance in this field, achieving remarkably stable transmission of single photons through optical fibre links. Their system exhibits exceptionally low timing jitter and enables a fidelity exceeding 99. 8% between two 2. 1 kilometre fibre links, representing a major step towards practical, scalable quantum networks. By adapting techniques from high-stability optical atomic clocks, the team effectively isolates quantum and classical communication channels, paving the way for networks that offer provable security advantages over existing technologies.

Remote Phase Synchronization Over Optical Fibers

This research details the successful demonstration of a system for synchronizing the phase of light between remote quantum network nodes using deployed optical fibers. The work addresses key challenges in building a scalable quantum internet, specifically maintaining coherence and synchronization over long distances. The researchers achieved stable phase synchronization between remote nodes using a frequency comb and advanced control techniques, paving the way for more complex quantum network applications. A crucial component of the system is an optical frequency comb, which precisely controls and compares the frequencies of light at different nodes.

The system utilizes existing deployed optical fibers, making it practical for real-world implementation. A sophisticated feedback loop, implemented using an FPGA, actively stabilizes the phase synchronization, compensating for noise and drift in the fiber link. High-stability lasers, exhibiting extremely low frequency noise, are critical for maintaining coherence throughout the process. This technology is crucial for applications like quantum key distribution, secure communication, distributed quantum computing, linking quantum computers to increase processing power, quantum metrology, improving the precision of measurements, and quantum telescopes, enhancing the resolution of telescopes. Future work will focus on improving stability and range, actively compensating for polarization drift and signal loss in the fiber, developing techniques to connect a larger number of nodes, and exploring advanced fiber types, such as hollow-core or multicore fibers, to reduce noise and improve performance. This research represents a significant step towards building a practical and scalable quantum internet by demonstrating a robust method for synchronizing quantum nodes over existing fiber infrastructure.

Stable Single-Photon Distribution Over Kilometer-Scale Fiber

Scientists have achieved a significant breakthrough in quantum networking by demonstrating the stable distribution of single-photon level pulses over a phase-stable fiber link. Experiments revealed an optical timing jitter of less than 100 as over a 10-minute data accumulation period, establishing a new benchmark for precision in quantum communication. This exceptional stability directly enables a fidelity greater than 0. 998 between two stabilized 2. 1km long deployed fiber links, paving the way for reliable long-distance quantum communication.

The research team employed techniques traditionally used in high-stability optical atomic clock signal distribution, adapting them to stabilize optical paths for quantum state distribution. This innovative approach utilizes time and frequency multiplexing to achieve an isolation between quantum and classical channels exceeding 10 10 , a critical factor in maintaining the integrity of quantum information. The data demonstrates a remarkable ability to separate the delicate quantum signals from background noise, ensuring high-fidelity entanglement distribution. Furthermore, the work establishes three key performance benchmarks for path-entangled quantum networking: optical timing jitter below 100 as over 10 minutes, indistinguishability between fiber paths exceeding 99.

6%, and isolation between classical and quantum channels greater than 8x 10 10 . These results are crucial for implementing advanced quantum protocols, such as quantum key distribution and distributed quantum computing, which rely on the precise and stable distribution of quantum information. The breakthrough delivers a necessary step towards scalable, high-rate quantum networks with a provable quantum advantage, opening new possibilities for secure communication and advanced computation.

Stable Quantum Links Over Kilometre Distances

Scientists have demonstrated a significant advance in quantum networking by successfully distributing single-photon pulses over deployed optical fiber links with exceptional phase stability, maintained for over ten minutes. The team achieved a fidelity exceeding 0. 998 between two stabilized 2. 1 kilometer fiber links, a crucial step towards practical, scalable quantum networks offering a demonstrable advantage over classical systems. This accomplishment builds upon established time and frequency control techniques, adapting them to achieve robust isolation between classical and quantum channels through time and frequency multiplexing.

The researchers overcame a key challenge in quantum communication by effectively suppressing noise and maintaining signal integrity over substantial distances. By carefully controlling optical phase and employing a novel signal multiplexing scheme, they achieved high channel isolation, essential for reliable quantum state transfer. While acknowledging limitations related to fiber phase noise and the current reliance on optical choppers, the authors outline promising avenues for future work, including passive shielding to reduce noise, active stabilization of fiber polarization and loss, and innovative techniques such as low-loss hollow-core fibers or multi-core fibers to further enhance isolation and reduce signal degradation. Ultimately, this work paves the way for multi-node quantum networks capable of sustaining high fidelity across numerous kilometer-scale links, bringing practical quantum communication closer to reality.