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Long-term phase stabilization for metropolitan-scale dark quantum channels

D. Ahn, Ivan A. Burenkov, M. V. Jabir, Mheni Merzouki, Paulina S. Kuo, Abdella Battou, Thomas Gerrits, Sergey V. Polyakov

Published Jan 1, 2026• 0 citations
Publisher

Abstract

We employ a frequency comb to demonstrate a multiband wavelength-division-multiplexing (WDM)-based phase stabilization over a metropolitan-scale fiber link while maintaining a dark quantum channel. We achieve phase stability over 100 seconds, a four orders of magnitude improvement of the current state of the art.
DOI
10.1364/cleo_fs.2026.fth2a.3

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Establishing phase-stable links over long-distance fiber infrastructure is crucial for phase-sensitive networking protocols. Faint-light and quantum networking present an additional challenge of keeping the payload channel dark. In this work, we present a scalable solution for phase stabilization of dark metropolitan-scale fiber links using wavelength-division-multiplexing (WDM). We implement a combined feed-back and feed-forward phase stabilization for a target channel by combining error signals from two separate reference channels over a 120 km deployed aerial fiber link. Residual phase noise in the dark target channel is reduced by two orders of magnitude compared to a WDM feedback-only strategy and reaches subfemtosecond root-mean-square (RMS) values. Observed cross-talk photon noise is ≈500 photons per second. Potentially, several dark target channels can be phase-stabilized without injecting additional reference channels. This solution is a major step toward scalable metropolitan-distance quantum networking applications, including single-rail entanglement distribution, quantum key distribution, coherent clock transfer, and coherent optical communication.
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Optical phase stabilization, tracking, and locking in long fiber links are pivotal for the functionality of many communication protocols and distributed sensors. However, conventional phase stabilization methods use strong optical probe signals that may contaminate and destroy fragile quantum signals propagating in the same fiber link. Here, we experimentally demonstrate phase locking of fiber spools up to 100 km and over 120 km deployed optical communication link using faint coherent states with an average of just photons per second detected at the receiving detector as a probe signal. The power spectral density of phase noise, integrated from 1 Hz to 50 kHz, yields an excess rms phase noise of 110 mrad (corresponding to timing jitter below 0.09 fs) for a 100 km fiber spool and 140 mrad (corresponding to rms timing jitter below 0.12 fs) for deployed fiber. The displacement-enhanced measurement protocol that we employ for phase stabilization yields Fisher information that is greater than that of homodyne and heterodyne measurement protocols for fiber links longer than 100 m and, therefore, exceeds the canonical shot noise limit of accuracy. By implementing this stabilization strategy with a 50% duty cycle, we enable time-division multiplexed faint light and quantum payloads to coexist within the same communication channel.