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arXiv:2609.02841

Exponential speedup of polarization stabilization for long distance DWDM quantum networks

Jinyi Du, En Teng Lim, Xingjian Zhang, Hongwei Gao, George F. R. Chen, Dawn T. H. Tan, Alexander Ling

quant-phphysics.app-phphysics.opticsSubmitted Sep 2, 2026• Announced Sep 3, 2026• 0 citations
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Abstract

Fibre-based quantum networks distributing polarization entanglement require a stable and uninterrupted transmission basis for reliable operation. Bright classical reference light enables rapid polarization feedback but can introduce noise into quantum channels. Entangled-photon-based feedback avoids this noise, but typically interrupts the target entanglement channel during calibration and becomes prohibitively slow over long distances due to the product loss of fibre links. Here, we overcome both limitations by combining wavelength-bracketed probing with switch-enabled path decomposition. Spectrally adjacent entangled-photon sidebands track the polarization response of the central distribution channel without interrupting its transmission, while optical switches and local reference fibres independently determine the signal and idler network transformations. Transferring the resulting compensation settings to the central channel eliminates calibration-induced downtime and changes the acquisition-time scaling from the product of the link losses to the sum of losses. We demonstrate the method on a 133 km fibre testbed and achieve continuous closed-loop stabilization for more than 24 hours without classical reference light. The decomposition of multi-link quantum feedback into single-link measurements provides a scalable stabilization strategy for wavelength-multiplexed quantum networks.

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Fibre-based quantum networks distributing polarization entanglement require a stable and uninterrupted transmission basis for reliable operation. Bright classical reference light enables rapid polarization feedback but can introduce noise into quantum channels. Entangled-photon-based feedback avoids this noise, but typically interrupts the target entanglement channel during calibration and becomes prohibitively slow over long distances due to the product loss of fibre links. Here, we overcome both limitations by combining wavelength-bracketed probing with switch-enabled path decomposition. Spectrally adjacent entangled-photon sidebands track the polarization response of the central distribution channel without interrupting its transmission, while optical switches and local reference fibres independently determine the signal and idler network transformations. Transferring the resulting compensation settings to the central channel eliminates calibration-induced downtime and changes the acquisition-time scaling from the product of the link losses to the sum of losses. We demonstrate the method on a 133 km fibre testbed and achieve continuous closed-loop stabilization for more than 24 hours without classical reference light. The decomposition of multi-link quantum feedback into single-link measurements provides a scalable stabilization strategy for wavelength-multiplexed quantum networks.
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.
Quantum networking continues to encode information in polarization states due to ease and precision. The variable environmental polarization transformations induced by deployed fiber need correction for deployed quantum networking. Here, we present a method for automatic polarization compensation (APC) and demonstrate its performance on a metropolitan quantum network. Designing an APC involves many design decisions as indicated by the diversity of previous solutions in the literature. Our design leverages heterodyne detection of wavelength-multiplexed dim classical references for continuous high-bandwidth polarization measurements used by newly developed multi-axis (non-)linear control algorithm(s) for complete polarization channel stabilization with no downtime. This enables continuous relatively high-bandwidth correction without significant added noise from classical reference signals. We demonstrate the performance of our APC using a variety of classical and quantum characterizations. Finally, we use C-band and L-band APC versions to demonstrate continuous high-fidelity entanglement distribution on a metropolitan quantum network with an average relative fidelity of 0.94 ± 0.03 for over 30 hrs.
To study channel-to-channel phase drift in broadband links, we implement a dense wavelength-division multiplexing (DWDM)-based phase stabilization testbed. We systematically study phase stabilization in adjacent DWDM channels of metropolitan-scale fiber links. A 25GHz frequency comb with a sub-Hz laser generated two DWDM-separable frequency components at 1550.12 nm and 1549.32 nm as signal and reference channels. One of these components is stabilized using a phase-locked-loop circuit, while the phase noise in the other channel is monitored. We demonstrate a significantly reduced phase noise in the monitored channel over a 100 km link. This approach mitigates phase noise in wavelength-detuned long-distance network links while wavelength-separating the classical probing signal and reducing its impact on the quantum channel through Raman scattering in a long fiber.
Entanglement distribution between spatially distant nodes is crucial for advancing the development of quantum networks. Achieving large-scale, practical quantum networks by leveraging existing fiber infrastructures and commercial technologies can greatly enhance cost efficiency in real-world applications. As such, the simultaneous transmission of quantum entanglement and classical signals within a single optical fiber is becoming increasingly desirable, yet it remains challenging due to the substantial intensity disparities between single-photon-level quantum light and classical traffic. In this study, we theoretically describe and experimentally implement polarization entanglement distribution using a continuous wave pumped integrated AlGaAs Bragg reflection waveguide quantum source, coexisting with classical light in optical fiber at the telecom C-band. The characteristics of noise photons are analyzed and evaluated under different levels of classical light power. Using the BBM92 protocol for quantum key distribution, we demonstrate that temporal and spectral filtering during coincidence detection, combined with optimized source brightness, effectively influences the secure key rate and achievable distance of the system. Polarization drift is measured across various fiber paths, with 35-hour stability tests on fibers ranging up to 55 km in length. Our results can guide the initial design of optimal entanglement distribution networks that are compatible with fiber-optic communication systems.
We present polarization entanglement distribution over an operator-owned, metropolitan-scale fiber network, employing standard wavelength-division multiplexing components to achieve coexistence of O-band photons and C-band classical light. First, we demonstrate dynamic path switching by alternating between 15-km and 30-km deployed fiber loops, enabled by automated polarization compensation achieving sub-second recovery and time-averaged CHSH S -values exceeding 2.65 over 21 hours. We then assess the impact of co-propagating 100-Gbit/s C-band classical signals by varying launch powers up to 6 dBm. Finally, by combining path switching and classical co-propagation, we achieve stable entanglement distribution over 30 km of deployed fiber, validating the feasibility of hybrid quantum–classical transmission and paving the way for entanglement-based quantum networking on existing metropolitan infrastructure.
Quantum measurements that use the entangled photons' polarization to encode quantum information require calibration and alignment of the measurement bases between spatially separate observers. Because of the changing birefringence in optical fibers arising from temperature fluctuations or external mechanical vibrations, the polarization state at the end of a fiber channel is unpredictable and time-varying. Polarization tracking and stabilization methods originally developed for classical optical communications cannot be applied to polarization-entangled photons, where the separately detected photons are statistically unpolarized, yet quantum mechanically correlated. We report here a fast method for automatic alignment and dynamic tracking of the polarization measurement bases between spatially separated detectors. The system uses the Nelder-Mead simplex method to minimize the observed coincidence rate between non-locally measured entangled photon pairs, without relying on classical wavelength-multiplexed pilot tones or temporally interleaved polarized photons. Alignment and control is demonstrated in a 7.1 km deployed fiber loop as well as in a controlled drifting scenario.