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arXiv:2512.01229 · EPJ Quantum Technology

Passive Polarization Stabilization for Robust Entanglement Distribution via Cross-Aligned Polarization Maintaining Fiber Pairs

Jin-Woo Kim, Minchul Kim, Jiho Park, Junsang Oh, Kyongchun Lim, Byung-Seok Choi, Chun Ju Youn

quant-phSubmitted Dec 1, 2025• Announced Dec 2, 2025• 0 citations• Revised Jun 19, 2026 (v2)

v1 · Dec 1, 2025 v2 · Jun 19, 2026

Abstract

Maintaining stable entanglement distribution through perturbed fiber links is essential for practical quantum-optics experiments, yet it remains challenging because of polarization fluctuations and phase or temporal-delay variations. We demonstrate stable entangled-photon transmission using a cross-aligned polarization-maintaining fiber (CAPMF) structure composed of two polarization-maintaining fiber sections with mutually orthogonal principal axes. The CAPMF configuration passively compensates polarization fluctuations without real-time active polarization control. We theoretically analyze the CAPMF structure and experimentally verify its stabilization performance under external mechanical perturbations. In the experiment, the single-mode fiber configuration yields an average visibility of $0.7655$ and a CHSH value of $S=1.7714$, whereas the CAPMF configuration maintains an average visibility of $0.9843$ and a CHSH value of $S=2.6838$. These results show that CAPMF offers a simple and robust architecture for stabilizing fiber-interface sections in practical entanglement-distribution systems.

Comments: 21 pages, 5 figures

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Maintaining stable entanglement distribution through perturbed fiber links is essential for practical quantum-optics experiments, yet it remains challenging because of polarization fluctuations and phase or temporal-delay variations. We demonstrate stable entangled-photon transmission using a cross-aligned polarization-maintaining fiber (CAPMF) structure composed of two polarization-maintaining fiber sections with mutually orthogonal principal axes. The CAPMF configuration passively compensates polarization fluctuations without real-time active polarization control. We theoretically analyze the CAPMF structure and experimentally verify its stabilization performance under external mechanical perturbations. In the experiment, the single-mode fiber configuration yields an average visibility of

0.7655

and a CHSH value of

S=1.7714

, whereas the CAPMF configuration maintains an average visibility of

0.9843

and a CHSH value of

S=2.6838

. These results show that CAPMF offers a simple and robust architecture for stabilizing fiber-interface sections in practical entanglement-distribution systems.

we experimentally demonstrate the distribution of polarization-entangled photons over a 62-km, partially-aerial fiber between the National Institute of Standards and Technology and the University of Maryland. With active polarization compensation applied to the fiber link, we observed a channel fidelity above 98%. Over the stabilized fiber link, we distributed approximately 1200 pairs of polarization-entangled photons per second over a course of 24 hours, with an average CHSH S-parameter value of 2.39 and a link up-time of 92.8% for entanglement distribution.

Entanglement distribution through optical fibers is essential for quantum communication networks; however, fiber transmission can alter photon polarization and modify the observed correlations of polarization-entangled states, necessitating polarization compensation to recover the desired entangled state in the measurement basis. Here, we experimentally evaluate two passive polarization compensation techniques: a free-space Quarter-Half-Quarter (QHQ) waveplate configuration and an in-fiber three-paddle Fiber Polarization Controller (FPC). The polarization transformation along each downconverted-photon path is independently compensated through a systematic path-by-path optimization procedure. Using both techniques, we recover high-quality polarization correlations and entanglement, with visibilities exceeding

93\%

in three mutually unbiased bases and fidelities above

94.5\%

. The results demonstrate comparable restoration using free-space waveplate-based and fiber-based control, establishing a systematic framework for laboratory and short-reach quantum communication links.

Distributing quantum entanglement over telecommunication fiber is a fundamental task in metropolitan-scale quantum networks, enabling advanced applications such as quantum-secured communication, quantum sensing, and distributed quantum computing. Although polarization entanglement is relatively easy to generate and manipulate, its distribution across deployed fibers is challenging because of the time-varying polarization transformations imposed by the fiber, which must be actively stabilized. In this work, we experimentally demonstrate the distribution of polarization-entangled photons over a 62 km, partially aerial fiber between the National Institute of Standards and Technology and the University of Maryland. With polarization stabilization applied to the fiber, we achieved a photon pair rate of approximately 1500s −1 and observed a time-averaged CHSH inequality parameter of S =2.34±0.37. During a continuous 24 h experiment, time-multiplexed polarization compensation sessions only cost 7.2% of the total link operation time, leaving an entanglement distribution uptime of 92.8%. Our results demonstrate the feasibility of distributing polarization-entangled photons over challenging fiber conditions, which is an important step toward the practical deployment of quantum networks.

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