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arXiv:2607.03202

Broadband characterization of polarization mode dispersion for quantum communication channels

Vadim Rodimin, Konstantin Kravtsov, Rui Ming Chua, Xingjian Zhang, Aleksei Ponasenko, Yury Kurochkin, Alexander Ling, James A. Grieve

quant-phphysics.opticsSubmitted Jul 3, 2026• Announced Jul 6, 2026• 0 citations• Revised Oct 28, 2026 (v3)

v1 · Jul 3, 2026 v2 · Sep 20, 2026 v3 · Oct 28, 2026

Abstract

We present a method for characterizing depolarization in fiber channels carrying broadband polarization-encoded optical signals. Wavelength-dependent polarization mode dispersion maps each input polarization to a trajectory on the Poincaré sphere; we show that the singular value decomposition of the band-averaged rotation matrix yields, in closed form, the optimal input states, the associated mutually unbiased measurement bases, and the resulting projection error probabilities. The three singular values provide a compact, bandwidth-dependent channel signature that distinguishes first- from higher-order dispersion, and yield the bandwidth at which the worst-case error probability reaches 3% as a practical filtering budget. We perform a second-order analysis of the band average that treats the two higher-order mechanisms: variation of the differential group delay at a fixed principal-state axis and wander of that axis with wavelength. We characterize 12 deployed fiber links in Masdar City and demonstrate mitigation by concatenating two channels through a single polarization controller, a step towards nonlocal compensation. The analysis is implemented in an openly available Python library for fiber-channel characterization.

Comments: 8 pages, 4 figures

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The polarization mode dispersion (PMD) in optical fibers poses a major challenge for maintaining the fidelity of quantum states for quantum communications. In this work, a comprehensive model linking the probability of quantum measurement errors (infidelity) to PMD is developed and validated by experimental measurements of differential group delay and quantum bit error rate (QBER). Our research proposes effective methods to mitigate PMD effects for broadband entangled photons and evaluates the impact of higher-order PMD effects. The model provides an experimentally verified framework for the optimization of commercial quantum key distribution systems in deployed fiber-optic lines.

The implementation of quantum communication protocols in long optical fibers is limited by several decoherence mechanisms. In this contribution we review this mechanisms and analysis their effect on the quantum states. Because of asymmetry in the real fibers, the two orthogonal polarization modes are propagated at different phase and group velocities. The difference in group velocities results in Polarization Mode Dispersion (PMD). It is created by random fluctuations of the residual birefringence in optical fibers, such that the State of Polarization (SOP) of an optical signal will turn randomly over time, in an unpredictable way. In optical quantum communication, quantum state measurement is necessary. The bottleneck for communication between far apart nodes is the increasing of the error probability with the length of the channel connecting the nodes. For an optical fiber, the value of absorption and depolarization of a photon (i.e., the qubit) increases exponentially with the length of the fiber.

Polarization-encoded quantum communication requires compensation of polarization transformations induced by the optical links. If the compensator is embedded between two channel segments, the transformations before and after the compensator must be treated separately. Moreover, standard three-wave-plate polarization controllers can become non-universal when their retardances deviate from their ideal values. To address these two challenges, we introduce a four-wave plate compensator that synthesizes arbitrary SO(3) polarization transformations over a broad wavelength range, and an eight-Stokes vector protocol that reconstructs the two link-segment Mueller matrices on either side of the compensator. Our experiment reveals that the four-plate sequence suppresses polarization-induced excess quantum bit error rate (QBER) to the sub-percent level at an operating wavelength more than 100 nm from the design wavelength without further optimization. Combined with two auxiliary wavelengths, our scheme tracks the temperature-driven drift of a strongly wavelength-sensitive fiber spool while keeping the excess QBER below 1%. These results support flexible compensator placement and wavelength channel selection, as well as non-interruptive polarization control in wavelength-division-multiplexed quantum optical links.

The performance of Quantum Key Distribution (QKD) protocols that encodeinformation in the polarization state of light can be affected by polarizationtransformations induced by the optical fiber channel. This thesis investigates whethera real, deployed fiber link can support polarization-based QKD, building on apolarization stabilization system developed in an earlier project course to activelycorrect the fiber’s polarization drift. To answer this, the channel-induced quantum biterror rate (QBER) is decomposed into a rotation-induced misalignment term,correctable by active polarization stabilization, and a depolarization term set by thefiber’s differential group delay (DGD) relative to the coherence time of thetransmitted photons, a term no stabilization system can correct. The stabilizationsystem’s performance and the fiber’s Polarization Mode Dispersion (PMD) vector,from which the DGD is extracted via a wavelength-swept Jones matrix reconstructionmethod, are both characterized experimentally on two deployed metropolitan fiberlinks in the Stockholm area. The results show the rotation term is fully within reachof the stabilization system, and reduce the remaining question — whether thedepolarization term is also acceptable — to a single, precisely defined externalparameter, the coherence time of the photon source eventually used for the quantumsignal, which this thesis shows exactly how to evaluate.

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.