Mode-resolved picosecond single-photon polarimetry maps modal dynamics in multimode fibers

Polarization dynamics in multimode optical fibers (MMFs) play a key role in applications ranging from high-capacity communication to distributed sensing. However, most approaches probe only a single polarization state, limiting polarimetric characterization and leaving orthogonal dynamics hidden. Here, we show that time-resolved single-photon avalanche diode (SPAD) arrays enable quasi-real-time observation of dual-polarization mode dynamics in few-mode fibers. Simultaneous detection of orthogonal polarization channels provides picosecond time-of-flight polarimetric readout, revealing spatio-temporal correlations between LP₀₁ and LP₁₁ modes under stress. Building on this capability, we introduce a SPAD-based single-photon polarimetric platform that reconstructs Stokes vectors across  ~ 1000 spatial channels with 55 ps resolution, visualizing complex modal dynamics in MMFs. Through Hilbert-transform analysis, the system provides mode-resolved Stokes retrieval with per-pixel minimum detectable modulation of 0.002 (0.2%, 3σ) and signal-to-noise ratios up to 33 dB. This scalable platform enables ultrafast Stokes polarimetry in MMFs, opening new opportunities in classical and quantum photonics.