Sub-picosecond inter-core skew characterization in multicore fibers via Hong-Ou-Mandel interference

Inter-core skew (ICS), the differential group delay between cores of a multicore fiber (MCF), is a critical parameter for both classical space-division multiplexed communications and quantum photonic networks. We present a high-precision measurement of ICS in commercially available four-core fibers using two-photon Hong-Ou-Mandel (HOM) interference in a fiber-integrated 4 × 4 multiport beam splitter, using both photon pairs from spontaneous parametric down-conversion (SPDC) and a weak-coherent-state source (a bimodal pulsed laser). By extracting the center position of HOM interference dips and peaks across all twelve core-pair combinations, we obtain individual ICS values whose precision is limited by the delay-stage positioning uncertainty. This first multi-length characterization of RMS ICS, spanning laboratory to field-deployed scales, finds results consistent with the expected L stochastic random-walk scaling, σ_τ(L)=κL+c with intrinsic coefficient κ=(39.80±0.003)ps/km. This measurement was made possible by HOM's immunity to first-order path fluctuations, which render classical interferometric methods impractical for long installed fibers. The demonstrated ±0.11 ps precision for the two-photon interference represents a ∼180-fold improvement over correlation optical time-domain reflectometry (C-OTDR), the standard method for long-fiber ICS characterization. Fisher information analysis establishes a fundamental Cramér-Rao precision limit in the few-femtosecond range, indicating further improvement is achievable with better delay control. The SPDC source provides this precision on laboratory-testbed fibers, while the weak-coherent-state source trades precision for loss tolerance, extending the measurement to a 1300 m field-deployed fiber with >10 dB losses. These results establish a practical platform for characterizing timing uniformity in MCF-based networks for both quantum and classical space-division multiplexed applications.