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 a commercially available four-core fiber using two-photon Hong--Ou--Mandel (HOM) interference in a fiber-integrated 4times4 multiport beam splitter. By extracting the center position of HOM interference dips and peaks across all twelve core-pair combinations, we obtain individual ICS values with a demonstrated precision of pm0.11 ps, limited by the delay-stage positioning uncertainty. The root-mean-square ICS grows as σ_τ(L) = κsqrt{L}+c with κ= 48.7 pm 2.5 ps/sqrt{km} and c = 9.76 pm 1.2 ps, over fiber lengths from 7.7 m to 1300 m. This first direct validation of the stochastic random-walk scaling across a length range spanning laboratory to field-deployed scales was made possible by HOM's immunity to first-order path fluctuations, which renders classical interferometric methods impractical for long installed fibers. The demonstrated pm0.11 ps precision represents a sim180-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 femtosecond range, indicating further improvement is achievable with better delay control. These results establish a practical platform for characterising timing uniformity in MCF-based networks for both quantum and classical space-division multiplexed applications.