Mode-tunable inter-core coupling of photon-number-resolved quantum light in a telecom multicore fiber
Multicore optical fibers (MCFs), developed to scale classical telecommunications, are gaining traction as a quantum platform for high-dimensional key distribution, entanglement, and photonic quantum walks. So far, quantum light has occupied these fibers only as single-mode path channels; the spatial-mode structure within each core has remained untouched. Here we operate a telecom-grade 39-core fiber far from its design wavelength, where each core supports exactly two LP mode groups (LP01, LP11), and measure heralded photon-number states |1> and |2>, resolving the photon number simultaneously with the intra-core spatial mode or with the core index. Displacing the launch prepares a coherent LP01-LP11 superposition whose single-sided intensity pattern shows that the intermodal phase is preserved over the fiber: a drift-resolved joint fit gives a pixel asymmetry (0.4% vs 47%) incompatible with an incoherent mode mixture. Because the two modes couple to neighboring cores at different rates (kappa_LP11/kappa_LP01 = 2.4), the launched mode content acts as a state-selectable coupling knob: the effective inter-core coupling of the passive fiber is set by the launch alone (3.9-13.1%), a tunability that otherwise requires electro-optic, thermo-optic, or mechanical reconfiguration. The propagation is photon-number independent: the per-photon crosstalk of heralded |2> states equals that of |1>, bounding any Fock-state dependence to |dx/x| < 6.1% (95% CL), and the imaged mode distributions of |1> and |2> agree to better than 1%. This measured linear-optics null, validated against exact Fock-space simulation, is the baseline against which genuine multiphoton interference in fiber quantum walks must be certified; we also show how coincidence-based event selection produces spurious photon-number-dependent crosstalk if accidentals are not subtracted.