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arXiv:2611.03304

Inline inter-core phase control in multi-core fiber with a commercial polarization controller

A. Contreras, A Muñoz, G. dos Santos, G. Cañas, G. Saavedra, C. Hermann-Avigliano, E. S. Gómez, G. Lima, S. P. Walborn

physics.opticsquant-phSubmitted Nov 2, 2026• Announced Nov 3, 2026
arXivHTMLPDF

Abstract

Inter-core phase control is a prerequisite for interferometry in multi-core fiber (MCF), but no compact inline element for this purpose is presently available, and existing approaches require fanout to single-mode fiber or free-space optics. We show that a commercial inline polarization controller, a low-loss and widely available component, can be operated as an inline inter-core phase shifter. Two-core interferometry on a jacketed four-core patchcord gives a reversible inter-core phase of $0.95 \pm 0.02$~rad from a single device, with an operating-point-independent lower bound of $0.38$~rad, driven through repeated load-unload cycles at a rate set by the actuator. A diametrically compressed disk model is used to predict the operating condition: with the input polarization aligned to a principal stress axis the device imparts a relative phase between cores without rotating the polarization state of either, which we verify by polarimetry on all four cores. The response is polarization dependent, falling to less than $0.58$~rad for the orthogonal input. The result is a practical, inline route to inter-core phase control and stabilization in all-MCF interferometers, requiring neither fanout to single-mode fiber nor free-space optics.

Comments
to appear in Journal of Optics

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Inter-core phase control is a prerequisite for interferometry in multi-core fiber (MCF), but no compact inline element for this purpose is presently available, and existing approaches require fanout to single-mode fiber or free-space optics. We show that a commercial inline polarization controller, a low-loss and widely available component, can be operated as an inline inter-core phase shifter. Two-core interferometry on a jacketed four-core patchcord gives a reversible inter-core phase of 0.95±0.020.95 \pm 0.020.95±0.02~rad from a single device, with an operating-point-independent lower bound of 0.380.380.38~rad, driven through repeated load-unload cycles at a rate set by the actuator. A diametrically compressed disk model is used to predict the operating condition: with the input polarization aligned to a principal stress axis the device imparts a relative phase between cores without rotating the polarization state of either, which we verify by polarimetry on all four cores. The response is polarization dependent, falling to less than 0.580.580.58~rad for the orthogonal input. The result is a practical, inline route to inter-core phase control and stabilization in all-MCF interferometers, requiring neither fanout to single-mode fiber nor free-space optics.
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 )= κ L + 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.
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.
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 4×44\times44×4 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 ±0.11 \pm0.11\,±0.11ps, limited by the delay-stage positioning uncertainty. The root-mean-square ICS grows as στ(L)=κL+cσ_τ(L) = κ\sqrt{L}+cστ​(L)=κL​+c with κ=48.7±2.5 ps/ ⁣kmκ= 48.7 \pm 2.5\,\mathrm{ps}/\!\sqrt{\mathrm{km}}κ=48.7±2.5ps/km​ and c=9.76±1.2 c = 9.76 \pm 1.2\,c=9.76±1.2ps, over fiber lengths from 7.7 7.7\,7.7m to 1300 1300\,1300m. 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 ±0.11 \pm0.11\,±0.11ps precision represents a ∼ ⁣180\sim\!180∼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 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.
Originally envisioned as a solution for the capacity crunch in telecommunications networks, multicore fibers (MCFs) are contributing to scientific fields beyond telecom, such as sensing and metrology. Confined within the same cladding, the cores of MCF have a high degree of noise correlation, which can be harnessed for a variety of applications. Here, we investigate MCF as a solution to the challenging problem of quantum and classical light co-existence in phase-stabilized quantum networks by operating the quantum and stabilization light in separate but highly correlated cores of a 7-core MCF. Over 40 km of spooled fiber, we achieved 100 attosecond integrated jitter on one core by using phase information derived from another core. This allows for 100% duty cycle on a quantum channel while maintaining a low spurious photon rate from optical crosstalk. With cycle-slip-free stabilization over 6 hours, frequency detuning between designated stabilization and quantum channels, and an additional 40 dB rejection of noise photons provided by the low optical crosstalk between cores, we achieved a Raman scattering-induced spurious photon rate of only 0.01 photons/s in 100 GHz bandwidth. Our results with MCF demonstrate a promising approach to ultra-stable quantum networks with 100% duty cycle on the quantum channel.
Rapid recent advances in quantum technology are giving rise to potential new applications in a wide range of fields. In particular, efficient generation and distribution of quantum entanglement is crucial for many use cases in quantum communication. In this field, multi-core optical fibers (MCF) are especially interesting, as they enable space-division multiplexing, increasing data transmission capacity of the channel. In this work, we present a flexible system for efficient and scalable quantum communication through MCF, using phase-only spatial light modulation (SLM). Firstly, we utilise SLM to enhance the coupling efficiency of near-infrared polarization-entangled photon pairs into a 19-core MCF. Secondly, we present a novel SLM-based system which corrects random polarization transformations incurred over propagation through fiber, for all MCF cores in parallel. The quality of the distributed entanglement is certified by high polarization visibility and violation of the Clauser–Horne–Shimony–Holt (CHSH) inequality for every core pair. Our results show potential for implementation in real-world quantum communication networks, as well as enabling further advanced quantum experiments.