Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Log inSign up
Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Blog
Job Board
Q3AS

© 2026 Aqora Quantum S.A.S.

TermsPrivacyLegal Notice

arXiv:2509.17989 · APL Photonics

Sub-femtosecond stabilization of multicore fiber for high-fidelity quantum networking at 100% duty cycle

Takuma Nakamura, Nazanin Hoghooghi, Nicolas K. Fontaine, Tetsuya Hayashi, Takuji Nagashima, Nicholas V. Nardelli, Dileep V. Reddy, Martin J. Stevens, Tara M. Fortier, Lynden K. Shalm, Franklyn Quinlan

Published Feb 1, 2026• 3 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1063/5.0308068

Related papers

Coexistence of Optical Frequency Transmission With Quantum Key Distribution Over Multi-Core Media

Lai Yu, Li Zhang, Lei Liu +8 more

The quantum-classical coexistence can be obtained using wavelength division multiplexing (WDM), however, the crosstalk between quantum and classical signals due to Raman noise must be handled with extra attention. Multi-core fiber (MCF) provides a good solution for the coexistence based on space division multiplexing (SDM). In this work, we propose and experimentally demonstrate hybrid transmission of ultrastable optical frequency reference and quantum key distribution (QKD) over homogeneous MCF. The scheme achieves optical frequency stabilization at the remote site by beating the loopback frequency signal with a local reference to extract link phase noise, which is then compensated via voltage-controlled oscillator (VCO) feedback. Prior to the experiment, we first analyze the critical challenges in MCFs including inter-core crosstalk and inter-core spontaneous Raman scattering. To minimize the crosstalk between the signals, the quantum signal and the optical frequency propagate through separate fiber cores. Successful hybrid transmission of optical frequency references and QKD is achieved across 20 km seven-core fiber. The frequency instability of$1.2 \times 10^{-17}$at 1 s and$4.2 \times 10^{-19}$at 10000 s, and secure key rate of 317.28 kbps can be realized in this integrated transmission system. The scheme benefits from two merits, ultrastable optical frequency offers high-precision clocks to QKD, and QKD provides one-time-pad keys to encrypt the time data. These results demonstrate the feasibility and tolerance of our QKD system with high-precision frequency reference and the viability of multi-core fibers for next-generation quantum networks.

Journal of Lightwave Technology•Jan 12, 2026•0 citations

Mode-tunable inter-core coupling of photon-number-resolved quantum light in a telecom multicore fiber

Chiran Wijesundara, Yasmin Sarhan, Henry Strong +3 more

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.

quant-pharXiv:2610.03493•Oct 5, 2026•0 citations

Co-Propagation of Quantum Time Synchronization and Optical Frequency Transfer over a 122 km Hollow-Core Fiber

Huibo Hong, Xiao Xiang, Runai Quan +15 more

The co-propagation of quantum and classical signals through shared optical fibers is crucial for scalable quantum networks. However, this coexistence is fundamentally limited by spontaneous Raman scattering (SpRS) from the bright classical light, which generates overwhelming noise that disrupts the single-photon-level quantum signals. Here, we overcome this long-standing challenge by leveraging the inherently ultralow nonlinearity of hollow-core fiber (HCF) to suppress SpRS noise. By operating both the quantum time synchronization (QTS) and classical optical frequency transfer (OFT) signals within the telecom C-band, separated by only ~10 nm, we successfully demonstrate their simultaneous transmission over a 122-km HCF link. With a classical OFT power of 1 mW, the QTS performance shows negligible degradation, maintaining sub-picosecond time stability at 2000 s, while the OFT achieves a fractional frequency instability of 10^-20. Near-sub-picosecond QTS stability is preserved even when the classical power is increased to 3 mW. Furthermore, simulations based on our experimental data indicate that with next-generation low-loss HCF, the platform can tolerate classical powers beyond 10 mW and extend the QTS range to over 500 km. By realizing a unified quantum-classical time-frequency distribution framework, this work establishes HCF as a highly capable and practical platform for future scalable quantum networks.

quant-pharXiv:2602.19013•Feb 24, 2026•0 citations

High-Isolation Stabilization with Multicore Fiber for Precise Timing in Quantum Networks

Takuma Nakamura, Nicolas K. Fontaine, Tetsuya Hayashi +3 more

We demonstrate link stabilization across 40 km of 7-core optical fiber, with stabilization and quantum channels in separate cores. Measured core-to-core time deviation is below 1×10-16 and estimated cross-talk to a quantum channel <0.1 photons/second.

•Jan 1, 2025•0 citations

Enabling phase stabilization of quantum networks via displacement-enhanced photon counting

M. V. Jabir, Daehyun Ahn, N. Fajar R. Annafianto +3 more

Optical phase stabilization, tracking, and locking in long fiber links are pivotal for the functionality of many communication protocols and distributed sensors. However, conventional phase stabilization methods use strong optical probe signals that may contaminate and destroy fragile quantum signals propagating in the same fiber link. Here, we experimentally demonstrate phase locking of fiber spools up to 100 km and over 120 km deployed optical communication link using faint coherent states with an average of just photons per second detected at the receiving detector as a probe signal. The power spectral density of phase noise, integrated from 1 Hz to 50 kHz, yields an excess rms phase noise of 110 mrad (corresponding to timing jitter below 0.09 fs) for a 100 km fiber spool and 140 mrad (corresponding to rms timing jitter below 0.12 fs) for deployed fiber. The displacement-enhanced measurement protocol that we employ for phase stabilization yields Fisher information that is greater than that of homodyne and heterodyne measurement protocols for fiber links longer than 100 m and, therefore, exceeds the canonical shot noise limit of accuracy. By implementing this stabilization strategy with a 50% duty cycle, we enable time-division multiplexed faint light and quantum payloads to coexist within the same communication channel.

Optica•Mar 21, 2025•8 citations

Telecommunication Compatibility Evaluation for Co-existing Quantum Key Distribution in Homogenous Multicore Fiber

Rui Lin, Aleksejs Udaļcovs, Oskars Ozoliņš +8 more

Quantum key distribution (QKD) is regarded as an alternative to traditional cryptography methods for securing data communication by quantum mechanics rather than computational complexity. Towards the massive deployment of QKD, embedding it with the telecommunication system is crucially important. Homogenous optical multi-core fibers (MCFs) compatible with spatial division multiplexing (SDM) are essential components for the next-generation optical communication infrastructure, which provides a big potential for co-existence of optical telecommunication systems and QKD. However, the QKD channel is extremely vulnerable due to the fact that the quantum states can be annihilated by noise during signal propagation. Thus, investigation of telecom compatibility for QKD co-existing with high-speed classical communication in SDM transmission media is needed. In this paper, we present analytical models of the noise sources in QKD links over heterogeneous MCFs. Spontaneous Raman scattering and inter-core crosstalk are experimentally characterized over spans of MCFs with different refractive index profiles, emulating shared telecom traffic conditions. Lower bounds for the secret key rates and quantum bit error rate (QBER) due to different core/wavelength allocation are obtained to validate intra- and inter-core co-existence of QKD and classical telecommunication.

IEEE Access•Jan 1, 2020•24 citations
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.
The quantum-classical coexistence can be obtained using wavelength division multiplexing (WDM), however, the crosstalk between quantum and classical signals due to Raman noise must be handled with extra attention. Multi-core fiber (MCF) provides a good solution for the coexistence based on space division multiplexing (SDM). In this work, we propose and experimentally demonstrate hybrid transmission of ultrastable optical frequency reference and quantum key distribution (QKD) over homogeneous MCF. The scheme achieves optical frequency stabilization at the remote site by beating the loopback frequency signal with a local reference to extract link phase noise, which is then compensated via voltage-controlled oscillator (VCO) feedback. Prior to the experiment, we first analyze the critical challenges in MCFs including inter-core crosstalk and inter-core spontaneous Raman scattering. To minimize the crosstalk between the signals, the quantum signal and the optical frequency propagate through separate fiber cores. Successful hybrid transmission of optical frequency references and QKD is achieved across 20 km seven-core fiber. The frequency instability of1.2×10−171.2 \times 10^{-17}1.2×10−17at 1 s and4.2×10−194.2 \times 10^{-19}4.2×10−19at 10000 s, and secure key rate of 317.28 kbps can be realized in this integrated transmission system. The scheme benefits from two merits, ultrastable optical frequency offers high-precision clocks to QKD, and QKD provides one-time-pad keys to encrypt the time data. These results demonstrate the feasibility and tolerance of our QKD system with high-precision frequency reference and the viability of multi-core fibers for next-generation quantum networks.
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.
The co-propagation of quantum and classical signals through shared optical fibers is crucial for scalable quantum networks. However, this coexistence is fundamentally limited by spontaneous Raman scattering (SpRS) from the bright classical light, which generates overwhelming noise that disrupts the single-photon-level quantum signals. Here, we overcome this long-standing challenge by leveraging the inherently ultralow nonlinearity of hollow-core fiber (HCF) to suppress SpRS noise. By operating both the quantum time synchronization (QTS) and classical optical frequency transfer (OFT) signals within the telecom C-band, separated by only ~10 nm, we successfully demonstrate their simultaneous transmission over a 122-km HCF link. With a classical OFT power of 1 mW, the QTS performance shows negligible degradation, maintaining sub-picosecond time stability at 2000 s, while the OFT achieves a fractional frequency instability of 10^-20. Near-sub-picosecond QTS stability is preserved even when the classical power is increased to 3 mW. Furthermore, simulations based on our experimental data indicate that with next-generation low-loss HCF, the platform can tolerate classical powers beyond 10 mW and extend the QTS range to over 500 km. By realizing a unified quantum-classical time-frequency distribution framework, this work establishes HCF as a highly capable and practical platform for future scalable quantum networks.
Optical phase stabilization, tracking, and locking in long fiber links are pivotal for the functionality of many communication protocols and distributed sensors. However, conventional phase stabilization methods use strong optical probe signals that may contaminate and destroy fragile quantum signals propagating in the same fiber link. Here, we experimentally demonstrate phase locking of fiber spools up to 100 km and over 120 km deployed optical communication link using faint coherent states with an average of just photons per second detected at the receiving detector as a probe signal. The power spectral density of phase noise, integrated from 1 Hz to 50 kHz, yields an excess rms phase noise of 110 mrad (corresponding to timing jitter below 0.09 fs) for a 100 km fiber spool and 140 mrad (corresponding to rms timing jitter below 0.12 fs) for deployed fiber. The displacement-enhanced measurement protocol that we employ for phase stabilization yields Fisher information that is greater than that of homodyne and heterodyne measurement protocols for fiber links longer than 100 m and, therefore, exceeds the canonical shot noise limit of accuracy. By implementing this stabilization strategy with a 50% duty cycle, we enable time-division multiplexed faint light and quantum payloads to coexist within the same communication channel.
Quantum key distribution (QKD) is regarded as an alternative to traditional cryptography methods for securing data communication by quantum mechanics rather than computational complexity. Towards the massive deployment of QKD, embedding it with the telecommunication system is crucially important. Homogenous optical multi-core fibers (MCFs) compatible with spatial division multiplexing (SDM) are essential components for the next-generation optical communication infrastructure, which provides a big potential for co-existence of optical telecommunication systems and QKD. However, the QKD channel is extremely vulnerable due to the fact that the quantum states can be annihilated by noise during signal propagation. Thus, investigation of telecom compatibility for QKD co-existing with high-speed classical communication in SDM transmission media is needed. In this paper, we present analytical models of the noise sources in QKD links over heterogeneous MCFs. Spontaneous Raman scattering and inter-core crosstalk are experimentally characterized over spans of MCFs with different refractive index profiles, emulating shared telecom traffic conditions. Lower bounds for the secret key rates and quantum bit error rate (QBER) due to different core/wavelength allocation are obtained to validate intra- and inter-core co-existence of QKD and classical telecommunication.