pith. sign in

arxiv: 2604.26791 · v1 · submitted 2026-04-29 · 🪐 quant-ph

Chip-to-chip entanglement distribution over 80-km multicore fiber link

Damien Roux , Giulia Guarda , Mujtaba Zahidy , Yunhong Ding , Siyan Zhou , Domenico Ribezzo , Battulga Munkhbat , Francesco Da Ros
show 2 more authors
Davide Bacco Caterina Vigliar
This is my paper

Pith reviewed 2026-05-07 13:17 UTC · model grok-4.3

classification 🪐 quant-ph
keywords entanglement distributionsilicon photonicspath-encoded entanglementquantum key distributionmulticore fiberBell state fidelityspontaneous four-wave mixing
0
0 comments X

The pith

Silicon photonic chips distribute path-encoded entangled states over 80 km of fiber with 85.7% Bell fidelity.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper demonstrates that fully integrated silicon photonic chips can generate telecom-band entangled photon pairs on-chip via spontaneous four-wave mixing and distribute them over an 80 km multicore fiber link while preserving path-encoded entanglement. Active stabilization of the dual-core link maintains the necessary phase coherence between spatial modes, which had previously limited such distribution to short ranges. This yields a measured Bell state fidelity of 85.7 percent and supports a secure key rate of 2.03 bits per second in the BBM92 quantum key distribution protocol. A sympathetic reader cares because the work shows integrated chips can serve as a scalable platform for long-distance entanglement-based quantum networks instead of relying on fragile bulk optics.

Core claim

Here, we report chip-to-chip distribution of path-encoded entangled states over 80 km between fully integrated silicon photonic transmitter and receiver chips. Telecom-band entangled photon pairs are generated via spontaneous four-wave mixing in on-chip spiral waveguides and distributed between chips over a dual-core, actively stabilized fiber link. Upon distribution, we measure a Bell state fidelity of 85.7 ± 0.2 %. Implementing the BBM92 protocol with the same source, we obtain a secure key rate of 2.03 bit/s in the infinite-key regime.

What carries the argument

The actively stabilized dual-core fiber link, which preserves phase coherence between the spatial modes that carry the path-encoded entanglement over the full 80 km distance.

If this is right

  • Silicon photonic chips become a viable platform for long-distance path-encoded entanglement-based quantum key distribution.
  • The on-chip source and distribution method supports secure key rates in the infinite-key regime for the BBM92 protocol.
  • Active stabilization overcomes the phase fragility that had restricted path encoding to short distances.
  • These results establish a path toward scalable device-independent quantum networks using integrated photonics.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The same stabilization approach might be tested on links longer than 80 km to determine its distance limit.
  • Combining this path-encoding method with other on-chip components could enable more complex multi-node quantum networks.
  • Future work could compare the robustness of path encoding against time-bin encoding under the same fiber conditions.

Load-bearing premise

Active stabilization of the dual-core fiber link is sufficient to preserve phase coherence between spatial modes over the full 80 km distance.

What would settle it

Repeating the 80 km distribution experiment with the stabilization system turned off and measuring a Bell state fidelity below 80 percent would falsify the claim that stabilization suffices to maintain the required coherence.

Figures

Figures reproduced from arXiv: 2604.26791 by Battulga Munkhbat, Caterina Vigliar, Damien Roux, Davide Bacco, Domenico Ribezzo, Francesco Da Ros, Giulia Guarda, Mujtaba Zahidy, Siyan Zhou, Yunhong Ding.

Figure 1
Figure 1. Figure 1: FIG. 1 view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2 view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3 view at source ↗
Figure 4
Figure 4. Figure 4: shows the calculated SKR for the two links, yielding 803 bit/s for the short link and 2.03 bit/s for the two-core fiber. Both values are in strong agreement with the theoretical simulation (dashed curve), which incor￾porates source rates, losses and detector parameters (see Supplementary Information). We also investigate the im￾pact of finite-size effects on the SKR evaluation. In par￾ticular, Table II rep… view at source ↗
read the original abstract

Long-range quantum entanglement is essential for building large-scale quantum networks and unconditionally secure cryptographic systems based on quantum key distribution (QKD). While photonic integrated circuits offer a highly scalable platform, the fragility of phase coherence between spatial modes has prevented the distribution of path-encoded entanglement over long distances. Here, we report chip-to-chip distribution of path-encoded entangled states over 80 km between fully integrated silicon photonic transmitter and receiver chips. Telecom-band entangled photon pairs are generated via spontaneous four-wave mixing in on-chip spiral waveguides and distributed between chips over a dual-core, actively stabilized fiber link. Upon distribution, we measure a Bell state fidelity of $85.7 \pm 0.2 \%$. Implementing the BBM92 protocol with the same source, we obtain a secure key rate of 2.03 bit/s in the infinite-key regime. These results establish silicon photonic chips as a viable platform for long-distance path-encoded entanglement-based quantum key distribution, paving the way toward scalable, device-independent quantum networks.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

1 major / 2 minor

Summary. The manuscript reports an experimental demonstration of chip-to-chip distribution of path-encoded entangled photon pairs generated via spontaneous four-wave mixing in on-chip spiral waveguides on fully integrated silicon photonic chips. The pairs are sent over an 80 km actively stabilized dual-core multicore fiber link, yielding a measured Bell state fidelity of 85.7 ± 0.2% and a secure key rate of 2.03 bit/s in the infinite-key regime under the BBM92 protocol.

Significance. If the results hold, this is a notable advance for quantum networks and QKD because it shows that path encoding, previously limited by phase fragility, can be preserved over long distances using scalable silicon photonic integrated circuits for both generation and reception. The concrete measured values with error bars and the use of a fully chip-based transmitter-receiver pair provide a practical benchmark that could accelerate device-independent protocols and large-scale entanglement distribution.

major comments (1)
  1. § on active stabilization of the dual-core fiber link (likely within the Experimental Setup or Methods section): the manuscript states that active stabilization is used to preserve phase coherence between spatial modes but provides no quantitative metrics such as residual phase-noise rms value, loop bandwidth, or long-term stability (e.g., Allan deviation) after 80 km. This is load-bearing for the central claim, because the reported 85.7% Bell fidelity requires the relative phase drift to remain below ~0.3 rad rms to avoid visibility loss; without these data the assertion that the fragility has been overcome cannot be verified from the presented evidence.
minor comments (2)
  1. The abstract and results section would benefit from an explicit statement of the total system loss budget and the contribution from the fiber link versus the chips to allow readers to assess scalability.
  2. Figure captions should include the integration time or number of coincidence events used for the fidelity and key-rate measurements to clarify statistical significance.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their careful and constructive review of our manuscript. We address the major comment below and will revise the manuscript to strengthen the presentation of the active stabilization results.

read point-by-point responses

  1. Referee: [—] § on active stabilization of the dual-core fiber link (likely within the Experimental Setup or Methods section): the manuscript states that active stabilization is used to preserve phase coherence between spatial modes but provides no quantitative metrics such as residual phase-noise rms value, loop bandwidth, or long-term stability (e.g., Allan deviation) after 80 km. This is load-bearing for the central claim, because the reported 85.7% Bell fidelity requires the relative phase drift to remain below ~0.3 rad rms to avoid visibility loss; without these data the assertion that the fragility has been overcome cannot be verified from the presented evidence.

    Authors: We agree that quantitative characterization of the stabilization performance is necessary to fully support the central claim. In the revised manuscript we will add a dedicated paragraph (or subsection) in the Experimental Setup section that reports the residual phase-noise rms value, the servo loop bandwidth, and long-term stability data (including Allan deviation) measured after the 80 km link. These metrics will be presented together with the observed Bell fidelity to allow direct verification that the phase drift remains below the threshold required for the reported visibility. revision: yes

Circularity Check

0 steps flagged

No circularity: pure experimental demonstration with direct measurements

full rationale

This is an experimental paper reporting measured Bell-state fidelity (85.7 ± 0.2 %) and secure key rate (2.03 bit/s) from a chip-to-chip entanglement distribution setup over 80 km of actively stabilized multicore fiber. No derivation chain, fitted parameters renamed as predictions, or self-citation load-bearing steps exist. The abstract and described results consist of direct experimental outcomes; the stabilization method is presented as an implemented technique rather than a mathematical reduction to prior inputs. The paper is self-contained against external benchmarks with no equations that equate outputs to inputs by construction.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The claim rests on established quantum optics and fiber transmission principles with no new free parameters, ad-hoc axioms, or invented entities required beyond standard assumptions about phase stabilization.

axioms (2)
  • standard math Standard quantum mechanics governs the generation and measurement of path-encoded Bell states in linear optical systems.
    Invoked for SFWM pair generation and Bell fidelity calculation.
  • domain assumption Active stabilization can maintain inter-core phase coherence over 80 km in multicore fiber.
    Central premise that enables the long-distance path encoding.

pith-pipeline@v0.9.0 · 5504 in / 1379 out tokens · 100953 ms · 2026-05-07T13:17:28.785737+00:00 · methodology

discussion (0)

Sign in with ORCID, Apple, or X to comment. Anyone can read and Pith papers without signing in.

Reference graph

Works this paper leans on

68 extracted references · 1 canonical work pages

  1. [1]

    & Hanson, R

    Wehner, S., Elkouss, D. & Hanson, R. Quantum internet: A vision for the road ahead.Science362, eaam9288 (2018)

    2018

  2. [2]

    Azuma, K.et al.Quantum repeaters: From quantum networks to the quantum internet.Reviews of Modern Physics95, 045006 (2023)

    2023

  3. [3]

    & Zeilinger, A

    Krenn, M., Hochrainer, A., Lahiri, M. & Zeilinger, A. Entanglement by path identity.Physical review letters 118, 080401 (2017)

    2017

  4. [4]

    Advances in optics and photonics12, 1012–1236 (2020)

    Pirandola, S.et al.Advances in quantum cryptography. Advances in optics and photonics12, 1012–1236 (2020)

    2020

  5. [5]

    Scarani, V.et al.The security of practical quantum key distribution.Reviews of modern physics81, 1301 (2009)

    2009

  6. [6]

    & Pan, J.-W

    Xu, F., Ma, X., Zhang, Q., Lo, H.-K. & Pan, J.-W. Se- cure quantum key distribution with realistic devices.Re- views of modern physics92, 025002 (2020)

    2020

  7. [7]

    Yin, J.et al.Entanglement-based secure quantum cryp- tography over 1,120 kilometres.Nature582, 501–505 (2020)

    2020

  8. [8]

    Wengerowsky, S.et al.Entanglement distribution over a 96-km-long submarine optical fiber.Proceedings of the National Academy of Sciences116, 6684–6688 (2019)

    2019

  9. [9]

    P., Buchner, A., Bulla, L., Bohmann, M

    Neumann, S. P., Buchner, A., Bulla, L., Bohmann, M. & Ursin, R. Continuous entanglement distribution over a transnational 248 km fiber link.Nature Communications 13, 6134 (2022)

    2022

  10. [10]

    & Thompson, M

    Wang, J., Sciarrino, F., Laing, A. & Thompson, M. G. Integrated photonic quantum technologies.Nature Pho- tonics14, 273–284 (2020)

    2020

  11. [11]

    Bao, J.et al.Very-large-scale integrated quantum graph photonics.Nature Photonics17, 573–581 (2023)

    2023

  12. [12]

    Wang, J.et al.Multidimensional quantum entanglement with large-scale integrated optics.Science360, 285–291 (2018)

    2018

  13. [13]

    J.et al.Quantum teleportation on a photonic chip.Nature photonics8, 770–774 (2014)

    Metcalf, B. J.et al.Quantum teleportation on a photonic chip.Nature photonics8, 770–774 (2014)

    2014

  14. [14]

    J., Rarity, J

    Politi, A., Cryan, M. J., Rarity, J. G., Yu, S. & O’brien, J. L. Silica-on-silicon waveguide quantum circuits.Sci- ence320, 646–649 (2008)

    2008

  15. [15]

    J.et al.Generating, manipulating and mea- suring entanglement and mixture with a reconfigurable photonic circuit.Nature Photonics6, 45–49 (2012)

    Shadbolt, P. J.et al.Generating, manipulating and mea- suring entanglement and mixture with a reconfigurable photonic circuit.Nature Photonics6, 45–49 (2012)

    2012

  16. [16]

    W.et al.On-chip quantum interference between silicon photon-pair sources.Nature Photonics8, 104–108 (2014)

    Silverstone, J. W.et al.On-chip quantum interference between silicon photon-pair sources.Nature Photonics8, 104–108 (2014)

    2014

  17. [17]

    W.et al.Qubit entanglement between ring-resonator photon-pair sources on a silicon chip.Na- ture communications6, 7948 (2015)

    Silverstone, J. W.et al.Qubit entanglement between ring-resonator photon-pair sources on a silicon chip.Na- ture communications6, 7948 (2015)

    2015

  18. [18]

    Tanzilli, S.et al.On the genesis and evolution of inte- grated quantum optics.Laser & Photonics Reviews6, 115–143 (2012)

    2012

  19. [19]

    Bogaerts, W.et al.Programmable photonic circuits.Na- ture586, 207–216 (2020)

    2020

  20. [20]

    Ma, C.et al.Silicon photonic transmitter for polarization-encoded quantum key distribution.Optica 3, 1274–1278 (2016)

    2016

  21. [21]

    Semenenko, H.et al.Chip-based measurement-device- independent quantum key distribution.Optica7, 238– 242 (2020)

    2020

  22. [22]

    Nature communications8, 13984 (2017)

    Sibson, P.et al.Chip-based quantum key distribution. Nature communications8, 13984 (2017)

    2017

  23. [23]

    Sibson, P.et al.Integrated silicon photonics for high- speed quantum key distribution.Optica4, 172–177 (2017)

    2017

  24. [24]

    Ding, Y.et al.High-dimensional quantum key distribu- tion based on multicore fiber using silicon photonic inte- grated circuits.npj Quantum Information3, 25 (2017)

    2017

  25. [25]

    & Kues, M

    Khodadad Kashi, A. & Kues, M. Frequency-bin-encoded entanglement-based quantum key distribution in a recon- figurable frequency-multiplexed network.Light: Science & Applications14, 49 (2025)

    2025

  26. [26]

    Tagliavacche, N.et al.Frequency-bin entanglement- based quantum key distribution.npj Quantum Infor- mation11, 60 (2025)

    2025

  27. [27]

    Yu, H.et al.Quantum key distribution implemented with d-level time-bin entangled photons.Nature Communica- 8 tions16, 171 (2025)

    2025

  28. [28]

    Bacco, D.et al.Characterization and stability measure- ment of deployed multicore fibers for quantum applica- tions.Photonics Research9, 1992 (2021)

    1992

  29. [29]

    Wang, J.et al.Chip-to-chip quantum photonic inter- connect by path-polarization interconversion.Optica3, 407–413 (2016)

    2016

  30. [30]

    Llewellyn, D.et al.Chip-to-chip quantum teleportation and multi-photon entanglement in silicon.Nature Physics 16, 148–153 (2020)

    2020

  31. [31]

    A.et al.High-dimensional path-encoded en- tanglement distribution between photonic chips enabled by multimode phase stabilisation (2025)

    Thomas, M. A.et al.High-dimensional path-encoded en- tanglement distribution between photonic chips enabled by multimode phase stabilisation (2025)

    2025

  32. [32]

    C., Vigliar, C., Santagati, R., Silverstone, J

    Adcock, J. C., Vigliar, C., Santagati, R., Silverstone, J. W. & Thompson, M. G. Programmable four-photon graph states on a silicon chip.Nature communications 10, 3528 (2019)

    2019

  33. [33]

    Paesani, S.et al.Generation and sampling of quantum states of light in a silicon chip.Nature Physics15, 925– 929 (2019)

    2019

  34. [34]

    Vigliar, C.et al.Error-protected qubits in a silicon pho- tonic chip.Nature Physics17, 1137–1143 (2021)

    2021

  35. [35]

    Zhang, M.et al.Generation of multiphoton quantum states on silicon.Light: Science & Applications8, 41 (2019)

    2019

  36. [36]

    Da Lio, B.et al.Stable transmission of high-dimensional quantum states over a 2-km multicore fiber.IEEE Jour- nal of Selected Topics in Quantum Electronics26, 1–8 (2019)

    2019

  37. [37]

    James, D. F. V., Kwiat, P. G., Munro, W. J. & White, A. G. Measurement of qubits.Physical Review A64 (2001)

    2001

  38. [38]

    H., Brassard, G

    Bennett, C. H., Brassard, G. & Ekert, A. K. Quantum cryptography.Scientific American267, 50–57 (1992)

    1992

  39. [39]

    Photonics Research11, 1314–1325 (2023)

    Liu, D.et al.Photonic-reconfigurable entanglement dis- tribution network based on silicon quantum photonics. Photonics Research11, 1314–1325 (2023)

    2023

  40. [40]

    Wen, W.et al.Realizing an entanglement-based mul- tiuser quantum network with integrated photonics.Phys- ical Review Applied18, 024059 (2022)

    2022

  41. [41]

    & Yvind, K

    Ding, Y., Peucheret, C., Ou, H. & Yvind, K. Fully etched apodized grating coupler on the SOI platform with- 0.58 db coupling efficiency.Optics letters39, 5348– 5350 (2014)

    2014

  42. [42]

    J.et al.High contrast 40Gbit/s optical modulation in silicon.Optics Express19, 11507 (2011)

    Thomson, D. J.et al.High contrast 40Gbit/s optical modulation in silicon.Optics Express19, 11507 (2011)

    2011

  43. [43]

    Wang, C.et al.Ultrabroadband thin-film lithium tan- talate modulator for high-speed communications.Optica 11, 1614–1620 (2024)

    2024

  44. [44]

    Xu, M.et al.High-performance coherent optical modula- tors based on thin-film lithium niobate platform.Nature communications11, 3911 (2020)

    2020

  45. [45]

    Sun, C.et al.Single-chip microprocessor that communi- cates directly using light.Nature528, 534–538 (2015)

    2015

  46. [46]

    H.et al.Integrating photonics with silicon nanoelectronics for the next generation of systems on a chip.Nature556, 349–354 (2018)

    Atabaki, A. H.et al.Integrating photonics with silicon nanoelectronics for the next generation of systems on a chip.Nature556, 349–354 (2018)

    2018

  47. [47]

    Optica1, 129 (2014)

    Grillanda, S.et al.Non-invasive monitoring and control in silicon photonics using CMOS integrated electronics. Optica1, 129 (2014)

    2014

  48. [48]

    Margalit, N.et al.Perspective on the future of silicon photonics and electronics.Applied Physics Letters118 (2021)

    2021

  49. [49]

    Ren, J.-G.et al.Ground-to-satellite quantum teleporta- tion.Nature549, 70–73 (2017)

    2017

  50. [50]

    Yin, J.et al.Satellite-to-ground entanglement-based quantum key distribution.Physical review letters119, 200501 (2017)

    2017

  51. [51]

    EPJ Quantum Technology6, 5 (2019)

    Bacco, D.et al.Field trial of a three-state quantum key distribution scheme in the florence metropolitan area. EPJ Quantum Technology6, 5 (2019)

    2019

  52. [52]

    In2023 23rd International Conference on Transparent Optical Networks (ICTON), 1–4 (IEEE, 2023)

    Guarda, G.et al.BB84 decoy-state QKD protocol over long-distance optical fiber. In2023 23rd International Conference on Transparent Optical Networks (ICTON), 1–4 (IEEE, 2023)

    2023

  53. [53]

    Ribezzo, D.et al.Deploying an inter-european quantum network.Advanced Quantum Technologies6, 2200061 (2023)

    2023

  54. [54]

    Fitzke, E.et al.Scalable network for simultaneous pair- wise quantum key distribution via entanglement-based time-bin coding.PRX Quantum3, 020341 (2022)

    2022

  55. [55]

    A.et al.Petabit-per-second data transmis- sion using a chip-scale microcomb ring resonator source

    Jørgensen, A. A.et al.Petabit-per-second data transmis- sion using a chip-scale microcomb ring resonator source. Nature Photonics16, 798–802 (2022)

    2022

  56. [56]

    Mateo, E. F. Multicore-fiber submarine systems [invited]. Journal of Optical Communications and Networking16, H1 (2024)

    2024

  57. [57]

    Da Lio, B.et al.Path-encoded high-dimensional quan- tum communication over a 2-km multicore fiber.npj Quantum Information7, 63 (2021)

    2021

  58. [58]

    Chi, Y.et al.A programmable qudit-based quantum processor.Nature Communications13(2022)

    2022

  59. [59]

    & Oxenløwe, L

    Cozzolino, D., Da Lio, B., Bacco, D. & Oxenløwe, L. K. High-dimensional quantum communication: bene- fits, progress, and future challenges.Advanced Quantum Technologies2, 1900038 (2019)

    2019

  60. [60]

    Paesani, S.et al.Near-ideal spontaneous photon sources in silicon quantum photonics.Nature communications 11, 2505 (2020)

    2020

  61. [61]

    Zahidy, M.et al.Quantum key distribution using deter- ministic single-photon sources over a field-installed fibre link.npj Quantum Information10(2024)

    2024

  62. [62]

    Xiong, C.et al.Active temporal multiplexing of indis- tinguishable heralded single photons.Nature Communi- cations7(2016)

    2016

  63. [63]

    I.et al.Improved heralded single-photon source with a photon-number-resolving superconducting nanowire detector.Physical Review Applied18(2022)

    Davis, S. I.et al.Improved heralded single-photon source with a photon-number-resolving superconducting nanowire detector.Physical Review Applied18(2022)

    2022

  64. [64]

    Ramelow, S.et al.Silicon-nitride platform for nar- rowband entangled photon generation.arXiv preprint arXiv:1508.04358(2015)

    work page Pith review arXiv 2015

  65. [65]

    & Bowers, J

    Xiang, C., Jin, W. & Bowers, J. E. Silicon nitride pas- sive and active photonic integrated circuits: trends and prospects.Photonics research10, A82–A96 (2022)

    2022

  66. [66]

    Appas, F.et al.Flexible entanglement-distribution net- work with an algaas chip for secure communications.npj Quantum Information7, 118 (2021)

    2021

  67. [67]

    Autebert, C.et al.Multi-user quantum key distribution with entangled photons from an algaas chip.Quantum Science and Technology1, 01LT02 (2016)

    2016

  68. [68]

    J., Shen, M., Castro, J

    Steiner, T. J., Shen, M., Castro, J. E., Bowers, J. E. & Moody, G. Continuous entanglement distribution from an algaas-on-insulator microcomb for quantum commu- nications.Optica Quantum1, 55–62 (2023)

    2023