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Cited by 842Quantum Information and CryptographyAdvanced Fiber Laser TechnologiesQuantum optics and atomic interactions

Generation of multiphoton entangled quantum states by means of integrated frequency combs

Open access
Reimer, Christian, Kues, Michael, Roztocki, Piotr, Wetzel, Benjamin, Grazioso, Fabio, Little, Brent E., Chu, Sai T., Johnston, Tudor +4 more·2016·Science
DOI 10.1126/science.aad8532
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Abstract

Entangled frequency combs: The ability to generate optical frequency combs in which the output light is made up of millions of sharp lines precisely spaced apart has been important for optical applications and for fundamental science. Reimer et al. now show that frequency combs can be taken into the quantum regime. They took individual teeth of the combs and quantum-mechanically entangled them to form complex optical states. Because the method is compatible with existing fiber and semiconductor technology, the results demonstrate a possible scalable and practical platform for quantum technologies. Science , this issue p. 1176

AI snapshot

Written from the paper’s full text.

TL;DR

Integrated Kerr frequency combs in a CMOS-compatible microring resonator generate broadband time-bin entangled photon pairs and four-photon entangled states via spontaneous four-wave mixing. The platform produces high-visibility two-photon entanglement and four-photon interference, with state tomography showing 96% fidelity for two-photon qubits and 64% for four-photon states.

Key findings

  • The microring resonator had a Q-factor of about 240,000, 803 MHz linewidth, and 200 GHz free spectral range, emitting photons across the S, C, and L ITU bands from 1470 to 1620 nm with pair production rates of 0.02–0.04 pairs per pulse.
  • Time-bin entangled qubits were generated across comb resonances; five selected channel pairs showed raw quantum-interference visibilities above 0.824, violating the CHSH/Bell-like inequality, and background-corrected visibilities above 93.2%.
  • Four-photon time-bin entangled states were generated by selecting two signal-idler pairs; at 1.5 mW pump power, a quadruple detection rate of 0.17 Hz was measured, corresponding to a calculated generation rate of 135 kHz, with four-photon interference visibility of 89%.
  • Quantum state tomography gave a two-photon fidelity of 96% and a four-photon fidelity of 64%; after adding 40 km of fiber, the two-photon fidelity was 87%.

Methods

  • The source was a four-port microring resonator in CMOS-compatible high-refractive-index glass, pumped by a 16.8 MHz repetition-rate mode-locked fiber laser filtered to a single ring resonance at 1556.2 nm with 570 ps pulses.
  • Photon pairs were generated by spontaneous four-wave mixing; time-bin entanglement was created by passing the pump through a stabilized unbalanced fiber interferometer with an 11.4 ns delay, producing double pulses with a defined relative phase.
  • Entanglement was characterized by quantum interference measurements with signal and idler interferometers, Bell-like inequality violation, four-photon interference, and quantum state tomography.

Limitations

  • The paper states that the demonstrated multi-photon entangled states are separable, generated as a product of bi-photon Bell states, and that non-separable multi-photon cluster states would require multiple excitation fields or controlled phase gates.
  • The four-photon fidelity of 64% was obtained without compensation for background noise or interferometer imperfections, and system/detection losses were 14.75 dB.
  • The text notes spectral asymmetry from Raman scattering, which could be reduced by cooling the chip, and that further device integration could reduce size and losses; the current setup uses fiber-connected components rather than a single chip.

Model-written summary. Check it against the paper before relying on it.

Integrity signals

Registry

Crossref · 10.1126/science.aad8532

Cites

39 of 39 references verified0 citation errors · 0 not checkable

Statistics

Nothing to recompute.

Claims the paper makes

0 of 22 cited sources checked · 0 supported · 0 flagged

  • Findingown data

    Quantum interference reveals the phase characteristic of the nonlinear generation process.

    “In addition to confirming time-bin entanglement, the quantum interference also reveals the phasecharacteristic of the nonlinear generation process.”
    Quote in the paperSubstantial claim
  • Findingown data

    The single photon spectrum emitted by the resonator covers the full S, C, and L ITU bands (1470 to 1620 nm).

    “It can be clearly seen that a very broad frequency comb of photons is emitted, covering the full S, C, and L International Telecommunication Union (ITU) bands (wavelengths ranging from 1470 to 1620 nm).”
    Quote in the paperSubstantial claim
  • Findingreasoning

    This coherence-length requirement is intrinsically fulfilled through the resonant characteristics (equal resonance bandwidths) of the ring cavity in combination with the excitation scheme described.

    “This requirement is intrinsically fulfilled through the resonant characteristics (i.e.equal resonance bandwidths) of the ring cavity in combination with the excitation scheme described above.”
    Quote in the paperSubstantial claim
  • Findingnone given

    By selecting two different signal-idler pairs, two-photon qubit states can be generated.

    “By selecting two different signal-idler pairs we can generate two-photon qubit states given by |Ψ , ⟩ = It is worth mentioning that for the generation of a four-photon state, the coherence length of both generated photon pairs has to be the same and matched to the excitation field's coherence time.”
    Quote in the paperSubstantial claim
  • Findingcitation

    The quantum frequency comb is generated in a CMOS-compatible high refractive index glass in a four-port microring resonator architecture.

    “Our quantum frequency comb is generated in a CMOS (Complementary Metal-Oxide-Semiconductor) compatible high refractive index glass in a four-port microring resonator architecture (details on device fabrication and characteristics are presented in (21)).”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [21]

      Couldn’t be matched to a paper in Crossref, so it can’t be checked.

  • May combine two claims. 23
  • Classified as probably not a substantial claim. 6

  • May combine two claims.

    Findingown data

    Five different frequency channel pairs within the ITU C band were selected, and quantum interference was recorded with raw visibilities above 0.824 > 1/√2 ≈ 0.71, confirming entanglement through violation of the CHSH inequality.

    “We selected 5 different frequency channel pairs within the ITU C band (marked in Fig. 1), and recorded quantum interference with raw visibilities above 0.824 > 1/√2 ≈ 0.71 (top inset of Fig. 1, and Fig. 3) thus confirming entanglement through the violation of the Clauser-Horne-Shimony-Holt (Bell-like) inequality (27).”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [27]

      Couldn’t be matched to a paper in Crossref, so it can’t be checked.

  • May combine two claims.

    Findingown data

    After adding 40 km of fiber, two-photon tomography measured a fidelity of 87%, demonstrating that the entanglement is still preserved after long fiber propagation.

    “We repeated the two-photon tomography measurement after adding 40 km of fiber, measuring a fidelity of 87% (Fig. S4, and (21) for more details), demonstrating that the entanglement is still preserved after long fiber propagation.”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [21]

      Couldn’t be matched to a paper in Crossref, so it can’t be checked.

  • May combine two claims.

    Findingcitation

    The four-photon entangled state has a fidelity of 64% without compensation for background noise or interferometer imperfections, comparable to the fidelity measured for non-integrated four-photon states used for practical applications.

    “For the four-photon entangled state (Fig. 4C,D), we obtained a fidelity of 64% without compensation for background noise or interferometer imperfections, which is comparable to the fidelity measured for non-integrated four-photon states used for practical applications (3).”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [3]

      Couldn’t be matched to a paper in Crossref, so it can’t be checked.

  • May combine two claims.

    Findingown data

    Two-photon qubits generated on comb lines symmetric with respect to the pump wavelength have a fidelity of 96%, confirming that the generated quantum states are of high quality and very close to the ideal entangled state.

    “We first measure the two-photon qubits generated on comb lines symmetric with respect to the pump wavelength (Fig. 4A,B) and find a fidelity of 96%, confirming that the generated quantum states are of high quality and very close to the ideal entangled state.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingcitation

    The ring resonator's high Q-factor of around 240,000 (803 MHz linewidth, 200 GHz free spectral range) allows high field-enhancement and low-power operation, while enabling direct generation of bright and narrow-bandwidth photons without spectral filtering.

    “The ring resonator's high Q-factor of around 240,000 (803 MHz linewidth, and 200 GHz free spectral range, see Fig. S1) allows for high field-enhancement and low-power operation, while simultaneously enabling the direct generation of bright and narrow-bandwidth photons, without the need for spectral filtering of the photons (22,23).”
    Quote in the paperSubstantial claim0 of 2 sources checked
    • cites [22]

      Couldn’t be matched to a paper in Crossref, so it can’t be checked.

    • cites [23]

      Couldn’t be matched to a paper in Crossref, so it can’t be checked.

  • May combine two claims.

    Findingreasoning

    Due to broad phase-matching, the emitted comb exhibits a flat and broadband spectrum with uniform pair production rates ranging from 0.02 to 0.04 pairs per pulse over the full measured comb.

    “Due to the broad phase-matching condition, achieved through the close-to-zero waveguide dispersion, the emitted comb exhibits a flat and broadband spectrum with uniform pair production rates, ranging from 0.02 to 0.04 pairs per pulse, over the full measured comb.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingown data

    Setting the pump power to 1.5 mW, a quadruple detection rate of 0.17 Hz was measured, leading to a calculated generation rate of 135 kHz accounting for system and detection losses of 14.75 dB.

    “Setting the pump power to 1.5 mW, a quadruple detection rate of 0.17 Hz was measured, leading to a calculated generation rate of 135 kHz -by taking into account the system and detection losses of 14.75 dB.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingown data

    The data clearly follows the expected relation, having a visibility of 89% without compensation for background noise or losses.

    “Our data clearly follows the expected relation, having a visibility of 89% without compensation for background noise or losses.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingown data

    These entangled qubits are generated over all microring resonances, leading to a quantum frequency comb of time-bin entangled photon pairs.

    “Most importantly, these entangled qubits are generated over all the microring resonances, thus leading to a quantum frequency comb of time-bin entangled photon pairs.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingown data

    The quantum frequency comb has intrinsic and simultaneous operation at many modes, high-purity photon pairs, high quality bi- and multi-photon entanglement shared among these modes, and intrinsic compatibility with fiber technology.

    “A key characteristic of our quantum frequency comb is the intrinsic and simultaneous operation at many modes, the high-purity photon pairs as well as high quality bi-and multi-photon entanglement shared among these modes, and an intrinsic compatibility with fiber technology.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingcitation

    The pump pulses are directly filtered by the resonator, resulting in a perfect match between the spectral bandwidth of the pump pulses and the excited resonator mode, leading to generation of pure single-mode photons per resonance.

    “We note that the pulses are directly filtered by the resonator, resulting in a perfect match between the spectral bandwidth of the pump pulses and the excited resonator mode.”
    • Also stated in: § Abstract · boosted
      “This in turn leads to the generation of pure single-mode photons per resonance (13), confirmed by single photon auto-correlation measurements (Fig. S2; see (21) for further details).”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [13]

      Couldn’t be matched to a paper in Crossref, so it can’t be checked.

  • May combine two claims.

    Findingown data

    The phase dependency is manifested at the single photon level with a clear difference between second- and third-order nonlinear interactions.

    “Here we show that this phase dependency is also manifested at the single photon level with a clear difference between second-and third-order nonlinear interactions.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingcitation

    For resonances closest to the excitation frequency, a photon coincidence rate of 340 Hz was measured, leading to an estimated pair production rate of 302 kHz per channel (0.018 pairs per double pulse), accounting for system and detection losses of 14.75 dB.

    “For the resonances closest to the excitation frequency a photon coincidence rate of 340 Hz was measured, leading to an estimated pair production rate of 302 kHz per channel (0.018 pairs per double pulse), accounting for system and detection losses of 14.75 dB (see (21) for more details).”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites [21]

      Couldn’t be matched to a paper in Crossref, so it can’t be checked.

  • May combine two claims.

    Interpretationreasoning

    The source can be implemented into both single-photon as well as entanglement-based quantum communication protocols.

    “For example, the source can be implemented into both single-photon as well as entanglement-based quantum communication protocols.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Interpretationnone given

    The temporal separation of the two pump pulses can be arbitrarily chosen as long as it is larger than the temporal duration of the single photons, offering significant flexibility.

    “The temporal separation of the two pulses can be arbitrarily chosen as long as it is larger than the temporal duration of the single photons, thus offering significant flexibility.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing). May combine two claims.

    Premisecitation

    The phase dependency of the nonlinear generation process has been well described in theory and exploited in optical squeezing.

    “Indeed, this phase dependency has been well described in theory (28) and has been exploited, for example, in optical squeezing (29).”
    Quote in the paperNot a substantial claim0 of 2 sources checked
    • cites [28]

      Couldn’t be matched to a paper in Crossref, so it can’t be checked.

    • cites [29]

      Couldn’t be matched to a paper in Crossref, so it can’t be checked.

  • Probably not a substantial claim: may not pass S2 (checkable).

    Interpretationreasoning

    Due to these features, the source can find versatile and immediate applications such as quantum communications and quantum computation.

    “Due to these features, it can find versatile and immediate applications such as quantum communications and quantum computation.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S4 (based). May combine two claims.

    Interpretationnone given

    The method is compatible with contemporary fiber and quantum memory infrastructures, as well as chip-scale semiconductor technology, enabling compact, low-cost, and scalable implementations.

    “Our method is compatible with contemporary fiber and quantum memory infrastructures, as well as chip-scale semiconductor technology, enabling compact, low-cost, and scalable implementations.”
    Quote in the paperNot a substantial claim
    • Rests on an assumption: The integrated Kerr frequency comb platform is compatible with quantum memory infrastructures.

  • May combine two claims.

    Premisereasoning

    For generation of a four-photon state, the coherence length of both generated photon pairs has to be the same and matched to the excitation field's coherence time.

    “It is worth mentioning that for the generation of a four-photon state, the coherence length of both generated photon pairs has to be the same and matched to the excitation field's coherence time.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Premisecitation

    For SFWM, quantum interference is expected to be of the form 1 - V cos(α + β - 2φ) for degenerate SFWM, or 1 - V cos(α + β - φ1 - φ2) for non-degenerate SFWM.

    “In contrast, for photons generated through SFWM (as we do in this work), quantum interference is expected to be of the form 1 -𝑉𝑐𝑜𝑠(𝛼 + 𝛽 -2𝜑) for degenerate SFWM, or 1 -𝑉𝑐𝑜𝑠(𝛼 + 𝛽 -𝜑 , -𝜑 .) for non-degenerate SFWM, where 𝜑 ,,. are the phases of the two pump fields (21).”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [21]

      Couldn’t be matched to a paper in Crossref, so it can’t be checked.

  • May combine two claims.

    Interpretationreasoning

    In third-order nonlinear processes, two photons generate the quantum state, enabling an additional control parameter (the relative phase between the two photons), which is easier to control.

    “In third-order nonlinear processes, however, two photons generate the quantum state, enabling an additional control parameter (i.e. the relative phase between the two photons, in turn much easier to control).”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Premisecitation

    Time-bin entanglement is particularly suitable for information processing and transmission because of its robustness (e.g., with respect to polarization fluctuations) and can be preserved over long propagation distances in standard fiber networks.

    “To demonstrate an entangled quantum frequency comb, we chose time-bin entanglement, which among several intrinsic advantages, is particularly suitable for information processing and transmission (25) because of its robustness (e.g. with respect to polarization fluctuations), and thus can be preserved even over long propagation distances in standard fiber networks (26).”
    Quote in the paperSubstantial claim0 of 2 sources checked
    • cites [25]

      Couldn’t be matched to a paper in Crossref, so it can’t be checked.

    • cites [26]

      Couldn’t be matched to a paper in Crossref, so it can’t be checked.

  • May combine two claims.

    Interpretationreasoning

    A decrease in losses by 5 dB would increase the four-photon detection rate by a factor of one hundred, while a loss reduction of 10 dB would increase it to the kHz range.

    “A very realistic decrease in losses by 5 dB would already increase the four-photon detection rate by a factor of one hundred, while an achievable loss reduction of 10 dB would increase it to the very useful kHz range.”
    Quote in the paperSubstantial claim
    • Rests on an assumption: A 5 dB reduction in losses is realistic and a 10 dB reduction is achievable.

  • May combine two claims.

    Interpretationreasoning

    Integrated quantum frequency comb sources based on third-order nonlinearities can open up new venues for the generation and control of complex quantum states, thus providing a scalable and practical platform for optical quantum information processing.

    “Our results indicate that integrated quantum frequency comb sources based on third-order nonlinearities can open up new venues for the generation and control of complex quantum states, thus providing a scalable and practical platform for optical quantum information processing.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    Two-photon time-bin entangled qubits have successfully been used for linear universal quantum computation.

    “Furthermore, two-photon time-bin entangled qubits have successfully been used for linear universal quantum computation (4,25), and the parallel generation and processing of multiple qubits can directly enhance such protocols, where the information capacity scales with the number of comb lines used, as theoretically predicted (25).”
    Quote in the paperNot a substantial claim0 of 2 sources checked
    • cites [4]

      Couldn’t be matched to a paper in Crossref, so it can’t be checked.

    • cites [25]

      Couldn’t be matched to a paper in Crossref, so it can’t be checked.

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    Four-photon cluster states have been used to demonstrate measurement-based quantum computation.

    “Indeed, four-photon cluster states have been used to demonstrate measurementbased quantum computation (3).”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites [3]

      Couldn’t be matched to a paper in Crossref, so it can’t be checked.

  • Probably not a substantial claim: may not pass S2 (checkable), S3 (load-bearing).

    Interpretationreasoning

    Further device integration of the frequency comb will lead to more compact and stable systems with higher performance in terms of detection rates.

    “Further device integration of the frequency comb will lead to more compact and stable systems with higher performance in terms of detection rates.”
    Quote in the paperNot a substantial claim
27 of 27

  • The paper relies on this without support.

    The integrated Kerr frequency comb platform is compatible with quantum memory infrastructures.

    “Our method is compatible with contemporary fiber and quantum memory infrastructures, as well as chip-scale semiconductor technology, enabling compact, low-cost, and scalable implementations.”

  • The paper relies on this without support.

    A 5 dB reduction in losses is realistic and a 10 dB reduction is achievable.

    “A very realistic decrease in losses by 5 dB would already increase the four-photon detection rate by a factor of one hundred, while an achievable loss reduction of 10 dB would increase it to the very useful kHz range.”
2 of 2

Self-check

Percentages, sums and reported differences re-computed from the paper’s own numbers.

Each contradiction between two passages listed here was confirmed by an independent check.

  • Four-photon measurement is assigned two different Fig. 3 panels

    Minor

    The setup description directs readers to Fig. 3E for the four-photon measurements, but the Fig. 3 caption identifies panel F as the four-photon measurement.

1 of 1

Cites

Flagged first, then by citations

Research works printed but not linked: 1

  1. SbaihM. A. A.SrourM. K.HamadaM. S.FayadH. M., Lie algebra and representation of SU(4). Electron. J. Theor. Phys. 10, 9–26 (2013).

Cited by

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Related

Authors
Reimer, C., Kues, M., Roztocki, P., Wetzel, B., Grazioso, F., Little, B. E., Chu, S. T., Johnston, T., Bromberg, Y., Caspani, L., Moss, D. J., & Morandotti, R.
Affiliations
  • Institut National de la Recherche Scientifique (CA)
  • University of Sussex (GB)
  • Chinese Academy of Sciences (CN)
  • City University of Hong Kong (HK)
  • Yale University (US)
  • RMIT University (AU)
  • University of Electronic Science and Technology of China (CN)
Corresponding author
  • Reimer, Christian
  • Kues, Michael
  • Moss, David J.
  • Morandotti, Roberto
ORCID
Venue
Science
Publisher
American Association for the Advancement of Science (AAAS)
Published
2016-03-11
Volume
351
Issue
6278
Pages
1176-1180
ISSN
  • 0036-8075 (ISSN-L)
  • 1095-9203
DOI
10.1126/science.aad8532
OpenAlex
W2316306129
Semantic Scholar
d26c9cf67937ee49f2c33eb30077a9a0e6eebcc3
Cited by
841
References
39
Citations by year
2026: 39 · 2025: 64 · 2024: 101 · 2023: 129 · 2022: 99 · 2021: 132 · 2020: 100 · 2019: 60 · 2018: 54 · 2017: 47 · 2016: 16
FWCI
48.51
Type
journal article
Language
en
Open access
Licence
cc-by-nc-nd
Locations
  • Science (publishedVersion)
  • PubMed (publishedVersion)
  • Institutional Repository of Leibniz Universität Hannover (Leibniz Universität Hannover) (submittedVersion, cc-by-nc-nd)
  • Figshare (submittedVersion, cc-by)
  • Institutional Repository of Xi'an Institute of Optics and Fine Mechanics, Chinese Academy of Sciences (Xian Institute of Optics and Precision Mechanics) (submittedVersion, cc-by-nc-sa)
  • IrInSubria (University of Insubria) (submittedVersion)
  • Swinburne figshare (Swinburne University of Technology) (submittedVersion)
  • Leibniz Universität Hannover
Funders
  • Natural Sciences and Engineering Research Council of Canada (NSERC)
  • Australian Research Council
  • FRQNT (Fonds de Recherche du Quebec–Nature et Technologies)
  • Research Executive Agency (REA) (625466)
  • REA (627478)
Topics
  • Physical Sciences › Computer Science › Artificial Intelligence › Quantum Information and Cryptography
  • Physical Sciences › Physics and Astronomy › Atomic and Molecular Physics, and Optics › Advanced Fiber Laser Technologies
  • Physical Sciences › Physics and Astronomy › Atomic and Molecular Physics, and Optics › Quantum optics and atomic interactions
Keywords
multi-photon entanglement, entangled qubits, photon pair generation, integrated photonics, quantum communication, quantum computing, quantum memory

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