Cited by 1,093Quantum Computing Algorithms and ArchitectureQuantum Information and CryptographyPhotonic and Optical Devices

Universal linear optics

Open access
Carolan, Jacques, Harrold, Christopher, Sparrow, Chris, Martín-López, Enrique, Russell, Nicholas J., Silverstone, Joshua W., Shadbolt, Peter J., Matsuda, Nobuyuki +8 more·2015·Science

Abstract

Complex quantum optical circuitry: Encoding and manipulating information in the states of single photons provides a potential platform for quantum computing and communication. Carolan et al. developed a reconfigurable integrated waveguide device fabricated in a glass chip (see the Perspective by Rohde and Dowling). The device allowed for universal linear optics transformations on six wave-guides using 15 integrated Mach-Zehnder interferometers, each of which was individually programmable. Functional performance in a number of applications in optics and quantum optics demonstrates the versatility of the device's reprogrammable architecture. Science , this issue p. 711 ; see also p. 696

AI snapshot

Written from the paper’s full text.

TL;DR

The paper experimentally demonstrates a six-mode universal linear optical processor (LPU) that can be reprogrammed to implement any linear optical protocol up to its circuit size. It uses the device to realize heralded quantum logic and entangling gates, boson sampling with verification, and six-dimensional complex Hadamard operations.

Key findings

  • A single photonic chip with 15 Mach-Zehnder interferometers and 30 thermo-optic phase shifters was programmed to implement 100 Haar random unitaries with average fidelity 0.999 ± 0.001, and could switch protocols in seconds.
  • Heralded gates were demonstrated: a Bell state generator with statistical fidelity F_s = 0.966 ± 0.004 and entanglement witness E = 0.673 ± 0.031, and a heralded CNOT with mean statistical fidelity F_s = 0.927 ± 0.004.
  • Boson sampling was implemented for 100 three-photon routines with mean statistical fidelity F_s = 0.950 ± 0.020; zero-transmission-law verification gave minimum violation ν_min = 0.319 ± 0.009 at the six-dimensional Fourier transform, and six-photon Bayesian model comparison reached confidence p = 0.998 after 15 events.
  • Complex Hadamard operations were distinguished: mean statistical fidelity F_s = 0.979 ± 0.007 for F^(2)_6 matrices, and F_s = 0.986 ± 0.001 and 0.998 ± 0.001 for G^(4)_6 and S^(0)_6 examples, respectively.

Methods

  • The device is a silica-on-silicon planar lightwave circuit with six modes, 15 MZIs, 30 directional couplers, and 30 thermo-optic phase shifters, controlled by custom electronics and cooled by a Peltier unit; photons are generated by SPDC in BiBO and detected with up to 12 single-photon avalanche diodes and time-correlated single-photon counting.
  • The LPU was programmed to implement heralded Bell state generation, heralded and unheralded CNOT gates, single-qubit gates, 100 Haar-random boson sampling circuits, zero-transmission-law tests, six-photon verification, and complex Hadamard matrices.
  • Analysis used statistical fidelity, quantum process tomography with maximum-likelihood estimation and semidefinite programming, unitary fidelity, Poissonian error propagation, and bootstrap resampling; effective phase accuracy was estimated as δφ = 0.035 rad.

Limitations

  • Stated: the LPU is not optimized for highly structured unitaries used in qubit operations, and multi-photon fidelity is primarily limited by photon-source imperfections such as reduced quantum interference and higher-order SPDC terms.
  • Stated: near-term improvements require higher-efficiency sources and detectors, while mid-term progress depends on integrating components with larger low-loss circuits.
  • Evident: the gates are probabilistic and rely on heralding or post-selection (Bell state p = 2/27, heralded CNOT p = 1/16, unheralded CNOT p = 1/9), and the six-mode size restricts protocols to at most six optical modes; boson-sampling verification remains circumstantial rather than a full classical check.

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

Integrity signals

Registry

Crossref · 10.1126/science.aab3642

Cites

71 of 71 references verified0 citation errors · 0 not checkable

Statistics

Nothing to recompute.

Claims the paper makes

0 of 39 cited sources checked · 0 supported · 0 flagged

  • Findingown data
    § Universal Linear Optics · p. 2

    The device was programmed to implement a new compact four-photon scheme suitable for measurement-based quantum computing, which generates the entangled state of two photonic qubits upon the detection of another two ancilla photons.

    “We programmed our device to implement a new compact four-photon scheme, suitable for measurementbased quantum computing, which generates the entangled state of two photonic qubits upon the detection of another two ancilla photons, schematically displayed in Fig. 2(a,b).”
    Quote in the paperSubstantial claim
  • Findingown data
    § Universal Linear Optics · p. 4

    The unheralded two-photon CNOT gate had process fidelity F_p = 0.909 ± 0.001.

    “The process fidelity was found to be F p = 0 . 909 ± 0 . 001, and the average gate fidelity F g = 0 . 927 ± 0 . 001 (see Appendix for details), greater than those previously reported [21, 33, 39].”
    Quote in the paperSubstantial claim
  • Findingown data
    § Universal Linear Optics · p. 1

    The paper experimentally demonstrates a single reprogrammable optical circuit that is sufficient to implement all possible linear optical protocols up to the size of that circuit.

    “Here we experimentally demonstrate the longstanding goal of a single reprogrammable optical circuit that is sufficient to implement all possible linear optical protocols up to the size of that circuit.”
    Quote in the paperSubstantial claim
  • Findingown data
    § Universal Linear Optics · p. 2

    The mean statistical fidelity between all four experimental and theoretical probability distributions for the Bell state measurements was F_s = 0.966 ± 0.004.

    “We implemented both types of measurement, data shown in Fig. 2(c), finding the mean statistical fidelity F s = ∑ i √ p exp i .p th i between all four experimental p exp and theoretical p th probability distributions to be F s = 0 . 966 ± 0 . 004 (see Methods for an explanation of all error analyses).”
    Quote in the paperSubstantial claim
    • Rests on an assumption: For Bell states, measurements in common bases are correlated while measurements in different bases are uncorrelated.
  • Findingown data
    § Universal Linear Optics · p. 4

    These are the first heralded two-qubit processes in integrated optics.

    “While these are the first heralded two-qubit processes in integrated optics, several examples of photonic chips specifically fabricated to implement unheralded (and hence not scalable) two-qubit gates have been reported [21, 33, 34].”
    Quote in the paperSubstantial claim0 of 3 sources checked
    • cites [21] source

      Not checked in the free run.

    • cites [33] source

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    • cites [34] source

      Not checked in the free run.

  • May combine two claims. 12
  • Classified as probably not a substantial claim. 9
  • May combine two claims.

    Findingown data
    § Universal Linear Optics · p. 1

    The system was programmed to implement heralded quantum logic and entangling gates.

    “We programmed this system to implement heralded quantum logic and entangling gates, boson sampling with verification tests, and six-dimensional complex Hadamards.”
    Quote in the paperSubstantial claim
  • May combine two claims.

    Findingcitation
    § Universal Linear Optics · p. 2

    The entanglement witness E calculated from these measurements was 0.673 ± 0.031, where E > 1/2 witnesses entanglement.

    “We used these measurements to verify the entanglement of our state by calculating E = 1 / 2( 〈 σ x ⊗ σ x 〉 + 〈 σ z ⊗ σ z 〉 ), finding a value of E = 0 . 673 ± 0 . 031 where E > 1 / 2 witnesses entanglement [30].”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [30] source

      Not checked in the free run.

  • May combine two claims.

    Findingown data
    § Universal Linear Optics · p. 1

    The system is electrically and optically interfaced for arbitrary setting of all phase shifters, input of up to six photons, and measurement with a 12 single-photon detector system.

    “Our six-mode universal system consists of a cascade of 15 MachZehnder interferometers with 30 thermo-optic phase shifters integrated into a single photonic chip that is electrically and optically interfaced for arbitrary setting of all phase shifters, input of up to six photons and their measurement with a 12 single-photon detector system.”
    Quote in the paperSubstantial claim
  • May combine two claims.

    Findingown data
    § Universal Linear Optics · p. 4

    For the Bell state generator, the heralded CNOT, and the unheralded CNOT, the unitary fidelities were F_U = 0.998 ± 0.002, 0.994 ± 0.008, and 0.988 ± 0.005 respectively.

    “For the Bell state generator, the heraldedand unheralded-CNOT respectively we found F U = 0 . 998 ± 0 . 002 , 0 . 994 ± 0 . 008 and 0 . 988 ± 0 . 005.”
    Quote in the paperSubstantial claim
  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Findingnone given
    § 2. Fabrication · p. 10

    The electric power required for a 2π phase shift is ∼0.8 W.

    “The electric power required for a 2 π phase shift is ∼ 0 . 8 W .”
    Quote in the paperNot a substantial claim
  • May combine two claims.

    Findingreasoning
    § 2. Heralded CNOT · p. 12

    Our photon source is based upon two independent SPDC events which produce the state ∑ η_i |i,i〉, such that in the four photon subspace |ψ4〉 ≈ |1111〉 + |2200〉 + |0022〉.

    “Our photon source is based upon two independent SPDC events which produce the state ∑ i η i | i, i 〉 , such that in the four photon subspace we have | ψ 4 〉 ≈ | 1111 〉 + | 2200 〉 + | 0022 〉 .”
    Quote in the paperSubstantial claim
  • May combine two claims.

    Findingown data
    § Methods: Circuit implementation & analysis · p. 7

    Recalculating statistical fidelities based on the recovered unitary U′ for three three-photon boson sampling unitaries increased the mean statistical fidelity and reduced the range Fmax_s - Fmin_s from 0.102 to 0.023.

    “This increased the mean statistical fidelity and reduced range F max s - F min s from 0 . 102 to 0 . 023”
    Quote in the paperNot a substantial claim
  • Probably not a substantial claim: may not pass S3 (load-bearing). May combine two claims.

    Premisecitation
    § Universal Linear Optics · p. 6

    The FT and F6^(2) are examples in the more general class of complex Hadamard matrices, which are related to mutually unbiased bases and are of fundamental interest in quantum information theory.

    “The FT and F (2) 6 are examples in the more general class of complex Hadamard matrices (CHMs), which are related to mutually unbiased bases [54] and are of fundamental interest in quantum information theory [55].”
    Quote in the paperNot a substantial claim0 of 2 sources checked
    • cites [54] I. Bengtsson, W. Bruzda, ˚ A. Ericsson, J.- ˚ A. Larsson, W. Tadej, and K. ˙ Zyczkowski, J. Math. Phys. 48 , 052106 (2007). source

      Not checked in the free run.

    • cites [55] R. F. Werner, J. Phys. A: Math. Gen. 34 , 7081 (2001). source

      Not checked in the free run.

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

    Premisecitation
    § Universal Linear Optics · p. 6

    The ZTL has been proposed as a certificate for the capability of a device to implement boson sampling.

    “Because large scale many-photon quantum interference is at the core of the ZTL, it has been proposed as a certificate for the capability of a device to implement boson sampling [52].”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites [52] M. C. Tichy, K. Mayer, A. Buchleitner, and K. Mølmer, Phys. Rev. Lett. 113 , 020502 (2014).

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S4 (based).

    Study design and conductnone given
    § 4. Circuit Characterisation · p. 10

    The triangular arrangement of our device allows each phase shifter to be isolated and characterised independently of any other.

    “The triangular arrangement of our device allows each phase shifter to be isolated and characterised independently of any other.”
    Quote in the paperNot a substantial claim
  • Classified as probably not a substantial claim. May combine two claims.

    Interpretationreasoning
    § 6. Phase accuracy · p. 11

    The δφ value lumps together error caused by the circuit, the control electronics and the calibration procedure, so is a good metric for our effective phase accuracy.

    “This value lumps together error caused by the circuit, the control electronics and the calibration procedure, so is a good metric for our effective phase accuracy.”
    Quote in the paperSubstantial claim
  • May combine two claims.

    Interpretationnone given
    § Methods: Circuit implementation & analysis · p. 7

    Detection in ancilla modes {3,4} of the compact Bell state generator occurs with probability p = 2/27 and heralds the state |Φ+〉 over the remaining modes with unit probability in the ideal case.

    “Detection in ancilla modes { 3 , 4 } occurs with probability p = 2 / 27 and heralds the state | Φ + 〉 = ( | 1 1 , 0 2 , 1 5 , 0 6 〉 + | 0 1 , 1 2 , 0 5 , 1 6 〉 ) / √ 2 (where | n i 〉 represents n photons in the i th mode) over the remaining modes with unit probability in the ideal case.”
    Quote in the paperSubstantial claim
  • Classified as probably not a substantial claim.

    Study design and conductown data
    § Universal Linear Optics · p. 6

    The ZTL verification protocol was implemented in the same device and with the same procedure as the boson sampling experiments.

    “Crucially, this verification protocol is implemented in the same device and with the same procedure as that for the boson sampling experiments above.”
    Quote in the paperSubstantial claim
  • Probably not a substantial claim: may not pass S3 (load-bearing). May combine two claims.

    Premisecitation
    § Universal Linear Optics · p. 6

    In the N = {2,3,5} case, all CHMs are equivalent to the respective FT matrix, while for N = 4 there exists a one parameter equivalence class.

    “In the N = { 2 , 3 , 5 } case, all CHMs are equivalent to the respective FT matrix, while for N = 4 there exists a one parameter equivalence class.”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites [56] W. Tadej and K. ˙ Zyczkowski, Open Syst. Inf. Dyn. 13 , 133 (2006). source

      Not checked in the free run.

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

    Interpretationreasoning
    § Methods: Circuit implementation & analysis · p. 7

    Compiling F(2)_6 onto our LPU requires unique phase shift values across the entire device for each implementation θ1, θ2 ∈ [0,2π).

    “compiling F (2) 6 onto our LPU requires unique phase shift values across the entire device for each implementation θ 1 , θ 2 ∈ [0 , 2 π ).”
    Quote in the paperNot a substantial claim
  • May combine two claims.

    Premisecitation
    § Methods: Circuit implementation & analysis · p. 7

    In the ideal case, detection of the two ancilla photons in the heralded CNOT occurs with probability p = 1/16 and guarantees the CNOT logic has been performed on the two photonic qubits.

    “In the ideal case, the detection of the two ancilla photons, which occurs with probability p = 1 / 16, guarantees the CNOT logic has been performed on the two photonic qubits.”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [32] source

      Not checked in the free run.

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

    Premisecitation
    § Universal Linear Optics · p. 6

    While a full classification of N = 6 CHMs is unknown, it is currently conjectured that the set consists of an isolated matrix S6^(0) which does not belong to any family, and a newly discovered four-parameter generic family G6^(4).

    “Whilst a full classification of N = 6 CHMs is unknown, it is currently conjectured that the set consists of an isolated matrix S (0) 6 which does not belong to any family [57], and a newly discovered four-parameter generic family G (4) 6 [58].”
    Quote in the paperNot a substantial claim0 of 2 sources checked
    • cites [57] source

      Not checked in the free run.

    • cites [58] F. Szollosi, J. London Math. Soc. 85 , 616 (2012). source

      Not checked in the free run.

17 of 17
  • The paper relies on this without support.

    For Bell states, measurements in common bases are correlated while measurements in different bases are uncorrelated.

    “For Bell states, measurements in common bases should be correlated while measurements in different bases should be uncorrelated.”
  • The paper relies on this without support.

    An essential requirement of boson sampling is indistinguishability among photons.

    “An essential requirement of boson sampling is the indistinguishability among photons.”
  • The paper relies on this without support.

    Any given unitary corresponds to a unique set of parameter values.

    “Any given unitary then corresponds to a unique set of parameter values.”
  • The paper relies on this without support.

    Up until now, observations of characteristic photonic quantum interference patterns have been too experimentally challenging for the six-dimensional case.

    “Up until now, these observations have been too experimentally challenging for the six-dimensional case.”
  • The paper relies on this without support.

    The unwanted photon terms from the non-ideal SPDC source can be removed by post-selection/detection without affecting the logical operation.

    “Operating the BSG with the non-ideal state from our SPDC source requires the Bell state to be detected to remove the unwanted photon terms”
    “Once again, the non-ideal input state requires a computational detection to remove unwanted terms.”
  • The paper relies on this without support.

    The 8th lossy balancing mode can be equivalently implemented as a reduction in detection efficiency on the lower ancilla mode.

    “The function of an 8th lossy balancing mode is implemented in post-processing as an equivalent reduction in detection efficiency on the lower ancilla mode”
  • The paper relies on this without support.

    The |2200〉 and |0022〉 terms do not produce output states in the computational subspace.

    “the other terms should not produce output states in the computational subspace”
7 of 7

Claims other papers make about this one

Each is checked against this paper’s own text: does it say what the citing paper says it does?

  • In the latter experiment, the circuit was too small to fully implement a heralded CNOT gate and only the classical XOR logic operations were demonstrated.

    “In the latter experiment, the circuit was too small to fully implement a heralded CNOT gate and only the classical XOR logic operations were demonstrated.”

    From Integrated-optics heralded controlled-NOT gate for polarization-encoded qubits (2018) · cites this paper as [26]

    The paper says the heralded CNOT was implemented, not that the circuit was too small to implement it (Universal Linear Optics, p. 3; Methods, p. 7). It reports a computational truth-table test of that gate, but also reports quantum process tomography of a separate, unheralded CNOT (p. 3), contrary to the claim that only classical XOR logic was demonstrated. (LLM, conf=0.84)

    Checked by a reasoning model — may be wrong.

    Experimental — accuracy not yet measured
  • Our work expands on the first demonstrations of unheralded integrated CNOT gates using polarization-encoding or path-encoding, combining the desirable features of heralding, integration, and polarization encoding.

    “Our work expands on the first demonstrations of unheralded integrated CNOT gates using polarization-encoding or pathencoding, [25][26][27] combining the desirable features of heralding, integration, and polarization encoding.”

    From Integrated-optics heralded controlled-NOT gate for polarization-encoded qubits (2018) · cites this paper as [26]

    The Unheralded CNOT section (p. 11) describes an integrated, unheralded CNOT using dual-rail (path-encoded) qubits. But Universal Linear Optics (p. 3) says earlier photonic chips had implemented unheralded two-qubit gates; it identifies this paper’s first integrated two-qubit processes as heralded ones. The source therefore supports the example of a path-encoded unheralded gate, but not its characterization as a first demonstration of that kind. (LLM, conf=0.88)

    Experimental — accuracy not yet measured
  • Multiphoton quantum walks have the potential to demonstrate an undisputed quantum advantage through the boson-sampling algorithm.

    “Further attention has been devoted to the computational complexity of multiphoton quantum walks, which has the potential to demonstrate an undisputed quantum advantage through the boson-sampling algorithm (13–19).”

    From Two-photon quantum walk in a multimode fiber (2016) · cites this paper as [18]

    In “Implementing boson sampling” (p. 4), the paper describes boson sampling with many photons as a potential route to computational capabilities beyond classical machines. It does not identify this as a result about multiphoton quantum walks. The “Verifying boson sampling” passage (p. 4) also says that checking large-scale results remains an open question, so the paper does not support calling the advantage “undisputed.” (LLM, conf=0.84)

    Experimental — accuracy not yet measured
  • The technological progress of integrated quantum photonics has enabled demonstrations of unheralded integrated CNOT gates, and recently also of a heralded CNOT gate.

    “The technological progress of integrated quantum photonics has enabled demonstrations of unheralded integrated CNOT gates, [25][26][27] and recently also of a heralded CNOT gate.26”

Cites

Flagged first, then by citations

Research works printed but not linked: 6

  1. N. K. Langford thesis University of Queensland (2007).
  2. Hurwitz A., Nachr. Akad. Wiss. Gött. I. Math. Phys. Kl. 1897, 71 (1897).
  3. Russell N. J. O’Brien J. L. Laing A. http://arxiv.org/pdf/1506.06220.pdf (2015). · arXiv
  4. Aaronson S., Arkhipov A., Quant. Inf. Comp. 14, 1383 (2014).
  5. P. W. Shor Proceedings of the 35th Annual Sympo-sium on Foundations of Computer Science (Society for Industrial and Applied Mathematics Philadelphia PA 1994) pp. 124–134.
  6. M. C. Tichy thesis Universitát Freiburg.

Cited by

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Authors
Carolan, J., Harrold, C., Sparrow, C., Martín-López, E., Russell, N. J., Silverstone, J. W., Shadbolt, P. J., Matsuda, N., Oguma, M., Itoh, M., Marshall, G. D., Thompson, M. G., Matthews, J. C. F., Hashimoto, T., O’Brien, J. L., & Laing, A.
Affiliations
  • University of Bristol (GB)
  • Nokia (United Kingdom) (GB)
  • Imperial College London (GB)
  • NTT (Japan) (JP)
Corresponding author
Laing, Anthony
Venue
Science
Published
2015-08-14
Volume
349
Issue
6249
Pages
711-716
ISSN
  • 0036-8075 (ISSN-L)
  • 1095-9203
OpenAlex
W2097100165
Cited by
1,093
References
71
Citations by year
2026: 67 · 2025: 91 · 2024: 104 · 2023: 102 · 2022: 110 · 2021: 118 · 2020: 116 · 2019: 127 · 2018: 92 · 2017: 86 · 2016: 71 · 2015: 8 · 2014: 1
FWCI
83.99
Type
journal article
Language
en
Open access
Locations
  • Science (publishedVersion)
  • PubMed (publishedVersion)
  • Bristol Research (University of Bristol) (publishedVersion)
  • arXiv (Cornell University) (submittedVersion)
  • UWA Profiles and Research Repository (UWA) (submittedVersion)
  • arXiv (Cornell University)
Funders
  • U.S. Air Force Office of Scientific Research (AFOSR)
  • U.S. Army Research Office (ARO) (W911NF-14-1-0133)
  • European Research Council (ERC)
  • Engineering and Physical Sciences Research Council (EPSRC)
  • PIQUE (FP7-PEOPLE-2013-ITN)
  • Centre for Nanoscience and Quantum Information (NSQI)
Topics
  • Physical Sciences › Computer Science › Artificial Intelligence › Quantum Computing Algorithms and Architecture
  • Physical Sciences › Computer Science › Artificial Intelligence › Quantum Information and Cryptography
  • Physical Sciences › Engineering › Electrical and Electronic Engineering › Photonic and Optical Devices
Keywords
Mach-Zehnder interferometer, thermo-optic phase shifters, photonic integrated circuits, entangling gates, boson sampling, Haar random unitaries, single-photon detection, quantum computing

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