CoChat
Sign Up
Cited by 262Quantum Information and CryptographyQuantum Computing Algorithms and ArchitectureQuantum Mechanics and Applications

Demonstration of a Simple Entangling Optical Gate and Its Use in Bell-State Analysis

Open access
Langford, N. K., Weinhold, T. J., Prevedel, R., Resch, K. J., Gilchrist, A., O’Brien, J. L., Pryde, G. J., White, A. G.·2005·Physical Review Letters
DOI 10.1103/physrevlett.95.210504
Open this paper in CoChat — free accountOpen PDF

Abstract

We demonstrate a new architecture for an optical entangling gate that is significantly simpler than previous realizations, using partially polarizing beam splitters so that only a single optical mode-matching condition is required. We demonstrate operation of a controlled-z gate in both continuous-wave and pulsed regimes of operation, fully characterizing it in each case using quantum process tomography. We also demonstrate a fully resolving, nondeterministic optical Bell-state analyzer based on this controlled-z gate. This new architecture is ideally suited to guided optics implementations of optical gates.

AI snapshot

Written from the paper’s full text.

TL;DR

The paper demonstrates a simplified optical entangling controlled-z gate using partially-polarising beamsplitters, requiring only one nonclassical mode-matching condition and no classical interferometers. It fully characterizes the gate in continuous-wave and pulsed regimes and uses it as a nondeterministic Bell-state analyser.

Key findings

  • The PPBS controlled-z gate uses three partially-polarising beamsplitters, two half-wave plates, no classical interferometers, and no ancilla photons; the controlled-z operation succeeds with probability 1/9 and is heralded by one photon in each output.
  • Quantum process tomography gives CW and pulsed process fidelities of 74.6±0.3% and 84.0±0.1%, and average gate fidelities of 79.7±0.2% and 87.2±0.1%; with modelled single-qubit unitary corrections these rise to 77.0±0.3% and 86.6±0.2% process fidelity, and 81.6±0.2% and 89.3±0.1% average gate fidelity.
  • As a Bell-state analyser, the average fidelity of measured output states with separable states is 79±3%, and the directly measured average probability of success is 78±3%.
  • The pulsed-gate PPBS reflectivities were η = 0.28, 0.28, and 0.29 (±0.01); modelling a gate with 0.28 reflectivities gives an optimum process fidelity of 96%, indicating the gate is relatively forgiving of exact splitting ratios.

Methods

  • Photon pairs were generated by spontaneous parametric downconversion in a BBO crystal using both continuous-wave and femtosecond-pulsed sources, collected into single-mode fibers, and injected into the PPBS controlled-z gate; a coincidence window of ∼5 ns was used with no correction for accidental counts.
  • The gate was characterized by quantum process tomography in both CW and pulsed regimes, using process fidelity and average gate fidelity metrics.
  • The Bell-state analyser was tested with entangled CW input states, including the four maximally entangled states |ψ′± and |φ′±, using quantum state tomography and a directly measured truth table.

Limitations

  • The paper states that PPBS beamsplitting ratios are fixed at manufacture, unlike schemes where a half-wave plate sets effective reflectivity; performance is limited almost exclusively by mode matching, primarily spatial.
  • The paper reports systematic fixed rotations of input and output polarisations, probably from birefringent effects in non-ideal PPBSs, which could be compensated with wave plates.
  • The Bell measurement is nondeterministic, and the paper notes that a protocol using this gate would be less efficient than ordinary classical communication; the measured output Bell states are unentangled but somewhat mixed (average linear entropy SL = 0.42 ± 0.07).

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

Integrity signals

Registry

Crossref · 10.1103/physrevlett.95.210504

Cites

24 of 24 references verified0 citation errors · 0 not checkable

Statistics

Nothing to recompute.

Claims the paper makes

0 of 8 cited sources checked · 0 supported · 0 flagged

  • Findingown data

    Temporal mode mismatch was not a significant factor in the gate's performance.

    “by comparing the two we find that temporal mode mismatch was not a significant factor in the gate's performance.”
    Quote in the paperNot a substantial claim
  • Findingown data

    The average tangle T = 0.04 ± 0.05 and linear entropy S_L = 0.42 ± 0.07 indicate the output states are unentangled, albeit somewhat mixed.

    “Their average tangle, T = 0.04 ± 0.05, and linear entropy, SL = 0.42 ± 0.07, indicates that they are unentangled, albeit somewhat mixed.”
    Quote in the paperSubstantial claim
  • Findingown data

    Systematic, fixed rotations of the input and output polarisations were observed, probably due to birefringent effects in non-ideal PPBSs.

    “In our experiment, we observed systematic, fixed rotations of the input and output polarisations, probably due to birefringent effects in non-ideal PPBSs.”
    Quote in the paperNot a substantial claim
  • Findingown data

    The PPBS cz gate can be used as a fully-resolving, nondeterministic optical Bell-state analyser.

    “We also demonstrate a fully-resolving, nondeterministic optical Bell-state analyser based on this controlled-z gate.”
    • Also stated in: § Abstract · boosted
      “we also demonstrate the use of this kind of gate for fully-resolving Bell measurements.”
    Quote in the paperSubstantial claim
    • Rests on an assumption: The four Bell states can be discriminated by the cz gate after a Hadamard rotation.
    • Rests on an assumption: An ideal cz gate takes the four maximally entangled states |ψ′±, |φ′± to the four separable orthogonal states |DD, |AD, |DA, and |AA respectively.
  • Findingreasoning

    The circuit of Fig. 1(b) performs a cz gate with additional x gates on the control and target qubits.

    “The circuit of Fig. 1(b) therefore performs a cz gate with additional x gates on the control and target qubits.”
    Quote in the paperSubstantial claim
  • May combine two claims. 15

  • May combine two claims.

    Findingown data

    The source state produced is of the form |HH + e^{iφ}|VV, with tangle T = 0.93 ± 0.01, linear entropy S_L = 0.05 ± 0.01, and fidelity with a maximally entangled state F = 98.0 ± 0.4%.

    “Due to the geometry of the source, and birefringence and geometric effects in the single mode fibers, the near-maximally entangled state produced is of the form |HH + e iϕ |V V .”
    • Also stated in: § Abstract · plain
      “The tangle T = 0.93 ± 0.01 and linear entropy S L = 0.05 ± 0.01 show that this state is highly entangled and highly pure, the fidelity with a maximally entangled state is F = 98.0 ± 0.4%.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingcitation

    The pulsed gate has a process fidelity of 84.0±0.1% and an average gate fidelity of 87.2±0.1%.

    “The CW and pulsed gates have process fidelities of 74.6±0.3% and 84.0±0.1% respectively; and average gate fidelities of 79.7±0.2% and 87.2±0.1% [20].”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [20] source

      Not checked in the free run.

  • May combine two claims.

    Findingown data

    The four experimentally produced Bell states have average fidelity F = 95.8 ± 0.7%, average tangle T = 0.94 ± 0.02, and average linear entropy S_L = 0.04 ± 0.01.

    “The four experimentally produced density matrices are shown in Fig. 3(e): the average of their fidelities is F = 95.8 ± 0.7%; the average of the tangles and linear entropies are T = 0.94 ± 0.02 and SL = 0.04 ± 0.01, respectively.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingown data

    Modelling single qubit unitary corrections increased the CW and pulsed process fidelities to 77.0±0.3% and 86.6±0.2%, and average gate fidelities to 81.6±0.2% and 89.3±0.1%.

    “To demonstrate this, we modelled their effect numerically, identifying single qubit unitary corrections which increased the CW and pulsed process fidelities to 77.0±0.3% and 86.6±0.2%respectively; and average gate fidelities to 81.6±0.2% and 89.3±0.1%.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingcitation

    The CW gate has a process fidelity of 74.6±0.3% and an average gate fidelity of 79.7±0.2%.

    “The CW and pulsed gates have process fidelities of 74.6±0.3% and 84.0±0.1% respectively; and average gate fidelities of 79.7±0.2% and 87.2±0.1% [20].”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [20] source

      Not checked in the free run.

  • May combine two claims.

    Findingown data

    The PPBSs for the CW gate were measured to be within ±0.01 of the required reflectivities; for the pulsed gate, the values were η = 0.28, 0.28 and 0.29 (±0.01).

    “While the PPBSs for the CW gate (optimized for 702.2 nm) were measured to be within ±0.01 of the required reflectivities, for the pulsed gate (820 nm), the values for the three PPBSs were η = 0.28, 0.28 and 0.29 (±0.01; normalised to output power).”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingown data

    The extra spatial filtering in the pulsed regime led to better gate operation, equivalent to the previous best demonstration.

    “Despite more stringent temporal modematching requirements in the pulsed regime, the extra spatial filtering led to better gate operation, equivalent to the previous best demonstration [7].”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [7] source

      Not checked in the free run.

  • May combine two claims.

    Findingown data

    The average fidelity between all combinations of the measured output states is 24 ± 5%, demonstrating that the states are close to orthogonal.

    “The average of the fidelities between all combinations of the measured output states is 24 ± 5% (ideally zero), demonstrating that the states are close to orthogonal.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingown data

    The directly measured average probability of success for the Bell-state analyser is 78 ± 3%, in agreement with tomography.

    “The results are shown in Fig. 3(c), and the average probability of success is 78 ± 3%, in agreement with the tomography results.”
    Quote in the paperSubstantial claim
    • Rests on an assumption: The four Bell states can be discriminated by the cz gate after a Hadamard rotation.
    • Rests on an assumption: An ideal cz gate takes the four maximally entangled states |ψ′±, |φ′± to the four separable orthogonal states |DD, |AD, |DA, and |AA respectively.

  • May combine two claims.

    Findingown data

    The average fidelity of the measured output states with the separable states is F = 79 ± 3%, meaning the Bell-state would be correctly identified with probability 79%.

    “The average fidelity of the measured output states with the above separable states is F = 79 ± 3%: if we analyzed the output of the circuit in this rotated basis, we would correctly identify the Bell-state with a probability of 79%.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Interpretationown data

    The cz gate is relatively forgiving of the exact splitting ratios, making it suitable for a PPBS architecture.

    “As originally anticipated in Ref. [13], the cz gate is relatively forgiving of the exact splitting ratios-making it an eminently suitable gate to be realised with a PPBS architecture.”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [13] source

      Not checked in the free run.

  • May combine two claims.

    Interpretationnone given

    The PPBS cz gate requires only three partially-polarising beamsplitters, two half-wave plates, no classical interferometers, and no ancilla photons.

    “This cz gate is the simplest entangling (or dis-entangling) linear optics gate realized to date, requiring only three partially-polarising beam splitters (PPBSs), two half-wave plates, no classical interferometers, and no ancilla photons.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Interpretationnone given

    The PPBS cz gate is nondeterministic and its success is heralded by detection of a single photon in each of the outputs.

    “It is nondeterministic and success is heralded by detection of a single photon in each of the outputs.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Interpretationcitation

    The gate as a Bell-state analyser has the advantage of higher success probability and no ancillae compared to alternative recent demonstrations.

    “Finally, we have demonstrated the operation of this gate as a Bell-state analyser which has the advantage of higher success probability and no ancillae compared to alternative recent demonstrations [9,27].”
    Quote in the paperSubstantial claim0 of 2 sources checked
    • cites [9] source

      Not checked in the free run.

    • cites [27]

      Not checked in the free run.

  • May combine two claims.

    Interpretationown data

    These systematic rotations have no effect on gate quality and could be compensated for with appropriate wave plates.

    “In practice, these have no effect on gate quality and, if necessary, could be compensated for with appropriate wave plates.”
    Quote in the paperSubstantial claim
15 of 15

  • The paper relies on this without support.

    The short pump pulses in pulsed parametric downconversion lead to more stringent temporal mode-matching requirements.

    “However, the short pump pulses lead to a larger bandwidth of downconverted photons, and to more stringent temporal mode-matching requirements.”

  • The paper relies on this without support.

    The four Bell states can be discriminated by the cz gate after a Hadamard rotation.

    “These are just the usual four Bell states, with the second qubit rotated by a Hadamard so that they can be discriminated by the cz gate.”

  • The paper relies on this without support.

    An ideal cz gate takes the four maximally entangled states |ψ′±, |φ′± to the four separable orthogonal states |DD, |AD, |DA, and |AA respectively.

    “An ideal cz gate would take the four maximally entangled states |ψ ′± , |φ ′± to the four separable orthogonal states: |DD , |AD , |DA , and |AA respectively.”
3 of 3

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?

  • An even simpler set-up becomes possible with polarization encoding (Kiesel et al., 2005b; Langford et al., 2005; Okamoto et al., 2005) which in turn could be already applied, e.g., to observe cluster states for one-way quantum computing (Kiesel et al., 2005a).

    “An even simpler set-up becomes possible with polarization encoding (Kiesel et al., 2005b;Langford et al., 2005;Okamoto et al., 2005) which in turn could be already applied, e.g., to observe cluster states for one-way quantum computing (see below, Kiesel et al., 2005a).”

    From Multiphoton entanglement and interferometry (2012) · cites this paper as Langford et al., 2005

  • These results show near-ideal algorithm performance, far better than expected given the known errors inherent in the logic gates.

    “These results show that we have near-ideal algorithm performance, far better than we have any right to expect given the known errors inherent in the logic gates [8,29].”

    From Experimental Demonstration of a Compiled Version of Shor’s Algorithm with Quantum Entanglement (2007) · cites this paper as [8]

  • The authors use a pair of compact optical gates, each operating on a dependent pair of photons, resulting in measured visibilities for both of V_r=98±2%.

    “We use a pair of compact optical gates [8,9,10], Fig 2c),e), each operating on a dependent pair of photons, resulting in measured visibilities for both of V r =98±2%.”

    From Experimental Demonstration of a Compiled Version of Shor’s Algorithm with Quantum Entanglement (2007) · cites this paper as [8]

Cites

Flagged first, then by citations

Cited by

Sorted by citations

Related

Authors
Langford, N. K., Weinhold, T. J., Prevedel, R., Resch, K. J., Gilchrist, A., O’Brien, J. L., Pryde, G. J., & White, A. G.
Affiliations
  • The University of Queensland (AU)
  • University of Vienna (AT)
ORCID
Venue
Physical Review Letters
Publisher
American Physical Society (APS)
Published
2005-11-18
Volume
95
Issue
21
Pages
210504-210504
ISSN
  • 0031-9007 (ISSN-L)
  • 1079-7114
DOI
10.1103/physrevlett.95.210504
OpenAlex
W1998141297
Semantic Scholar
d9688239d7e905dd0699832575c8889049b0add1
Cited by
262
References
24
Citations by year
2026: 7 · 2025: 6 · 2024: 6 · 2023: 9 · 2022: 9 · 2021: 11 · 2020: 8 · 2019: 6 · 2018: 14 · 2017: 9 · 2016: 19 · 2015: 13 · 2014: 18 · 2013: 12 · 2012: 11
FWCI
13.75
Type
journal article
Language
en
Open access
Licence
link.aps.org/licenses/aps-default-license (vor)
Locations
  • Physical Review Letters (publishedVersion)
  • PubMed (publishedVersion)
  • arXiv (Cornell University) (submittedVersion)
  • Griffith Research Online (submittedVersion)
  • CERN Document Server (European Organization for Nuclear Research) (submittedVersion)
  • Explore Bristol Research (submittedVersion)
  • arXiv (Cornell University)
Funders
  • Austrian Science Fund
  • Queensland Government
  • Agricultural Research Development Agency
Topics
  • Physical Sciences › Computer Science › Artificial Intelligence › Quantum Information and Cryptography
  • Physical Sciences › Computer Science › Artificial Intelligence › Quantum Computing Algorithms and Architecture
  • Physical Sciences › Physics and Astronomy › Atomic and Molecular Physics, and Optics › Quantum Mechanics and Applications
Keywords
controlled-Z gate, quantum process tomography, continuous-wave operation, pulsed operation, guided optics

About these checks

Registry, retraction, Cites and statistics results are computed, not judged, with one exception: a printed reference that no index lists is read once by a model before it counts as a citation error. Claims are extracted by a model and kept only when their quote appears verbatim in the text; each cited source is then checked, with a model’s help, for whether it says what the claim relies on. Those verdicts can be wrong — read the claim’s quote and the cited source before relying on one.

Claims read by deepseek-v4.1-flash under definition version 3 (extract-v3) on 2026-10-10.

Classified by jev-1.13-20260917 (gold-guideline-v0.2) on 2026-10-10.

Statistics and arithmetic are recomputed, not judged.

Confidence bands are shown only for fields calibrated on our annotated test set. None is calibrated yet, so no band is shown.

What counts as a claim ›How we measure confidence ›How we check this ›

Are you an author, or do you see a problem with this page? Request a review