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REVIEW 3 major objections 4 minor 61 references

Closing objectivity loophole in Bell tests on a public quantum computer

T0 review · 3 major / 4 minor · reviewed 2026-08-07 · deepseek-v4-flash

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Pith's one-line read Public quantum computers pass a Bell test in which three unanimous observers at each side must confirm the outcome.

desk verdict Honest, careful three-friend Bell test on cloud hardware, but the central passing claim is undercut by unquantified signaling that could explain the violations. read the letter →

arxiv 2506.08940 v2 pith:2BAMKIYE submitted 2025-06-10 quant-ph

Adam Bednorz , Josep Batle , Tomasz Bia{l}ecki , Jaros{l}aw K. Korbicz This is my paper
classification quant-ph MSC 81P4081P1581P68 PACS 03.65.Ud03.67.-a03.65.Ta
keywords BelltestCHSHinequalityobjectivityunanimityconditionno-signalingquantumcomputerentanglementbenchmarkWigner'sfriend
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

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

The reading

This paper reports an extended Bell test run on public quantum computers in which each party consists of three 'friend' qubits that must all agree on the outcome (+1 or -1) for the event to be counted, while any non-unanimous outcome is assigned 0. The test asks whether entanglement survives a unanimity cut across a seven-qubit chain, addressing the objectivity loophole in Bell tests, the question of when a measurement result can be considered macroscopically confirmed. The authors claim that IBM Quantum devices violated the CHSH inequality by more than 10 standard deviations in 2 of 6 qubit groups on ibm_brisbane, 5 of 6 on ibm_sherbrooke, and all 6 on ibm_kingston, and that IonQ Forte Enterprise 1 gave $2.569 \pm 0.0024$ against the classical bound 2. They also report statistically significant residual signaling that violates no-signaling on several devices, especially ibm_sherbrooke, while ibm_kingston shows no detectable signaling. If the claim holds, it demonstrates that these devices can produce nonclassical correlations that survive a strict three-observer unanimity requirement, and it provides a benchmark for how far entanglement spreads across a processor's qubit chain.

What carries the argument

The central object is the extended CHSH expression (4), $B = \langle A_0B_0\rangle - \langle A_1B_0\rangle - \langle A_0B_1\rangle - \langle A_1B_1\rangle$, evaluated with the unanimity rule: each side's observable is $\bar A = |000\rangle\langle 000| - |111\rangle\langle 111|$, so an event counts only if all three friend qubits on a side read the same value. The state is prepared as an entangled pair $|00\rangle - i|11\rangle$ between $A_0$ and $B_0$, the setting is encoded by local rotations $S_\alpha$ and $S_\beta$, and information is copied to the friends by controlled-X gates before measurement, keeping the measurement fixed while varying the local state so that exactly the same information reaches all three observers. This machinery turns the Bell test into an objectivity test: a violation with unanimity means the outcome is collectively confirmed by several independent copies, and the same circuit simultaneously tests no-signaling through the differences in friend-outcome probabilities under the two settings.

What would settle it

Use the measured signaling differences to build an explicit local realistic model in which one party's setting shifts the other party's outcome distribution, optimize that shift to maximize the CHSH combination, and check whether the maximum reaches the observed values such as 2.569 or the reported IBM numbers; if it does, the violation can be reproduced without entanglement.

Watch

Extended reading notes

Core claim

The central claim is that the CHSH-Bell inequality, strengthened by an objectivity condition in which three unanimous simulated observers at each party must confirm the outcome, is violated on public superconducting and trapped-ion quantum computers. The violation is achieved by applying a setting-dependent rotation $S_\alpha, S_\beta$ to the shared entangled pair before copying the information to the friends with controlled-X gates, then reading only the unanimous patterns $000$ and $111$ and assigning $0$ to non-unanimous ones. The measured Bell numbers reach values such as $2.569 \pm 0.0024$ on IonQ and exceed the classical bound 2 by more than 10 standard deviations in most tested qubit groups on the newest IBM device, ibm_kingston. The paper interprets this as evidence that entanglement is spread across the tested qubit chains with sufficient quality to pass the objectivity-style test, while acknowledging that communication loopholes remain open and that residual signaling is present and statistically significant.

Load-bearing premise

The measured signaling is not itself generating the Bell violation: no model or bound is given showing that the signaling contribution to the CHSH combination stays below 2, so if the crosstalk is even partially responsible the pass would not demonstrate entanglement or objectivity.

Editorial extensions

If this is right

  • If the devices pass the unanimity-required Bell test, entanglement can be generated and measured across a chain of at least seven qubits on a superconducting processor, making the test a practical benchmark of entanglement spread.
  • A violation of CHSH under the objectivity condition with no detectable signaling, as on ibm_kingston, would certify nonclassical correlations without the usual postselection-based detection loophole, since all events are recorded.
  • The statistically significant signaling on ibm_brisbane, ibm_sherbrooke, and IonQ means those particular results cannot be read as demonstrating no-signaling-compatible nonlocality, and the devices need crosstalk countermeasures.
  • The test can be extended to more than three friends per party, setting limits on how many observers can confirm the same outcome before the violation degrades.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • A non-unanimous outcome is assigned 0 rather than discarded, so this is not postselection in the usual sense; but the trit encoding still lets a classical signaling model concentrate its outputs on the unanimous patterns, so the strength of the no-signaling check is what separates a genuine Bell violation from a communication artifact.
  • The objectivity framing makes the Bell test a device benchmark: the same data that certify entanglement also expose crosstalk, so the protocol could serve as a standard diagnostic for how publicly accessible quantum processors handle multi-qubit information broadcasting.
  • A natural extension is to test with four or more friends per party and longer qubit chains; one would predict a monotone decrease of the Bell number with chain length, and the crossover point would map the device's useful entanglement range.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

3 major / 4 minor

Summary. The paper reports an extended Bell-type test on IBM Quantum (ibm_brisbane, ibm_sherbrooke, ibm_torino, ibm_kingston) and IonQ Forte, in which each of the two parties is represented by three 'friend' qubits and an outcome is counted only when all three friends agree (unanimity condition). The ideal-state derivation in Section II shows that the CHSH combination (4) should reach 2√2 under the unanimity rule. The measurements give CHSH values above 2 with reported statistical errors of order 3e-4 in several qubit groups, and all six groups on ibm_kingston violate the inequality; the IonQ run gives B = 2.569 ± 0.0024. The same data show statistically significant no-signaling violations in many groups and on IonQ. The authors conclude that public quantum devices are sufficiently accurate to pass an objectivity-extended Bell-type test, while acknowledging communication loopholes and residual signaling.

Significance. If the central claim were fully established, this would be a useful benchmark: it would show that entanglement can survive a three-observer unanimity cut across a seven-qubit chain on current cloud quantum hardware, and the public scripts and data [52] are a reproducible resource. The ideal-state calculation in Section II is simple and parameter-free, and no free parameters are fitted to the data. However, the paper's interpretation of the Bell violations in signaling-affected groups is not yet justified: without a bound on how much the detected signaling can contribute to the CHSH functional, a classical model with signaling can reproduce B > 2. Only the signaling-free ibm_kingston groups currently escape this concern. For those groups the result is a genuine, if modest, demonstration; for the remaining groups the 'pass' claim is conditional on an untested assumption.

major comments (3)
  1. [Section IV, Tables IV and V, Figs. 8-12] The paper never quantifies the maximum contribution that the detected signaling could inject into the CHSH combination (4). The statement that signaling is 'much smaller than Bell violation' compares marginal-probability differences to the Bell value, but the relevant object is the largest CHSH excess achievable by any classical model whose signaling is constrained by the observed marginal distributions; this is the kind of bound provided by Hall (refs. [39,40]), which is cited but never applied here. For example, sherbrooke group 3 has B = 2.176 ± 0.0026 (Table IV) while Fig. 9 shows δP values up to about 3e-2. Without an explicit bound showing that the signaling-induced contribution to (4) is below 2, the observed B > 2 in that group does not demonstrate entanglement. The pass claim in the abstract should therefore either be restricted to groups with no detected signaling (at minimum ibm_kingston) or supplemented by a Hall-type bound for every group with significant signaling, including the IonQ result in Section V.
  2. [Section IV, Table IV] The text states that the inequality was violated by more than 10 standard deviations in 2 groups on ibm_brisbane, but Table IV lists three groups above 2 + 10ΔB: group 1 (2.259 ± 2.75e-4), group 4 (2.034 ± 2.67e-4), and group 6 (2.194 ± 2.79e-4). This discrepancy affects the quantitative summary of the main result and should be corrected or explained.
  3. [Section II, Eq. (17) and Eq. (21)] The statistical treatment of the signaling estimators is incompletely specified. Eq. (21) gives the single-shot Bernoulli variance P(1−P), but the quantities δPa∗(A) and δP∗b(B) defined in Eq. (17) are differences of two sample proportions, so their standard errors involve sums of P(1−P)/N terms, possibly with different N for the two settings. The text says results are 'scaled by the number of repetitions,' but the formula as written does not display this scaling or the difference structure. Please give the explicit estimator and standard error, and state whether any multiple-comparison correction was used in flagging violations 'beyond 5 standard deviations' in Tables IV and V.
minor comments (4)
  1. [Section III, first paragraph] The sentence 'For single qubit rotation we can essentially use (7) which is realize natively' contains a grammar error and should read 'is realized natively'; similar typos appear elsewhere (e.g., 'we tak' in Section III).
  2. [Section II, Eq. (4)] The displayed inequality appears as '... − ⟨A1B1⟩11 ≤ 2'; the stray '11' should be removed.
  3. [Tables IV and V captions] The symbols A→B, A←B, and A↔B are used but not defined in the captions; please specify which marginal difference and which direction each symbol denotes.
  4. [References] References [39] and [40] appear to be the same paper; one duplicate should be removed. Also, the Zenodo reference [52] should include the direct dataset identifier if possible.

Circularity Check

0 steps flagged · score 0.0 of 10

No circularity: the Bell and no-signaling statistics are direct measurements with no fitted parameters, and the cited self-work is contextual rather than load-bearing.

full rationale

The paper's central claim is an experimental result: raw count statistics from fixed circuits are converted via Born-rule probabilities (Eq. 15) into the CHSH combination (Eq. 4), and the measured Bell values in Tables IV and V are compared with the classical bound 2. No parameter is fitted to the data and no 'prediction' is generated from a fitted input, so the derivation is self-contained. The objectivity criterion is an operational definition (unanimity among three friends, Eq. 11), not a conclusion imported from prior work; the identity of the multi-observer correlation with the standard CHSH expression follows from the explicit state (Eq. 10) and the stated projection rule. Self-citations to the authors' earlier objectivity framework (refs. [26,27,59,60,61]) and to a prior no-signaling study (ref. [43]) provide background and motivation, but the pass/fail verdict does not depend on the correctness or uniqueness of those works. The acknowledged residual signaling is a validity limitation — the paper states 'we are unable to point out a direct cause' — and the skeptic's concern that signaling might contaminate the CHSH functional is a loophole/correctness issue, not a circular reduction, because the signaling magnitudes are also direct measurements. The derivation chain from circuit to Bell value is therefore not equivalent to its inputs.

Assumptions & free parameters 0 free parameters · 4 assumptions · 0 invented entities

The protocol introduces no fitted parameters: measurement angles, qubit groups, and job sizes are fixed before data taking. The main assumptions are standard quantum measurement theory, device readout fidelity, the authors' unanimity definition of objectivity, and independent-shot statistics. No new physical entities are postulated.

assumptions (4)
  • standard math The Born rule for projective measurements and the CHSH inequality are used to derive the ideal probabilities (Eq. 15) and the classical bound 2.
    Standard results invoked in Section II without proof; they underpin the expected correlations and the bound whose violation is the central claim.
  • domain assumption Computational-basis readout of the cloud quantum processors realizes the projective measurements in Eq. (15) without state-dependent detector bias.
    All observed bitstrings are treated as projective measurement outcomes; if readout is biased, the unanimity statistics and Bell estimates would be distorted.
  • domain assumption Unanimity among the three friend qubits is a faithful operational implementation of the objectivity condition (mapping (11)).
    This definition is chosen by the authors in Section II; it is not derived, and different definitions of objectivity could change the test.
  • domain assumption Runs are independent Bernoulli trials, so the error formulas (20) and (21) apply.
    The sigma values for Bell and signaling violations assume statistical independence across shots and jobs; drift or autocorrelation would change the error bars.

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Cite this review

Pith. "Pith review of Closing objectivity loophole in Bell tests on a public quantum computer." pith.science (2026). https://pith.science/paper/2BAMKIYE

@misc{pith2026250608940,
  author       = {Pith},
  title        = {Pith review of: Closing objectivity loophole in Bell tests on a public quantum computer},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/2BAMKIYE}},
  note         = {Machine review of arXiv:2506.08940}
}
read the original abstract

We have constructed and run a Bell test of local realism focusing on the objectivity criterion. The objectivity means that the outcomes are confirmed macroscopically by a few observers at each party. The IBM Quantum and IonQ devices turn out to be sufficiently accurate to pass such an extended Bell-type test, although at the price of communication loopholes and residual but statistically significant signaling. The test also serves as the benchmark of entanglement spread across larger sets of qubits.

Figures

Figures reproduced from arXiv: 2506.08940 by the authors.

Figure 1
Figure 1. FIG. 1. Bell test results with objectivity condition i.e. all [PITH_FULL_IMAGE:figures/full_fig_p004_1.png] view at source ↗
Figure 2
Figure 2. FIG. 2. The circuit of the Bell test with friends. The [PITH_FULL_IMAGE:figures/full_fig_p005_2.png] view at source ↗
Figure 3
Figure 3. FIG. 3. The connections in the objectivity test. the middle [PITH_FULL_IMAGE:figures/full_fig_p005_3.png] view at source ↗
Figures from the paper (13 more)
Figure 13
Figure 13. Figure 13: FIG. 13. The notation of the ECR gate in the convention [PITH_FULL_IMAGE:figures/full_fig_p005_13.png]
Figure 4
Figure 4. Figure 4: FIG. 4. The placement of all the tested groups on the [PITH_FULL_IMAGE:figures/full_fig_p006_4.png]
Figure 5
Figure 5. Figure 5: FIG. 5. The placement of all the tested groups on the [PITH_FULL_IMAGE:figures/full_fig_p006_5.png]
Figure 6
Figure 6. Figure 6: FIG. 6. The placement of all the tested groups on the [PITH_FULL_IMAGE:figures/full_fig_p007_6.png]
Figure 7
Figure 7. Figure 7: FIG. 7. The circuit of the Bell test with friends on IonQ. The [PITH_FULL_IMAGE:figures/full_fig_p007_7.png]
Figure 8
Figure 8. Figure 8: FIG. 8. Signaling between qubits [PITH_FULL_IMAGE:figures/full_fig_p008_8.png]
Figure 9
Figure 9. Figure 9: FIG. 9. Signaling between qubits [PITH_FULL_IMAGE:figures/full_fig_p008_9.png]
Figure 10
Figure 10. Figure 10: FIG. 10. Signaling between qubits [PITH_FULL_IMAGE:figures/full_fig_p009_10.png]
Figure 11
Figure 11. Figure 11: FIG. 11. Signaling between qubits [PITH_FULL_IMAGE:figures/full_fig_p009_11.png]
Figure 12
Figure 12. Figure 12: FIG. 12. Signaling between qubits [PITH_FULL_IMAGE:figures/full_fig_p010_12.png]
Figure 14
Figure 14. Figure 14: FIG. 14. The [PITH_FULL_IMAGE:figures/full_fig_p011_14.png]
Figure 15
Figure 15. Figure 15: FIG. 15. The [PITH_FULL_IMAGE:figures/full_fig_p011_15.png]
Figure 16
Figure 16. Figure 16: FIG. 16. The [PITH_FULL_IMAGE:figures/full_fig_p011_16.png]

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Pith tools

Reviewed August 7, 2026 · model on record in the stance chip above.