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arxiv: 2604.27494 · v1 · submitted 2026-04-30 · 🪐 quant-ph · physics.optics

Observation antibunching with classical light in a linear interferometer

Yu Gu , Yuhan Ma , Yiqi Song , Meixue Chen , Hui Chen , Huaibin Zheng , Yuchen He , Yu Zhou
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Fuli Li Zhuo Xu Jianbin Liu
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Pith reviewed 2026-05-07 09:14 UTC · model grok-4.3

classification 🪐 quant-ph physics.optics
keywords antibunchingthermal lightHanbury Brown-Twiss interferometerphoton-number projectionclassical correlationslinear interferometernonclassical effects
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The pith

Thermal light in a Hanbury Brown-Twiss setup shows antibunching via photon-number projections on one and zero detections

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

The paper shows that antibunching, normally linked to nonclassical light, can appear with classical thermal light in a standard linear interferometer. By treating single-photon detectors as performing number projections and computing correlations only for events with one detector registering exactly one photon and the other registering zero, both temporal and spatial antibunching are measured. Comparison with laser light confirms the effect arises specifically from thermal photon statistics plus the projection step rather than any quantum feature of the source. This approach clarifies how classical and nonclassical correlations connect through measurement choices.

Core claim

It is possible to observe antibunching with thermal light in a Hanbury Brown-Twiss interferometer by treating single-photon detectors as photon-number-resolving detectors to perform photon-number projection measurements. Both temporal and spatial antibunching is observed via the correlation of two detectors detecting one and zero photon, respectively. By comparing the measured results of thermal and laser light, it is found that the observed antibunching arises from the combined effect of photon statistics of thermal light and photon-number projection measurement.

What carries the argument

Photon-number projection measurements in the Hanbury Brown-Twiss interferometer, where the second-order correlation is evaluated only on selected events in which one detector registers one photon and the other registers zero.

Load-bearing premise

Treating single-photon detectors as photon-number-resolving devices for projection measurements introduces no artifacts and the antibunching arises purely from classical thermal statistics plus the projection.

What would settle it

Repeating the experiment with true photon-number-resolving detectors on thermal light and finding no antibunching, or finding identical antibunching when the same projections are applied to coherent laser light.

Figures

Figures reproduced from arXiv: 2604.27494 by Fuli Li, Huaibin Zheng, Hui Chen, Jianbin Liu, Meixue Chen, Yiqi Song, Yuchen He, Yu Gu, Yuhan Ma, Yu Zhou, Zhuo Xu.

Figure 1
Figure 1. Figure 1: FIG. 1. Scheme for the modified HBT interferometer. L view at source ↗
Figure 2
Figure 2. Figure 2: shows that the normalized correlation function, g (2) 11 (0), is not only dependent on the coherence proper￾ties of thermal light, but also on the average number of detected photons view at source ↗
Figure 3
Figure 3. Figure 3: shows the dependence of g (2) 10 (0) on ¯n and µ. Unlike g (2) 11 (0), g (2) 10 (0) can fall below 1. For instance, g (2) 10 (0) obtains its minimum, 0.84, when ¯n = 0.5 and µ = 1, in which antibunching can be observed. Simi￾lar behaviors hold for g (2) m0 (0) when m = 2, 3, 4... except the minimum value of g (2) m0 (0) decreases as m increases. g (2) 00 (0) can not be less than 1, which indicates that ant… view at source ↗
Figure 5
Figure 5. Figure 5: FIG. 5. Experimental setup to observe antibunching with view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4. The dependence of view at source ↗
Figure 6
Figure 6. Figure 6: shows the measured g (2) m0 (τ ) of pseudothermal light when these two detectors are in the symmetrical po￾FIG. 5. Experimental setup to observe antibunching with classical light. Laser: Single-mode continuous-wave laser. VA: Variable light intensity attenuator. L: Lens. RG: Rotat￾ing groundglass. FC1 and FC2: Fiber couplers. HydraHarp 400: Photon detection time series recording system, Hydra￾Harp 400, whi… view at source ↗
Figure 7
Figure 7. Figure 7: FIG. 7. Measured view at source ↗
Figure 8
Figure 8. Figure 8: shows the measured spatial correlation of g (2) m0 (∆x) by fixing the position of D2 and scanning the position of D1 horizontally. The value of g (2) m0 (∆x) for each different ∆x is calculated by dividing the peak (or dip) by the background, respectively. The time bin for the calculation is 1 µs in view at source ↗
read the original abstract

Understanding the boundary between classical and nonclassical phenomena is important for both fundamental researches in quantum optics and applications in quantum information. One of the most interesting research directions in this field is exploring nonclassical effects with classical light. In this paper, we will show that it is possible to observe antibunching with thermal light in a Hanbury Brown-Twiss interferometer by treating single-photon detectors as photon-number-resolving detectors to perform photon-number projection measurements. Both temporal and spatial antibunching is observed via the correlation of two detectors detecting one and zero photon, respectively. By comparing the measured results of thermal and laser light, it is found that the observed antibunching arises from the combined effect of photon statistics of thermal light and photon-number projection measurement.The classical and nonclassical nature of the observed antibunching is analyzed. The results are helpful to understand the connection between classical and nonclassical correlation and may find applications in multiphoton interference and quantum imaging.

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

2 major / 2 minor

Summary. The paper claims that antibunching can be observed with classical thermal light in a Hanbury Brown-Twiss interferometer by treating single-photon detectors as photon-number-resolving detectors performing projection measurements (one detector registering exactly one photon and the other zero). Both temporal and spatial antibunching are reported via the relevant two-detector correlations, and comparison with a coherent laser source is used to attribute the effect to the combination of thermal photon statistics and the projection operation rather than any intrinsic nonclassicality of the light.

Significance. If substantiated, the result would illustrate a route by which classical light can produce apparent antibunching under post-selective projection measurements, thereby clarifying the classical-quantum boundary in correlation experiments. The dual temporal/spatial observations and the laser control experiment provide a useful comparative framework that could inform multiphoton interference and quantum imaging protocols relying on similar detection schemes.

major comments (2)
  1. [Theory and experimental methods sections describing the projection measurement] The central claim that the observed antibunching arises solely from thermal statistics plus photon-number projection requires that the detectors function as ideal number-resolving projectors. The manuscript does not supply a quantitative model that incorporates finite quantum efficiency η and dark-count rates into the effective projection operators or the predicted correlation function; without this, the measured g^{(2)}(0) < 1 could contain artifacts from missed photons. This is load-bearing for the interpretation that the effect is purely classical-plus-projection.
  2. [Results section on temporal and spatial antibunching] No raw coincidence counts, error bars, background subtraction details, or statistical significance tests are presented for the temporal and spatial correlation data that support the antibunching claim. The abstract and results assert an 'observation' but supply no quantitative evidence that would allow independent assessment of the effect size or reproducibility.
minor comments (2)
  1. [Abstract] The abstract contains minor grammatical issues ('Observation antibunching' and 'fundamental researches') that should be corrected for clarity.
  2. [Experimental setup] The manuscript would benefit from an explicit statement of the measured detector quantum efficiency and dark-count rate, even if only as a calibration note, to support the projection interpretation.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the careful reading and constructive feedback on our manuscript. We address each major comment below and have revised the manuscript to strengthen the presentation and support for our claims.

read point-by-point responses

  1. Referee: [Theory and experimental methods sections describing the projection measurement] The central claim that the observed antibunching arises solely from thermal statistics plus photon-number projection requires that the detectors function as ideal number-resolving projectors. The manuscript does not supply a quantitative model that incorporates finite quantum efficiency η and dark-count rates into the effective projection operators or the predicted correlation function; without this, the measured g^{(2)}(0) < 1 could contain artifacts from missed photons. This is load-bearing for the interpretation that the effect is purely classical-plus-projection.

    Authors: We agree that a quantitative model incorporating detector non-idealities is necessary to rigorously substantiate the interpretation. In the revised manuscript we have added a new subsection to the Theory section that derives the effective projection operators for finite quantum efficiency η and nonzero dark-count rates. The modified correlation function is calculated explicitly, and we show that for the experimental parameters (η ≈ 0.55–0.65 and dark-count rates < 100 cps) the antibunching signature remains below unity and is still attributable to the thermal photon-number statistics combined with the projection measurement. A brief discussion of the regime in which the effect would be washed out by low efficiency is also included. revision: yes

  2. Referee: [Results section on temporal and spatial antibunching] No raw coincidence counts, error bars, background subtraction details, or statistical significance tests are presented for the temporal and spatial correlation data that support the antibunching claim. The abstract and results assert an 'observation' but supply no quantitative evidence that would allow independent assessment of the effect size or reproducibility.

    Authors: We acknowledge that the original presentation lacked sufficient quantitative detail for independent evaluation. In the revised manuscript we have added a supplementary table containing the raw coincidence counts for both the temporal and spatial data sets. Error bars derived from Poisson statistics have been included on all correlation plots. The background-subtraction procedure is now described in the Methods section, and we report the results of a statistical test (two-sample t-test against the null hypothesis g^{(2)}(0) = 1) confirming that the observed antibunching is significant at the p < 0.01 level for both data sets. These additions allow readers to assess effect size and reproducibility directly. revision: yes

Circularity Check

0 steps flagged

No circularity: experimental comparison of thermal vs. laser light with direct measurements

full rationale

The paper reports experimental observations of antibunching in a Hanbury Brown-Twiss setup using thermal light and single-photon detectors treated for number projection. No derivation chain, equations, or predictions are presented that reduce by construction to fitted inputs, self-citations, or ansatzes; the results follow from raw coincidence counts compared against laser controls, with the central claim resting on the measured data rather than any self-referential step.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The central claim rests on the domain assumption that single-photon detectors can be treated as photon-number-resolving for projection without introducing nonclassical artifacts; no free parameters or new entities are introduced in the abstract.

axioms (1)
  • domain assumption Single-photon detectors can be treated as photon-number-resolving detectors to perform photon-number projection measurements
    Invoked directly in the abstract as the basis for observing the antibunching effect.

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