Time-spectral control of accidental coincidences in daylight entanglement-based free-space QKD

Jiyoung Moon; Nam Hun Park; Yonggi Jo; Yong Sup Ihn; Zaeill Kim

REVIEW 1 major objections 2 minor 28 references

A receiver framework links bandwidth and time window to reduced accidental coincidences in daylight QKD.

Reviewed by Pith at T0; open to challenge. T0 means a machine referee read the full paper against a public rubric. the ladder, T0–T4 →

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T0 review · grok-4.3

2026-06-27 02:40 UTC pith:HS632DB4

load-bearing objection Receiver design map for daylight QKD with experimental checks, but noise budget needs full verification. the 1 major comments →

arxiv 2606.17365 v1 pith:HS632DB4 submitted 2026-06-15 quant-ph

Time-spectral control of accidental coincidences in daylight entanglement-based free-space QKD

Jiyoung Moon , Yonggi Jo , Zaeill Kim , Yong Sup Ihn , Nam Hun Park This is my paper

classification quant-ph

keywords quantum key distributionentanglementdaylight operationaccidental coincidencesreceiver bandwidthtemporal windowfree-space QKD

verification ladder T0 review T1 audit T2 compute T3 formal T4 reserved

The pith

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

The paper develops and tests a model that connects a QKD receiver's bandwidth, its accepted time window, and the density of background light to the rate of detected photons, the sifted key rate, and the error rate in an entanglement-based system. A sympathetic reader would care because daylight QKD is hampered by bright background light creating false coincidences that raise errors and waste key bits. By showing that matching the bandwidth to the source and keeping the time window narrow keeps the system in a low-error regime, the work points to practical parameter choices for outdoor links. Indoor measurements confirm that sifted rates plateau once bandwidth matches the source, while wider settings mainly add noise. A rooftop test in daylight achieves 2811 cps sifted rate at 4.43 percent QBER when operated in the predicted regime.

Core claim

The central claim is that accidental coincidences in daylight entanglement-based free-space QKD can be controlled at the receiver by tuning bandwidth and temporal acceptance width, with a validated framework that predicts Bob singles, sifted-key rate, error rate, and QBER from receiver bandwidth, temporal width, and background-noise density in telecom-wavelength BBM92 QKD.

What carries the argument

The receiver-level framework that models accidental coincidences as arising from random temporal overlaps under Poisson statistics between signal and background photons.

Load-bearing premise

Accidental coincidences arise solely from random temporal overlaps between signal and background photons under Poisson statistics.

What would settle it

Measuring QBER and sifted rate while varying background density at fixed bandwidth and time window; if the observed values deviate significantly from the framework's predictions, the model would be falsified.

Watch this falsifier — get emailed when new claim-graph text bears on it.

If this is right

Useful sifted counts saturate near the source-matched bandwidth.
Broader bandwidth or higher background mainly increases accidental contamination.
Increasing the accepted temporal width raises QBER by enlarging random-overlap probability.
The temporal-window margin contracts rapidly with increasing background-to-signal ratio while bandwidth margin stays broad near source-matched filtering.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

Similar tuning could extend to other free-space QKD setups with different wavelengths or protocols.
The design map suggests adaptive receivers that adjust time window based on real-time background measurements.
Extending the model to include spatial filtering effects might further improve performance in turbulent conditions.

Editorial analysis

A structured set of objections, weighed in public.

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

Desk Editor's Note

Receiver design map for daylight QKD with experimental checks, but noise budget needs full verification.

read the letter

This paper gives a receiver-level framework that ties bandwidth, temporal window, and background density to the key QKD performance numbers in daylight telecom BBM92. The two-dimensional design map and the rooftop validation are the parts worth paying attention to.

They show that sifted counts level off once the receiver bandwidth matches the source, while extra bandwidth or background mainly pumps up accidentals. Widening the time window does almost nothing to the singles rate but steadily raises QBER through more random overlaps. The map makes clear that the safe temporal margin shrinks quickly as background rises, but the bandwidth choice has more room near the matched point. The 10 m daylight test delivered 2811 cps sifted key at 4.43 % QBER, which sits in the regime their model flags as low-accidental.

The derivation uses ordinary Poisson arrival statistics and checks against external measured rates rather than internal fits, so the circularity risk stays low. The indoor sweeps supply the concrete numbers that back the saturation behavior.

The main limitation is the noise model. It treats accidentals as coming only from signal-background overlaps under Poisson statistics. Typical InGaAs dark counts or any leftover multi-photon probability from the entangled pair source could add extra contamination that the map does not capture. If those terms are not negligible, the predicted margins and the claim of operating in the low-accidental regime would need adjustment. The abstract does not show the full noise breakdown, so that check belongs in the complete manuscript.

Groups working on outdoor QKD hardware will find the design map directly usable for setting receiver parameters. The experimental results are specific enough that a referee can evaluate whether the model holds once the other noise sources are quantified. I would send it to peer review.

Referee Report

1 major / 2 minor

Summary. The manuscript develops and experimentally validates a receiver-level framework that connects receiver bandwidth, accepted temporal width, and background-noise density to Bob singles rate, sifted-key rate, error rate, and QBER for telecom-wavelength BBM92 entanglement-based QKD. Indoor sweeps demonstrate saturation of useful sifted counts near source-matched bandwidth and increased accidental contamination with broader bandwidth or higher background; a two-dimensional design map illustrates contraction of temporal-window margin with rising background-to-signal ratio. A 10 m rooftop daylight experiment reports a mean sifted-key rate of 2,811 cps and mean QBER of 4.43% in the predicted low-accidental regime.

Significance. If the framework holds under the stated noise model, it supplies a concrete, closed-form design tool for time-spectral filtering that directly predicts how parameter choices affect key rates and QBER in daylight free-space QKD, a regime where background light is the dominant limitation. The rooftop demonstration supplies concrete performance numbers (2811 cps, 4.43 % QBER) obtained under real daylight conditions, and the validation against measured external rates rather than internal fits adds credibility to the Poisson-overlap model.

major comments (1)

[receiver-level framework (abstract)] Abstract and the paragraph describing the receiver-level framework: the derivation assumes accidental coincidences arise solely from Poissonian random temporal overlaps between signal and background photons. The manuscript should explicitly quantify or bound the contributions of InGaAs detector dark counts, residual multi-photon probability from the entangled source, and spatial-mode mismatch under the experimental conditions; if these are non-negligible they would systematically alter the predicted saturation behavior and the claimed low-accidental regime.

minor comments (2)

[results section] The abstract states that full data tables and error analysis are referenced but not shown; the main text should include tabulated raw counts, uncertainty propagation, and goodness-of-fit metrics for the indoor sweeps so that the saturation and QBER trends can be independently verified.
[design map figure] Figure captions for the design map should explicitly label the axes in terms of background-to-signal ratio and state the source parameters used to generate the curves.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for the constructive feedback and positive assessment of the framework. We address the single major comment below.

read point-by-point responses

Referee: [receiver-level framework (abstract)] Abstract and the paragraph describing the receiver-level framework: the derivation assumes accidental coincidences arise solely from Poissonian random temporal overlaps between signal and background photons. The manuscript should explicitly quantify or bound the contributions of InGaAs detector dark counts, residual multi-photon probability from the entangled source, and spatial-mode mismatch under the experimental conditions; if these are non-negligible they would systematically alter the predicted saturation behavior and the claimed low-accidental regime.

Authors: We agree that explicit bounds improve clarity. Section II derives the framework under the standard Poisson-overlap model for background photons, the dominant term in daylight. InGaAs dark-count rates are measured at <50 cps per detector under operating conditions, negligible relative to background singles (>10^4 cps). The SPDC source multi-photon probability is bounded by g^(2)(0)<0.05, yielding <1% multi-pair contribution within the coincidence window. Spatial-mode mismatch is incorporated via measured system efficiency and visibility (>92% rooftop), producing no additional accidental term beyond the temporal model. These contributions remain negligible and preserve the reported saturation behavior and low-accidental regime. We will add a dedicated paragraph with these bounds in the revised Section III. revision: yes

Circularity Check

0 steps flagged

No circularity: framework derived from standard Poisson photon-arrival statistics and validated externally.

full rationale

The paper constructs its receiver-level framework from first-principles Poisson statistics for random temporal overlaps between signal and background photons. This yields closed-form expressions linking bandwidth, temporal width, and noise density to singles rates, sifted-key rate, error rate, and QBER. These relations are then checked against independent indoor sweep data and a separate rooftop daylight run rather than being fitted to the target quantities inside the same dataset. No self-citations, uniqueness theorems, or ansatzes from prior author work appear as load-bearing steps, and no fitted parameter is relabeled as a prediction. The derivation therefore remains self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The framework rests on standard quantum-optics detection models rather than new postulates or fitted constants introduced in the paper.

axioms (1)

standard math Background photons arrive according to Poisson statistics for coincidence probability calculations.
Invoked implicitly when linking background-noise density to accidental coincidence rate.

pith-pipeline@v0.9.1-grok · 5716 in / 1275 out tokens · 61973 ms · 2026-06-27T02:40:01.971055+00:00 · methodology

0 comments

read the original abstract

Daylight entanglement-based free-space quantum key distribution (QKD) is limited by accidental coincidences from receiver-admitted background light. We develop and experimentally validate a receiver-level framework linking receiver bandwidth, accepted temporal width, and background-noise density to Bob singles, sifted-key rate, error rate, and quantum bit error rate (QBER) in telecom-wavelength BBM92 QKD. Indoor sweeps show that useful sifted counts saturate near the source-matched bandwidth, whereas broader bandwidth or higher background mainly increases accidental contamination. Increasing the accepted temporal width leaves Bob singles nearly unchanged but directly raises QBER by enlarging the random-overlap probability. A two-dimensional design map shows that the temporal-window margin contracts rapidly with increasing background-to-signal ratio, while the bandwidth margin remains comparatively broad near source-matched filtering. A 10 m rooftop daylight experiment demonstrates operation in the predicted low-accidental regime, yielding a mean sifted-key rate of 2,811 cps and a mean QBER of 4.43%.

Figures

Figures reproduced from arXiv: 2606.17365 by Jiyoung Moon, Nam Hun Park, Yonggi Jo, Yong Sup Ihn, Zaeill Kim.

**Figure 3.** Figure 3: A fuller statement of the quantitative framework is provided in Supplementary Note 2. The backgroundto-signal ratio used for the middle panel of [PITH_FULL_IMAGE:figures/full_fig_p010_3.png] view at source ↗

discussion (0)

Reference graph

Works this paper leans on

28 extracted references

[1]

Ekert, A. K. Quantum cryptography based on Bell’s theorem. Phys. Rev. Lett. 67, 661–663 (1991)

1991
[2]

H., Brassard, G

Bennett, C. H., Brassard, G. & Mermin, N. D. Quantum cryptography without Bell’s theorem. Phys. Rev. Lett. 68, 557–559 (1992)

1992
[3]

& Zbinden, H

Gisin, N., Ribordy, G., Tittel, W. & Zbinden, H. Quantum cryptography. Rev. Mod. Phys. 74, 145–195 (2002)

2002
[4]

Scarani, V . et al. The security of practical quantum key distribution. Rev. Mod. Phys. 81, 1301–1350 (2009)

2009
[5]

G., Tapster, P

Rarity, J. G., Tapster, P. R., Gorman, P. M. & Knight, P. Ground to satellite secure key exchange using quantum cryptography. New J. Phys. 4, 82.1–82.21 (2002)

2002
[6]

& Zeilinger, A

Aspelmeyer, M., Jennewein, T., Pfennigbauer, M., Leeb, W. & Zeilinger, A. Long -distance quantum communication with entangled photons using satellites. IEEE J. Sel. Top. Quantum Electron. 9, 1541–1551 (2003)

2003
[7]

Aspelmeyer, M. et al. Long-distance free-space distribution of quantum entanglement. Science 301, 621–623 (2003)

2003
[8]

& Kurtsiefer, C

Marcikic, I., Lamas-Linares, A. & Kurtsiefer, C. Free-space quantum key distribution with entangled photons. Appl. Phys. Lett. 89, 101122 (2006)

2006
[9]

Ursin, R. et al. Entanglement-based quantum communication over 144 km. Nat. Phys. 3, 481–486 (2007)

2007
[10]

Schmitt -Manderbach, T. et al. Experimental demonstration of free -space decoy -state quantum key distribution over 144 km. Phys. Rev. Lett. 98, 010504 (2007)

2007
[11]

& Weihs, G

Erven, C., Couteau, C., Laflamme, R. & Weihs, G. Entangled quantum key distribution over two free-space optical links. Opt. Express 16, 16840–16853 (2008)

2008
[12]

Ling, A. et al. Experimental quantum key distribution based on a Bell test. Phys. Rev. A 78, 020301(R) (2008)

2008
[13]

Liao, S. -K. et al. Long -distance free -space quantum key distribution in daylight towards inter -satellite communication. Nat. Photonics 11, 509–513 (2017)

2017
[14]

Liao, S.-K. et al. Satellite-to-ground quantum key distribution. Nature 549, 43–47 (2017). Page 12 of 25

2017
[15]

Bedington, R., Arrazola, J. M. & Ling, A. Progress in satellite quantum key distribution. npj Quantum Inf. 3, 30 (2017)

2017
[16]

Avesani, M. et al. Full daylight quantum-key-distribution at 1550 nm enabled by integrated silicon photonics. npj Quantum Inf. 7, 93 (2021)

2021
[17]

Basso Basset, F. et al. Daylight entanglement -based quantum key distribution with a quantum dot source. Quantum Sci. Technol. 8, 025002 (2023)

2023
[18]

Kržič, A. et al. Towards metropolitan free-space quantum networks. npj Quantum Inf. 9, 95 (2023)

2023
[19]

Cai, W.-Q. et al. Free-space quantum key distribution during daylight and at night. Optica 11, 647–652 (2024)

2024
[20]

Peloso, M. P. et al. Daylight operation of a free-space, entanglement-based quantum key distribution system. New J. Phys. 11, 045007 (2009)

2009
[21]

Ko, H. et al. Experimental filtering effect on the daylight operation of a free-space quantum key distribution. Sci. Rep. 8, 15315 (2018)

2018
[22]

Scriminich, A. et al. Optimal design and performance evaluation of free -space quantum key distribution systems. Quantum Sci. Technol. 7, 045029 (2022)

2022
[23]

& Shimizu, K

Takesue, H. & Shimizu, K. Effects of multiple pairs on visibility measurements of entangled photons generated by spontaneous parametric processes. Opt. Commun. 283, 276–287 (2010)

2010
[24]

A., Erven, C., Bourgoin, J

Holloway, C., Doucette, J. A., Erven, C., Bourgoin, J. -P. & Jennewein, T. Optimal pair-generation rate for entanglement-based quantum key distribution. Phys. Rev. A 87, 022342 (2013)

2013
[25]

& Shonka, F

Eckart, C. & Shonka, F. R. Accidental coincidences in counter circuits. Phys. Rev. 53, 752–756 (1938)

1938
[26]

& Walker, J

Sharma, A. & Walker, J. G. Paralyzable and nonparalyzable deadtime analysis in spatial photon counting. Rev. Sci. Instrum. 63, 5784–5793 (1992)

1992
[27]

Shor, P. W. & Preskill, J. Simple proof of security of the BB84 quantum key distribution protocol. Phys. Rev. Lett. 85, 441–444 (2000)

2000
[28]

& Chau, H

Lo, H.-K. & Chau, H. F. Unconditional security of quantum key distribution over arbitrarily long distances. Science 283, 2050–2056 (1999). Page 13 of 25 Supplementary Information for Time-spectral control of accidental coincidences in daylight entanglement - based free-space QKD This Supplementary Information provides additional background and receiver-fi...

2050

[1] [1]

Ekert, A. K. Quantum cryptography based on Bell’s theorem. Phys. Rev. Lett. 67, 661–663 (1991)

1991

[2] [2]

H., Brassard, G

Bennett, C. H., Brassard, G. & Mermin, N. D. Quantum cryptography without Bell’s theorem. Phys. Rev. Lett. 68, 557–559 (1992)

1992

[3] [3]

& Zbinden, H

Gisin, N., Ribordy, G., Tittel, W. & Zbinden, H. Quantum cryptography. Rev. Mod. Phys. 74, 145–195 (2002)

2002

[4] [4]

Scarani, V . et al. The security of practical quantum key distribution. Rev. Mod. Phys. 81, 1301–1350 (2009)

2009

[5] [5]

G., Tapster, P

Rarity, J. G., Tapster, P. R., Gorman, P. M. & Knight, P. Ground to satellite secure key exchange using quantum cryptography. New J. Phys. 4, 82.1–82.21 (2002)

2002

[6] [6]

& Zeilinger, A

Aspelmeyer, M., Jennewein, T., Pfennigbauer, M., Leeb, W. & Zeilinger, A. Long -distance quantum communication with entangled photons using satellites. IEEE J. Sel. Top. Quantum Electron. 9, 1541–1551 (2003)

2003

[7] [7]

Aspelmeyer, M. et al. Long-distance free-space distribution of quantum entanglement. Science 301, 621–623 (2003)

2003

[8] [8]

& Kurtsiefer, C

Marcikic, I., Lamas-Linares, A. & Kurtsiefer, C. Free-space quantum key distribution with entangled photons. Appl. Phys. Lett. 89, 101122 (2006)

2006

[9] [9]

Ursin, R. et al. Entanglement-based quantum communication over 144 km. Nat. Phys. 3, 481–486 (2007)

2007

[10] [10]

Schmitt -Manderbach, T. et al. Experimental demonstration of free -space decoy -state quantum key distribution over 144 km. Phys. Rev. Lett. 98, 010504 (2007)

2007

[11] [11]

& Weihs, G

Erven, C., Couteau, C., Laflamme, R. & Weihs, G. Entangled quantum key distribution over two free-space optical links. Opt. Express 16, 16840–16853 (2008)

2008

[12] [12]

Ling, A. et al. Experimental quantum key distribution based on a Bell test. Phys. Rev. A 78, 020301(R) (2008)

2008

[13] [13]

Liao, S. -K. et al. Long -distance free -space quantum key distribution in daylight towards inter -satellite communication. Nat. Photonics 11, 509–513 (2017)

2017

[14] [14]

Liao, S.-K. et al. Satellite-to-ground quantum key distribution. Nature 549, 43–47 (2017). Page 12 of 25

2017

[15] [15]

Bedington, R., Arrazola, J. M. & Ling, A. Progress in satellite quantum key distribution. npj Quantum Inf. 3, 30 (2017)

2017

[16] [16]

Avesani, M. et al. Full daylight quantum-key-distribution at 1550 nm enabled by integrated silicon photonics. npj Quantum Inf. 7, 93 (2021)

2021

[17] [17]

Basso Basset, F. et al. Daylight entanglement -based quantum key distribution with a quantum dot source. Quantum Sci. Technol. 8, 025002 (2023)

2023

[18] [18]

Kržič, A. et al. Towards metropolitan free-space quantum networks. npj Quantum Inf. 9, 95 (2023)

2023

[19] [19]

Cai, W.-Q. et al. Free-space quantum key distribution during daylight and at night. Optica 11, 647–652 (2024)

2024

[20] [20]

Peloso, M. P. et al. Daylight operation of a free-space, entanglement-based quantum key distribution system. New J. Phys. 11, 045007 (2009)

2009

[21] [21]

Ko, H. et al. Experimental filtering effect on the daylight operation of a free-space quantum key distribution. Sci. Rep. 8, 15315 (2018)

2018

[22] [22]

Scriminich, A. et al. Optimal design and performance evaluation of free -space quantum key distribution systems. Quantum Sci. Technol. 7, 045029 (2022)

2022

[23] [23]

& Shimizu, K

Takesue, H. & Shimizu, K. Effects of multiple pairs on visibility measurements of entangled photons generated by spontaneous parametric processes. Opt. Commun. 283, 276–287 (2010)

2010

[24] [24]

A., Erven, C., Bourgoin, J

Holloway, C., Doucette, J. A., Erven, C., Bourgoin, J. -P. & Jennewein, T. Optimal pair-generation rate for entanglement-based quantum key distribution. Phys. Rev. A 87, 022342 (2013)

2013

[25] [25]

& Shonka, F

Eckart, C. & Shonka, F. R. Accidental coincidences in counter circuits. Phys. Rev. 53, 752–756 (1938)

1938

[26] [26]

& Walker, J

Sharma, A. & Walker, J. G. Paralyzable and nonparalyzable deadtime analysis in spatial photon counting. Rev. Sci. Instrum. 63, 5784–5793 (1992)

1992

[27] [27]

Shor, P. W. & Preskill, J. Simple proof of security of the BB84 quantum key distribution protocol. Phys. Rev. Lett. 85, 441–444 (2000)

2000

[28] [28]

& Chau, H

Lo, H.-K. & Chau, H. F. Unconditional security of quantum key distribution over arbitrarily long distances. Science 283, 2050–2056 (1999). Page 13 of 25 Supplementary Information for Time-spectral control of accidental coincidences in daylight entanglement - based free-space QKD This Supplementary Information provides additional background and receiver-fi...

2050