Phase coding semi-quantum key distribution system based on the Single-state protocol

Jin-Dong Wang; Nai-Da Mo; Qin-Cheng Hou; Si-Ying Huang; Tian-Ming Zhao; Ya-Fei Yu; Zheng-JunWei; Zhi-Ming Zhang

arxiv: 2405.07469 · v3 · pith:FNTUJFWPnew · submitted 2024-05-13 · 🪐 quant-ph · physics.optics

Phase coding semi-quantum key distribution system based on the Single-state protocol

Si-Ying Huang , Qin-Cheng Hou , Tian-Ming Zhao , Jin-Dong Wang , Nai-Da Mo , Zheng-JunWei , Ya-Fei Yu , Zhi-Ming Zhang This is my paper

Pith reviewed 2026-05-24 01:02 UTC · model grok-4.3

classification 🪐 quant-ph physics.optics

keywords semi-quantum key distributionsingle-state protocolphase encodingtagged attackselective modulationinterference contrastquantum bit error rate

0 comments

The pith

Selective modulation implements a phase-encoded semi-quantum key distribution system that resists tagged attacks.

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

The paper sets out to establish that a selective modulation scheme can realize the single-state semi-quantum key distribution protocol in a phase-encoded optical system. This addresses both the experimental difficulty of measurement and resend operations by a classical user and the theoretical risk of tagged attacks. A sympathetic reader would care because the approach lets one party operate with only two optical devices rather than full quantum hardware, potentially widening the reach of quantum key distribution in networks. The work supplies both a quantum-state-evolution proof of security against tagging and experimental data showing 97.45 percent interference contrast, 1.20 percent average error rate, and 88 kilobits per second raw key rate at 100 megahertz with mean photon number 0.1.

Core claim

The selective modulation method allows the classical user to perform the operations required by the single-state protocol while the overall system remains secure against tagged attacks, as shown by analysis of the quantum state evolution; the same scheme is realized in a phase-encoded setup that experimentally delivers 97.45 percent interference contrast, 1.20 percent average quantum bit error rate, and 88 Kbps raw key rate at 100 MHz with mean photon number 0.1, using only two optical devices on the classical side.

What carries the argument

The selective modulation scheme, which lets the classical user apply the single-state protocol operations with two optical devices while the quantum-state evolution analysis rules out tagged attacks.

If this is right

The classical user needs only two optical devices and no full quantum measurement capability.
The system operates at 100 MHz with mean photon number 0.1 while maintaining 97.45 percent interference contrast.
Tagged attacks are ruled out by the quantum state evolution analysis of the selective modulation scheme.
The approach supplies a concrete experimental platform for studying semi-quantum key distribution security.

Where Pith is reading between the lines

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

The reduced hardware requirement on the classical side could allow integration of semi-quantum links into existing classical fiber networks with minimal upgrades.
The same modulation principle might be tested in other phase or polarization encodings to check whether the tagged-attack resistance generalizes.
Longer fiber distances or higher repetition rates would provide a direct test of whether the reported contrast and error rates remain stable outside the laboratory conditions shown.

Load-bearing premise

The selective modulation method exactly reproduces the single-state protocol steps without introducing side channels or deviations that the state-evolution analysis fails to capture.

What would settle it

Detection of a successful tagged attack on the experimental setup or a measurable deviation between the predicted and observed quantum states under selective modulation would falsify the security claim.

Figures

Figures reproduced from arXiv: 2405.07469 by Jin-Dong Wang, Nai-Da Mo, Qin-Cheng Hou, Si-Ying Huang, Tian-Ming Zhao, Ya-Fei Yu, Zheng-JunWei, Zhi-Ming Zhang.

**Figure 1.** Figure 1: Phase encoding of the states. difference between two pulses being 0 or π. Conversely, we define the X basis as the phase difference between two pulses being π 2 or 3π 2 . Quantum states under each basis are illustrated in [PITH_FULL_IMAGE:figures/full_fig_p003_1.png] view at source ↗

**Figure 2.** Figure 2: The scheme of phase encoding semi-quantum [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗

**Figure 3.** Figure 3: The relationship between the response of the [PITH_FULL_IMAGE:figures/full_fig_p004_3.png] view at source ↗

**Figure 5.** Figure 5: The change of interference contrast within 30 [PITH_FULL_IMAGE:figures/full_fig_p005_5.png] view at source ↗

**Figure 7.** Figure 7: Quantum states on the Bloch sphere. |+⟩B can be represented as: |+⟩B = 1 + i 2 |0⟩B + 1 − i 2 |1⟩B . (1) It means the |+⟩B , |0⟩B and |1⟩B can be defined as poles on the equator of Bloch sphere (as shown in [PITH_FULL_IMAGE:figures/full_fig_p006_7.png] view at source ↗

read the original abstract

Semi-quantum key distribution (SQKD) allows sharing random keys between a quantum user and a classical user, which significantly saves user resources, especially when using the Single-state protocol. However, the operation of the classical user, which involves measurement and resending using the Single-state protocol, presents technical difficulties in experiment and there is a security vulnerability of "tagged" attack in theory. To solve these problems, in our work, based on the Single-state protocol, we propose the "selective modulation" method and successfully implement a phase-encoded semi-quantum key distribution system. The system operates at a frequency of 100MHz and an average photon number of 0.1. The interference contrast achieved 97.45%, the average quantum bit error rate was 1.20%, and the raw key rate reached 88Kbps. Our experimental results demonstrate the feasibility and stability of the proposed phase-encoded SQKD system. Furthermore, we conducted an analysis of the "selective modulation" scheme in terms of quantum state evolution to assess the security of our system and ultimately proved that it can resist "tagged" attack. The classical user of our system requires only two optical devices and operates without relying on full quantum capabilities, thereby enhancing its application potential in quantum networks. This work validates the feasibility of SQKD experiments and provides ideas for future research on SQKD experiments and security studies.

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.

Desk Editor's Note private letter to a colleague

This paper shows a phase-encoded single-state SQKD experiment that cuts the classical side to two devices via selective modulation, with decent 100 MHz performance numbers and a state-evolution argument against tagged attacks.

read the letter

The main point is a working experimental demonstration of phase-coded SQKD based on the single-state protocol. The authors introduce selective modulation so the classical user only needs two optical devices instead of full measurement-and-resend hardware. They run the system at 100 MHz with mean photon number 0.1 and report 97.45% interference contrast, 1.20% average QBER, and 88 kbps raw key rate. That is a concrete data point for practical SQKD setups. The security section uses quantum state evolution to argue that the scheme resists tagged attacks, which directly addresses a known theoretical concern for this protocol. The experimental numbers look reasonable for an incremental hardware paper and show the system can be made stable enough to produce keys. The hardware reduction itself is the clearest practical step forward. The security argument is tied to the specific modulation method rather than left at the abstract protocol level. The main soft spot is whether the state-evolution analysis fully accounts for the measured visibility, the Poisson photon statistics at μ=0.1, and any side channels that selective modulation might introduce in real hardware. The abstract gives no error bars or raw data, so independent checks on those points would need the full manuscript. This is useful for researchers building or analyzing semi-quantum systems that want to minimize quantum resources on one side. It is not a broad theoretical result, but the combination of experiment and targeted security check is solid enough to merit referee time. I would send it for peer review with attention to the completeness of the tagged-attack bound under realistic conditions.

Referee Report

2 major / 2 minor

Summary. The manuscript proposes a selective modulation scheme to implement a phase-encoded semi-quantum key distribution (SQKD) system based on the single-state protocol. The classical user is realized with only two optical devices. An experiment at 100 MHz repetition rate and mean photon number 0.1 reports 97.45% interference contrast, 1.20% average QBER and 88 kbps raw key rate. A quantum-state-evolution argument is presented to show that the scheme resists tagged attacks.

Significance. If the security argument survives incorporation of the measured visibility and photon-number statistics, the work supplies a concrete hardware simplification for the classical party in SQKD together with the first reported phase-encoded implementation at these performance levels. The two-device classical-user architecture and the explicit experimental metrics constitute the main advance.

major comments (2)

[Security analysis] Security analysis (quantum state evolution section): the proof that selective modulation resists tagged attacks is performed under ideal operations; it must be shown whether the calculation incorporates the experimental visibility of 97.45% and the Poisson photon-number distribution at μ=0.1, because any deviation can reopen a tagging loophole.
[Experimental results] Experimental results: the reported 1.20% QBER and 88 kbps rate are given without error bars, number of samples, or description of how the QBER is extracted from the interference fringes; these omissions prevent assessment of statistical significance of the feasibility claim.

minor comments (2)

[Abstract] Abstract: missing spaces (“theSingle-state”, “quantumbit”) and minor grammatical issues should be corrected for readability.
[Methods] Notation: the mean photon number is stated as 0.1 but the precise definition (e.g., whether it is the mean at the input or after loss) is not repeated in the methods; a single consistent symbol and definition would help.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the constructive comments. We address each major comment below and indicate the revisions planned for the manuscript.

read point-by-point responses

Referee: [Security analysis] Security analysis (quantum state evolution section): the proof that selective modulation resists tagged attacks is performed under ideal operations; it must be shown whether the calculation incorporates the experimental visibility of 97.45% and the Poisson photon-number distribution at μ=0.1, because any deviation can reopen a tagging loophole.

Authors: The quantum-state-evolution argument demonstrates that selective modulation prevents an eavesdropper from obtaining usable tagging information, because any attempt to tag introduces a disturbance detectable through the protocol's measurement outcomes. This structural property holds independently of specific device parameters. To directly address the concern, the revised manuscript will incorporate the measured visibility of 97.45% and the Poisson statistics at μ=0.1 into an updated security bound, confirming that the tagging loophole remains closed under the experimental conditions. revision: yes
Referee: [Experimental results] Experimental results: the reported 1.20% QBER and 88 kbps rate are given without error bars, number of samples, or description of how the QBER is extracted from the interference fringes; these omissions prevent assessment of statistical significance of the feasibility claim.

Authors: We agree that the experimental section requires additional statistical detail for full assessment. The revised manuscript will report error bars on both the QBER and raw key rate, state the total number of samples (or integration time) used for each measurement, and describe the QBER extraction procedure, including how the interference fringes are processed to obtain the quoted 1.20% value. revision: yes

Circularity Check

0 steps flagged

No circularity: results rest on experiment and independent state-evolution analysis

full rationale

The paper's claims derive from direct experimental measurements (97.45% contrast, 1.20% QBER, 88 Kbps rate at 100 MHz, μ=0.1) and a separate quantum state evolution analysis that proves tagged-attack resistance for the selective-modulation scheme. No equations reduce a prediction to a fitted input by construction, no self-citation chains bear the central security or performance claims, and the single-state protocol operations are implemented and analyzed without renaming or smuggling ansatzes. The derivation chain is self-contained and externally falsifiable via the reported lab data.

Axiom & Free-Parameter Ledger

1 free parameters · 1 axioms · 0 invented entities

The work relies on standard quantum-optics assumptions for coherent-state interference and state evolution; the mean photon number 0.1 is an operating choice rather than a fitted parameter derived from the target result.

free parameters (1)

mean photon number = 0.1
Chosen operating intensity (0.1) for weak-coherent-pulse implementation; standard value in QKD experiments, not fitted to the reported key rate or contrast.

axioms (1)

standard math Standard quantum mechanics governs phase encoding, interference visibility, and state evolution under selective modulation.
Invoked implicitly when analyzing tagged-attack resistance via state evolution.

pith-pipeline@v0.9.0 · 5802 in / 1150 out tokens · 25560 ms · 2026-05-24T01:02:30.297025+00:00 · methodology

discussion (0)

Lean theorems connected to this paper

Citations machine-checked in the Pith Canon. Every link opens the source theorem in the public Lean library.

IndisputableMonolith/Cost/FunctionalEquation.lean washburn_uniqueness_aczel unclear

?

unclear
Relation between the paper passage and the cited Recognition theorem.

the selective modulation process ... can be fully represented by a unitary operator corresponding to rotation around the y-axis ... ˆRy(δ)

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