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arxiv: 2606.07206 · v1 · pith:C6ZCI45Fnew · submitted 2026-06-05 · 🪐 quant-ph · physics.optics

Experimental Demonstration of Free-Space Unidimensional Continuous-Variable Quantum Key Distribution Under High Detector Noise

Rachita Nandan , Jayanth Ramakrishnan , Shashi Prabhakar , R. P. Singh This is my paper

Pith reviewed 2026-06-27 22:03 UTC · model grok-4.3

classification 🪐 quant-ph physics.optics
keywords continuous-variable quantum key distributionunidimensional modulationfree-space quantum communicationtrusted detector noiseelectronic noisesecret key rate
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The pith

Free-space unidimensional CV-QKD generates positive secret key rates only when detector electronic noise is treated as trusted.

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

The paper demonstrates an experimental free-space unidimensional continuous-variable quantum key distribution system that operates with co-propagating signal and local oscillator in orthogonal polarizations. Under high detector electronic noise of 1.4 shot-noise units, the untrusted detector model produces no positive secret key rate at any modulation variance. The trusted detector model, however, permits secure key generation over a finite range of modulation variances, reaching a maximum rate of 270 kbps at variance 11.57. The work shows that high channel transmittance is required for security under these noise conditions and identifies detector trust as the decisive factor separating zero from positive rates.

Core claim

The experiment establishes that a free-space Gaussian-modulated UD-CVQKD link with polarized coherent states achieves a maximum secret key rate of 270 kbps at modulation variance 11.57 when electronic noise is modeled as trusted and independent of any eavesdropper; the same data yield zero rate under the untrusted detector model, and secure operation further requires high-transmittance channels.

What carries the argument

The trusted detector (TD) versus untrusted detector (UTD) noise model distinction, which treats electronic noise as eavesdropper-independent only under TD and thereby enables finite secret key rates.

If this is right

  • Secure key rates exist only for a bounded interval of modulation variances under the TD model.
  • High channel transmittance is required to maintain positive rates when detector noise reaches 1.4 shot-noise units.
  • The co-propagating orthogonal-polarization geometry stabilizes interference without active phase locking.
  • Maximum rate occurs at modulation variance 11.57 under the reported noise level.

Where Pith is reading between the lines

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

  • Systems with comparable detector noise will need either lower-loss channels or explicit trust assumptions to reach usable distances.
  • The zero-rate result under UTD underscores that practical CV-QKD deployments may require hardware-level isolation of detector noise from the quantum channel.
  • Extending the same setup to lower noise detectors would widen the usable modulation range and increase achievable distance.

Load-bearing premise

Electronic detector noise can be treated as independent of any eavesdropper.

What would settle it

An experimental demonstration that detector electronic noise is correlated with an eavesdropper's measurements would eliminate all positive key rates under the TD model.

Figures

Figures reproduced from arXiv: 2606.07206 by Jayanth Ramakrishnan, Rachita Nandan, R. P. Singh, Shashi Prabhakar.

Figure 1
Figure 1. Figure 1: FIG. 1. (a) Physical and secure regions in the ( [PITH_FULL_IMAGE:figures/full_fig_p006_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2. Behavior of the mutual information [PITH_FULL_IMAGE:figures/full_fig_p006_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3. Schematic of the experimental setup for Gaussian-modulated UD-CVQKD system over a free-space channel. HWP: [PITH_FULL_IMAGE:figures/full_fig_p007_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4. Calibration of the amplitude modulation and de [PITH_FULL_IMAGE:figures/full_fig_p008_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: FIG. 5. Quadrature probability distributions for Alice and [PITH_FULL_IMAGE:figures/full_fig_p008_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: The modulated X-quadrature exhibits a clear linear correlation, while the unmodulated P-quadrature remains narrowly distributed. A linear regression of the X-quadrature data yields a correlation coefficient of ap￾proximately 0.98, confirming stable signal transfer. FIG. 6. Correlation between Alice’s modulation voltage and Bob’s measured output. A clear linear relationship is observed for the modulated X-q… view at source ↗
Figure 7
Figure 7. Figure 7: FIG. 7. Bob’s measured [PITH_FULL_IMAGE:figures/full_fig_p009_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: FIG. 8. Dependence of the TD secret key rate [PITH_FULL_IMAGE:figures/full_fig_p010_8.png] view at source ↗
Figure 9
Figure 9. Figure 9: FIG. 9. Secret key rate (TD) as a function of channel loss [PITH_FULL_IMAGE:figures/full_fig_p010_9.png] view at source ↗
read the original abstract

Continuous-variable quantum key distribution (CV-QKD), which uses quadratures of the electromagnetic field, enables practical quantum communication using standard telecommunication technologies. Unidimensional CV-QKD (UD-CVQKD) simplifies the implementation by restricting modulation to a single quadrature. In this work, we experimentally demonstrate a free-space Gaussian-modulated UD-CVQKD system operating under a high detector electronic-noise regime (1.4 shot-noise units). The system employs polarized coherent states with signal and local oscillator co-propagating in the same spatial mode in orthogonal polarizations, ensuring stable interference. System security is analyzed under both untrusted (UTD) and trusted (TD) detector noise models. While no positive secret key rate is obtained under the UTD model, the TD model enables secure key generation over a finite range of modulation variances, highlighting the critical role of detector trust in high-noise conditions. A maximum secret key rate of 270 kbps is achieved at an optimal modulation variance of 11.57. Furthermore, secure operation requires high-transmittance (low-loss) channels under such noise conditions. This study demonstrates the practical feasibility of free-space UD-CVQKD in realistic high electronic-noise detection constraints and highlights detector electronic noise as a key limiting factor in practical systems.

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 manuscript experimentally demonstrates a free-space unidimensional CV-QKD system using Gaussian-modulated polarized coherent states with co-propagating signal and local oscillator in orthogonal polarizations. It operates under high detector electronic noise (1.4 SNU) and reports a maximum secret key rate of 270 kbps at modulation variance 11.57 under the trusted detector (TD) noise model, while the untrusted detector (UTD) model yields zero key rate for the same data. The work analyzes security under both models and concludes that secure operation requires high-transmittance channels, highlighting detector trust as critical in high-noise regimes.

Significance. If the TD model is valid, the result shows practical feasibility of UD-CVQKD in free-space links with realistic high electronic noise, achieving usable key rates when detector noise can be treated as trusted. The experimental choice of co-propagating orthogonal-polarization beams for stable interference is a clear strength. The data explicitly contrast TD and UTD outcomes, underscoring detector noise as a limiting factor. The significance is reduced by the absence of independent verification for the trust assumption and by missing uncertainty quantification on the reported rates.

major comments (2)
  1. [Security Analysis] Security Analysis section: The positive key rate (270 kbps) under the TD model rests entirely on the assumption that the 1.4 SNU electronic noise is independent of any eavesdropper. No experimental test (e.g., injected-probe correlation or side-channel characterization) is described to support this premise for an open free-space link in which the detector is physically accessible. This assumption is load-bearing because the same quadrature statistics produce zero key rate under the UTD model.
  2. [Results] Results and Abstract: The reported maximum secret key rate of 270 kbps at modulation variance 11.57 is given without error bars, statistical uncertainties, or raw quadrature data. No detailed steps of the security proof (e.g., explicit covariance matrix elements or finite-size corrections) are supplied, preventing independent assessment of the TD-model claim.
minor comments (2)
  1. [Abstract] The manuscript would benefit from explicit units and a brief definition when stating the optimal modulation variance of 11.57 in the abstract and results.
  2. Figure captions (e.g., key-rate versus modulation variance) should include both TD and UTD curves on the same plot for direct visual comparison.

Simulated Author's Rebuttal

2 responses · 1 unresolved

We thank the referee for the careful review and constructive comments on our manuscript. We address the major comments point by point below.

read point-by-point responses

  1. Referee: [Security Analysis] Security Analysis section: The positive key rate (270 kbps) under the TD model rests entirely on the assumption that the 1.4 SNU electronic noise is independent of any eavesdropper. No experimental test (e.g., injected-probe correlation or side-channel characterization) is described to support this premise for an open free-space link in which the detector is physically accessible. This assumption is load-bearing because the same quadrature statistics produce zero key rate under the UTD model.

    Authors: We acknowledge that the TD model relies on the standard assumption that electronic noise is internal to the detector and inaccessible to an eavesdropper. No additional experimental tests such as injected-probe correlation were performed in this work. We will revise the manuscript to include an expanded discussion of the conditions and limitations under which the TD model is applied in free-space channels. revision: partial

  2. Referee: [Results] Results and Abstract: The reported maximum secret key rate of 270 kbps at modulation variance 11.57 is given without error bars, statistical uncertainties, or raw quadrature data. No detailed steps of the security proof (e.g., explicit covariance matrix elements or finite-size corrections) are supplied, preventing independent assessment of the TD-model claim.

    Authors: We agree that the presentation would benefit from additional details. In the revised manuscript we will add statistical error bars to the reported key rates, provide the explicit covariance matrix elements used in the security analysis, and clarify the finite-size corrections. The full raw quadrature dataset will be made available upon reasonable request. revision: yes

standing simulated objections not resolved
  • Absence of independent experimental verification (e.g., injected-probe correlation or side-channel characterization) for the trusted detector noise assumption in the open free-space link.

Circularity Check

0 steps flagged

No circularity; experimental key rates from measured data via standard formulas

full rationale

The paper is an experimental demonstration that applies established CV-QKD security formulas (under TD/UTD models) to measured quadrature statistics. The positive key rate under TD is a direct consequence of the modeling assumption that electronic noise is trusted, not a derivation that reduces to fitted inputs or self-citations by construction. No load-bearing self-citation chains, self-definitional steps, or renamed known results are present. The result is self-contained against external benchmarks (standard CV-QKD literature) and receives the default low score for experimental work.

Axiom & Free-Parameter Ledger

1 free parameters · 1 axioms · 0 invented entities

Experimental paper; central claim rests on measured noise value and standard CV-QKD security formulas rather than new axioms or invented entities. No free parameters are introduced beyond the chosen modulation variance; no new particles or forces postulated.

free parameters (1)
  • modulation variance
    Optimal value 11.57 chosen to maximize key rate; reported as experimental operating point rather than derived constant.
axioms (1)
  • domain assumption Standard Gaussian-modulated coherent-state CV-QKD security bounds apply under the stated channel and detection model.
    Invoked when converting measured quadratures into secret key rate under TD and UTD models.

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