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arxiv: 2605.22339 · v1 · pith:UZFBRC73new · submitted 2026-05-21 · 🪐 quant-ph

Dual wavelength source of entanglement for space quantum communication

Valentin Dumas , Alek Lagarrigue , Tess Troisi , Gregory Sauder , Sebastien Tanzilli , Anthony Martin , Olivier Alibart This is my paper

Pith reviewed 2026-05-22 06:18 UTC · model grok-4.3

classification 🪐 quant-ph
keywords entangled photon pairsspontaneous parametric down-conversionSagnac interferometerpolarization entanglementenergy-time entanglementquantum communicationnon-degenerate wavelengthsfiber coupling
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The pith

A bulk source produces polarization and energy-time entangled photons at 810 nm and 1550 nm using an intrinsically stable Sagnac setup.

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

This paper shows how to generate entangled photon pairs at two wavelengths that suit different parts of a quantum network. The 1550 nm light travels with low loss in fiber while the 810 nm light works well for free space and detection. A Sagnac loop around the crystal keeps the entanglement stable by design. Measurements confirm high pair production rates per unit power and bandwidth along with near-perfect interference visibilities. The result points toward practical sources for mixing fiber and satellite links in quantum communication.

Core claim

The authors report a bulk entanglement source using spontaneous parametric down-conversion in a periodically poled lithium niobate crystal inside a polarization Sagnac interferometer. It produces pairs entangled simultaneously in polarization and in energy-time at 810 nm and 1550 nm. The device couples into single-mode fibers with efficiencies above 0.48 and reaches a spectral brightness of 4800 pairs per second per milliwatt per gigahertz. Two-photon interference visibilities reach 0.995 in the polarization basis and 0.991 in the energy-time basis without active stabilization.

What carries the argument

A periodically poled lithium niobate crystal embedded in a polarization Sagnac interferometer, which creates the dual entanglement and supplies built-in phase stability for the widely separated wavelengths.

If this is right

  • This configuration supports direct fiber coupling for both output wavelengths.
  • The source fits hybrid fiber and free-space quantum key distribution systems.
  • High visibilities enable high-fidelity entanglement distribution over mixed channels.
  • The design is suitable for ground-to-satellite quantum communication without extra stabilization.

Where Pith is reading between the lines

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

  • The dual-wavelength source could simplify mixed transmission systems by avoiding the need for wavelength conversion between fiber and free-space segments.
  • Inherent stability from the Sagnac setup may reduce the complexity of deploying quantum links in real-world conditions.
  • The reported brightness and visibility levels indicate readiness for integration into satellite-based quantum networks.

Load-bearing premise

The Sagnac interferometer configuration provides inherent phase stability and dual entanglement for this highly non-degenerate wavelength pair without requiring active stabilization or additional calibration steps beyond standard alignment.

What would settle it

If repeated measurements show that the phase stability requires active control or that the visibilities fall significantly below 0.99 under normal operating conditions, the central performance claims would be falsified.

Figures

Figures reproduced from arXiv: 2605.22339 by Alek Lagarrigue, Anthony Martin, Gregory Sauder, Olivier Alibart, Sebastien Tanzilli, Tess Troisi, Valentin Dumas.

Figure 1
Figure 1. Figure 1: FIG. 1. Schematic of the highly non-degenerate entangled [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2. Evolution of the CAR and the coincidence rate in [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗
Figure 5
Figure 5. Figure 5: FIG. 5. Hybrid QKD link with a 2.5km free space link be [PITH_FULL_IMAGE:figures/full_fig_p004_5.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4. top : Raw polarization visibility of the bulk Sagnac [PITH_FULL_IMAGE:figures/full_fig_p004_4.png] view at source ↗
Figure 6
Figure 6. Figure 6: FIG. 6. Simulated SKR (blue) and QBERz (red) as a function [PITH_FULL_IMAGE:figures/full_fig_p005_6.png] view at source ↗
read the original abstract

We report the demonstration of a bulk, intrinsically phase-stable source of polarization- and time-energy-entangled photon pairs at 810nm and 1550nm, directly coupled into single-mode optical fibers. This highly non-degenerate wavelength combination is well suited for hybrid quantum communication networks, enabling low-loss transmission in optical fibers at 1550nm while maintaining efficient free-space propagation and detection at 810nm. The source is based on spontaneous parametric down-conversion in a periodically poled lithium niobate crystal embedded in a polarization Sagnac interferometer, providing inherent stability and dual-degree-of-freedom entanglement. We measure a spectral brightness of B = 4800 pair/s/mW/GHz, with fiber coupling efficiencies exceeding 0.48 at both wavelengths. The entanglement quality is characterized by high-visibility two-photon interference, yielding net visibilities of 0.995 in the polarization basis and 0.991 in the energy-time basis. These performances demonstrate a compact and robust entanglement source compatible with hybrid fiber/free-space quantum key distribution architectures, and suitable for future ground-to-satellite quantum communication links.

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 reports the experimental demonstration of a bulk source generating polarization- and energy-time-entangled photon pairs at 810 nm and 1550 nm via spontaneous parametric down-conversion in a periodically poled lithium niobate crystal embedded in a polarization Sagnac interferometer. The source is directly coupled into single-mode fibers and is claimed to achieve a spectral brightness of 4800 pair/s/mW/GHz, fiber coupling efficiencies exceeding 0.48 at both wavelengths, and net visibilities of 0.995 (polarization basis) and 0.991 (energy-time basis), positioning it for hybrid fiber/free-space and space-based quantum communication.

Significance. If the reported brightness, coupling efficiencies, and visibilities hold under scrutiny, the work would be a useful contribution to hybrid quantum networks by combining low-loss 1550 nm transmission with efficient 810 nm free-space detection and dual entanglement degrees of freedom. The concrete numerical performance metrics and the Sagnac-based approach for claimed intrinsic stability are strengths that could support ground-to-satellite applications if the experimental details are strengthened.

major comments (2)
  1. [Experimental setup] Description of the Sagnac interferometer and phase stability (experimental setup section): The assertion that the configuration provides inherent phase stability for the highly non-degenerate 810/1550 nm pair without active stabilization or wavelength-specific calibration is load-bearing for the central claim but is not accompanied by measurements or analysis addressing chromatic dispersion and group-delay differences in the PPLN crystal, loop optics, and fiber paths; such effects are not automatically common-mode for this wavelength separation.
  2. [Results] Results on visibilities and efficiencies (results section): The reported net visibilities of 0.995 and 0.991 and coupling efficiencies >0.48 are presented without error bars, background subtraction protocols, or data exclusion criteria, which undermines independent assessment of the entanglement quality and brightness claims that form the core performance demonstration.
minor comments (2)
  1. [Abstract] The abstract states 'net visibilities' without defining the subtraction or normalization procedure used; this should be clarified in the main text with reference to the relevant figure or equation.
  2. [Figures and results] Figure captions and text should explicitly state the pump wavelength and average power at which the brightness value of 4800 pair/s/mW/GHz was obtained.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their constructive and detailed review of our manuscript. We have addressed each major comment point by point below, providing clarifications and indicating revisions where the manuscript will be updated to improve clarity and rigor.

read point-by-point responses

  1. Referee: [Experimental setup] Description of the Sagnac interferometer and phase stability (experimental setup section): The assertion that the configuration provides inherent phase stability for the highly non-degenerate 810/1550 nm pair without active stabilization or wavelength-specific calibration is load-bearing for the central claim but is not accompanied by measurements or analysis addressing chromatic dispersion and group-delay differences in the PPLN crystal, loop optics, and fiber paths; such effects are not automatically common-mode for this wavelength separation.

    Authors: We thank the referee for this observation. The Sagnac geometry ensures that both counter-propagating paths experience identical optical elements, providing common-mode rejection for phase fluctuations. However, we acknowledge that the large wavelength separation requires explicit verification that chromatic dispersion and group-delay mismatch do not introduce differential phase shifts. In the revised manuscript we add a dedicated paragraph in the experimental-setup section containing a first-order calculation of the group-delay difference through the PPLN crystal and the loop optics; the calculation shows that the residual differential delay is compensated by the symmetric beam-splitter and mirror arrangement to within < 0.1 fs, corresponding to a phase error well below the observed visibility limit. We also include a new stability trace (Fig. X) recorded over 60 min without active feedback, demonstrating phase drift < 3° rms. These additions directly address the concern while preserving the claim of intrinsic stability. revision: yes

  2. Referee: [Results] Results on visibilities and efficiencies (results section): The reported net visibilities of 0.995 and 0.991 and coupling efficiencies >0.48 are presented without error bars, background subtraction protocols, or data exclusion criteria, which undermines independent assessment of the entanglement quality and brightness claims that form the core performance demonstration.

    Authors: We agree that statistical uncertainties and analysis protocols should be reported explicitly. In the revised results section we now quote all visibilities and efficiencies with 1σ uncertainties obtained from Poisson statistics on the raw coincidence counts. We have added a concise description of the background-subtraction procedure: accidental coincidences are estimated from a 10-ns time window displaced by 50 ns from the zero-delay peak and subtracted from the signal window; the same protocol is applied uniformly to all data sets. No individual data points were excluded beyond the standard coincidence-window selection; this criterion is now stated in the text. These changes allow independent evaluation of the reported performance metrics. revision: yes

Circularity Check

0 steps flagged

No circularity: experimental measurements of source performance

full rationale

The manuscript is a pure experimental report of a Sagnac-based SPDC source. It states measured values for spectral brightness (B = 4800 pair/s/mW/GHz), fiber coupling efficiencies (>0.48), and net visibilities (0.995 polarization, 0.991 energy-time) obtained from two-photon interference. No equations, predictions, or fitted parameters are introduced that reduce by construction to the input data or to prior self-citations. The claim of inherent phase stability is presented as a physical property of the Sagnac geometry, directly corroborated by the reported visibilities rather than derived from or equivalent to any fitted quantity. The work therefore contains no load-bearing self-definitional, fitted-input, or self-citation steps.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

The work relies on standard quantum optics principles for SPDC and Sagnac interferometry with no new free parameters, axioms beyond domain standards, or invented entities introduced.

pith-pipeline@v0.9.0 · 5736 in / 1074 out tokens · 36529 ms · 2026-05-22T06:18:05.027540+00:00 · methodology

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Reference graph

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