Echoes of Entanglement: Amplifying Quantum Correlations in Dynamic Media

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Author: Denis Avetisyan

New research reveals how carefully controlling light scattering can significantly boost the strength of quantum correlations between photons, even in complex and rapidly changing environments.

Entangled photon pairs are generated via spontaneous parametric down-conversion within a nonlinear crystal following illumination by a pump beam and subsequent diffusion, then directed toward a rotating diffuser and mirror system where coincidence events—detected by static and scanning single-photon detectors—reveal correlations dependent on the scattering angle $\theta_0$.
Entangled photon pairs are generated via spontaneous parametric down-conversion within a nonlinear crystal following illumination by a pump beam and subsequent diffusion, then directed toward a rotating diffuser and mirror system where coincidence events—detected by static and scanning single-photon detectors—reveal correlations dependent on the scattering angle $\theta_0$.

This study demonstrates enhanced two-photon correlations arising from the interplay of pump and photon pair scattering within dynamically disordered media, highlighting the sensitivity to scattering sequence and providing insights into coherent effects in diffusive systems.

While quantum correlations are typically degraded by disorder, understanding their behavior within complex media remains a significant challenge. This work, ‘Enhanced quantum correlations from joint pump and photon pair scattering’, investigates the propagation of entangled photon pairs generated via spontaneous parametric down-conversion through a dynamically scattering medium, crucially considering the scattering of both the pump and down-converted photons. We demonstrate surprisingly robust two-photon correlations, persisting even when pair generation occurs within the disordered environment, and reveal a sensitivity to the sequence of scattering events. These findings offer new insights into coherent effects in complex media – and suggest potential pathways for harnessing quantum light in challenging environments.

Entangled Light: Beyond Independent Photons

Traditional light scattering studies often simplify analysis by treating photons as independent entities, neglecting the subtle correlations that exist between them. This limits the characterization of complex media where multiple scattering introduces intricate relationships. An emerging approach utilizes entangled photon pairs to overcome these limitations. Entangled photons exhibit strong correlations, making them exquisitely sensitive to a medium’s structure. By analyzing changes in their entanglement, researchers can extract information inaccessible to conventional techniques.

Entangled photon pairs can be generated either after traversing a random medium following spontaneous down conversion, before entering the medium from a scattered pump field, or with both the pump and entangled pairs undergoing scattering within the medium, as demonstrated by simulations utilizing nonlinear crystals and random media.
Entangled photon pairs can be generated either after traversing a random medium following spontaneous down conversion, before entering the medium from a scattered pump field, or with both the pump and entangled pairs undergoing scattering within the medium, as demonstrated by simulations utilizing nonlinear crystals and random media.

These pairs can be generated through spontaneous down-conversion or by scattering both the pump and entangled photons within the medium. Each configuration offers advantages for probing specific structural characteristics. The sensitivity of entangled photons suggests that information, like light, may not always travel in straight lines, but through interwoven connections.

Generating Entanglement: The Nonlinear Path

Spontaneous Parametric Down-Conversion (SPDC) is a prominent technique for generating entangled photon pairs, fundamental to quantum optics. This process involves the nonlinear interaction of a high-intensity pump beam with a carefully selected nonlinear crystal. The crystal’s properties dictate the efficiency and characteristics of the generated photons.

Efficient SPDC relies on satisfying energy and momentum conservation. The pump photon splits into two lower-energy photons within the crystal. The crystal’s refractive index and phase-matching conditions maximize this down-conversion, increasing photon pair generation rates. Higher efficiency reduces data acquisition needs, enabling investigations into subtle quantum phenomena. Optimization of crystal properties, pump beam characteristics, and collection optics is paramount.

Revealing Structure: Entanglement as a Probe

The interaction of entangled photons with a scattering medium modifies their correlation properties, providing a pathway to characterize opaque materials. By analyzing two-photon correlation, researchers can map features within the medium, including the correlation depth – a parameter defining the extent of light scattering influence.

Two-dimensional coincidence mapping reveals a distinct peak in backscattering from a random medium, with the transverse momentum expressed as angular separation between detectors, while simultaneous single-count registration by the scanning detector demonstrates a homogeneous distribution across the scanned region, and a one-dimensional scan of the peak confirms these observations with enhanced resolution using smaller core fibers.
Two-dimensional coincidence mapping reveals a distinct peak in backscattering from a random medium, with the transverse momentum expressed as angular separation between detectors, while simultaneous single-count registration by the scanning detector demonstrates a homogeneous distribution across the scanned region, and a one-dimensional scan of the peak confirms these observations with enhanced resolution using smaller core fibers.

Modeling light propagation requires sophisticated techniques. A Fresnel kernel accurately simulates wave-like behavior. Further refinement is achieved with random phase screens, introducing phase distortions to account for scattering dynamics. These computational approaches enable a detailed understanding of light-matter interaction. For complex, three-dimensional environments, a volumetric scatterer model provides a more accurate representation of light propagation, extending two-dimensional models to all three spatial dimensions.

Quantifying Coherence: The Signature of Correlation

Two-Photon Coincidence Backscattering (CBS) reveals an enhancement in backscattering linked to the coherence of scattered photons. This isn’t simply increased signal, but a manifestation of correlated photon pairs, providing a unique signature for characterizing the medium. The strength of this effect is sensitive to angular relationships, necessitating a detailed understanding of angular correlation.

The Full Width at Half Maximum (FWHM) of the CBS peak serves as a sensitive measure of scattering strength. Analysis demonstrates a non-monotonic behavior, peaking at $|z/z_0| \approx 2.1$, where $z$ represents diffuser position and $z_0$ is a characteristic length scale. This peak indicates an optimal scattering configuration. The coincidence amplitude decreases to approximately 0.5 as $z$ increases, suggesting reduced scattering volume or loss of coherence at greater distances.

Numerical simulations and theoretical predictions, normalized by $\theta_0$, demonstrate a strong correlation between the full-width-at-half-maximum of enhancement terms $\Gamma_{ba}^{\pm}$ and the amplitude of $\Gamma_{ba}^{\pm}$ at $\theta = 0$, with the simulations closely aligning with predictions derived from equations (5), (7), and (III.2) after background subtraction.
Numerical simulations and theoretical predictions, normalized by $\theta_0$, demonstrate a strong correlation between the full-width-at-half-maximum of enhancement terms $\Gamma_{ba}^{\pm}$ and the amplitude of $\Gamma_{ba}^{\pm}$ at $\theta = 0$, with the simulations closely aligning with predictions derived from equations (5), (7), and (III.2) after background subtraction.

These observations highlight that backscattered light isn’t just a measure of scattering potential, but a delicate interplay of coherence and geometry. The medium doesn’t merely reflect light; it orchestrates a subtle dance of photons, revealing its character through their correlations.

The study meticulously navigates the intricacies of two-photon correlations within disordered systems, revealing a sensitivity to the order of scattering events. This echoes Louis de Broglie’s sentiment: “It is in the interplay between waves and particles that the true nature of reality is revealed.” The research demonstrates that the medium’s dynamic nature isn’t merely a source of noise, but actively shapes the correlations, offering a pathway to control and enhance these quantum effects. It reinforces the idea that observation isn’t passive; the act of measurement, or in this case, the sequence of scattering, fundamentally alters the observed phenomena, aligning with de Broglie’s holistic view of wave-particle duality and its implications for understanding the quantum world.

Where This Leads

The observed sensitivity to scattering sequence is not merely a technical detail. It suggests a deeper principle: complex media do not simply obscure correlations, they re-sculpt them. Abstractions age, principles don’t. Future work must move beyond characterizing the degree of enhancement, and focus on controlling the type of correlation generated. Every complexity needs an alibi.

Current methods rely on dynamic scattering, a useful but imprecise lever. Direct manipulation of the scattering potential—through spatially resolved control, or tailored metamaterials—offers a path toward deterministic control of two-photon states. This is not about ‘better’ statistics, but about designing environments that actively participate in quantum processes.

The long-term implications extend beyond fundamental studies. Harnessing coherent effects within disordered systems could lead to novel imaging techniques, or even robust quantum communication channels. Simplicity remains the ultimate goal. The challenge lies not in adding layers of complexity, but in distilling the essential physics.

Original article: https://arxiv.org/pdf/2511.08105.pdf

Contact the author: https://www.linkedin.com/in/avetisyan/

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2025-11-13 02:15