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Physical Review A

Spatial extra-bunching effect in classical light

Chen‐Xin Ding, Ruiqing He, Zhiyuan Ye, Xue‐Jiao Men, Wanting Hou, Yi Cui, Hongchao Liu, Hai-Bo Wang, Jun Xiong

Published Aug 15, 2025• 1 citation
Publisher

Abstract

Thermal light by nature exhibits the well-known photon bunching effect with the zero-delay second-order correlation coefficient ${g}^{(2)}$ ideally equal to 2. The Photon extra-bunching effect, as a special class of the superbunching effect (${g}^{(2)}>2$), describes the enhanced photon correlation (${g}^{(2)}=3$) through the interference of bright twin beams produced in the spontaneous parametric down-conversion. In this work, we report that this effect can exist in classical beams and that ${g}^{(2)}$ can approach 4 in some extended extreme models. We experimentally demonstrate the nontrivial extra-bunching correlation ($\ensuremath{\sim}1.4\ifmmode\times\else\texttimes\fi{}$ gain) in pairwise-generated holographic thermal light beams with a Dove prism-embedded balanced Mach-Zehnder interferometer. The interaction of such a pair of conjugated thermal light beams can generate a pair of uncorrelated superthermal light beams, enabling ghost imaging and Young's intensity interference with enhanced visibility. The quantum-inspired interference-induced spatial extra-bunching effect provides novel insights into inducing superbunching correlation in classical light and holds the potential for application in the coherent two-photon Lidar system to enhance sensitivity.

DOI
10.1103/2r44-md7c

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Thermal light by nature exhibits the well-known photon bunching effect with the zero-delay second-order correlation coefficient g(2){g}^{(2)}g(2) ideally equal to 2. The Photon extra-bunching effect, as a special class of the superbunching effect (g(2)>2{g}^{(2)}>2g(2)>2), describes the enhanced photon correlation (g(2)=3{g}^{(2)}=3g(2)=3) through the interference of bright twin beams produced in the spontaneous parametric down-conversion. In this work, we report that this effect can exist in classical beams and that g(2){g}^{(2)}g(2) can approach 4 in some extended extreme models. We experimentally demonstrate the nontrivial extra-bunching correlation (\ensuremath∼1.4\ifmmode×\else\texttimes\fi\ensuremath{\sim}1.4\ifmmode\times\else\texttimes\fi{}\ensuremath∼1.4\ifmmode×\else\texttimes\fi gain) in pairwise-generated holographic thermal light beams with a Dove prism-embedded balanced Mach-Zehnder interferometer. The interaction of such a pair of conjugated thermal light beams can generate a pair of uncorrelated superthermal light beams, enabling ghost imaging and Young's intensity interference with enhanced visibility. The quantum-inspired interference-induced spatial extra-bunching effect provides novel insights into inducing superbunching correlation in classical light and holds the potential for application in the coherent two-photon Lidar system to enhance sensitivity.
Superposition of multiple indistinguishable two-photon paths is introduced by means of a digital wave front encoding technique to design a spatially phase-correlated wave front of a light field, which could result in a superbunching effect with a bunching peak much larger than 2, the theoretical bunching peak of thermal light. Experimentally, we demonstrated a bunching peak of $11.72\ifmmode\pm\else\textpm\fi{}0.05$ with the digitally designed phase-correlated wave front of classical light, which can be improved further on. A significant improvement of the visibility of the second-order interference fringes and ghost imaging was also experimentally demonstrated. Our method provides a convenient digitally controllable way to implement superposition of multiple different but indistinguishable two-photon paths, and therefore to modify the optical coherence properties of photons. Such a superbunching effect may have potential applications in improving the visibility of correlation interference and imaging, and enhancing the efficiency of two-photon nonlinear effects such as two-photon absorption effect.
We proposed to boost the photon bunching effect of classical light via a modified Fabry-Pérot cavity with one of the cavity mirrors being replaced by a spatial light modulator, through which multiple independent transverse modulations, each was to encode independently spatially correlated phase on the wave front of the light field, was cascaded in sequence in the longitudinal propagation direction in the cavity. The two-photon bunching peak and the three-photon bunching peak of the generated superbunching light field were measured to be 36.9±2.036.9\pm2.036.9±2.0 and (2.46±0.12)×103\rm (2.46\pm0.12)\times10^{3}(2.46±0.12)×103, respectively, which, as compared to those of thermal light, were improved by one-order and three-order of magnitude, respectively. Correspondingly, the ghost image visibility reconstructed with the superbunching light was demonstrated experimentally to be improved by ∼40\sim 40∼40 times as compared to that with the pseudo-thermal light. Such classical superbunching light field is of essential importance not only for fundamental physics but also for potential applications such as correlated imaging, ghost interference and nonlinear optics.
Two-photon bunching and superbunching are important to understanding the physics of ghost imaging and second-order interference of light. With the help of the recently proposed superbunching pseudothermal light, the physics of bunching and superbunching in superbunching pseudothermal light is studied in detail. A special two-photon coincidence count detection system is designed to measure the required two-photon coincidence counts. It is concluded that the two-photon superbunching effect cannot be observed without taking the intensity changing process into account. A similar conclusion holds for two-photon bunching of thermal light. The conclusions are helpful for understanding of the physics of two-photon bunching and superbunching in intensity or photon number fluctuation correlation theory.
Spatial anti-bunching, in contrast to the well-known bunching behavior observed in classical light sources, describes a situation where photons tend to avoid each other in space, resulting in a reduced probability of detecting two or more photons in proximity. This anti-bunching effect, a hallmark of nonclassical light, signifies a deviation from classical intensity fluctuations and has been observed not only in free electrons and entangled photon pairs but also in chaotic-thermal light. This work investigates the generation mechanism of spatial anti-bunching correlation in random light fields, leveraging the wandering of light centers to induce a second-order coherence degree below unity. Unlike traditional Gaussian Schell-model partially coherent light, this work predicts the emergence of two fundamentally distinct orbital angular momentum states, arising from the inherent differences between rigid body rotation and fluid-like rotation in the wandering light. These predictions are supported by experimental evidence. Based on these findings, we propose a resolution-enhanced lensless ghost imaging system as an application example. Experimental results indicate that the adoption of the anti-bunching correlated light field as the illumination source enables the imaging system to attain super-resolution imaging capabilities. Our findings provide new insights into the utilization of anti-correlated light fields for precision imaging and detection applications.
Since Hanbury Brown and Twiss revealed the photon bunching effect of a thermal light source in 1956, almost all studies in correlation optics have been based on light’s intensity fluctuation, regardless of fact that the polarization fluctuation is a basic attribute of natural light. In this work, we uncover the veil of the polarization fluctuation and corresponding photon correlations by proposing a new light source model, termed pseudo-natural light, embodying both intensity and polarization fluctuations. Unexpectedly, the strong antibunching and superbunching effects can be simultaneously realized in such a new source, whose second-order correlation coefficient g ( 2 ) can be continuously modulated across 1. For the symmetric Bernoulli distribution of the polarization fluctuation, particularly, g ( 2 ) can be in principle from 0 to unlimitedly large. In pseudo-natural light, while the bunching effects of both intensity and polarization fluctuations enhance the bunching to superbunching photon correlation, the antibunching correlation of the polarization fluctuation can also be extracted through the procedure of division operation in the experiment. The antibunching effect and the combination with the bunching one will arouse new applications in quantum imaging. As heuristic examples, we carry out high-quality positive or negative ghost imaging, and devise high-efficiency polarization-sensitive and edge-enhanced imaging. This work, therefore, sheds light on the development of multiple and broad correlation functions for natural light.
Ghost imaging and interference using squeezed thermal light are investigated. We show that squeezed thermal light can have a very strong photon-bunching effect in the region of very weak squeezing and thermal excitation. As a result, the visibility of the image and interference pattern can be greatly enhanced and raised to be much higher than the limitation 1/31/31/3 to thermal light. In the squeezed vacuum case, especially, the visibility can approach to the level of entangled photon pairs from spontaneous parametric down-conversion. The present investigation may open a new potential application of squeezed light.