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arXiv:2603.01764 · Optics Express

Distinguishing thermal and pseudothermal light by testing the Siegert relation

Xi Jie Yeo, Justin Yu Xiang Peh, Darren Ming Zhi Koh, Christian Kurtsiefer, Peng Kian Tan

physics.opticsquant-phSubmitted Mar 2, 2026• Announced Mar 3, 2026• 0 citations
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Abstract

Thermal light, including blackbody radiation and spontaneous emission, exhibits photon bunching. Thermal light sources, however, typically yield low spectral densities, limiting their practical utility. Pseudothermal light sources with higher brightness and longer coherence time are often employed instead. While pseudothermal light also exhibits photon bunching, this property may not suffice to fully replicate the behavior of genuine thermal light. Here we demonstrate a method to directly test the Siegert relation for two sources of photon-bunched light, laser light scattered from a rotating ground glass and spontaneously emitted light from a gas discharge lamp, probing a fundamental criterion expected of thermal light.

Journal reference
Optics Express 34, pp. 16904-16910 (2026)
DOI
10.1364/OE.597030
Comments
4 pages, 4 figures

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Thermal light, including blackbody radiation and spontaneous emission, exhibits photon bunching. Thermal light sources, however, typically yield low spectral densities, limiting their practical utility. Pseudothermal light sources with higher brightness and longer coherence time are often employed instead. While pseudothermal light also exhibits photon bunching, this property may not suffice to fully replicate the behavior of genuine thermal light. Here we demonstrate a method to directly test the Siegert relation for two sources of photon-bunched light, laser light scattered from a rotating ground glass and spontaneously emitted light from a gas discharge lamp, probing a fundamental criterion expected of thermal light.
Pseudothermal light exhibits photon bunching, like thermal light, but it does not originate from randomly phased emission. As photon bunching is common in both pseudothermal and thermal light, the analysis of photon bunching is insufficient to differentiate pseudothermal light and thermal light. However, thermal light obeys the Siegert relation, which connects the interferometric visibility ∥g(1)(τ )∥ and second-order photon correlation g(2)(τ ). In this work, we present a direct test test for Siegert relation by a single-shot measurement. Using our technique, we demonstrate that laser light scattered off a rotating ground glass, violates the Siegert relation, which provides further evidence that it is a pseudothermal light source.
Thermal light sources exhibiting photon bunching have been suggested for sensing applications that exploit timing correlations of stationary light, including range finding, clock synchronization, and non-line-of-sight imaging. However, these proposals have remained unrealized in practice because available sources of photon bunching either possess coherence times too short to be timing resolved by photodetectors, or produce brightness levels too low to tolerate realistic return losses. In this work, we demonstrate a low-loss method for generating photon bunching with a conversion efficiency nearly 9 orders of magnitude higher than that achieved by many other bunching processes.
A simple superbunching pseudothermal light source is introduced based on common instruments such as laser, lenses, pinholes, and ground glasses. ${g}^{(2)}(0)=3.66\ifmmode\pm\else\textpm\fi{}0.02$ is observed in the suggested scheme by employing two rotating ground glasses. Quantum and classical theories are employed to interpret the observed superbunching effect. It is predicted that g(2)(0){g}^{(2)}(0)g(2)(0) can reach 2N{2}^{N}2N if NNN rotating ground glasses were employed. These results are helpful to understand the physics of superbunching. The proposed superbunching pseudothermal light may serve as a type of light to study the second- and higher-order coherence of light and have potential application in improving the visibility of thermal light ghost imaging.
Thermal light such as blackbody radiation including starlight has been known to exhibit photon bunching behaviour. This is the characteristic property of thermal photons to propagate closer together than as described by random Poissonian timing statistics. Although first theorised in the 1960s, attempts to directly probe the temporal coherence of starlight has remained challenging due to the very short timescales required. This work aims to address that by using narrowband spectral filtering to increase the timescale sufficiently for a direct measurement. We present calibration tests using laboratory light sources towards an observatory measurement.
Light from thermal blackbody radiators such as stars exhibits photon bunching behavior at sufficiently short timescales. However, with available detector bandwidths, this bunching signal is difficult to observe directly. We present an experimental technique to increase the photon bunching signal in blackbody radiation via spectral filtering of the light source. Our measurements reveal strong temporal photon bunching from blackbody radiation, including the Sun. This technique allows for an absolute measurement of the photon bunching signature g (2) (0), and thereby a direct statement on the statistical nature of a light source. Such filtering techniques may help revive the interest in intensity interferometry as a tool in astronomy.
Two-photon excited fluorescence (TPEF) has become a standard technique in modern microscopy but it is still affected by photo-damage of the probe owed to high excitation intensities. This work reports on utilizing the photon bunching effect in thermal light to enhance the efficiency of TPEF under continuous wave excitation. A super-luminescence diode (SLD) was used as thermal light source and compared it to coherent excitation from a DFB diode laser emitting at the same wavelength. The degree of second order coherence g(2)of both sources was measured. The SLD showed photon bunching with g(2)= 1.90±0.2 as expected for thermal light, while the DFB diode laser emitted coherent light exhibiting no photon bunching at g(2)= 1.