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Correlations in Intensity Fluctuations From a Quasi-Thermal Light Source: An Application of Hanbury Brown & Twiss

Christopher Watanabe

Published Jan 1, 2015• 0 citations
OpenAlex

Abstract

In this experiment, the observation by Hanbury Brown and Twiss that light intensity fluctuations from an extended broadband source are spatially correlated is demonstrated with a quasi-thermal light source. Laser light is used to illuminate a ground glass screen and generate a diffraction pattern, also known as a speckle pattern. The speckle pattern is recorded using an electronic camera and analyzed. Calculated auto-correlations of the intensity fluctuations agree with theoretical results originating from speckle analysis.

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In this experiment, the observation by Hanbury Brown and Twiss that light intensity fluctuations from an extended broadband source are spatially correlated is demonstrated with a quasi-thermal light source. Laser light is used to illuminate a ground glass screen and generate a diffraction pattern, also known as a speckle pattern. The speckle pattern is recorded using an electronic camera and analyzed. Calculated auto-correlations of the intensity fluctuations agree with theoretical results originating from speckle analysis.
The observation of the Hanbury Brown and Twiss (HBT) effect with thermal light marked the birth of quantum optics. All the thermal sources considered to date did not feature quantum signatures of light, as they consisted of independent emitters that emit uncorrelated photons. Here, we propose and demonstrate an incoherent light source based on phase-randomized spatially entangled photons, which we coin thermal biphotons. We show that in contrast to thermal light, the width of the HBT peak for thermal biphotons is determined by their correlations, leading to violation of the Siegert relation and breakdown of the speckle-fluctuations interpretation. We further provide an alternative interpretation of the results by drawing a connection between the HBT effect and coherent backscattering of light. Finally, we discuss the role of spatial entanglement in the observed results, deriving a relation between the Schmidt number and the degree of violation of the Siegert relation under the double-Gaussian approximation of spontaneous parametric down conversion. Our work reflects new insights on the coherence properties of thermal light in the presence of entanglement, paving the way for entanglement certification using disorder averaged measurements.
We report on the experimental observation of interference pattern of a distributed double-slit, consisting of a single-slit and a wire spatially separated, illuminated by two beams from a pseudothermal light source, in Hanbury Brown and Twiss–like configuration. The interference pattern is obtained performing a classical correlation with the intensities of the light beams scattered by each part of the double-slit. A theoretical analysis confirms our experimental results, showing that classical correlations can also mimic non-local properties analogue to those using twin photons for quantum images.
Introduction The degree of first-order temporal coherence, a function denoted by g (1) ( τ ), provides information about the coherence length and the power spectral density of a light source. However, without additional information, g (1) ( τ ) has no bearing on intensity fluctuations and higher-order statistics of the emitted light. A quasi-monochromatic laser beam and the beam of light from an incandescent light bulb, provided that the latter is properly filtered to match the spectral line-shape of the former, will have identical degrees of first-order coherence. Any interferometric experiment involving the splitting and superposition of amplitudes would yield identical results for the laser beam and the (properly filtered) thermal light. Therefore, on the basis of such experiments alone, there is no way to distinguish the two light sources. It turns out, however, that the intensity fluctuations of laser light are fundamentally different from those of thermal light. The two sources can, therefore, be distinguished based on their second-order coherence properties. An ideal photodetector produces an electrical signal proportional to the “cycle-averaged intensity” of the E -field (or B -field) of the light beam at the location of the detector. Assuming that the electrical bandwidth of the detector (including all associated circuitry) is greater than the bandwidth of the incident light wave by at least a factor of 2, the output of the detector should accurately represent the intensity fluctuations of the light beam as a function of time.
In traditional Hanbury Brown--Twiss (HBT) schemes, thermal intensity-intensity correlations (IICs) are phase insensitive. Here we propose a modified HBT scheme with phase conjugation to demonstrate phase-sensitive and nonfactorizable features of thermal IIC speckles. It shows that the novel phase-sensitive features originate from the correlation between the original and phase-conjugated thermal light. Our numerical simulation confirms the possibility of experimental realization, even for the cases of the imperfect phase conjugation. Unlike two-photon speckles, our scheme uses thermal light sources, not the entangled two-photon sources. Our result provides deeper insight into thermal correlations and may lead to significant applications in imaging and speckle technologies.
Two-photon correlation phenomena, including the historical experiment of Hanbury Brown and Twiss, may have to be described quantum mechanically, regardless of whether the source of radiation is classical or quantum. Supporting this point, we present a ghost imaging type of second-order spatial correlation experiment of chaotic light to show that the classical understanding based on the concept of statistical intensity fluctuations does not give a correct interpretation for the observation. From a practical point of view, this experiment demonstrates the possibility of having high contrast lensless two-photon imaging with chaotic light, suggesting imaging applications for radiations for which no effective lens is available.