Several imaging techniques, such as ghost imaging, are based on the use of classical and quantum correlated light states. This fact has encouraged the search for new strategies to produce light states more correlated than the thermal states that are typically used. In this work, we produce and characterize classical states of light with “more than thermal” statistics. Such states are obtained by means of a sequence of two rotating ground-glass disks and by appropriately selecting the speckle field produced at the output of each disk. The experimental results are in excellent agreement with the developed theoretical model, suggesting the potential of this kind of light for imaging applications.
Several imaging techniques, such as ghost imaging, are based on the use of classical and quantum correlated light states. This fact has encouraged the search for new strategies to produce light states more correlated than the thermal states that are typically used. In this work, we produce and characterize classical states of light with “more than thermal” statistics. Such states are obtained by means of a sequence of two rotating ground-glass disks and by appropriately selecting the speckle field produced at the output of each disk. The experimental results are in excellent agreement with the developed theoretical model, suggesting the potential of this kind of light for imaging applications.
We investigate the nature of correlations in Gaussian light sources used for ghost imaging. We adopt methods from quantum information theory to distinguish genuinely quantum from classical correlations. Combining a microscopic analysis of speckle-speckle correlations with an effective coarse-grained description of the beams, we show that quantum correlations exist even in 'classical'-like thermal light sources, and appear relevant for the implementation of ghost imaging in the regime of low illumination. We further demonstrate that the total correlations in the thermal source beams effectively determine the quality of the imaging, as quantified by the signal-to-noise ratio.
When a laser beam passes through a rotating ground glass (RGG), the scattered light exhibits thermal statistics. This is extensively used in speckle imaging. This scattering process has not been addressed in the photon picture and is especially relevant if nonclassical light is scattered by the RGG. We develop the photon picture for the scattering process using the Bose statistics for distributing N photons in M pixels. We obtain an analytical form for the P distribution of the output field in terms of the P distribution of the input field. In particular, we obtain a general relation for the nth-order correlation function of the scattered light, i.e., ${g}_{\text{out}}^{(n)}\ensuremath{\simeq}n!\phantom{\rule{0.16em}{0ex}}{g}_{\text{in}}^{(n)}$, which holds for any order n and for arbitrary input states. This result immediately recovers the classical transformation of coherent light to pseudothermal light by RGG.
Thermal light, such as sunlight, is usually considered classical light. In a macroscopic picture, classical theory successfully explained the first-order coherence phenomena of thermal light. The macroscopic theory, based on the statistical behavior of light intensity fluctuations, however, can only phenomenologically explain the second- or higher-order coherence phenomena of thermal light. This thesis introduces a microscopic quantum picture, based on the interferences of a large number of randomly distributed and randomly radiated subfields, wavepackets or photons, to the study of high-order coherence of thermal light. This thesis concludes that the second-order intensity fluctuation correlation is caused by nonlocal interference: a pair of wavepackets, which are randomly paired together, interferes with the pair itself at two distant space-time coordinates. This study has the following practical motivations: (1) to simulate N-qbits. Practical quantum computing requires quantum bits(qubits) of N-digit to represent all possible integers from 0 to 2 N -1 simultaneously. A large number of independent particles can be prepared to represent a large set of N orthogonal |0> and |1> bits. In fact, based on our recent experiments of simulating the high-order correlation of entangled photons, thermal radiation is suggested as a promising source for quantum information processing. (2) to achieve sunlight ghost imaging. Ghost imaging has three attractive non-classical features: (a) the ghost camera can see targets that can never be seen by a classic camera; (2) it is turbulence-free; and (3) its spatial resolution is mainly determined by the angular diameter of the light source. For example, a sunlight ghost image of an object on earth may achieve a spatial resolution of 200 micrometer because the angular diameter of sun is 0.53 degree with respect to Earth. Although ghost imaging has been experimental demonstrated by using entangled photon pairs and pseudo-thermal light, we still have difficulties to make sunlight ghost image. From the picture of macroscopic local statistics of intensity fluctuations, we may never find the problem that causes sunlight ghost imaging so difficult comparing with that of pseudo-thermal light. To achieve the goals in both fundamental understanding of physics and practical engineering applications, this thesis aims at studying the high-order quantum coherence of thermal light. The first successful attempt of this study is to invent a novel measurement scheme — Positive-Negative Intensity Fluctation Correlation(PNFC) protocol, to measure the intensity fluctuation correlation(or photon number fluctuation correlation) of thermal light. This new photodetection measurement protocol distinguishes positive and negative intensity fluctuations(or photon number fluctuations) of two or more photodetectors within a sequence of short time windows, and then calculates the statistical correlation between these individual fluctuations. With the help of this invention, an anti-correlation of thermal light in a typical Hanbury Brown and Twiss(HBT) interferometry was discovered, which cannot be understood from the view point of traditional macroscopic classical theory. In the microscopic quantum coherence picture, the observation is successfully explained as a nonlocal two-photon interference phenomenon. We also successfully simulated the behavior of Bell-state with thermal light. By manipulating polarization, a thermal Bell-like state was built from two incoherent and orthogonally polarized thermal radiations. With intensity fluctuation correlation measurement, a sinusoidal function with 100% visibility, which suggests thermal light as a feasible source for preparing qubits. Besides the fundamental significance, PNFC protocol also brings incredibly high contrast for quantum imaging, such as 100% visibility for ghost imaging. Above all, we invented a 100% Intensity Fluctuation Correlation of Chaotic-thermal Light and Turbulence-free Imaging System, which provides high contrast as well as high resolution. Based on the experiments of the simulations of quantum interference in thermal light and the dramatically improvement of the contrast in quantum imaging, this study of the high-order coherence of thermal light has a great significance both for fundamental physics and applications.
Ghost imaging is a novel non-conventional technique allowing to generate high resolution images by correlating the intensity of two light beams, neither of which independently contains sufficient information about the spatial distribution and shape of the object. The first demonstration of ghost imaging used light in double photon state, obtained from spontaneous parametric down-conversion. Owing to the entanglement of the source photons, the proposed theory required quantum descriptions for both the optical source and its photo-detection statistics1. However, subsequent experimental and theoretical considerations2,3 demonstrated that ghost imaging can be performed also with thermalized light, utilizing either CCD detector arrays or photon-counting detectors, thus admitting to a semi-classical description, employing classical fields and shot-noise limited detectors. This has generated increasing interest4-6 in establishing a unifying theory that characterizes the fundamental physics of ghost imaging and defines the boundary between classical and quantum domains. In this view, we exploited recent progress obtained through the application of Fourier Transform Techniques to demonstrate ghost imaging in the frequency domain, in order to measure a continuous spectrum by using a highly brilliant and coherent monochromatic source. In particular, we demonstrate the application of this ghost imaging technique to broadband spectroscopic measurements by means of interaction free photon detection. The experimental apparatus and the collected data are described in a dedicated work7. In this paper, we consider the theoretical aspects underlying the proposed Spectroscopic technique. In particular, two alternative theoretical models are presented. In one case, a statistical approach (semi-classical) is applied, where the states of the sampling beam are considered, whereas in the other case a pure quantum treatment is carried on, by describing the interaction of vacuum states generated by photon conversion processes. Both theoretical models, though carried on by means of a complementary formalism, lead to equivalent results and offer a physical interpretation of the collected experimental data. The application of these results offer novel perspectives for remote sensing in low light conditions, or in spectral regions where sensitive detectors are lacking.
The quantum ghost imaging phenomena, experimentally demonstrated a decade ago, exploited the apparent spooky action at a distance of entangled photon pairs and offered a novel approach toward imaging. Can ghost imaging effects be produced by ``classical'' light sources, such as separable systems of photon pairs or thermal light? If so, can these sources achieve the same accuracy achieved by entangled states? In order to answer these questions, we formulate the different physics behind entangled and separable systems in terms of a set of inequalities derived from the historical argument of Einstein, Podolsky, and Rosen. We show that the ghost images produced by separable sources are subject to the standard statistical limitations. However, entangled states offer the possibility of overcoming such limitations. Imaging can, therefore, achieve its fundamental limit through the high spatial resolution and nonlocal behavior of entangled systems.