THE HIGH-ORDER QUANTUM COHERENCE OF THERMAL LIGHT
Hui Chen
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.
•Jan 1, 2014•1 citation