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PhD Defense - Sanjit Karmakar

Sanjit successfully defended his PhD dissertation on February 1, 2012.

Ghost Imaging with Sunlight

The main result of this dissertation is the first successful experimental demonstration of ghost imaging using the sun as a light source. This result supports the quantum theory of near-field thermal light ghost imaging and also clarifes the physics of near-field thermal light ghost imaging from the fundamental level. The quantum theory of two-photon interference is the key to understanding non-local ghost imaging with thermal light sources. Two-photon interference occurs between two different yet indistinguishable probability two-photon amplitudes, nonclassical entities produced by the joint-detection between two distant photo-detectors. On the other hand, the classical theory considers the reason behind thermal light ghost imaging to be an intensity fluctuation correlation. Interestingly, the physics of intensity fluctuation correlation was misled by the speckle-to-speckle picture.

The experimental demonstration of ghost imaging with sunlight suggests that the nonlocal ghost-imaging effect of thermal light is caused by quantum-mechanical two-photon interference and it also proves that the idea of 'speckles" is unnecessary in near-field thermal light ghost imaging. Most importantly, the sun does not make any speckle and it is a near-field source. The experimental studies on sunlight-based ghost imaging is discussed in two steps: (1) an experimental demonstration as well as a quantum mechanical explanation of the nontrivial intensity correlation with the sun, a natural thermal source, as a light source and (2) the demonstration of the experimental observation of ghost imaging with sunlight with its quantum-mechanical explanation. These observations with their theoretical explanation are very helpful to understanding the physics of ghost imaging from a fundamental level. From the application point of view, sunlight-based ghost imaging may achieve a spatial resolution equivalent to that of a classical imaging system taking pictures at a distance of 10 km with a lens of 92 m size.

So far ghost imaging using thermal light with one color are demonstrated. This dissertation also reports an experimental study of two-color, biphoton ghost imaging using an entangled photon pair source. The result of this experimental observation shows a ghost image with enhanced angular resolving power by means of a greater field of view compared with that of classical imaging. The experience gained in the two-color ghost imaging experiment with entangled photon pairs will be helpful to get a real color ghost image with sunlight. A proposal to achieve sunlight-based ghost imaging with real colors is also reported here. Potential real color sunlight-based ghost imaging with its nonlocal behavior and turbulence-free nature gives us a promise for its applications in distant imaging.