Tese de Doutorado #299: Hakob Avetisyan

The thesis deals with theoretical aspects of atmospheric propagation of correlation beams, the most practical tool in quantum communication protocols. One can prepare two-qudit states to implement communication with large alphabets. We analytically calculated the atmospheric two-photon joint detection probability when the pump represents a coherent Hermite- or Laguerre Gaussian mode of any order as well as a partially coherent beam from a Gaussian-Schell model source. Our results show that the expressions for fourth-order correlation functions are similar to those of intensities of classical beams: a manifestation of the concept of the correlation beam. To quantify the crosstalk between different modes we made a mode analysis of the correlation beam. An operational procedure to implement projective measurements on two-photon multimode states is also given. We discussed the role of entanglement with a short survey in the area. We compared the single photodetections probability with the two-photon joint detection probability. The former probability degrades rapidly after short distances of propagation, whereas the latter one is considerable even after five or more kilometers of propagation. Another aspect of the problem is the correction of the corrupted two-photon wave phase. This is done by using Zernike polynomials. The inference of atmospheric parameters by measuring the correlation beam is also addressed. The parameters that can be inferred only with fourth order statistics are easier to do using correlation beams, which is a fourth-order phenomenon.