De Bettin, Federico (2018) Optimization of a Pulsed Source of Entangled Photons for the Violation of the "CHSH" Inequality. [Laurea triennale]
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Optimization of a Pulsed Source of Entangled Photons for the Violation of the CHSH Inequality This thesis focuses on my work carried out in the Quantum Communications laboratory with the Quantum Future research group of the Department of Information Engeneering, in which I worked on the creation and optimization of a pulsed source of polarization entangled photons. The main focus in the laboratory was to optimize the experimental setup to prove the entangled nature of the photons produced. This was done by violating the Bell inequality in the form presented by Clauser, Horne, Shimony and Holt (CHSH) in 1969. The present work's objective is to provide an overview of all the crucial physical phenomena exploited in the experiment to obtain the wanted results. In section 1 a presentation of the phenomenon of entanglement and of Bell's argument proving the incapability to describe its properties with a both real and local theory is given, along with the derivation of the ``CHSH" inequality and the possibility to violate it with polarization entangled photons. Sections 2 and 3 focus on the properties of the Gaussian beam used in the experiment and its source: the mode-locked Mira laser. A brief discussion about mode-locked lasers and their properties is provided. In section 4 the nonlinear optical phenomena of second harmonic generation and spontaneous parametric down conversion are discussed, the former from a classical standpoint and the latter with a few quantum mechanical considerations. Section 5 focuses on polarization optics and the both classical and quantum mechanical formalism of the Jones vectors. A discussion of the effects of the birefringent devices used in the experiment on the photons' state is provided. Finally, sections 6 and 7 display a complete overview of the experimental setup and the results obtained for the violation of the inequality. A systematic explanation of the experimental optimization procedure is provided in section 6 as it may be useful for the future experimenters, since it proved to be tricky. Throughout all the work, various characteristics of the devices used in the experimental setup are given, hopefully making the most important and peculiar aspects of the experiment clear.
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