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Measuring properties of entangled photons using the Hong-Ou-Mandel effect

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Mili Barai

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2021

We model Joint Spectral Intensities of two entangled photons created by spontaneous parametric down conversion using three crystals, beta-Barium Borate, Calcium Carbonate and Lithium Iodate. For an entangled photon pair, by changing the property of one photon, one can also change the property of the other photon in the pair. We can model Joint Spectral Intensities by considering the effect of crystal length and the angle at which the crystal is oriented with respect to the pump laser polarization. We learn that on increasing length of the crystal, peaks of Joint Spectral Intensities get narrower. On increasing the angle of tilt of the crystal, the singular probability peak divides into two peaks and moves out of the visible range. We then model the Hong-Ou-Mandel effect to measure properties of the two photons by modelling the photons passing through a beam splitter after going through two separate paths, with one path having a time delay and getting detected by the detectors at the beam splitter outputs. We model the rate of coincidence, or number of times both photons are detected at different ports simultaneously for different lengths of crystals. We find that on increasing the length of the crystal, the width of the Hong-Ou-Mandel dip increases as well. This project is important because it helps us learn about the properties of entangled photons, which have numerous applications such as quantum computing, quantum cryptography, and secure communications.