Polarization entanglement based cryptography uses the photon pairs generated in a spontaneous paramet- ric down-conversion process and guarantees the security through the violation of Bell’s inequality. Certain experimental parameters affects the entanglement fidelity and leads to a possible information leakage. The optical path difference of photons born at different crystals contributes to reduced fidelity, due to the extra birefringence of the nonlinear crystal. Although previous studies suggested methods to erase the distinguishability of photons by introducing compensation crystals, the phase difference, which is due to the lateral ray distribution is not studied. We used two commercially available collection optics; an aspheric and an achromatic lens. With these collection optics, the effect of collection optics on the entanglement fidelity is studied. We developed a simulation of such a system and found that aspheric collection optics is more suitable to achieve high fidelity.
We present a novel method to measure the range of an object in a lab setting. This is realised by utilizing the phase insensitive cross-correlation analysis of the probe and reference photons generated in a spontaneous parametric down-conversion process. In our technique, the comparative measurement of time-correlated photons after one is transmitted to the target direction and the copy of it retained at the lab allows to single out the signal from the noise lead to sense, range and imaging the sample.
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