Effect of Aperture shape on Visibility of Quantum Cryptography System

Abstract

The main goal in Quantum Cryptography is highsecurity and this can be achieved by using singlephoton sources, thus we have studied the entangledphoton source and how the coincident count rateand visibility could be affected by the opticalelements of the system setup.The present work is a theoretical analysisdevoted to investigate the interference pattern ofbiphoton amplitude generated by spontaneousparametric down conversion (SPDC) in a nonlinearcrystal (BBO) pumped by femto-second opticalpulses. We have studied the visibility as a functionof optical path delay for different parameters, suchas the crystal length, aperture diameter, size andshape of the apertureThe shape of interference pattern can be enhancedby using different shapes & sizes of aperture. So inthe case of continuous wave ( CW )pumping, whenusing circular aperture the visibility is highest andsymmetry shape occurs at lower aperture diameter(b=0.5mm). Asymmetry and wide dip pattern isoccurred at larger aperture diameter (b=10mm),when using shifting ring, a negative peak wasappeared. In case of pulsed pump asymmetry,patterns occur and increase with increase of crystallength and decreasing aperture diameter. Whenusing slit aperture (vertical and horizontaldirection), the coincident count is more symmetricin horizontal with dimension (1x7) mm. All theresults of this work are based on several numericaltechniques and different programming codes. Forexample, adaptive Simpson quadrator method havebeen used to numerically evaluate the doubleintegral in some equations. And adaptive Lobattoquadrator method has been used to numericallyevaluate a single integrals. The later method is moreefficient when required a high accuracy or asmooth integral.