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In their attempt to create a quantum random number generator, K. Muhammed Shafi et al. used a continuous-wave diode laser to fire photons at a periodically-poled potassium titanyl phosphate (PPKTP) nonlinear crystal. A plano-convex lens ______ the laser on the center of the 10-millimeter-long crystal ensured a spot size (a measure of the beam's diameter) of 85 micrometers.