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Schematic of alignment/balancing interferometer and HOM interferometer. The components to the left of the apertures drawn with black lines denote sources/optics for the interferometer alignment/balancing. The gray lines denote spontaneous parametric downconversion sourced HOM components. The broken lines represent the common optical paths shared by the visible/HOM interferometers. The flip mirror switches between HeNe and Hg illumination of the Mach–Zehnder interferometer. The deflecting mirrors and roof mirrors precede the beam splitter . The coincidence counter records the coincident detection events between the SPADs.
HOM interferometer coincidence data are shown by square data points with error bars conveying the Poissonian uncertainty. The solid line fit to the data is derived from the Fourier transform of the spectral filters. The feature confined within the HOM dip is the Hg interferogram; the center position of this feature differs from that of the HOM dip minimum by . The He–Ne interferogram is displaced from the central region of the figure for clarity, and the inset shows the interferogram with the position axis rescaled.
How the cube beam-splitter alignment technique may be applied to various experimental schemes; the graduations correspond to 10 mm intervals in the main figure and 1 mm intervals in the insets. The black broken lines represent the alignment beams while the gray broken lines represent the paths of the pump and downconverted fields. The downconverted and alignment fields are displaced for clarity. (a) In the experiment of this paper, the cube beam splitter was positioned in place of the downconverter. (b) For smaller emission angles, or situations where the downconverter cannot be removed for the alignment, the cube beam splitter may be placed a distance in front of the crystal. (c) By changing the tilt of the beam splitter and its position perpendicular to the pump, the alignment beams may be made to overlap with downconverted beams having different angles about the pump beam.
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