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This technique, proposed and first implemented by A. G. White (Univ. of Queensland, Australia), attempts to eliminate some of the loss due to high reflectivity (40%) spectral filters. By placing a QWP between the measurement PBS and the filter-detector assemblies, any reflected light will travel through the quarter waveplate, be reflected at the PBS, be retroreflected by a mirror, and travel back through the PBS toward the filter-detector assembly for a second chance at detection. The effective transmission of the filter is thus increased from 0.6 to ~ (0.6+0.4�?0.6) = 0.84

We have not studied the effects of phase compensation in conjunction with single-mode collection optics. However, it is the untested conjecture of the authors that rather than causing decoherence, angle-dependent phases may cause the coupling efficiency of the |HH> and |VV> terms to vary independently. The compensators presented here would cause them to vary jointly, with specially designed compensators allowing optimal and balanced coupling.

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