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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.

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

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