coefficients of reflection and transmission, respectively) deviates from its ideal
performance, but this gives rise to a negligible correctio n factor [110] 2RT/
(R
2
þ T
2
) ’0.994. (ii) The spatial-mode mismatch of the two photon wavepackets
at the BS results in some distinguishing information between the two coincidence-
generating probability amplitudes. A simple calculation [119] shows that a small
angular mismatch of around 30 mrad can bring the HOM dip visibi lity down
to 94%. (iii) Some remaining
2002
component due to nonideal alignment of the
50/50 Sagnac loop may lead to a degradation of the dip visibility [102]. (iv)
Existence of unsuppressed noise photons, such as Raman photons and FWM
photons induced by a single-frequency pump, may also