Spectral Correlation Measurements at the Hong-Ou-Mandel Interference Dip
  We present an efficient tool capable of measuring the spectral correlations between photons emerging from a Hong-Ou-Mandel interferometer. We show that for our spectrally factorizable spontaneous downconversion source the Hong-Ou-Mandel interference visibility decreases as the photons frequency spread is increased to a maximum of 165 nm. Unfiltered, we obtained a visibility of $92.0 \pm 0.2 \%$. The maximum visibility was $97 \pm 0.2 \%$ after applying filtering. We show that the tool can be useful for the study of spectral correlations that impair high-visibility and high-fidelity multi-source interference applications. 
Thomas Gerrits, Francesco Marsili, Varun Verma, Lynden Shalm, Matthew   Shaw, Richard Mirin, Sae Woo Nam
quant-ph