Correlated photon-pair generation in a periodically poled MgO doped   stoichiometric lithium tantalate reverse proton exchanged waveguide
  We demonstrate photon-pair generation in a reverse proton exchanged waveguide fabricated on a periodically poled magnesium doped stoichiometric lithium tantalate substrate. Detected pairs are generated via a cascaded second order nonlinear process where a pump laser at wavelength of 1.55 $\mu$m is first doubled in frequency by second harmonic generation and subsequently downconverted around the same spectral region. Pairs are detected at a rate of 42 per second with a coincidence to accidental ratio of 0.7. This cascaded pair generation process is similar to four-wave-mixing where two pump photons annihilate and create a correlated photon pair. 
M. Lobino, G. D. Marshall, C. Xiong, A. S. Clark, D. Bonneau, C. M.   Natarajan, M. G. Tanner, R. H. Hadfield, S. N. Dorenbo...
quant-ph