  In the process of spontaneous parametric downconversion, photons from a pump
field are converted to signal and idler photon pairs in a nonlinear crystal.
The reversed process, or upconversion of these pairs back to single photons in
a second crystal, is also possible. Here, we present experimental measurements
of the upconversion rate with a controlled time delay introduced between the
signal and idler photons. As a function of delay, this rate presents a full
width at half maximum of 27.9 fs under our experimental conditions, and we
further demonstrate that group delay dispersion of the photon pairs broadens
this width. These observations are in close agreement with our calculations,
thus demonstrating an ultrafast, nonclassical correlation between the signal
and idler waves.
