  Two-color second-order correlations of the light scattered near-resonantly by
a quantum dot were measured by means of spectrally-filtered coincidence
detection. The effects of filter frequency and bandwidth were studied under
monochromatic laser excitation, and a complete two-photon spectrum was
reconstructed. In contrast to the ordinary one-photon spectrum, the two-photon
spectrum is asymmetric with laser detuning and exhibits a rich structure
associated with both real and virtual two-photon transitions down the "dressed
states" ladder. Photon pairs generated via virtual transitions are found to
violate the Cauchy-Schwartz inequality by a factor of 60. Our experiments are
well described by the theoretical expressions obtained by del Valle et al. via
time-and normally-ordered correlation functions.
