  The second-order correlation function of light $g^{(2)}(\tau)$ constitutes a
pivotal tool to quantify the quantum behavior of an emitter and in turn its
potential for quantum information applications. The experimentally accessible
time resolution of $g^{(2)}(\tau)$ is usually limited by the jitter of
available single photon detectors. Here, we present a versatile technique
allowing to measure $g^{(2)}(\tau)$ from a large variety of light signals with
a time resolution given by the pulse length of a mode-locked laser. The
technique is based on frequency upconversion in a nonlinear waveguide, and we
analyze its properties and limitations by modeling the pulse propagation and
the frequency conversion process .We measure $g^{(2)}(\tau)$ from various
signals including light from a quantum emitter - a confined exciton-polariton
structure - revealing its quantum signatures at a scale of a few picoseconds
and demonstrating the capability of the technique.
