Quantum frequency-resolved optical gating measurement for ultranarrow temporal correlation of twin beams

Intense twin beams generated by high-gain parametric downconversion exhibit simultaneous correlations in time and frequency. To characterize the simultaneous correlations, it is important to determine the temporal correlation profile and the corresponding phase. Although sum frequency generation (SFG) is often used to characterize the temporal correlation of the twin beams, the SFG spectra reconstruct the pump intensity spectrum shape used to generate the twin beams, so useful information for phase reconstruction cannot be obtained from the SFG spectra. We propose and demonstrate frequency-resolved optical gating measurement for ultranarrow temporal correlation of twin beams (referred to as quantum FROG measurement). Quantum FROG measurement for twin beam correlations is realized using a cross-correlator based on four-wave mixing (FWM). We show theoretically that although the quantum FROG trace exhibits a different functional form obtained by the conventional FROG trace for ultrashort pulses, it is sensitive to phase dispersion. In experiments, we observed a shift of the FWM frequency, i.e., frequency chirping, within a correlation time of hundreds of femtoseconds, and the sign of the chirp was also identified.