Measurement of Ultrashort Biphoton Correlation Times with an Integrated Two-Color Broadband SU(1,1)-Interferometer

The biphoton correlation time, a measure for the conditional uncertainty in the temporal arrival of two photons from a photon pair source, is a key performance identifier for many quantum spectroscopy applications, with shorter correlation times typically yielding better performance. Furthermore, it provides fundamental insight into the effects of dispersion on the biphoton state. Here, we show that a characteristic dependence of the width of the temporal interferogram can be exploited to obtain insights into the amount of second-order dispersion inside the interferometer and to retrieve actual and Fourier-limited ultrashort biphoton correlation times of around $100\text{fs}$. In the presented scheme, we simultaneously measure spectral and temporal interferograms at the output of an $\mathrm{SU}(1,1)$ interferometer based on an integrated broadband parametric down-conversion source in a ${\text{Ti:LiNbO}}_3$ waveguide.