SU(1,1) or nonlinear interferometers that is, Mach-Zehnder-like interferometers that replace the beam splitters with quantum light sources, have become increasingly relevant for applications. Examples include super-resolving quantum measurements and spectroscopy and imaging with undetected photons. A central performance benchmark of an SU(1,1) interferometer is the correlation time of the quantum light source, which describes the simultaneity of two time-frequency entangled photons from the source.
In this work, we present a method to measure the correlation time in an SU(1,1) interferometer based on parametric down-conversion in a dispersion-engineered waveguide. We measure the correlation time by adding known amounts of dispersion inside the SU(1,1) interferometer and obtain both the actual and minimal correlation time, which lie around 100 fs.
Our method is readily applied to other parametric photon-pair sources and paves a way towards a more reliable source characterization, optimized experiments and, ultimately, ultrafast quantum spectroscopy with optimum performance.
