Measuring laser chirp rate at single-emitter excitation energies

We present a simple and direct method for measuring laser chirp rate, i.e., group delay dispersion (GDD) of ultrashort laser pulses at power levels compatible with single-quantum-emitter excitation. Traditional pulse characterization techniques rely on nonlinear optical processes that require high peak powers, making them unsuitable for the attojoule regime relevant to on-chip quantum photonics. Our approach utilizes a wavelength-to-time mapping method in which the arrival times of spectrally filtered components of a broadband pulse are recorded using a single-photon detector and correlated via a high-resolution time-tagging system. The resulting linear relationship between wavelength and arrival time directly yields the dispersion parameter and, subsequently, the GDD. We call this technique "WAvelength-Time mapping for CHirp measurement (WATCH)." Beyond single-emitter excitation, this technique can be applied in areas such as single-photon spectroscopy, ultralow-power optical communications, and time-domain quantum control, where linear and non-destructive dispersion characterization is essential.