Femtosecond spectroscopy with paired single photons: Emulating a double-slit experiment in the time-frequency domain

Time-resolved spectroscopy has long served as a cornerstone technique for investigating the photophysics of materials. However, conventional methods based on intense optical pulses and ensemble measurements often fail to resolve quantum dynamics due to spectral overlap, coherent artifacts, and multiphoton effects. To overcome these limitations, single-photon transient stimulated emission (SP-TSE) has been proposed as a femtosecond spectroscopy technique using quantum light and detection principles. Here, we theoretically demonstrate frequency-resolved SP-TSE. Unlike semiclassical techniques, SP-TSE reveals a strong dependence of the time-resolved spectrum on the frequency-resolving method. With a tunable optical filter, the observed spectrum reflects a quantum double-slit interference in the time-frequency domain. In contrast, interferometric detection produces spectra resembling semiclassical results, but the signal emerges specifically from the second harmonic frequency window due to quantum interference. This framework provides insight into ultrafast quantum processes and suggests a foundation for future applications in quantum spectroscopy and technologies.