Two-dimensional fluorescence spectroscopy with quantum entangled photons: Idler-referenced timing without pump detection

TL;DR

This paper proposes an idler-referenced timing scheme for two-dimensional fluorescence spectroscopy using entangled photons, simplifying experimental setup and enabling effective time-resolved measurements without pump detection.

Contribution

It introduces a new protocol that defines reference timing via idler photons, reducing experimental complexity and broadening practical application of entangled photon spectroscopy.

Findings

Idler-referenced scheme works with negative or negligible frequency correlations.

Eliminates the need for pump-timing channel, simplifying setup.

Requires entangled photon sources with specific temporal characteristics.

Abstract

Entangled photons have attracted increasing interest as resources for developing time-resolved spectroscopic techniques. Theoretical studies suggest that their non-classical correlations enable time-resolved spectroscopy with monochromatic pumping and can selectively isolate specific Liouville pathways in nonlinear optical signals. In an earlier study, we proposed a fluorescence detection scheme that could, in principle, be implemented using existing single-photon detectors [Y. Fujihashi et al., arXiv:2502.02073 (2025)]. In that design, the time origin was defined by detecting the arrival of the pulsed laser used to pump the nonlinear crystal for spontaneous parametric down-conversion, a requirement that made the overall experiment cumbersome. This study theoretically examines an alternative protocol that defines the reference time based on the arrival of idler photons. We demonstrate that this idler-referenced scheme functions effectively when the entangled photons exhibit either negative or negligible frequency correlations. Eliminating the pump-timing channel simplifies the optical layout and lowers the experimental barrier to realizing time-resolved two-dimensional fluorescence spectroscopy with entangled photons. Although the photons may exhibit frequency correlations in isolation, their frequency-time degrees of freedom can behave as effectively uncorrelated when considered over the full measurement timescale. Therefore, fully exploiting non-classical correlations requires an entangled photon source whose temporal characteristics are carefully matched to the overall timescale of the experiment.