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arXiv:2511.22198 · The Journal of Chemical Physics

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

Yuta Fujihashi, Akihito Ishizaki

Published Jun 9, 2026• 0 citations
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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., Sci. Adv. 12, eaed7026 (2026)]. 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.

DOI
10.1063/5.0325960

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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., Sci. Adv. 12, eaed7026 (2026)]. 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.
Recent theoretical studies highlight how nonclassical photon correlations in entangled photon pairs can selectively address nonlinear optical pathways. However, the resulting signals are typically too weak for practical time-resolved experiments. Here, we propose two-dimensional (2D) time-resolved fluorescence spectroscopy that exploits these correlations and operates with current single-photon detectors. The method provides two advantages over conventional 2D electronic spectroscopy: (i) It yields 2D spectra without phase-stable multipulse control, relying instead on heralded twin-photon correlations, and (ii) it simplifies spectra by isolating the contribution that is spectroscopically equivalent to stimulated emission, thereby suppressing ground-state bleaching and excited-state absorption. Numerical calculations for a natural pigment-protein complex-inspired trimer show that this pathway selectivity enables the extraction of rich information on energy transfer dynamics. These results indicate a feasible route to real-time observation of molecular dynamics using entangled photon pairs.
Recent advances in techniques for generating quantum light have stimulated research on novel spectroscopic measurements using quantum entangled photons. One such spectroscopy technique utilizes non-classical correlations among entangled photons to enable measurements with enhanced sensitivity and selectivity. Here, we investigate the spectroscopic measurement utilizing entangled three photons. In this measurement, time-resolved entangled photon spectroscopy with monochromatic pumping [A. Ishizaki, J. Chem. Phys. 153, 051102 (2020)] is integrated with the frequency-dispersed two-photon counting technique, which suppresses undesired accidental photon counts in the detector and thus allows one to separate the weak desired signal. This time-resolved frequency-dispersed two-photon counting signal, which is a function of two frequencies, is shown to provide the same information as that of coherent two-dimensional optical spectra. The spectral distribution of the phase-matching function works as a frequency filter to selectively resolve a specific region of the two-dimensional spectra, whereas the excited-state dynamics under investigation are temporally resolved in the time region longer than the entanglement time. The signal is not subject to Fourier limitations on the joint temporal and spectral resolution, and therefore, it is expected to be useful for investigating complex molecular systems in which multiple electronic states are present within a narrow energy range.
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Classical time-resolved optical spectroscopy experiments are performed using sequences of ultrashort light pulses, with photon fluxes incident on the sample which are many orders of magnitude higher than real-world conditions corresponding to sunlight illumination. Spectroscopy and microscopy schemes that use quantum states of light have been widely described theoretically with fewer experimental demonstrations that typically require very long measurements that can extend for hours or more. Here, we show that time-resolved spectroscopy with quantum light can be performed without compromising measurement time or wavelength tunability, recording a fluorescence lifetime trace in biological samples in less than a second with acceptable signal-to-noise ratio. Starting from spontaneous parametric down-conversion driven by a continuous-wave laser, we exploit the temporal correlation between randomly generated signal/idler pairs to obtain temporal resolution, and their spectral correlation to select the excitation frequency. We also add spectral resolution in detection, using a 'photon-efficient' Fourier transform approach which employs a common-path interferometer. We demonstrate the potential of this approach by resolving, at the single-photon level, excitation energy transfer cascades from LH2 to LH1 in the photosynthetic membrane and disentangling the lifetimes of two dyes in a mixture. Our results provide a new approach to ultrafast optical spectroscopy, where experiments are performed under illumination intensity conditions comparable to real-world sunlight illumination.
Ultrafast optical spectroscopy is a powerful technique for studying the dynamic processes of molecular systems in condensed phases. However, in molecular systems containing many dye molecules, the spectra can become crowded and difficult to interpret owing to the presence of multiple nonlinear optical contributions. In this work, we theoretically propose time-resolved spectroscopy based on the coincidence counting of two entangled photons generated via parametric down-conversion with a monochromatic laser. We demonstrate that the use of two-photon counting detection of entangled photon pairs enables the selective elimination of the excited-state absorption signal. This selective elimination cannot be realized with classical coherent light. We anticipate that the proposed spectroscopy will help simplify the spectral interpretation of complex molecular and material systems comprising multiple molecules.
High-dimensional encoding in time-frequency and photon-number degrees of freedom promises photon-efficient quantum communication, but requires detectors that are both temporal-mode selective and photon-number resolving. Here we demonstrate time-correlated single-photon detection combining both capabilities. A train of shaped pump pulses drives quantum frequency up-conversion to select a chosen temporal mode, and the converted photons are counted with a photon-number-resolving detector. Applying to a photon-pair source by spontaneous parametric downconversion into signal and idler bands, we demonstrate direct sampling of the high-dimensional correlated quantum states on compound time-frequency and photon number bases. Our measurements show that the up-converted photons in each band exhibit thermal photon-number statistics, indicating strong single-mode selection. We reconstruct joint temporal intensities conditioned on one and two photon pairs, revealing signal-idler correlations in both time-frequency and photon number. Together, these results establish a unified technology framework linking high-dimensional quantum correlations, mode-selective photon measurement, and photon-number resolved detection. This approach provides a route to characterizing multimode photon-pair sources and to larger time-frequency and photon-number alphabets for high-dimensional quantum communication and information processing.