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Science Advances

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

EunHo Hong, EunSeo Jang, JunWoo Kim

Published Oct 17, 2025• 5 citations
Publisher

Abstract

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.

DOI
10.1126/sciadv.adw9759

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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.
Recent progress in single-photon femtosecond spectroscopy has highlighted the crucial role of entangled photons in exploring material properties beyond the reach of traditional semiclassical approaches. In this work, we introduce a theoretically proposed technique named single-photon transient stimulated emission (SP-TSE), a new method in femtosecond time-resolved spectroscopy. SP-TSE uniquely enables the selective probing of single-excitation superposition states and utilizes the quantum mechanical characteristics of photons for the efficient acquisition of transient response data, enhancing the practicality of SP-TSE experiments. A fundamental feature of SP-TSE is its employment of coincidence counting techniques to selectively detect two-photon states produced by stimulated emission. Our theoretical framework, bolstered by numerical simulations, demonstrates its effectiveness in elucidating the pure decoherence dynamics of a model molecular ensemble, thereby revealing quantum mechanical properties that are otherwise challenging to observe using conventional femtosecond time-resolved experiments.
Recent advances in single-photon femtosecond spectroscopy have highlighted the power of entangled photons in probing the properties of materials, previously inaccessible through semiclassical spectroscopic approaches. In this study, we theoretically propose a new single-photon-based femtosecond time-resolved spectroscopy technique termed single-photon transient stimulated emission (SP-TSE). SP-TSE not only enables the selective investigation of singly excited superposition states but also harnesses the quantum mechanical nature of photons for the efficient data acquisition of transient responses, thereby supporting the feasibility of experimental realization of SP-TSE. The key aspect of SP-TSE is the selective detection of two-photon states produced through stimulated emission using coincidence counting techniques. Our theoretical framework, supported by numerical simulations, demonstrates the efficacy in capturing the pure decoherence dynamics of a model molecular cavity, highlighting its potential to reveal quantum mechanical properties that are difficult to observe with semiclassical femtosecond time-resolved experiments.
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.
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.
Quantum entanglement has emerged as a great resource for spectroscopy and its importance in two-photon spectrum and microscopy has been demonstrated. Current studies focus on the two-photon absorption, whereas the Raman spectroscopy with quantum entanglement still remains elusive, with outstanding issues of temporal and spectral resolutions. Here we study the new capabilities provided by entangled photons in coherent Raman spectroscopy. An ultrafast frequency-resolved Raman spectroscopy with entangled photons is developed for condensed-phase molecules, to probe the electronic and vibrational coherences. Using quantum correlation between the photons, the signal shows the capability of both temporal and spectral resolutions not accessible by either classical pulses or the fields without entanglement. We develop a microscopic theory for this Raman spectroscopy, revealing the electronic coherence dynamics even at timescale of 50fs. This suggests new paradigms of optical signals and spectroscopy, with potential to push detection below standard quantum limit.
Quantum correlations of ultrafast quantum optical states in the time–frequency domain manifest prominently on femtosecond time scales, an elusive challenging regime. We develop a new method to probe the time–frequency mode structure of ultrafast two-photon states utilizing dual-comb-based asynchronously overlapping pulses with single-photon frequency upconversion through femtosecond optical sampling. This method enables quick, highly precise joint temporal intensity measurements over a broad temporal dynamic range with only a single linear scanning stage. We demonstrate full characterization of time–frequency entangled two-photon states in both time and frequency domains. We quantified the time–frequency mode structure in both time and frequency domains through Schmidt decomposition, resulting in temporal and spectral purities of 87.7% and 76.9%, respectively, for heralded single-photons without any spectral filtering. Furthermore, through the simple control of pump spectral bandwidth, we demonstrate the ability to tailor time–frequency entanglement, verifying the two-dimensional Fourier duality. This method paves the way for a substantial upgrade to ultrafast quantum optical techniques, enabling a rapid, detailed, and multi-dimensional characterization of entangled states with potential applications in quantum metrology and photon-deficient fluorescence imaging.