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Quantum Effects and Measurement Techniques in Biology and Biophotonics

Practical femtosecond time-resolved spectroscopy using paired single photons

JunWoo Kim

Published Mar 21, 2025• 0 citations
Publisher

Abstract

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.

DOI
10.1117/12.3038131

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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.
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.
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.
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.
Efficient excitation of an ensemble of atoms and molecules to a desired state is of fundamental importance not only for spectroscopy but also for quantum control of molecular dynamics in chemical and biological processes. The latter usually occurs on a much shorter time scale (in the femtosecond range) giving rise to steadily advanced problem how to excite one of the nearby levels, even when they are well within the broad fs pulse spectrum. We propose a new effective method for selective excitation of atoms based on the atomic coherence and discuss a sensitive method to verify experimentally the proposed mechanism using the technique of quantum beating in pump-probe experiments.