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arXiv:2403.00857

Two-colour photon correlations probe coherent vibronic contributions to electronic excitation transport under incoherent illumination

Charlie Nation, Valentina Notararigo, Hallmann Óskar Gestsson, Luca Sapienza, Alexandra Olaya-Castro

Published Feb 29, 2024• 0 citations
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Abstract

Identifying signatures of quantum coherent behaviour in photoactive systems that are maintained in stationary states away from thermal equilibrium is an open problem of wide interest in a variety of physical scenarios, including single photosynthetic complexes subjected to continuous incoherent illumination. Here we consider a prototype light-harvesting heterodimer exhibiting coherent and collective exciton-vibration interactions and show that the second-order frequency-filtered correlations of fluorescence photons provide insightful information on the influence of such coherent interactions for different transitions, thereby yielding fundamentally different photon-counting statistics. Furthermore, we show that coherent vibronic mechanisms strongly affect the asymmetries characteristic of time-resolved photon cross-correlations and manifest themselves in a time-dependent violation of the Cauchy-Schwarz inequality bounding cross-correlations for classically fluctuating fields. We finally discuss how such second-order correlation asymmetry establishes important connections between coherent vibronic interactions, directional exciton population transport, and violation of quantum detailed balance. Our work then indicates that measurement of two-colour photon correlation asymmetry can be an important avenue to investigate quantum behaviour of single photoactive biomolecular and chemical systems under incoherent illumination conditions.

DOI
10.48550/arxiv.2403.00857

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We theoretically investigate polarization-filtered two-photon correlations for the light emitted by a multichromophoric system undergoing excitation transport under realistic exciton-phonon interactions, and subject to continuous incoherent illumination. We show that for a biomolecular aggregate, such as the Fenna-Matthews Olson (FMO) photosynthetic complex, time-asymmetries in the cross-correlations of photons corresponding to different polarizations can be exploited to probe both quantum coherent transport mechanisms and steady-state coherence properties, which are not witnessed by zero-delay correlations. A classical bound on correlation asymmetry is obtained, which FMO is shown to violate using exact numerical calculations. Our analysis indicates that the dominant contributions to time-asymmetry in such photon cross-correlations are population to coherence transfer for Frenkel-Exciton models. Our results therefore put forward photon correlation asymmetry as a promising approach to investigate coherent contributions to excited-stated dynamics in molecular aggregates and other many-site quantum emitters.
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In quantum aggregates, delocalized exciton states across energy manifolds interact with phonon modes, making state-resolved spectroscopic monitoring of dynamics challenging. We propose a scheme that combines photon-entanglement-enhanced narrowband excitation of two-exciton states with time-frequency-filtered two-photon coincidence counting, which allows high-resolution probing of dissipative two-exciton dynamics spread across multiple spectral and temporal windows. We demonstrate that entangled photon pairs can be used to prepare narrowband two-exciton population distributions, circumventing transport in the mediating one-exciton manifold, and the redistributed two-exciton population can be monitored using time-frequency-filtered two-photon coincidence counting. Numerical simulations for a light-harvesting aggregate highlight the ability of this protocol to suppress or amplify specific pathways under a realistic scenario. Combining entangled photonic sources with multidimensional photon correlation techniques enables promising applications in spectroscopy and sensing.
The frequency- and time-resolved second order quantum coherence function (g(2)(τ)) of single-molecule fluorescence has recently been proposed as a powerful new quantum light spectroscopy that can reveal intrinsic quantum coherence in excitation energy transfer in molecular systems ranging from simple dimers to photosynthetic complexes. Yet, no experiments have been reported to date. Here, we have developed a single-molecule fluorescence g(2)(τ) quantum light spectroscopy (SMFg2-QLS) that can simultaneously measure the fluorescence intensity, lifetime, spectra, and g(2)(τ) with frequency resolution, for a single molecule in a controlled environment at both room temperature and cryogenic temperature. As a proof of principle, we have studied single molecules of IDTBT (indacenodithiophene-co-benzothiadiazole), a semiconducting donor-acceptor conjugated copolymer with a chain-like structure that shows a high carrier mobility and annihilation-limited long-range exciton transport. We have observed different g(2)(τ=0) values with different bands or bandwidths of the single molecule IDTBT fluorescence. The general features are consistent with theoretical predictions and suggest non-trivial excited state quantum dynamics, possibly showing quantum coherence, although further analysis and confirmation will require additional theoretical calculations that take into account the complexity and inhomogeneity of individual IDTBT single molecular chains. Our results demonstrate the feasibility and promise of frequency- and time-resolved SMFg2-QLS to provide new insights into molecular quantum dynamics and to reveal signatures of intrinsic quantum coherence in photosynthetic light harvesting that are independent of the nature of the light excitation.