Abstract
Ultrafast, multi-dimensional spectroscopic measurements of photosynthetic light-harvesting complexes have revealed quantum coherences with timescales comparable to those of energy-transfer processes. These observations have led to a debate regarding the states that give rise to the coherences and whether the presence of the coherences has implications for photosynthetic light harvesting. In these experiments, laser pulses create a coherent superposition of quantum states with a defined phase relationship across an ensemble, which gives rise to the quantum coherence and associated quantum beating signal. Dephasing of these quantum coherences, seen as a decay of the beating signal, is among the most sensitive probes of the interactions between a system and its surrounding environment. In this Review, we discuss the proposed origin and assignment of the observed quantum coherences in photosynthetic systems as electronic, vibronic or vibrational. We describe the latest experimental efforts towards unravelling the nature of the coherences, in particular ultrafast, two-dimensional electronic spectroscopy, as well as the accompanying theoretical and computational results. We discuss how measuring coherences can inform us about the excited-state dynamics of biophysical and chemical systems relevant to natural light harvesting and how these measurements reveal electronic structure beyond that captured by simplistic models.
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Acknowledgements
The authors thank Karen Watters for the scientific comments and scientific editing of the manuscript. This work is supported by the Vannevar Bush Fellowship (N00014-16-1-2513) and the Air Force Office of Scientific Research (FA9550-18-1-0099). G.S.E. acknowledges support from the Qatar National Research Foundation exceptional grant (NPRP-X-107-1-027). Additional support was provided by the University of Chicago Materials Research Science and Engineering Centers (MRSEC), which is funded by the National Science Foundation (NSF) through grant DMR-1420709. M.A.A. acknowledges support from an Arnold O. Beckman Postdoctoral Fellowship from the Arnold and Mabel Beckman Foundation.
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Glossary
- Density matrix
-
An operator in quantum mechanics that contains all the physically relevant information about the statistical state of a quantum ensemble.
- Decoherence
-
Fundamental quantum process that underlies dephasing and can lead to loss of coherences, even in a system without static disorder. The system can interact with its bath to exchange or transfer quantum information. The coherence is destroyed when such information cannot be regained by the system. The specifics of decoherence in a molecular system depends upon the coupling between electronic and nuclear degrees of freedom.
- Dephasing
-
Decay of the macroscopic coherence across the ensemble. Includes both static and dynamic contributions. Because of differences in local environments, different members of the ensemble will accumulate phase at different rates, weakening the amplitude of measured quantum beats. Static disorder that leads to energy differences across the ensemble also contributes to the loss of quantum coherences. The overall dephasing envelope of these coherences is an exceptionally sensitive probe of ensemble system–bath interactions.
- Reduced density matrix
-
A version of the density matrix operator that focuses specifically on the system of interest. This reduced density matrix requires a trace to be taken over the degrees of freedom that are defined as outside of the system of interest. The states that have been traced over are commonly referred to as bath degrees of freedom, or simply ‘the bath’.
- Exciton basis
-
In pigment–protein complexes, coupling between molecules causes excitations to delocalize over several chromophores, forming Frenkel excitons that are the eigenstates of the system Hamiltonian.
- Adiabatic dynamics
-
Dynamics that can be described by interactions within a single potential energy surface.
- Nonadiabatic dynamics
-
Dynamics that need to consider interactions between several coupled potential energy surfaces.
- Liouville space
-
A linear vector space, analogous to Hilbert space, that allows for the calculation of quantum dynamics. It is of higher dimensionality than Hilbert space and uses the density matrix to natively represent the state of the system. This allows for facile calculations of the statistical ensemble physics contained in the density operator.
- Population states
-
Individual state or multiple states of the system when no coherent superposition between different quantum states exists and the system can be described as propagating along a single potential energy surface (that is, the excited state or the ground state). This is in contrast to coherences in which superpositions of prescribed phase are present.
- Sliding window Fourier transform
-
A Fourier transform algorithm that employs some window size (N), where N is less than the total number of points (P) in the dataset to be Fourier transformed. This window can then be moved to find the Fourier transform of different sections of the full dataset, P. This technique can be useful to determine if different frequencies appear at different times in (or, more generally, in different parts of) the dataset, P.
- Third-order nonlinear spectroscopy
-
A spectroscopic measurement that involves three interactions of the electric field of light with matter, thus generating a microscopic polarizability that has terms to the third order. In practice, this requires three laser pulses. Examples include 2DES and pump–probe spectroscopy. This measurement is also known as four-wave mixing (three input, one output) in the literature.
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Wang, L., Allodi, M.A. & Engel, G.S. Quantum coherences reveal excited-state dynamics in biophysical systems. Nat Rev Chem 3, 477–490 (2019). https://doi.org/10.1038/s41570-019-0109-z
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DOI: https://doi.org/10.1038/s41570-019-0109-z
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