Abstract
We report a conceptually straightforward witness that distinguishes coherent electronic oscillations from their vibronic-only counterparts in nonlinear optical spectra of molecular aggregates. Coherent oscillations as a function of waiting time in broadband pump/broadband probe spectra correspond to coherent electronic oscillations in the singly excited manifold. Oscillations in individual peaks of 2D electronic spectra do not necessarily yield this conclusion. Our witness is simpler to implement than quantum process tomography and potentially resolves a long-standing controversy on the character of oscillations in ultrafast spectra of photosynthetic light harvesting systems.
Highlights
There has been considerable interest in long-lived quantum superpositions of electronic states in photosynthetic molecular aggregates and their potential role in efficient energy transport in biological conditions.1–7. Evidence for such electronic coherences stems from time oscillations in peaks of two-dimensional electronic spectra (2D-ES), which persist for over 600 fs
There is additional evidence to support the interpretation that the oscillations are due to electronic states, unambiguous tools to experimentally unravel the nature of these oscillations are required
The rephasing 2D-ES of the paradigmatic Fenna-Matthews-Olson (FMO) complex exhibits oscillations in both diagonal and off-diagonal peaks, indicating that systems of interest may lie in the regime of strong coupling to vibrations29 or exhibit vibronic coherences only
Summary
There has been considerable interest in long-lived quantum superpositions of electronic states in photosynthetic molecular aggregates and their potential role in efficient energy transport in biological conditions. Evidence for such electronic coherences stems from time oscillations in peaks of two-dimensional electronic spectra (2D-ES), which persist for over 600 fs. coherences between vibronic levels involving a single electronic state (and without coherence between excited electronic states) exhibit similar signatures in 2D-ES (Refs. 12–15 and 61) and have been shown to nontrivially affect energy transfer too. there is additional evidence to support the interpretation that the oscillations are due to electronic states (beating frequencies and comparison with all-atom simulations8,26), unambiguous tools to experimentally unravel the nature of these oscillations are required. A big step has been the observation that, under weak coupling to vibrations and negligible coherence transfer processes, electronic coherences imply oscillations in off-diagonal peaks of rephasing 2D-ES and in diagonal peaks of their non-rephasing counterparts, whereas general vibronic coherences show up as oscillations in any region of either spectra.. A big step has been the observation that, under weak coupling to vibrations and negligible coherence transfer processes, electronic coherences imply oscillations in off-diagonal peaks of rephasing 2D-ES and in diagonal peaks of their non-rephasing counterparts, whereas general vibronic coherences show up as oscillations in any region of either spectra.28 Another important proposal to address the problem in this regime has been recently suggested in Ref. 62. Which gives the intuitive understanding that the number of photons gained by the probe P due to the action of the pump P is proportional to the norm of the wavepacket “dumped” into |g
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