Nature does not rely on long-lived electronic quantum coherence for photosynthetic energy transfer

TL;DR

This study shows that electronic quantum coherence in photosynthetic energy transfer is short-lived, lasting about 60 femtoseconds, and does not play a functional role in the process, challenging previous claims of long-lived coherence.

Contribution

The paper provides experimental evidence that contradicts claims of long-lived electronic quantum coherence in photosynthesis, supporting the classical incoherent hopping model.

Findings

Electronic quantum coherence decays within 60 femtoseconds.

No evidence of long-lived coherence in 2D photon echo spectra.

Supports the orthodox incoherent energy transfer model.

Abstract

During the first steps of photosynthesis, the energy of impinging solar photons is transformed into electronic excitation energy of the light-harvesting biomolecular complexes. The subsequent energy transfer to the reaction center is understood in terms of exciton quasiparticles which move on a grid of biomolecular sites on typical time scales less than 100 femtoseconds (fs). Since the early days of quantum mechanics, this energy transfer is described as an incoherent Forster hopping with classical site occupation probabilities, but with quantum mechanically determined rate constants. This orthodox picture has been challenged by ultrafast optical spectroscopy experiments with the Fenna-Matthews-Olson protein in which interference oscillatory signals up to 1.5 picoseconds were reported and interpreted as direct evidence of exceptionally long-lived electronic quantum coherence. Here, we show that the optical 2D photon echo spectra of this complex at ambient temperature in aqueous solution do not provide evidence of any long-lived electronic quantum coherence, but confirm the orthodox view of rapidly decaying electronic quantum coherence on a time scale of 60 fs. Our results give no hint that electronic quantum coherence plays any biofunctional role in real photoactive biomolecular complexes. Since this natural energy transfer complex is rather small and has a structurally well defined protein with the distances between bacteriochlorophylls being comparable to other light-harvesting complexes, we anticipate that this finding is general and directly applies to even larger photoactive biomolecular complexes.