Photosynthesis and Quantum Coherence

A recent paper in Nature by Gregory Engel and co-workers presents direct evidence from two-dimensional Fourier Transform electronic spectroscopy that quantum coherence plays a role in energy transport within the Fenna-Matthews-Olson bacteriochlorophyll complex. Popular accounts of the research appear in various venues such as Wired magazine and The Scientist.

So what does that mean?

Photosynthetic light-harvesting is an incredibly efficient process, which seems at odds with the great complexity of the chain of molecules involved. The “wire” along which the excitation energy signal travels is sensitive to the precise geometric relationship between orbitals on all the molecules in the chain between the light-harvesting antenna protein and the reaction center, which in turn is sensitive to thermal agitations at the operating temperature of the photosynthetic system.

Past work on attempting to understand these processes has focused on semiclassical models involving (classical) “hopping” of energy between individual excited (quantum) states. The new experiment is a laser “pump-probe” measurement using ultrafast (sub-picosecond) laser pulses to excite one part of the system and detect changes remote from the original excitation. The key finding is the existence of “quantum beats”, correlations in phase between the wavefunction on one part of the system and another separated from it by a distance of thousands of atoms. These are illustrated below.

So, what’s the significance of all this?

The “beats” imply that the energy transport is wavelike, in other words truly quantum-mechanical (or “coherent”). The quantum state enveloping the whole photosynthetic system allows energy transport to occur along a superposition of multiple pathways, ensuring that the energy reaches its destination efficiently, with some degree of protection from the randomizing effects of thermal jostling. The energy travels along all possible pathways, so whatever the circumstances it travels along the “best” pathway (as well as all the others).

This is a remarkable result given the large size of the photosynthetic system. Such quantum effects are not observed in the macroscopic world, where “decoherence” scrambles the phase of the wavefunction, considerably impairing our ability to be in two places at once.

Full disclosure: I was first alerted to this paper via the blog Cosmic Variance (direct link via image above), where you can also see video of Drew Brees throwing a football at an archery target with rather disturbing accuracy.