Wednesday, July 14, 2010

The limited relevance of quantum coherence in photosynthesis

Things that I think need to be born in mind.

In just a few very specific photosynthetic systems quantum coherence has been observed to last less than a psec at liquid nitrogen temperatures. It is far from a universal phenomenon. People have to go to exotic locales (such as the bottom of hot springs in Yellowstone) to find the relevant proteins.

Claims that coherence exists at room temperature are based on curve fitting with an excessive number of free parameters.

Biomolecules can only perform highly specific functions because they have specific properties. However, the converse does not necessarily apply. i.e., The existence of a specific property in a biomolecule does not imply that the property is then necessary for the function of the molecule. Many photosynthetic systems harvest light without quantum coherence.

Experimental signatures of entanglement (such as violation of Bell inequalities) have not yet been observed in photosynthetic systems.
I fear a reason why people talk about entanglement in photosynthesis is because of its compelling "sex appeal", particularly to people in the quantum information community.
Were such speculations necessary to get the first paper on this subject into Nature?

It is debatable whether quantum coherence in the one exciton sector itself is a signature of entanglement.

The fact that quantum coherence of excitons between a pair of chromophores imbedded inside the relevant membrane proteins can exist for hundreds of femtoseconds (even at room temperature) is actually not that surprising, being consistent with simple model calculations with physically realistic parameters.

1 comment:

  1. I think concerns about the relevance of quantum coherence in photosynthesis are certainly warranted, but in a few ways you are overstating the case.

    I would argue that coherence has been only shown in a very limited set of photosynthetic complexes because only a very limited set of complexes has been extensively studied. Isolating, crystallizing and characterizing a new complex is hard, never-mind performing the 2D spectroscopy experiments. That said, recent experiments on the LHC II complex of higher plants (about 50% of the world's chlorophyll) show the long lasting quantum coherence, too. Unstudied systems may or may not show such quantum coherence, but I have yet to hear of a negative result.

    Experiment results in this field (especially at high temperature) are indeed often subject to questionable amounts of curve fitting, but the methods used to verify coherence don't dependent on the particular shapes of the curves but rather general trends such as anti-correlated peaks. To me, a more compelling result is in a recent PNAS paper from Greg Engel's group which shows similar beating in the same complex across a range of temperatures. I think this evidence is pretty convincing for the possibility of long lasting coherence at room temperature -- at least long enough for a coherent oscillation or two.

    As for biological function, I absolutely agree. It remains the major challenge for theorists in this field to show that such coherences could actually lead to some plausible biological advantage. It remains entirely possible in my view that no such advantage exists at all, and that coherence is actually only a by-product of strong coupling and complexes well isolated by their protein environment.

    It seems popular to cite some recent work showing that decoherence is essential for function to make the argument that coherence has function, but to date I only know of speculation about mechanisms through which coherence could have a function.