This is an idea that has been bouncing around for a while, but is now apparently seen in experiments: real-world photosynthesis taking advantage of quantum mechanics. (Story in Wired, via @symmetrymag. Here’s the Nature paper on which it’s all based.)
The idea is both simple and awesome: you want to transport energy through an “antenna protein” in a plant cell to the “reaction-center proteins” where it is chemically converted into something useful for the rest of the plant. Obviously you’d like to transport that energy in the most efficient way possible, but you’re in a warm and wet environment where losses are to be expected. But the plants somehow manage the nearly impossible, of sending the energy with nearly perfect efficiency through the judicious use of quantum mechanics.
We can think about this in terms of Feynman’s way of talking about quantum mechanics: rather than a particle taking a unique path between two points, as in classical mechanics, a quantum particle takes every possible path, with simple paths getting a bit more weight than complicated ones. In the case of the protein, different paths for the energy might be more or less efficient at any particular moment, but this bit of quantum trickery allows the energy to find the best possible route at any one time. Imagine at rush hour, if your car could take every possible route from your home to the office, and the time it officially took would be whatever turned out to be the shortest path. How awesome would that be?
The reason you can’t do that is that your car is a giant macroscopic object that can’t really be in two places at once, even though the world is governed by quantum mechanics at a deep level. And the reason for that is decoherence — even if you tried to put your car into a superposition of “take the freeway” and “take the local roads,” it is constantly interacting with the outside world, which “collapses the wave function” and keeps your car looking extremely classical.
Proteins in plants aren’t as big as cars, but they’re still made of a very large number of atoms, and they’re constantly bumping into other molecules around them. That’s why it’s amazing that they can actually maintain quantum coherence long enough to pull off this energy-transport trick. Previous studies had hinted at the possibility, but only by cooling the proteins down and shielding them from external jiggling. This new work happens at room temperature in the context of marine algae, so it seems to indicate that it can happen in real environments.
One step closer to building my teleportation machine. Get to work, quantum engineers!
I’ve been pretty skeptical of this hypothesis, since it’s not easy to imagine a cactus in Death Valley, using macromolecules, can pull off what physicists struggle to do with a handful of atoms in a vacuum near absolute zero.
But just maybe it can!
And, yeah, the implications of this for materials science and engineering are pretty damn staggering.
Dr. Sean said
“The reason you can’t do that is that your car is a giant macroscopic object that can’t really be in two places at once, even though the world is governed by quantum mechanics at a deep level. And the reason for that is decoherence — even if you tried to put your car into a superposition of “take the freeway” and “take the local roads,” it is constantly interacting with the outside world, which “collapses the wave function” and keeps your car looking extremely classical.”
I don’t enough, so my question may look naive, but If this is the situation with the car, why some scientists are confused that Schrödinger’s cat
(macroscopic object) is dead or alive? Shouldn’t we also be sure that the cat must be in one state as the car, since both are macroscopic objects?
If Schrödinger’s cat were interacting with its environment (which it almost certainly would be, in the real world), then indeed it would have its wave function collapsed all the time. But it’s just a thought experiment, so we are free to make idealizations.
On the downside: The “How could evolution possibly create an eye?” people, who became the “How could evolution possibly create a flagellum?” people, will now become the “How could evolution possibly create a quantum photosynthesis thingamajig?” people.
I still take issue with your phrasing that “a quantum particle takes every possible path, with simple paths getting a bit more weight than complicated ones.” I consider the *equal* weight of every path to be one of the truly remarkable (and fundamental) aspects of Feynman’s formalism. So I might instead phrase your point along the following lines: “a quantum particle takes every possible path, but complicated paths cancel each other out so that only the simple ones remain.”
@Lonely flower
The Schroedinger’s Cat experiment is a metaphor , and is clearly not the case in a macroscopic world.
Sean: “Proteins in plants aren’t as big as cars, but they’re still made of a very large number of atoms, and they’re constantly bumping into other molecules around them. That’s why it’s amazing that they can actually maintain quantum coherence long enough to pull off this energy-transport trick.”
Can you explain a bit more why this is so unexpected?
Naively I would think that all proteins need to do is restrict the excited states they work with to such which are unlikely to decay due to environmental perturbations – for example their energy may be in the range which cannot easily be absorbed by surrounding molecules. Since reaction centers can be buried deep within proteins, meaning their immediate surrounding can be tailored as needed, I was thinking it should be doable.
Is there something in QM which makes such “shielding” hard or impossible?
Hello Prof. Carroll. It looks like your Nature link is to the older Engel, et.al paper from a couple of years ago. The Wired article cites the Scholes team work in the current issue and also notes a new arxiv preprint from the Engel team from last week.
http://www.nature.com/nature/journal/v463/n7281/full/nature08811.html
http://arxiv.org/abs/1001.5108
Impressive. Though I seem to recall some previous work showing that not just light is at work at the quantum level in enzymes, in some cases proton transfers have kinetics that can only be explained if their mechanism of action is through tunnelling (but I may well misremember).
Thanks for the DVDs! I’d almost forgotten about them, so I was rather surprised when the taxman sent me a bill.
All proteins have a pretty high absorbance at 230nm due to the peptide bonds. I think tyrosines absorb pretty readily at 280nm. I should think that any protein exposed to sunlight should be constantly interacting quite readily with UVB and C photons that make it through the ozone. Plus, a polypeptide at room temperature has an average quaternary structure, but it is far from rigid, and my understanding is that rotations and flexions about any number of bonds at any given time for any given protein are frequent, and proportional to the thermal energy in the environment. That’s just light on either end of the visible spectrum, forget about all the matter flying around. I’m no biophysicist, but wouldn’t that imply that proteins, out in the sun, at around 20 deg. C or more are for all purposes in constant interaction with their environment? I’m not sure it’s at all expected that a particular pocket or other domain in a protein would or could be shielded from thermal energy, or higher energy photons. Perhaps I’m wrong, my recollection from IR spectroscopy is that one can expect that every amino acid in a polypeptide is probably jiggling around quite a bit at anything near physiological temperatures. So, again, my non-expert perspective is that these results, if they hold up, are really quite amazing. Maybe I’m too easily impressed, I dunno.
its chemistry. its all quantum.
“Long lived” quantum coherence in photosynthesis means for perhaps 500fs, compared to naive predictions of 100fs or so. Transport through these complexes still takes at least several picoseconds, so the the entire process is not coherent, just part of it.
I would hesitate before making the leap from the compelling evidence for quantum coherence to unsupported notion that some sort of check-all-pathways at once quantum search is going on. The jury is still out on whether coherence is plays any sort of functional role in photosynthesis, and a quantum search seems particularly far-fetched. For more details, see my arXiv preprint, accepted last week for publication by New Journal of Physics: http://arxiv.org/abs/0910.1847
Is there another unrelated super-efficient process called “photosynthesis”? Because the one most people call “photosynthesis” has low-single-digit-% efficiency.
Numbers here: http://en.wikipedia.org/wiki/Photosynthetic_efficiency (or in any biochemistry textbook)
My understanding is that not all pathways are actually ‘taken’ at once in any real(ism) sense but rather that all possible pathways are ‘evaluated’ and the least required ‘actions’ determine the more likely favored pathways or outcomes in general. What’s weird for me is to think of the possible paths as probabilistic if the optimum path always wins out. So i model it in my mind as though there were an electric field potential across all the possible paths like between the plates of a capacitor and the shortest past is then the one with the best field strength so that’s the route actually taken.
Tomasz — The overall process of photosynthesis has low efficiency in converting solar into chemical energy, but the process of capturing and transferring energy from individual photons in these antennae complexes is extremely efficient. The loss of energy happens after it is converted from electronic excitations when it triggers chemical reactions. The super-efficient process is just the first stage that takes place in the complexes studied in these papers.
My favorite handy example a macroscopic system whose wavefunction is a superposition of opposite states is a blackbody { perfect absorber + perfect radiator }. But a perfect skating rink also fits that superposition in that if you run onto one, it perfectly allows your ingress and then your egress off the other end.
Maybe we just need to recognize, philosophically, that the superposition in macroscopic systems is the fact that the macroscopic system consists of the superposition of all its subsystems which in turn are superpositions of further subsystems all the way down to the most highly probabilistic quantum level we are used to talking about. The outermost nested shell doesn’t look like superposition very much because it’s in a perpetually collapsed state from the many ongoing interactions with its subsystems in a kind of Zeno effect.
Here’s an article in 2007
So does this make the idea of quantum processes as a crucial part of the functioning of neurons any more plausible?
The cat is, as far as we observers are concerned, enjoyed being in the superposed states of alive and dead, and it is only when we interact with its system to observe it that the states collapse to one or the other.
But.
For my variation, I cannot but help note that cats are also astute observers.
Not quite the same observation as objecting that a cat is a macro object.
@ Wondering
sounds like the Invariant Set Postulate
@GreyGaffer
There is no cat.
@Low Math, Meekly Interacting:
Yes, atoms in proteins are certainly jiggling around quite a bit due to thermal motions but I suspect such atomic motions are much to slow to cause decoherence in time it takes for the energy to reach it’s destination. Thermal radiation would be fast enough but in this case the wavelength is most likely too long to cause collapse.
For example AFAIR in the electron double slit experiment the photons do not destroy coherence (and interference pattern) if their wavelength is much larger then separation between paths as this makes them unable to identify and carry away “which way” information. Only photons whose wavelength is comparable or shorter then the separation will destroy coherence since this allows them to differentiate between the two cases (electron taking path 1 and electron taking path 2).
I assume the same principle holds also for molecular superpositions and therefore EM radiation would have to have a wavelength comparable to the distance on which energy is transported to threaten coherence of this process. Judging from the structure of chloroplasts this distance in photosynthesis is probably smaller then 100nm (roughly the size of one thylakoid structure within chloroplast) which interestingly enough is comparable to the shortest wavelength of UV C. (Perhaps it is not a coincidence and the shortest UV C wavelength actually determines the maximum size of thylakoids still able to utilize coherent energy transport).
All this leads me to think that thermal radiation or sunlight should not prevent coherent energy transfer on distances of up to a few tens of nanometers, unfortunately as I am a molecular biologist not a physicist my knowledge of QM is insufficient to be sure this makes sense or that I am not missing something obvious which alters this picture.
Perhaps someone with more expertise could clarify what are the primary mechanism responsible for decoherence in such conditions?
#14 Wondering – see Feynman’s “QED: the strange theory …”
That is the clearest explanation I have ever seen, and better
than his earlier explanations. All particles are oscillators and
hence the phase must be taken into account. He derives the
principle of Least Time.
Interesting points, Paul7.
Surprising (to me) is the authors’ speculation that the covalent linkage of the bilin chromophores to the proteins facilitates the unexpectedly long state of coherence. I’m sure this is just my ignorance at work, but I would have guessed naively that sharing electrons, instead of “floating” in some pocket or other structure due to electrostatic attractions/repulsions (mediated only by the exchange of virtual photons, I think) would make the chromophores more susceptible to perturbations from the immediate environment, not less.
Anyway, little blurb in SciAm for those who are interested
http://www.scientificamerican.com/article.cfm?id=shining-a-light-on-plants-quantum-secret
It is easy to forget that cosmologically the entire universe is a quantum entity. “macroscopic” and “microscopic” are simply ways of observing information embedded in the manifold. Likewise, the “arrow of time” is a frame of reference phenomenon. We cannot ever divorce ourselves from the process of (electromagnetic) observation…in fact, the scientific method itself is dependent on the “process” of observation.
Sean’s little thread is excellent! We need to remind ourselves that in quantum mechanical determinisim, a cat is both road pizza and purring on our lap…permanently and really (not a “thought experiment”). I exist both as ashes in an urn and a very much alive person, permanently embedded as both, and eternally being both depending on taking a cerrtain observational frame or sets of frames.
I’m reminded of the work of Fred Hoyle with thermonuclear fusion inside stars. When he discovered that stars produce Carbon in just the quantities necessary to make life possible, and that the thermonuclear processes necessary to bring that combination about were a part of the information stored in the manifold, he exclaimed that life “monkey’s around”…
and “monkey around” life indeed does. Consciousness is entangled right with the rest of the information. There are many kinds of horizons in this universe. We have a light cone. When we experience death we also move over a certain horizon. Any observer, anywhere in the universe…even beyond our light cone, would observe the big bang as having occurred 13.7BY ago. Everything is just there, and works together to create the whole of existence- the Uni-verse…the one song.