Sunday, April 4, 2010

Everywhere in a Flash: The Quantum Physics of Photosynthesis


By hitting single molecules with quadrillionth-of-a-second laser pulses, scientists have revealed the quantum physics underlying photosynthesis, the process used by plants and bacteria to capture light’s energy at efficiencies unapproached by human engineers.

The quantum wizardry appears to occur in each of a photosynthetic cell’s millions of antenna proteins. These route energy from electrons spinning in photon-sensitive molecules to nearby reaction-center proteins, which convert it to cell-driving charges.

Almost no energy is lost in between. That’s because it exists in multiple places at once, and always finds the shortest path.

pc645-view4“The analogy I like is if you have three ways of driving home through rush hour traffic. On any given day, you take only one. You don’t know if the other routes would be quicker or slower. But in quantum mechanics, you can take all three of these routes simultaneously. You don’t specify where you are until you arrive, so you always choose the quickest route,” said Greg Scholes, a University of Toronto biophysicist.

Scholes’ findings, published Wednesday in Nature, are the strongest evidence yet for coherence — the technical name for multiple-state existence — in photosynthesis.

Two years ago, researchers led by then-University of California at Berkeley chemist Greg Engel found coherence in the antenna proteins of green sulfur bacteria. But their observations were made at temperatures below minus 300 degrees Fahrenheit, useful for slowing ultrafast quantum activities but leaving open the question of whether coherence operates in everyday conditions.

The Nature findings, made at room temperature in common marine algae, show that it does. Moreover, similar results from an experiment on another, simpler light-harvesting structure, announced by Engel’s group last Thursday on the pre-publication online arXiv, suggest that photosynthetic coherence is routine.

The findings are wondrous in themselves, adding a new dimension to something taught — incompletely, it now seems — to every high school biology student. They also have important implications for designers of solar cells and computers, who could benefit from quantum physics conducted in nonfrigid conditions.

“There’s every reason to believe this is a general phenomenon,” said Engel, now at the University of Chicago. He called Scholes’ finding “an extraordinary result” that “shows us a new way to use quantum effects at high temperatures.”

Scholes’ team experimented on an antenna protein called PC645, already imaged at the atomic scale in earlier studies. That precise characterization allowed them to target molecules with laser pulses lasting for one-quadrillionth of a second, or just long enough to set single electrons spinning.

quantumphotographBy analyzing changes to a laser beam sent through the protein immediately afterwards, the researchers were able to extrapolate what was happening inside — an ultra-high-tech version of shadows on a screen. They found that energy patterns in distant molecules fluctuated in ways that betrayed a connection to each other, something only possible through quantum coherence.

“It’s the same as when you hit two tuning forks at the same time, and hear a low-pitched oscillation in the background. That’s the interference of sound waves from the forks. That’s exactly what we see,” said Scholes.

According to Scholes, the physics of photosynthetic proteins will be further studied and used to improve solar cell design. Engel suggested their use in long-promised but still-unworkable quantum computing. “This allows us to think about photosynthesis as non-unitary quantum computation,” he said.

Quantum-physical processes have been observed elsewhere in the biological realm, most notably in compass cells that allow birds to navigate by Earth’s geomagnetic fields. Researchers have also proposed roles for quantum physics in the animal sense of smell and even in the brain. Engel predicts the emergence of an entire field of quantum biology.

“There are going to be some surprises,” said Scholes. “Who knows what else there is to discover?”

Images: 1. Bùi Linh Ngân/Flickr
2. Antenna protein: Light-harvesting molecules are red./Greg Scholes
3. Graph of energy wave interference inside the antenna protein/


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