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.
“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 Engels’ 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.
By 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. Engels 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. Engels 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?”
Awakening Sun
As the sun emerges from a long lull in activity, the star’s emissions in the radio band of the spectrum have also picked up. And from a shed on three acres of land outside Santa Fe, New Mexico, amateur radio astronomer Thomas Ashcraft is making recordings of them available for download.
“The Sun has become hyper-dynamic the past few days,” Ashcraft wrote on his website Sunday, along with links to four “specimens” of radio bursts, as he calls them.
The sun is crackling with solar flares now as a very large sunspot continues to circle our star. The recent solar activity almost assuredly signals the end of the solar minimum. Only 5 percent of the days in 2010 have seen a blank sun. In 2008 and 2009, more than 70 percent of the days had no sunspot activity.
Not all bursts sound the same, though. Another kind, Type V, is generally shorter and sharper. They happen to be Ashcraft’s favorites.
“I like that one because they are very strong and very fast,” Ashcraft told Wired.com. “They are only short lived, only a minute or two minutes. You can get a rush out of it. You can get high off of it. You can trip on it a little bit.”
The physics of solar radio emissions are quite complicated, but Ashcraft just likes to listen to the radio static out in the shed on his property. It gives him a feel for what the sun is doing, he said. He held up the phone to his speakers where the standard hiss of the radio, speckled by cosmic background radiation, constantly plays.
“I have that playing at a low level. I’m able to hear when there are sudden fluctuations,” Ashcraft said. “That makes me hypersensitive to the sun. I consider my antennas, which are mostly dipole antennas, I consider them my hyperextended nervous systems, so I can feel subtle solar movements.”
When he processes the recordings, Ashcraft likes to track one frequency (say, 21 megahertz) in one channel and another (say, 24 megahertz) in the other channel. It tends to give his specimens what he calls “spatiality” and a kind of pulsating effect. That’s because he isn’t just trying to record the sun, he’s trying to make it into something with which people can connect.
“I sort of see it as a possible musical form of the future. You know? An energetic form,” Ashcraft said. “Maybe the word isn’t even art anymore, it’s almost nutritional to the nervous system in a way that I don’t know about, but I’m groping towards, kind of as an artist.”
After almost 20 years of studying the sun, Ashcraft said his view of being a human has actually changed.
“I’m very conscious of myself as an organism, an electroreceptor sensing the sun,” Ashcroft said. “It’s human, but the human is a subset of being an organism.”