Quantum 'weirdness' used by plants, animals
Bird navigation, plant photosynthesis and the human sense of smell all represent ways living things appear to exploit the oddities of quantum physics, scientists are finding.
Quantum mechanics is the branch of physics dealing with the strange behaviour of very tiny things like elementary particles and atoms, and is extremely different from the physics that humans experience every day.
"Down at that level, everything is pretty darn weird," Seth Lloyd said before giving a lecture about quantum aspects of biology Wednesday evening in Waterloo, Ont.
"Electrons can be in two places in once, or five places at once, or a thousand places at once. And then there are these funky quantum effects like 'entanglement,' where two different systems can have more information about each other than they have any right to have under classical mechanics," said Lloyd, a professor of quantum mechanical engineering at the Massachusetts Institute of Technology.
"When things get bigger, certainly on the scale of human beings or even at the scale of bacteria, then this kind of quantum weirdness tends to go away."
Sensor, solar cell lessons
Seth Lloyd titled his talk "Quantum Hanky-Panky," partly because living things exploit quantum mechanics to ultimately better their chances of surviving, having sex and reproducing, he said, and partly because "quantum mechanics is weird and strange and it seems like it's doing something it shouldn't be doing."
The lecture, organized by the Perimeter Institute for Theoretical Physics, was scheduled for 7 p.m. ET on Nov. 3 at Waterloo Collegiate Institute.
Afterward, it was to be posted online in the Perimeter Institute's public lecture archive.
It has only been in the past three or four years that scientists have started figuring out how quantum mechanics is exploited by animals, plants and bacteria to give some of them a keen sense of smell and others a very efficient way to harness energy from the sun, said Lloyd.
That means humans might be able to develop technology for more accurate sensors or far more efficient solar cells by mimicking the way living things use quantum physics, he said. "When you want to make an ultimate sensor, something that senses things at the limits allowed by the laws of physics — well, at that level, everything is quantum mechanical."
In the case of human smell, it appears that receptors are triggered in part by "phonons" — tiny vibrational phenomena that provide the extra energy needed to create a signal that we recognize as a scent.
"This process, which is called phonon-assisted tunnelling, is a purely quantum mechanical process," Lloyd said. "It can't be explained by ordinary classical models."
Lloyd said quantum mechanics likely plays a key role in vision as well, because vision systems in animals are sensitive to individual quantum particles of light called photons.
Birds use quantum compass
Meanwhile, birds like the European robin have an internal compass that helps them navigate, and it appears to make use of quantum entanglement — a linkage of two or more very small objects so that any change to one is immediately experienced by another, no matter how far apart they are.
Lloyd said birds can sense the orientation of the Earth's magnetic field, but can't tell the difference between north or south, and become disoriented by oscillating magnetic fields like microwaves.
Physics experiments show that certain entangled electrons are also very sensitive to the orientation of weak magnetic fields, and the birds' behaviour suggests they are using that to navigate.
Lloyd's biological research, funded by the U.S. Defense Advanced Research Projects Agency, looks at how living things use quantum computation.
Lloyd said he got into the area about 3½ years ago when someone in his lab found an article in the New York Times about researchers in Berkeley, Calif., who claimed green sulphur bacteria were performing a type of quantum calculation called a quantum search process while using photosynthesis to turn sunlight into energy.
"We thought that was really hysterical," he recalled. "It's like, 'Oh my God, that's the most crackpot thing I've heard in my life!'" But after looking into it, he realized that although the bacteria weren't performing quantum search, they were doing a different type of quantum computation.
When sunlight hits the part of the bacteria that collects sunlight, it creates a quantum particle of energy called an exciton. That exciton must travel through a complex that Lloyd likens to a gigantic forest in order to get to a place where it can be turned into chemical energy.
"If you look at kind of a classical way of getting through some forest in the dark, surrounded by trees with no notion of what the direction is you're supposed to go, then you just wander around at random … and you just get completely lost," Lloyd said. Consequently, scientists were puzzled about how the exciton ever arrived at its destination.
"If it uses quantum mechanics, it's not limited to just taking just one path through the forest," Lloyd said. "It can take all possible paths simultaneously. In quantum computing, this is what's called a quantum walk."
As it turns out, plants use the same trick to achieve photosynthesis.
Lloyd had spent his research career studying quantum computation and designing quantum computers. For humans, that's a relatively new field. "It turns out," Lloyd said, "that bacteria have been up to quantum computation for hundreds of millions of years."