The model focuses on the production and manipulation of pairs of spin-entangled radical molecules in the retinas of birds' eyes. These molecules could theoretically sense Earth's magnetic field and pass this information to the bird's brain to allow for navigation.
But the original proof-of-concept for this model predicted the production rate of the entangled radical pairs to be too slow to be biologically useful, and researchers have long struggled to find a way around this problem.
In their new study, published today in Nature, a team of scientists from the University of California, Berkeley; Caltech; and the Australian National University in Canberra, Australia, developed a new approach that overcomes this speed limitation.
"Our work provides a path toward a quantum mechanical-based biological compass," said lead author Peter Hore, an emeritus professor of physical chemistry at the University of Oxford and visiting scholar in residence at UC Berkeley.
Biologists have known since the early 1970s that certain migratory birds have magnetite, an iron-oxide mineral that is slightly magnetic, in specialized cells in their beaks. One explanation is that the birds have a quantum compass, in which the electrons in magnetite are arranged in a very specific way that allows detection of Earth's magnetic field.
However, this magnetite-based model faces two major challenges, said UC Berkeley chemistry professor Adam Willard, a co-author of the paper. First, magnetite alone doesn't offer an explanation for how birds can sense Earth's weak magnetic field, about one ten-thousandth as strong as the field from a refrigerator magnet. Second, magnetite doesn't explain how certain long-distance migratory birds can sense the direction of the magnetic field with enough precision to migrate north or south.
A more promising explanation is based on quantum mechanics—a phenomenon of the natural world that happens at the level of atoms and subatomic particles. Quantum coherence, a specific type of quantum effect that involves the behavior of pairs of particles becoming linked or "entangled," has been shown in photosynthesis and other biological processes, and is currently being explored in the field of quantum computing.
Quantum entanglement is also the basis for radical pair recombination—a way in which energy can be transferred between two molecules when their electrons are entangled.
Researchers have focused on one specific type of entangled radical pair, formed between two cryptochrome proteins, that's found in various organisms including animals and plants. These radical pairs might interact with Earth's magnetic field in such a way that their properties become slightly different depending on the orientation of the molecule with respect to the field, which could then serve as a type of compass.
The original proof-of-concept for cryptochrome-based magnetoreception suffered from a crucial flaw, said Hore. Birds must be highly sensitive to any changes in the magnetic field, and the amount of change caused by the interaction of a single radical pair with Earth's weak magnetic field would be minute. In addition, the researchers calculated the number of radical pairs that could be formed during a bird's flight time and found it to be far too slow to be of use in magnetoreception.
In the new study, the researchers solved both of these problems. First, they found they could amplify the signal from the radical pairs by manipulating the radical pairs' chemical surroundings, which effectively increases the strength of the interaction between the magnetic field and the molecule.
The team also came up with a way to speed up the production rate of the entangled radical pair. They propose using light pulses from the bird's inner ear to directly excite the production of thousands of radical pairs, instead of relying on the thermal and chemical processes that lead to radical pair formation in plants. Because many more excited states can also produce radical pairs, this would overcome the problem of slow radical-pair production.
"The biological feasibility of these solutions is supported by the existence of both visual pigments and cryptochromes in the retinas of birds, and the demonstrated sensitivity of cryptochrome to blue or ultraviolet light," the researchers wrote in the paper.
To test the hypothesis, the researchers are performing experiments on cryptochromes in bird species such as European robins and zebra finches, as well as homing pigeons.
Willard said a practical application of this work could be a new, more sensitive compass based on quantum entanglement.
Hore added: "This work may also provide insights into other biologically relevant behaviors, such as the remarkable time sense of certain insects, in which an internal circadian clock must somehow interact with the external environmental cues."
The research was supported by the U.S. National Science Foundation, the W.M. Keck Foundation, the U.S. Army Research Office, and the Australian Research Council.
