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Physics + MathPhysics & Math

A bird’s eye view of quantum entanglement

Scientists have long wondered how birds “read” Earth’s magnetic field to navigate. Some think entangled particles in birds’ eyes play a role.

ByKatherine J. WuNOVA WondersNOVA Wonders

European robins make harrowing, cross-continental migrations to escape harsh winters. To guide their way, these birds and others may use quantum entanglement to "read" Earth's magnetic field. Image Credit: Frank Vassen, flickr

In a bare, windowless room, a lone robin stretches her wings. The chamber is silent and dark, illuminated only by a dim artificial light source. But even with no apparent connection to the outside world, the small bird repeatedly flutters in the same direction, guided by an invisible force. It’s a scene scientists have watched play out again and again.

That force, it turns out, is Earth’s magnetic field, which arises from the electric currents in our planet’s molten, metallic core. Scientists have long known that birds are equipped with a mysterious ability to tune in to these orienting forces, but exactly how this works hasn’t been clear. But some researchers say they have a good working theory—and it shows that avian evolution has tapped into quantum mechanics.

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Earth’s magnetic field is perhaps the most reliable navigational guide in nature, constant even when neither sunlight nor starlight is available, or landmarks fail to guide the way.

“Birds have to use whatever information they can get from their environment when they’re migrating,” says Roswitha Wiltschko, a biologist at Goethe University Frankfurt who, together with her husband Wolfgang, was among the first to study this phenomenon in the 1970s. “If they want to use the sun or the stars, they have to interpret what they see. But the direction of the magnetic field is direct.”

Despite its ubiquity, however, the magnetic field isn’t very strong, making it difficult to imagine how it could affect something as robust as an animal’s senses.

The key, it seems, isn’t the strength of the magnetic field, but the sensitivity of the system that’s detecting it. And in the eyes of some birds, there appears to exist such a system, built around a light-sensing protein called cryptochrome. Cryptochrome is common in both plants and animals, but researchers believe that birds make a special variant of this protein that operates as a molecular compass.

The current working theory is this: When a particle of light, or photon, hits bird cryptochrome, its energy can perturb molecules within the protein. The disturbance catapults a pair of molecules into an unstable state so fragile that it can be affected by even the subtle energetic pulse of Earth’s magnetic field.

That’s the bird’s eye view of things. To understand the role of quantum mechanics in this system—and how even quantum entanglement makes a brief cameo—we have to go a bit deeper.


Migrating birds use a combination of cues from their environment to find their way from point A to point B, including the sun, stars, landmarks, and Earth's magnetic field. Image Credit: Mark Round, flickr

The energy from that photon is specifically disturbing the electrons within cryptochrome molecules. If an electron is knocked out of place—perhaps exiting one molecule and joining another—this creates what’s called a radical pair, or a duo of molecules that each has an odd number of electrons. But this radical pair is also somewhat unusual: Because both radicals were generated at the same time, the fates of the two molecules involved are actually linked, locking them into a strange, delicate state known as quantum entanglement.

The consequences of this are a little mind-bending. Even after entangled molecules have been physically separated, the molecules are completely synchronized: Fiddling with the properties of one molecule instantaneously alters its partner in exactly the same way. When it comes to entangled pairs, “you can only describe the two [molecules] together,” says Erik Gauger, a quantum scientist at Heriot-Watt University who has studied magneto-sensing in birds. “You can’t describe just one by itself.”

The entangled, radical pair state is a precarious one to be in, and it’s not built to last. Eventually, the two molecules will recover and settle back into their original configuration. But until they do, the pair, still intertwined, will actually flip-flop back and forth between two distinct chemical states. And this is our link back to navigation: The amount of time the duo spends in one state versus the other is thought to affect how the bird’s eye relays information to the brain about the surrounding environment.

This is where the magnetic field comes in: Scientists think it biases the proportion of time that this radical pair spends in one chemical state versus the other—and thus what’s communicated between eye and brain. Scientists are still unclear on how exactly the information is passed along, but one possibility is this: One of these states helps produce a specific chemical, while the other does not. The buildup (or absence) of this chemical might then influence how signals are received by the brain’s visual cortex.

Of course, that’s still a far chirp from “hey, turn left.” There are still a lot of puzzle pieces missing, and the theory is just that—a theory.

Even so, there’s evidence that radical pairs are behind the avian magnetic compass. Because cryptochrome plays many roles in animal bodies, it’s a little hard to manipulate without causing a lot of other effects. But researchers have found a different way to mess with this delicate system: exposing birds to radio frequencies that interfere with their perception of Earth’s magnetic field. Even if these artificial sources of energy are 3,000 times weaker than Earth’s natural magnetic field, migratory birds get reliably discombobulated.

That’s an astounding level of sensitivity, Gauger says. Only a radical pair system that maintained entanglement for a fairly long time would be sensitive enough to be derailed by such a weak radiofrequency field. To put this in perspective, think of the radical pair system as a camera, on which the exposure time needs to be dialed way up to capture a very dim source of light—that is, the artificial radio frequency.

In entanglement terms, “a fairly long time” is still only about 100 microseconds (or one ten-thousandth of a second). Even in ideal laboratory conditions, which usually involve powerful vacuums or astoundingly icy temperatures, artificial quantum entanglement can unravel in just nanoseconds. And yet, in the wet, messy environment of a bird’s eye, entanglement holds. “It seems nature has found a way to make these quantum states live much longer than we’d expect, and much longer than we can do in the lab,” Gauger says. “No one thought that was possible.”


Scientists believe they've found a cryptochrome gene in zebra finches that may be the basis for these birds' ability to sense Earth's magnetic field. Image Credit: Peripitus, Wikimedia Commons

Baffling though these ideas may be, there aren’t really any credible alternative explanations for the phenomenon. As such, the radical pair theory remains the most widely accepted one in the field. And the evidence for it continues to build.

Completely depriving birds of all light shuts down the compass function (but it still works at night in nature; even after the suns sets, it’s not entirely pitch black). And just last year, two independent research groups proposed that a specific variant of cryptochrome called Cry4 is the long-awaited basis of the molecular compass. Cry4 is made in the right kinds of light-sensing cells, and birds even seem to produce more of it when they’re gearing up for big, cross-continental trips. “All this goes hand in hand with how we expect that protein to work,” says Lauren Jarocha, a chemist at the University of Oxford who is studying the chemical basis of magneto-sensitivity in animals.

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Researchers may have also identified the region of the brain where the magnetic information is probably processed—a region called Cluster N that’s located in the visual center of the brain. What traverses the mysterious gap between Cry4 and Cluster N isn’t clear yet.

But if the researchers’ theory is right, it means that, for millennia, birds have been inadvertently utilizing quantum mechanics—a field we humans still struggle to understand and put to use. The system might not even be restricted to migratory birds: Wiltschko believes that it could even play a role in the day-to-day putterings of the domestic chicken.

Other, non-avian animals, like mole rats and bats, also appear to have their own ways of using Earth’s magnetic field, though likely through a different system. And while Wiltschko says there’s no real evidence (yet) of an internal magnetic compass in humans, researchers are searching for ways to build those abilities with quantum technology. “If we could make our own molecular compass, there’s so much we could do with it,” Jarocha says. One option might be an advanced GPS that relies on chemistry rather than satellites.

Studying these systems in animals, Jarocha says, is what will ultimately help us understand how we can control the quantum world. After all, Gauger says, “nature is an ingenious architect.”

Major funding for NOVA Wonders is provided by the National Science Foundation, the Gordon and Betty Moore Foundation, the Alfred P. Sloan Foundation and public television viewers, with additional funding for “Are We Alone?” and “What’s the Universe Made Of?” provided by the John Templeton Foundation.

This material is based upon work supported by the National Science Foundation under Grant No. DRL-1420749. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

Major funding for NOVA is provided by the David H. Koch Fund for Science, the Corporation for Public Broadcasting, and PBS viewers. Additional funding is provided by the NOVA Science Trust.