Flying home with quantum physics
Quantum mechanics is usually associated with weird and counterintuitive phenomena we can't observe in real life. But it turns out that quantum processes can occur in living organisms, too, and with very concrete consequences. Some species of birds, for example, use quantum mechanics to navigate. And as Plus found out at a conference on quantum physics and the nature of reality, which took place in Oxford in September, studying these little creatures' quantum compass may help us achieve the holy grail of computer science: building a quantum computer.
At the conference Plus editor Rachel Thomas met up with the physicists Simon Benjamin and Erik Gauger, both from the University of Oxford, who were intrigued by research done with European Robins by biologists in Frankfurt, Germany. European robins spend their summers in Scandinavia, but avoid the chilly winter by migrating to North Africa in the autumn. Biologists believe that the birds' sense of direction comes from an internal quantum compass in the bird's eye which consists of two electrons and a quantum property called spin. Effectively, each electron behaves like a tiny bar magnet, which can point either up or down. (For a more detailed explanation of electron spin read this entertaining blog by Chad Orzel, which includes a demonstration by his toddler!)
"The two electrons [in the bird compass] are correlated with each other, with their spins pointing in different directions, " explains Benjamin. "They get excited when a photon is absorbed in the bird's eye. The two electron spins then move apart from each other. The way they behave afterward, whether they stay correlated as they were originally, or the correlations change, depends on the Earth's magnetic field." Thus able to sense the Earth's magnetic field, the birds know which direction to fly in.
Biologists have known about this theoretical model of the birds' navigation system, called the radical-pair model, for around thirty years. It's quantum mechanical, since spin is a quantum mechanical concept, but not sufficiently so to interest hard-core quantum physicists like Benjamin and Gauger. What caught their interest was some recent research by Roswitha and Wolfgang Wiltschko, from the Goethe University, into how easily the birds' quantum compass could be disrupted.
To test the bird compass, the researchers had kidnapped some birds on their way down to North Africa and subjected them to a weak oscillatory electromagnetic field, that is a field whose strength jitters backwards and forwards about a million times a second. "That's an incredibly weak oscillatory field," says Benjamin. "Not only could it not possibly harm the birds, but it would be amazing if the birds could even tell that there was this [oscillation]."
Surprisingly, though, this weak signal was enough to disrupt the birds' sense of direction. "The researchers found that at a particular speed of oscillation — 1.3 MHz — suddenly the birds were no longer able to orientate themselves," says Benjamin. "The direction they wanted to go in became random, no longer pointing to Africa."
Intrigued that such a tiny perturbation should have an effect on the birds, Benjamin and Gauger looked at the mathematics describing what goes on in the birds' quantum compass. They were particularly interested to see how long it would take for the effect of the field to kick in, since basic physics suggests that detecting signals as weak as that takes some time. "There must be time for this tiny effect to build up and make a difference for the bird," says Benjamin. Using their equations Benjamin and Gauger calculated that it would take at least 120 microseconds for the birds' compass to get jammed by the field. That's very fast, certainly a time period like that can't be detected by humans, but in terms of quantum processes it's rather slow.
And this is where quantum computers come into to the picture. As the name suggests, quantum computers work using quantum processes. No-one has as yet been able to build a useful working quantum computer, but once we do, these machines will be way faster and more powerful than ordinary computers.
Electrons and their spins form the basic components of quantum computers. "In [quantum computing] you care about in which direction electron spins point and how they correlate with each other," says Benjamin. "But in order to make a quantum computer work , you must insulate these electron spins, the tiny magnets, from the rest of the world. For that reason people have been trying to come up with molecules that can protect electron spins, to isolate them from the rest of the world."
A nitrogen-doped C60 molecule — an atom of nitrogen trapped in a carbon cage. Image Quantum Spin Dynamics group at the University of Oxford.
What Benjamin and Gauger realised is that the same goes for the the birds. For the bird compass to work, interference from the outside world must be kept down to a very low level. "Otherwise it would mess up such a long lasting sensing process," says Benjamin. Since it takes the bird at least 120 microseconds to detect the oscillatory field, it must be able to insulate its quantum compass from the outside world for at least that length of time, perhaps more. That's compared to the record of 80 microseconds that's so far been achieved in the lab. "It seems that the way the bird protects the pair of two tiny magnets is better than the best we can do," says Benjamin.
What's more, the exotic molecule used to insulate quantum systems in the lab — a nitrogen atom trapped inside a carbon cage, called N@C60 — is incredibly hard to make (and costs around £7 million a gramme). The birds certainly don't have access to this material, so the question is how they achieve their insulation and if we can copy their method to build quantum computers. "It's a series of ifs," says Benjamin. "Various things could be wrong. The experimental results could be wrong, or our basic idea of [how the quantum compass works] might be wrong. But if all the ingredients are correct and there really is this extraordinary protection of quantum information in the birds, then it's conceivable that we can work out what chemical it is and we might learn a thing or two."
You can listen to the podcast of our interview with Simon and Erik, as well as our podcast from the conference on Quantum Physics and the Nature of Reality. You can also learn more about quantum mechanics from Simon in Caging Schrödinger's Cat, his series of audio and video podcasts about quantum nanotechnology.