Bird Navigation Breaks Entanglement Record

In the race to own the golden goose that is quantum information processing, quantum physicists are scrabbling to find ways of storing and manipulating quantum information. That turns out to be hard. Quantum information is fragile stuff: sneeze and you lose it.

But while researchers have been puzzling over this problem for a few years now, nature has had 4.5 billion years to work on it using the tools of natural selection. Various scientists have pointed out that photosynthesis and bird navigation must rely on quantum effects (we’ve looked at them on this blog here and here). So it’s just possible that the solution to this problem of handling quantum information has been staring us in the face.

Now Vladko Vedral at the University of Oxford and a few pals have calculated just how good nature could be at this game. The answer is very good: it looks as if nature has worked out how to preserve entanglement at body temperature over time scales that physicists can only dream about.

The system that Vedral and co have studied is a model that describes how birds navigate using the earth’s magnetic field. The most recent thinking is that birds have molecules at the back of their eyes that are sensitive to both photons and the orientation of the earth’s magnetic field. When one of these molecules absorbs a photon, an electron pair is split, and one of these electrons is transferred to another part of the molecule. These electrons then form a “radical pair” that are entangled.

In the absence of a magnetic field, this pair would recombine to form the original molecular state. But the earth’s magnetic field can flip the spin of one of these electrons, allowing them to recombine in a different way and leaving the molecule in an alternative chemical state that the bird can sense. The result is that the bird “sees” the earth’s magnetic field as it flies.

This raises an interesting question: how long does this entangled state last?

Vedral and co have done the numbers and say that it lasts for at least 100 microseconds. That’s an extraordinary figure. The best that humans have measured is 80 microseconds for so-called electron spin relaxation in C60 buckyballs.

Curiously, entanglement is not being put to work in magnetoreception; it is simply a by-product of the process. It also seems to play a nonspeaking role in photosynthesis too, as we saw here. Given that nature seems to have created the conditions in which entanglement thrives, the big question now is whether there are any natural systems that exploit it.

Ref: arxiv.org/abs/0906.3725: Quantum Coherence and Entanglement in the Avian Compass