Code-breaking’s quantum leap

On 12 February, physicists in Shanghai announced a big breakthrough. They have linked eight subatomic particles in a way that might
one day allow them to break the world's most secret codes.

The link is called entanglement and it's a strange phenomenon. It occurs only in systems that follow the rules of quantum theory: things such as atoms, electrons or the particles of light known as photons. Entanglement creates links between certain of their properties. Do something
to one particle and you can instantaneously change the properties of the other even if it is half a universe away.

One application of this is in "quantum computing". Quantum computers encode numbers in the properties of photons or single atoms. Thanks to the weird nature of the quantum world, if you entangle these photons or atoms, you can encode many different numbers at once. This allows researchers to use entangled particles to perform many simultaneous computations, as if they had connected lots of computers.

With the latest milestone - eight entangled photons - researchers can, in theory, perform 256 simultaneous computations. And 256 is not
a number to be sniffed at.

If you could entangle 256 quantum particles, for instance, you would be able to do as many simultaneous computations as there are atoms in the universe. That is why national security agencies worry about (and sponsor) research into quantum computing. If you had a half-decent quantum computer, you would have the number-crunching power to break every encryption now in use.

Scientists don't pose a threat as yet, though. That is because the entanglement is too fragile. To perform experiments with entangled quantum particles, you have to work in isolated laboratories, usually at cryogenic temperatures. It has taken decades to get to eight entangled particles. But there is growing hope (or fear, depending on your perspective) that entanglement can be made robust.

It turns out that the warm, wet and messy natural world appears to be using entanglement to create its little miracles. Smell seems to be a quantum sense. Fruit flies can smell a difference between two identically shaped forms of the molecule acetophenone. The only satisfactory explanation for this invokes quantum entanglement between electrons involved in the flies' nerve signals.

The European robin also seems to have a quantum sense. Its sensitivity to magnetic fields, vital for migration journeys, seems to rely on an entanglement between photons in sunlight and electrons in the back of the bird's eye.

Chain gang

Most intriguing are the links between entanglement and energy production. Our bodies run on adenosine triphosphate, a chemical made in cells when electrons move through a chain of intermediate molecules. The process happens much faster than scientists can explain - apparently because of entanglement between the electrons.

That would certainly tie in with the way in which algae and bacteria process sunlight. The solar energy appears to travel simultaneously across all possible paths through the light-gathering molecules used for photosynthesis. The result is a high-efficiency energy transfer, similar to the efficiency a quantum computer achieves when it computes using all possible input numbers at once.

It's a long way from studying marine algae to undermining national security. However, it might not be as long as some would like - especially now
that the Chinese have become the world leaders in the field.

Michael Brooks's "Free Radicals: the Secret Anarchy of Science" is published by Profile Books (£12.99)