Friday, 1 February 2013

Quantum vampires

The title of this piece may sound like the latest Young Adult bestseller (and I reserve all rights, thank you very much) but I was thinking of something a little more down to earth... yet at the same time rather more exciting. Even though it has been out for a while, I get more emails about my book on quantum entanglement, The God Effect than almost any other. I think it is because the subject is mind-boggling even to physicists (the whole business really started when Einstein wrote a paper to the effect of 'this entanglement stuff is so weird, quantum theory must be wrong'... but it was Einstein who was proved to be in error), and because some of the applications are amazing, notably quantum teleportation, which produces an effect like a Star Trek transporter on the scale of quantum particles.

I just thought I'd give a taster for the subject by using a little extract from The God Effect where the scientists head for the sewers:

By 2004, [Anton] Zeilinger and his team had achieved teleportation over significantly greater distances – in fact across the river Danube. A year after their ground-breaking long range transmission of entangled photons across the Danube, the Austrian team was back in the sewers, this time achieving teleportation from one side of the river to the other. (Quantum entanglement experimenters seem to have a functional relationship with the sewage system rivaled only by utility workers and Buffy the Vampire Slayer.)

As always with teleportation there are two “channels”, one carrying the entangled particles, the other transmitting the conventional information that will be used to complete the teleportation process. Entangled photons were pumped along a fiber optic cable running through the sewer system under the Danube, while the conventional information was beamed by microwave for 600 meters across the river. This may not seem ground breaking, but as their paper in Nature commented they had “demonstrated quantum teleportation over a long distance and with high fidelity under real world conditions outside a laboratory”. 

This is a significant blow to those critics who have said that teleportation could only occur under highly controlled laboratory conditions. The team points out that it’s also possible that this technique could be used as an alternative approach to make quantum repeaters that would enable entanglement to be shared anywhere around the world, as teleporting an entangled particle transfers the particle’s state, including its entanglement.

As this demonstrates, even if there never can be “real” teleportation of physical objects, it doesn’t mean that this isn’t a development of great importance. Teleportation even its limited form will prove vitally useful in making quantum computers real. Quantum computers rely on qubits, where information is stored in the quantum state of a particle. This may be very powerful, but it is also difficult to transfer that quantum state safely from place to place within the computer – or even between two quantum computers.

Teleportation means that, provided a supply of entangled particles is available, something that is now relatively easy to achieve, a qubit can be teleported from one place to another using only a conventional link. So a satellite pumping out entangled photons to two locations could enable quantum computers in two locations to swap qubits over the Internet.

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