Source: The Electronic Telegraph via UASR
Date: Nov 26, 1998

Teleportation has at last been demonstrated . . . but we are light years away from Star Trek technology, says Roland Pease

We could make you a quantum computer

IT started as an intellectual battle between two of the scientific giants of this century, Albert Einstein, and Niels Bohr, the Danish father of nuclear physics. But in the past weeks it has ended up in even the pages of a tabloid newspaper.

The world has witnessed the first full demonstration of teleportation, a theoretical idea resembling Star Trek's transporter dreamt up just five years ago by Charles Bennett of IBM and an international group of theoretical physicists.

In Star Trek, Scotty's transporter room dematerialises crew members such as Mr Spock and reconstitutes them down on some planetary surface. The new quantum teleporter doesn't quite do that. But it can send the atomic "vital statistics" you'd need to put all of Mr Spock's body parts in the right place and in an identical condition. Or it would if it could handle more than a single atom at a time.

In radio transmissions, there is a connection in the form of radio waves. Yet in this feat of quantum magic, information is transmitted instantaneously and the communication channel is immaterial.

The connection with Einstein lies in quantum theory, the science dreamt up by Bohr and his acolytes in the Twenties to explain the behaviour of light and atoms. So powerful is the theory in explaining the properties of matter that even this year, the Nobel prize in chemistry went to researchers who found new ways to use it.

But there was a price, which Einstein refused to accept. It was the loss of all certainty. The laws of quantum theory allow atoms to make apparently random choices. Any measurement of a quantum property is liable to change that property in the process. That also means, for example, that the more accurately you know the position of a subatomic particle, the less accurately you can know its speed at the same time. Atoms can also be in two places at once, or behave in two different ways simultaneously.

Indeed, if it wasn't for the fact that every theoretical prediction of quantum theory had been proved precisely correct it would have been abandoned long ago for being too ridiculous. Einstein fought to the end against what he saw as an assault on reality.

Teleportation encapsulates both the problem with quantum theory, and its magic. The problem is that for Scotty to send a complete instruction set on how to rebuild one of the Enterprise crew, he would need to measure the quantum state of every one of his atoms, in other words its energy, its position, its state of rotation.

But quantum theory says that any attempt to read the complete state would scramble it. That would seem to be the end of it, and time for a rewrite of the series.

But Charles Bennett and his colleagues at IBM found a way out back in 1993. And it works because you can send the quantum information so long as you do not know the details of what you are sending.

This bizarre solution has its roots in a thought-experiment Einstein and two sidekicks described in the mid-Thirties to highlight a profound flaw in quantum theory. They noted that the theory applied not just to single atoms but to molecules made of many atoms as well. So, for example, a molecule containing two atoms could be described by a simple quantum expression, called its wavefunction, so that one could know a certain amount about the molecule, but next to nothing about its constituent parts.

Einstein realised that if you then separated these two atoms to a great distance, they would both be described by the same single wavefunction. But the moment you measured something about one atom, the state of the other atom would instantaneously be transformed in a quite specific way. And that, he said, would violate his theory of relativity, which says that nothing can travel faster than light.

To put the idea to the test was beyond the technology of the Thirties. But when the experiment was done, quantum theory's bizarre prediction was borne out.

Because tweaking one atom means automatically tweaking the other, Bennett and his co-workers realised the pairs of atoms in effect made a "quantum phoneline" across space. This opened up the possibility of transmitting the kind of atomic data the Star Trek transporter needs to send.

And that is the idea that has now been demonstrated by Jeff Kimble of the Caltech, along with Samuel Braunstein of the University of Wales at Bangor and others.

First, the sender and receiver, usually called Alice and Bob, share a pair of atoms (actually a pair of atoms of light, called photons) like the ones described by Einstein - "entangled", in the parlance of quantum theorists.

Now Alice wants to transmit the quantum state of a sample photon she has in her pocket. What she has to do is make a measurement comparing her sample photon with her half of the entangled pair. She knows the state of neither, so making this joint measurement tells her nothing about the actually state of her photon.

But it does tweak the entangled pair so that the photon at Bob's end is now ready to be transformed into a copy of Alice's original. The measurement also gives her a number which she then sends to Bob by ordinary means - for example by telephone. With this additional information, Bob can give his photon one further tweak, transforming it into a copy of Alice's original.

Remarkable as this is, it is still a long way in reality to building anything like a Star Trek transporter. Astrophysicist Lawrence Krauss calculated that it would take more than the age of the Universe to get Mr Spock's vital statistics down to the planetary surface.

But the idea does have its uses. Closely related processes have been shown to yield totally secure ways of sending coded information. Scientists at British Telecom's research laboratories have been testing a quantum-based system that could, for example, send sensitive financial transactions with guaranteed secrecy round the fibre optic networks in the City.

Samuel Braunstein, one of the creators of the new teleporter, says it could be used in memory devices for futuristic computers based on the principles of quantum theory (see below) offering a huge increase in computing power. According to Braunstein, it would be possible to preserve these precious "qubits" of information by teleporting them into super-cold clouds of atoms, trapped in magnetic fields. "By reversing the process the data could be read out," he adds.

Meanwhile, Swiss researchers still concerned to test Einstein's ideas to the limit have created the largest quantum state ever measured by separating entangled photons by 10 kilometres over the public phone networks.

This unlikely meeting between the workaday and the esoteric has inflated quantum theory a million million times beyond its natural realm, the atomic world. But still Einstein's objections fall and quantum theory triumphs. The intellectual debate may be all over, but the practical benefits are only just becoming apparent.

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