From here to there

Will we ever be able to teleport people to faraway places? It all depends on the strange uncertainties of the quantum world

To a classically trained physicist, perhaps the most galling aspect of the quantum world is the way that nothing seems real until it is measured. Suppose you want to know something about a quantum particle - say the polarisation of a photon of light. Before you make the measurement, the photon isn't really polarised in a particular direction. Instead, it has a ghostly range of possible polarisations, each with some probability of being measured. When you measure the photon's polarisation you will get a definite answer. But the snag is, all the other ghostly possibilities will vanish in the process, and the original, indefinite state is lost forever.

So the act of measuring a particle actually destroys some of the information about its pristine state. That would seem to make it practically impossible to copy such particles and reproduce them elsewhere. But ironically, one of the quantum world's strange tricks turns this argument on its head. It turns out that you can re-create an unmeasured quantum state - as long as you are prepared to sacrifice the original. The trick exploits the very uncertainty that makes quantum measurements so puzzling in the first place.

It was the physicist Charles Bennett from IBM's labs at Yorktown Heights in New York who introduced the world to the idea of quantum teleportation in 1993 - in theory that is. Bennett and his co-workers found a way for an imaginery character called Alice to teleport a particle to her friend Bob, some distance away. What happens is that Bob creates a particle in exactly the same state as Alice's original particle - even though Alice never knew what that state was.

Suppose Alice and Bob wanted to copy a photon. Alice can't just measure her photon and send the results to Bob, because that would destroy some of the information Bob needs. Fortunately, quantum theory has a more subtle means of communication. An extra pair of "entangled" photons opens up the teleportation channel between Alice and Bob.

According to quantum theory, you can tangle up a pair of photons so that their properties are inextricably linked. This holds true even if you send them to opposite ends of the Earth: measure one photon at the North Pole and you immediately determine the state of the other photon at the South Pole.

Perplexed? You're in good company. In fact, Albert Einstein and his younger colleagues Boris Podolsky and Nathan Rosen dreamt up this photon scenario to show just how absurd and unacceptable quantum mechanics was. It appears to demand the impossible - measurements in one place producing an instantaneous effect somewhere else. Ironically, experiments have shown that "EPR" pairs of particles do indeed communicate in this "spooky" (Einstein's word) fashion. More of this later. For now there's Alice and Bob's problem to solve.

Alice has an unmeasured, pristine photon that she wants to teleport to Bob. First, she creates a pair of linked EPR photons, keeping one with her while sending the other off to Bob. She then arranges for her unmeasured photon to interact with her EPR photon, measures the result of that interaction, and sends the answer on to Bob the old-fashioned way - by phone, e-mail, fax or carrier pigeon.

And now the mysterious part. Bob receives Alice's message, and depending on what it says, performs some prearranged operation on his EPR photon - the other half of the entangled pair that Alice created. For instance, he might change the polarisation of his photon by an amount that depends on the information Alice sent him. At the end of this procedure, Bob's photon has become an exact replica of Alice's original unmeasured photon. The quantum state of that photon - although not the photon itself - has been teleported from Alice to Bob.

What's going on here? Well, to re-create the photon - to teleport it - you need to transmit two kinds of information about its state. One kind is the ordinary, everyday information. That's the easy part. You can measure it and send the details by an ordinary route. But what about the quantum information - the stuff that disappears when you make your measurement? The trick to transmitting this lies in the secret, spooky connection between Alice and Bob's EPR photons. By persuading her unknown photon to interact with her EPR photon, Alice made Bob's EPR photon, the other half of the entangled pair, interact with the unknown photon as well.

Via the spooky EPR channel, Bob therefore receives some strange, quantum information about the state of the photon Alice wants to teleport. That's not the whole story though, because Alice also has to measure something about the interaction of her two photons, and send the result to Bob. But if all is done correctly, Bob receives a combination of spooky quantum information and plain old classical information that allows him to reproduce Alice's original unknown photon.

Not surprisingly, this is an extremely tricky experiment to get right. The joint measurement that Alice makes on the unknown photon and her EPR photon must be carefully designed and executed. Alice and Bob have to ensure the EPR photons remain absolutely untouched by any unwanted external interaction. If either photon were to bump into a stray atom somewhere along the way, for instance, that would destroy their spooky connection. But last year two teams of scientists, one at the University of Innsbruck and the other at the University of Rome, managed to teleport a photon. From one side of their lab to another, anyway.

There are a few interesting provisos about this process. First, Alice has to send Bob the results of her measurements by a standard, slower-than-light means, so even though the spooky part of teleportation is instantaneous, the equally essential non-spooky part is not. Quantum teleporting can't happen faster than light, something Einstein would be pleased to learn. Second, Alice's measurement destroys the quantum state of her original photon. Third, neither Alice nor Bob will ever actually know what that original quantum state was. Directly measuring a quantum state will always destroy information about it in an unpredictable way. Alice can teleport a quantum state to Bob only on the strict understanding that neither party can ever know exactly what state they teleported.

But what does all of this mean for Star Trek style teleportation, where an entire person is transported from one place to another? Certain difficulties spring to mind. To teleport a collection of atoms, rather than just a single photon, the EPR channel of communication has to transmit not just one item of spooky quantum information but a whole package of it. This requires not just a large number of individual EPR pairs, which would be bad enough, but a single EPR complex encompassing a huge number of particles. It would be well nigh impossible to construct such a state, let alone send it off into the ether without destroying its integrity.

And that's not at all. To teleport Jean-Luc Picard, you would have to send a complete specification of the collective quantum state of every last electron and atom in his body - all in an instant. A tall order. Alice would have to devise a single, instantaneous measurement that would ensnare all this information at once, and Bob would have to perform a similarly complex reconstruction at the other end. And suppose you destroy Jean-Luc Picard's quantum representation in one place and re-create it in another. Would the reconstruction be, in every respect, the same man? Would it act as the original would have acted? That's for you to figure out...