Better than fiction

Will we ever be able to teleport?

If Anton Zeilinger succeeds with his latest proposal to the European Space Agency (ESA), some of Europe's most eminent scientists will soon be bouncing photons (light particles) off satellites in experiments to discover whether quantum entanglements hold over long distances. Eventually, they will demonstrate global quantum networking using teleportation.

Zeilinger, professor of experimental physics at the University of Vienna, performed one of the very first quantum teleportations on a photon over a decade ago. Since then, many experiments have replicated and improved the quality, distances and types of systems.

In 2004, David Wineland's team at the US National Institute of Standards and Technology and an Austrian group led by Rainer Blatt teleported the internal spin state of a trapped ion - the first example of teleporting the state of a massive particle.

In 2006, Eugene Polzik's group at the University of Copenhagen teleported the information stored in a beam of light into a cloud of atoms. The same year, Sam Braunstein of the University of York and Akira Furusawa at the University of Tokyo combined quantum cloning and teleportation by sending information about a laser beam to two remote locations in a single step.

Zeilinger's Space-Quest proposal to ESA promises to take research to the next stage with a star-studded consortium of European research groups, backed by industrial partners including Contraves Space, TESAT and Austrian Aerospace.

Space-Quest builds on recent successes distributing quantum entanglements by fibre optic cable under the river Danube and via a free-space link at distances of 144km between La Palma and Tenerife, helped by Professor John Rarity of Bristol University. The plan is to extend experiments to full-scale teleportation in a few years as part of the larger space programme.

Quantum reality

Teleportation in sci-fi movies is a fabulous time-saving notion for sending solid objects such as humans, aliens and cups of tea around the universe without the costs involved in landing on planets. Quantum teleportation, in contrast, is a way of harnessing the peculiarities of quantum mechanics in new types of secure cryptographic systems and communication schemes that, theoretically, will be able to send large amounts of data via handfuls of photons.

Today's digital systems use information coded into nice solid classical bits, realised as macroscopic quantities like voltages or electric charges. Classical bits are either zeros or ones and they don't disappear into an existential sulk if you try to pin them down with a measurement or copy or delete them. But if we want to code information at the atomic scale - where Moore's Law miniaturisation is taking us - we have to deal with quantum rules.

A quantum bit (qubit) e.g. the spin of an electron or the polarisation of a light photon, is an indecisive entity that exists as a wave function, i.e. simultaneously in an infinite number of energy states from zero to one and every fraction in-between but with different probabilities for each. An arbitrarily large amount of information can be encoded in the state of a qubit (hence the appeal for information processing) but if you try to measure it, the wave function promptly collapses into one particular state while destroying the original one.

As a marker for information tampering, this collapse is one of the central security features of quantum cryptography as we are always able to find out if anybody has tampered with the transmissions. But it makes quantum information hard to handle. The excitement about teleportation is that it lets us transport delicate quantum states robustly using the quantum property of 'entanglement'.

Quantum theory says that two entangled particles can only be described by their joint properties so, for instance, you can never say which particle is red and which is blue, they are always 'red plus blue'.

Teleportation exploits the fact that measuring one half of an entangled pair gives random results, but the results of both particles will always be correlated - no matter how far apart the two particles are. If you think of two imaginary entangled dice, although each throw shows a random result between one and six, the second would always give the same result.

Practical teleporting

One of this year's noteworthy developments was the first integrated quantum gate from professor Jeremy O'Brien and John Rarity's group at Bristol University. The silica-on-silicon C-NOT (controlled-not) gate can be used for making the Bell measurements which, up until now, required bulk optics on an optical bench. A C-NOT gate has two inputs for a target qubit and a control qubit, which for the teleportation protocol would be the photon to be teleported and half the entangled photon pair. The gate flips the target qubit only if the control qubit is 'one'. It does nothing if the control qubit is 'zero'.

But quantum teleportation is far from being a push-button affair and a lot of brainpower is currently focused on improving its predictability and quality.

"If we create an entangled photon pair and send something we think might contain the unknown photon we want to teleport, it's down to chance that all three turn up at the right time for measurements to be made and for us to see all three photons as clicks in our detectors," explains Rarity, part of the Space Quest consortium. "The count rates in a very good experiment, for example, would only be a few hundred quantum bits teleported per second," he says.

Various groups have boosted the success rate by confining the entangled pairs and the unknown states into separate short laser pulses, so they arrive simultaneously. The limiting factor then becomes the reliability of the entangled pair generators, which sometimes generate one pair, sometimes two or more and often none.

Entanglement generators are typically based on non-linear optical crystals that work by a kind of optical down conversion. So, for example, you pour in blue laser light and the crystal splits it into pairs of entangled, lower frequency red photons.

Anton Zeilinger's group has recently published a way of increasing the probability of generating polarisation-entangled pairs of photons by sandwiching crystals between mirrors, which has the effect of recycling the pumped laser beam. "We can easily create one million entangled photon pairs per second, which was impossible two years ago," he says. Rarity's group has used the confinement of light in photonic crystal fibres to achieve a similar brightness.

Meanwhile, researchers at Oxford University, who are using intensity-entangled photons, have been honing a method for counting photons emitted from entangled pair generators.

Directly measuring an entangled beam destroys the entanglement, but it can be done indirectly by sending one of the two entangled beams through a half silvered mirror, which splits off a tiny bit of light.

"The light you split off may be more than one photon. So if you put a second beam-splitter behind the first and split off more light, that will contain less photons," explains professor Ian Walmsley, head of the atomic and laser physics group at Oxford. By cascading beam splitters with an array of detectors, it's possible to count the photons by how many detectors fire. The latest implementation is very compact and uses one detector and a series of fibre optic loops.

Walmsley (also in the Space-Quest consortium), is involved in another project with Martin Plenio, professor of quantum physics at Imperial College, to develop a high quality quantum repeater; a device that uses teleportation to compensate for loss
in optical fibres thus extending the distances quantum information can be sent through fibre optic cables.

A quantum repeater works by distributing pairs of entangled photons between successive repeater stations. In a three-station repeater, Station Two would end up with two photons; one entangled with a photon at Station Three and the other entangled with a photon at Station One.

"In practise, if you try to distribute entangled pairs and store them, they suffer from absorption and noise and become imperfectly correlated. So you need to be able to improve the correlation," explains Plenio. "Our method involves sending lots of damaged photon pairs and then 'distilling' them to generate one high quality entangled pair."

"Normally, entanglement between two photons is obtained by emitting them simultaneously from the same source. But we've shown that entanglement can be transferred, or swapped, onto two particles that originated from quite different sources and were formerly completely independent," says Plenio.

Walmsley and Plenio hope to build one of these distillation stations this year. The next step is to concatenate several together.

Fundamental physics

At one level, research into teleportation is about the practicalities of quantum information transmission, which is interesting from an engineering viewpoint and a commercial one, too. But what's exciting is how the associated technological developments are re-kindling interest in testing the foundations of quantum mechanics and, in turn, allowing us to do new science.

"There have always been things in quantum mechanics that have been unsettling - the weirdness doesn't seem to go away. Even if you push it around, it pops up somewhere else," says Braunstein.

The idea that we will shortly be able to send entanglement into space to test whether this 'weirdness' holds out even to intra-planetary distances is better than science fiction.

"Like many theories in physics, we may find it's only an approximation and we'll have to move onto something bigger and better," he adds. And who knows what technologies may then emerge.

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