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Optical Science Gets Spookier

Light Reading
Out of the Lab
Light Reading
2/5/2003
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Optical science turns up the weirdest effects -- none more so than "quantum teleportation."

In the latest advance in the field, scientists from the Group of Applied Physics (GAP) at the University of Geneva in Switzerland have taken photons, destroyed them, and resurrected identical copies 2 kilometers away. They claim this is the longest distance that photons have ever been teleported.

Their work is interesting because the scientists are looking at how quantum teleportation can be combined with optical communications. "The marriage between quantum optics and telecoms wavelengths allows us to send photons further," says Prof. Nicolas Gisin, who leads the research.

The work was reported in last week's edition of the journal Nature. (Non-subscribers can view a copy of it here.)

Teleportation involves making a precise replica of an object. It differs from simple copying because the original is destroyed at the same time the replica is produced.

Until ten years ago, teleportation was thought to be impossible because it requires making an exact copy of every atom in an object, which goes against the Heisenberg's uncertainty principle of quantum mechanics. According to the principle, the very act of measuring a particle alters its properties, so an exact replica can never be made.

A way around this problem was found with the discovery of so-called "entangled photons." Called "spooky action at a distance" by Einstein, entangled photons are somehow connected, so that everything that happens to one photon also affects the other, no matter how far apart they are. So teleportation is possible, provided you're a photon.

And what use is this? One potential application -- probably the only possible application in the foreseeable future, says Gisin -- would be to extend the reach of quantum cryptography, a technique used to transmit unbreakable codes via the quantum states of individual particles of light, rather than more conventional mathematical means (see Quantum Cipher Sent by Fiber).



Currently there is no way to boost the signal as its intensity falters due to the attenuation in optical fiber. A network for quantum cryptography cannot contain Erbium Doped-Fiber Amplifiers (EDFAs), which would interfere with the signal, rendering it unreadable by the intended recipient. Quantum teleportation could provide an alternative way of regenerating the signal every 50 km or so.

Beam me up, Alice

The details of the Swiss experiment highlight the fact that real life can be stranger than fiction -- even Star Trek.

The Swiss researchers created entangled photons by shining a bright laser beam on a non-linear crystal (ed. note: a dilithium crystal?). The apparatus was configured so that, when entangled photons are created, one half of the pair has a wavelength around 1310 nm, while the other has a wavelength of roughly 1550 nm. This allows the two photons to be physically separated by a simple WDM splitter.

The actual information to be teleported is encoded on a third stream of photons, also at 1310 nm. To complete the teleportation process, Alice (the sender of quantum information is always called Alice), needs both the coded 1310nm photon, and the entangled 1310 nm photon. One photon is used to measure the other. In this case, the measurement simply involved passing the pair of photons through a beam splitter.

By now, the 1550nm photon has been sent down a spool of fiber to Bob who is waiting in another room (the receiver of quantum information is always called Bob). And in theory, teleportation has already taken place because, by measuring it, the coded photon has been destroyed, according to the uncertainty principle. When it was destroyed, it affected the entangled 1310nm photon, which immediately passed the information on to its 1550nm twin.

However, Bob needs to read the information from his photons. To do that he needs to know the outcome of the measurements made with the beam splitter. And the only way he can get these results is by classical communications, meaning that this data must be sent over a standard Internet connection or telephone line. When this data arrives, then he knows what kind of measurement to make on the 1550nm photons in order to get the coded signal out.

In theory that's it. But being the skeptics they are, the researchers need to check how well the teleportation process works. Errors can creep in as a result of noise in the system, which can be created in the non-linear crystal, or in produced at the detector. Gisin says that the fidelity of the system -- a measure of how many photons are good copies of the original -- is 85 percent. "For the quantum optics community, it is a good result," says Gisin. "But it still means there is a 15 percent chance of losing the photon."

A fidelity of 85 percent may be good enough for commercial applications -- no one really knows, says Gisin. There's a more pressing problem to solve first, which is reliability. "Sometimes it works, sometimes it doesn't," he notes. "You cannot simply press a button."

Transporting people is still science fiction, of course. Apart from anything else, the sheer amount of information about all the atoms in a human being would take a lifetime to transmit. And imagine the disappointment when, after such a long journey, you discover that 15 percent of you went missing along the way.

[No cats were killed in the conduct of this experiment.]

— Pauline Rigby, Senior Editor, Light Reading

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