Teleporting What Matters
One small step for an atom, but a giant leap for quantum computing
In the common parlance of science fiction and quack spiritualism, teleportation means moving a person or an object by means of some mysterious, magical energy. In quantum communications and computing, it refers to transferring the state of one atomic or subatomic particle to another over a distance and without direct physical contact. Those states--say, the energy levels of the electrons around a nucleus--can be used like the on-off states of transistors, to encode information and do computation.
But, because in the weird world of quantum physics atoms and particles can exist in two different states simultaneously, with a single state manifesting itself only upon measurement, computers storing data as quantum states can calculate along many parallel paths simultaneously. In a flash they could solve intractable problems, such as the factoring of large numbers, which is essential to electronic cryptography. What's more, particles can be "entangled," so that when one is observed, fixing it into a particular state, the other is instantaneously fixed into a related state, whether the particles are separated by micrometers or light-years. Einstein famously called it "spooky action at a distance."
Exploiting entanglement , physicists in Austria and the United States have shown, for the first time, how to teleport the properties of one atom to another. In future quantum computers, this trick could move the results of a calculation from one area of the computer to another, acting, in effect, as the computer's wiring.
A classic example of teleportation involves the joint creation of two photons in a single process, whereby they are polarized in an undetermined yet interdependent way. As they part ways from each other, if the polarization of one is measured as, say, vertical, the other's is automatically defined as horizontal. That kind of teleportation was first demonstrated in 1997 by Anton Zeilinger and his team at Innsbruck University in Austria. (Zeilinger has since moved to the University of Vienna.) Its limitation in computation is that photons are slippery creatures that rarely interact with each other and are not easily held in place. Atoms and ions should be superior in computational systems because they can be fixed in place and manipulated with lasers.
At the National Institute of Standards and Technology in Boulder, Colo., a team of teleporters led by David Wineland trapped three beryllium ions. The ions--beryllium atoms with missing electrons and hence a positive charge--are subjected to forces from electric fields so that they can be held in what is called an ion trap. Here, radio waves and electric fields from a set of electrodes hold the ions in a line but also allow them to be moved from one segment of the trap to the other [see photo, " Tiny Teleporter"].
In this way these ions could be brought over to the different laser beams used for entanglement and for the measurement of quantum states. Two atoms can be entangled by simultaneously undergoing changes in quantum state when hit with a specific series of laser pulses. Their state can then be determined by whether or not they disgorge a photon in response to a pulse from a different laser. Through a series of entanglements and measurements, the quantum state of one ion is teleported to another ion, which is entangled with a third.
The same teleportation protocol was used by another team at Innsbruck University, this one led by Reiner Blatt. But the team used a different type of trap, one with no segments. Instead of moving the ions to the lasers, Blatt and his colleagues were able to aim the lasers well enough so that they could move the laser beams to the ions. The latter was quite a technical feat, since the trapped ions were only about 5 micrometers apart. "Controlling the three ions took us the better part of a year," says Blatt. Both of the Innsbruck groups are now preparing experiments with traps containing four to five ions.
It is likely that tiny traps on silicon chips will form the core of quantum computers, but miniaturizing the lasers and optics that must be built around the traps is still a problem. "The optics could become part of the chip, but that is still far off,"says Andrew Steane, an Oxford University physicist.
Jonathan Dowling, an expert on optical quantum computers at Louisiana State University, in Baton Rouge, thinks that simple trap systems are very promising and may first be applied in a specialized computer called a quantum repeater. This kind of device, though still the stuff of mere theory, would extend the range of wiretap-proof quantum communication systems that encode messages in the polarization of photons. "You would store the photons in ions, do the error correction in the ions, and then convert them back into photons again," he says.