Ion Teleportation Scheme Could Scale Up Quantum Computers

Scientists have teleported the quantum state of one trapped ion onto another a meter away

PHOTO: Laboratory of Physical Sciences

23 January 2009—A team of scientists is announcing today in the journal Science that in one of those bizarre demonstrations of quantum mechanics it has managed to teleport the quantum state of one ion onto another across a distance of a meter. Though we’re accustomed to thinking of the Star Trek version of teleportation, what physicists call teleportation is the exact mapping of one particle’s quantum characteristics to another distant particle. That matters because future quantum computers and quantum cryptography networks need some way of storing data and moving it around.

In the past decade, physicists have shown that teleportation is possible with magnetic fields, photons, and even atoms. What makes the new results—by Christopher Monroe of the University of Maryland and his colleagues—interesting is that the team uses a hybrid approach involving both atoms and photons that fits well with quantum information networks and quantum computers. Theoretically, Monroe says, the technique they have invented can be extended to distances as great as thousands of kilometers, although all they have demonstrated so far is one meter.

Raymond Laflamme, director of the Institute for Quantum Computing, at the University of Waterloo, in Canada, called it ”a very neat experiment and important milestone, demonstrating very good quantum control and bringing quantum teleportation one step nearer to practical applications.”

At the heart of teleportation lies a quantum mechanics effect known as entanglement. That phenomenon allows two particles—such as photons, atoms, or ions—to be linked in such a way that if someone measures the quantum state of one object, the state of the other becomes known as well. Entangled photons are often used in experimental quantum information networks. But while photons are easy to transmit (after all, they move with the highest speed in the universe), they are very difficult to store. On the other hand, atoms and ions preserve entanglement for a long time, but being massive, they are much harder to move from place to place.

The beauty of Monroe’s approach to teleportation is that it is an intelligent combination of the strengths of photons and ions. His team used two ytterbium ions confined in electromagnetic ion traps and cooled by lasers. The goal is to teleport the quantum state of one ytterbium ion to the other. Both ions are prepared for entanglement by microwave pulses and then zapped by ultrafast laser pulses. Each ion subsequently gives off a photon, which is entangled with the ion’s state. Through a complex series of steps, the system transfers the quantum state of one ytterbium ion to the other.

Norbert Lütkenhaus, of the Institute for Quantum Computing, says Monroe’s approach ”make sense.” He says that ”this technique allows them to couple ion traps in this optical way.”

Monroe says the hybrid approach will enable the creation of quantum repeaters—still-theoretical devices needed to make large-scale quantum cryptography networks—and will also be useful for making quantum computers. His group’s design for scalable quantum computers is to build multiple ion traps on a chip. The ions act as quantum bits and can perform computations when placed close together in a trap. However, to get the result of one trap to a computation in another trap without some sort of teleportation would require the difficult task of moving ions around on the chip. He says teleportation ”may well be the most scalable approach” to building ion-chip computers.

Dick Slusher of the Georgia Tech Research Institute, isn’t so sure. ”I think that in principle it is true that this teleportation process will facilitate scaling of quantum computation,” he says. ”However, there are many ways to accomplish this scaling, including ion transport and error-correcting pulse sequences. Teleportation may well turn out to be the key process for scaling, but I think it is far too early to be sure of this.”

About the Author

Saswato R. Das is a science writer based in New York City. In October 2008 he reported on the launch of the world’s largest quantum cryptography network.

To Probe Further

The paper in Science describing the teleportation breakthrough can be found here.

Monroe wrote a feature article for the August 2007 issue of IEEE Spectrum about ion-trap chip-scale quantum computers.

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