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See Inside March 2008

Here and There

Why quantum teleportation is nothing like getting beamed up by Scotty

The sci-fi dream (or utter fantasy) of getting from one place to another instantaneously continued this February 14, with the opening of Doug Liman’s film Jumper, based on the novel by Steven Gould. We asked quantum physicist H. Jeff Kimble of the California Institute of Technology to explain how physicists understand quantum teleportation, which turns out to be more relevant to computing than to commuting.

What’s the biggest misconception about teleportation in physics?
That the object itself is being sent. We’re not sending around material stuff. If I wanted to send you a Boeing 757, I could send you all the parts, or I could send you a blueprint showing all the parts, and it’s much easier to send a blueprint. Teleportation is a protocol about how to send a quantum state—a wave function—from one place to another.

Is that hard to do?
The most straightforward way to do it would be to imagine it was an electron: just shoot the electron from point A to point B, and it takes its quantum state with it. But that’s not always so good, because the state gets messed up in the process.

How does teleportation get around that problem?
The special resource that enables teleportation is entanglement. You’re Alice (location A), and I hand you an electron in an unknown quantum state. Your job is to send the quantum state (not the electron) to location B, which is Bob. If you try to measure it directly, you necessarily disturb it.

You and Bob also share a pair of electrons—you have one, Bob has the other—and they’re in an entangled state such that if yours is spinning up, his is spinning down, and conversely. You make a joint measurement of two electrons—the one I handed you and the one you’re sharing with Bob. And that gives you two bits of information. You call up Bob on the cell phone and give him those two bits, and he uses them to manipulate his electron. And bingo, in the ideal case he can perfectly re-create the state of the electron that I handed you.

Why would you want to transmit a quantum state? What are the applications?
Imagine you want to build a quantum computer. The quantum memory’s got to talk to the quantum processor. Teleportation is just a fancy quantum wire.

So how has quantum teleportation advanced since SciAm’s 1997 story on it?
In 1998 my team demonstrated teleportation of a beam of light. I would say that was the first bona fide demonstration. A few years ago [in 2004] a group led by ­David J. Wineland at the National Institute of Standards and Technology in Boulder, Colo.—and simultaneously with that, a group led by Rainer Blatt in Innsbruck, Austria—teleported the internal spin of a trapped ion. It’s the first time teleportation had been done with the state of a massive particle. More recently [in 2006], the group of Eugene S. Polzik at the University of Copenhagen teleported the quantum state of light directly into a material system.

Do these demonstrations have any practical value?
It has practical implications, because a quantum computer is going to be a hybrid system. Light is good for propagating from one place to the other with very low loss, but it’s really hard to store light.

Switching gears—this new movie Jumper is about a kid, and some other people, who teleport from place to place.
I didn’t know that.

If you saw X-Men 2, with Nightcrawler...
I haven’t seen X-Men either.

Do you watch Heroes on NBC?
No. I watch some of the football play-offs.

But you know Captain Kirk...
I have some advice. Just don’t talk about teleporting people in your story. There’s a really incredibly exciting frontier in science that didn’t exist 15 or 20 years ago, and it’s this quantum information science, which brings together traditional computer science and quantum mechanics. There’s stuff going on that is just titillating.

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