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. Note: This is an expanded version of a Q&A published in the March 2008 print edition of Scientific American.
Scientific American: What's the biggest misconception about teleportation?
Jeff Kimble: 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 transmitting a quantum state 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 the disruption of the quantum state?
The special resource that enables teleportation is entanglement. You're Alice [in 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 electron—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 doesn't Alice just copy the quantum state and store the copy?
There are uncertainty relations like Heisenberg's uncertainty principle. When I hand my electron to Alice, what she might think to do is just keep a copy—clone it. The more information she tries to get about the state, the less good is the teleportation. If she tries to keep a perfect copy, then Bob would create a state that is perfectly random.
Why would you want to transmit a quantum state? What are the applications?
Imagine you want to build a quantum computer. A quantum computer is going to have parts just like a computer on your desktop. They have to be wired together quantum mechanically. The quantum memory's got to talk to the quantum processor. Teleportation is just a fancy quantum wire.
Why not just shoot electrons around?
If I carry this electron from the memory to the processor and I make a mistake—say it collides with some impurity in the wire—then I've lost more than just the state of that one electron. That one spin is entangled, potentially, with all the spins in the computer.
How has the field advanced since the first demonstrations of quantum teleportation in 1997?
All the initial work was done with light. In 1998 my team demonstrated teleportation of a beam of light. I would say that was the first bona fide demonstration. A beam of light came in, and a beam of light came out. In the experiments your magazine covered, there was never a moment you could say, aha, the state has emerged and been teleported.
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. The quantum state of one ion was teleported to a second ion using a third ion in the middle as an intermediate.
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. All the other experiments had been teleportation from an atom or a photon to exactly the same kind of particle.
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. Some quantum information protocols require you to take light and map it into some material system, where you can store it for a long time. Then if you want to communicate across your computer or across the country, you map it back into light.
I should tell you one other experiment. A scientist named Akira Furusawa* at the University of Tokyo teleported entanglement. He had one beam of light that was entangled to a second beam; he teleported the first beam and he could show it was still entangled with the partner that wasn't teleported.
How would keeping a teleported object entangled with its unteleported partner be useful?
The quantum computer's working on thousands of entangled spins and from time to time I need to teleport the state of the 561st electron to another place. Well that's not as simple as just thinking it's that one electron.
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, with Nightcrawler…
I haven't seen X-Men either.