One quantum leap
Posted: Thursday, January 22, 2009 6:38 PM by Alan Boyle
Univ. of Maryland |
This graphic shows the apparatus set up for the
quantum teleportation experiment.
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Researchers have successfully teleported information from one trapped atom to another one sealed up in a container sitting 3.3 feet (1 meter) away. That's one small step for teleportation, and one quantum leap for code-makers and code-breakers.
But if you're waiting for the kind of teleporter that can beam Captain Kirk down from the Starship Enterprise ... well, don't hold your breath.
"The term 'teleportation' is a little weird," research team leader Christopher Monroe told me today. "When people see that word they think of Captain Kirk, and that's a big problem."
That's not to say that this kind of teleportation is ho-hum physics: Albert Einstein called it "spooky action at a distance" and thought it couldn't be done. But quantum teleportation, as in the transfer of information from one place to the other without passing through any physical medium, has been in the works for more than a decade.
Over the years, teleportation experiments have demonstrated that quantum states - for example, the spin of a particle or the polarization of a photon - can be teleported using a variety of methods. But the researchers behind the latest experiment, reported in Friday's issue of the journal Science, claim that this is the first time information has been teleported between two separate atoms in unconnected enclosures.
That's the kind of setup that makes the most sense for super-secure communication systems, as well as for super-smart computers that could break today's cryptographic codes or sort through huge databases.
"Our system has the potential to form the basis for a large-scale 'quantum repeater' that can network quantum memories over vast distances," Monroe, a physicist at the University of Maryland, said in a news release issued today. "Moreover, our methods can be used in conjunction with quantum bit operations to create a key component needed for quantum computation."
The experiment was run by Monroe and other researchers at the Joint Quantum Institute, a partnership between the University of Maryland and the University of Michigan. If you don't need to know the details about how the feat was done, and you don't want to risk getting your brain twisted in a knot (as mine was), skip the next section and resume reading about "the next giant leaps."
How the job was done
The team started out by trapping ytterbium ions in electromagnetic fields, inside separate vacuum chambers. Let's call the two ions A and B. (This chart shows the setup.)
Each of the ions could be in either of two energy states that were designated as the "1" and the "0" of a binary quantum bit. Unlike classical bits of information, quantum bits (or qubits) can be put into a state of superposition - that is, they can be in a combined 1-and-0 state until a measurement is made.
Ion A was zapped with a specially tailored burst of microwaves to put it into a desired state of superposition - in effect, entering information into A's "memory." Then, both ions were excited with laser pulses lasting just a trillionth of a second. That excitation sparked each ion to give off just a single photon that corresponded to each ion's energy state. (This chart explains the process.)
The photons were directed to a beam splitter that would set off a pair of detectors only if the energy states of each ion are entangled in such a way that they're complementary: If one is in the "1" state, the other has to be in the "0" state. It might take thousands of tries to get the right combination, and scientists wouldn't know which ion is in which state. But once the two detectors were activated at the same time, scientists could be confident that the entanglement is in force. (This chart shows how it works.)
With the ions in an entangled state, the scientists measured ion A - collapsing the quantum state out of superposition and making the original information vanish. Now A's energy state is definitely either "1" or "0." That would tell the scientists what kind of microwave burst to apply to ion B in order to read out the information that was originally entered into ion A.
No information was sent directly from A to B. Instead, quantum entanglement was used to put the information into ion A and get it out again through ion B. (This chart shows the final steps of the experiment.)
Future giant leaps
Monroe admitted that the experimental setup might seem rather clunky compared to today's classical computers. But physicists are still in the small-step phase of quantum computation.
"There's a lot of engineering that has to be done," he said, "but if you've ever seen the first solid-state transistor in 1957, it looked like this. It looked like it came out of a physics lab."
Monroe would like to boost the reliability of the system for entangling atoms, as well as the reliability of the system for reading out the results. In the experiment reported in Science, the information could be read out accurately about 90 percent of the time.
"For teleportation, that's very good," Monroe said. "We'd like to go up to 99 percent. But for quantum computing, you'd probably need three nines - 99.9 - so we have our work cut out for this in all dimensions."
Eventually, Monroe and other researchers in the field hope to establish networks of quantum communication devices that can send data across the globe. Quantum communication would be more secure than present-day communication, because if someone tried to eavesdrop on the signal, it would just collapse into random gobbledygook.
Theoretically, quantum computers would be much better than classical computers at sorting through huge databases to find the right information. One of the leading applications would be to find the prime factors of large numbers, which are the key to today's cryptographic systems.
A quantum computing system would be a godsend to spies - and that may be why the research reported in Science was supported by the federal government's Intelligence Advanced Research Projects Activity, or IARPA, as well as by the National Science Foundation.
Beam me up? Not so fast
But if information can be teleported without sending something between A and B, doesn't that mean something could be transmitted faster than the speed of light? And wouldn't that break the laws of physics? Well, not really. Even though the quantum entanglement operates at a distance, the information required to interpret the results has to be transmitted classically.
"Something happens faster than the speed of light," Monroe said. "It's just not information. ... But there is something weird nevertheless."
Monroe and other researchers hope to delve into some of that weirdness, including a phenomenon called nonlocal communication, in future experiments. Over at the University of Washington, physicist John Cramer is taking a different approach to the same kind of weirdness through an experiment that could investigate backward causality. (The last time I checked, Cramer was still working the bugs out of the lab apparatus.)
As for Captain Kirk ... physicists emphasize that the brand of quantum teleportation they work with isn't like the instant matter teleportation that's been so much a part of science fiction, from "Star Trek" to the 2008 movie "Jumper."
Theoretically, I suppose it's possible to entangle every single atom in Kirk's body with atoms down on the surface of the planet Vulcan. But in order to reconstruct the information at his destination, Kirk would have to be destroyed atom by atom on the Enterprise. And right now, even William Shatner wouldn't want to put that much faith in physics.
"There's always one kicker," Monroe said, speaking about the science in general rather than Kirk's fate in particular. "No matter what you do in quantum mechanics, there's always going to be a kicker somewhere."
Click through our interactive presentation, titled "Cats and Qubits," to learn more about how you get from quantum mechanics to next-generation computers.
