Quantum Weirdness: Two Times Zero Doesn't Always Equal Zero

Researchers think they can extract quantum information from two noisy channels that are individually useless

In a surprising discovery released this week, physicists have announced that two times zero does not always equal zero.

The new theoretical research examines transmissions of individual quantum states, such as sending a single photon down a fiber-optic cable and reading off its polarization on the far end. Rather than shipping the lone photon down a clean and undisturbed line, the researchers considered sending information down two lines that contained too much static to transmit anything reliably. When the lines were examined alone, each noisy channel proved as useless as a dead telephone jack. However, the researchers calculated that someone on the far end of two noisy channels used together could in fact extract actual information from the individually worthless lines.

The finding, while still purely theoretical, nevertheless promises to open up new methods of both strengthening quantum cryptography and assisting in the elusive quest to build a quantum computer.

The counterintuitive nature of the announcement stems from the peculiar kind of information being studied. Unlike the classical bit, which is simply either zero or one, the quantum bit can exist in an infinite number of intermediate states between zero and one. It also contains a feature that befuddled even Albert Einstein: measurements of one quantum bit affect the information carried by another quantum bit with which it has previously been in contact. (Most troubling to the legendary physicist was the fact that two ”entangled” quantum bits could theoretically lie on opposite sides of the universe from each other—and yet a measurement performed on one would still instantaneously affect its twin.)

The new finding, emerging from IBM’s Thomas J. Watson Research Center, in Yorktown Heights, N.Y., and Los Alamos National Laboratory, in New Mexico, compounds paradox upon paradox. In essence, the spooky form of information whose behavior no one completely understands—but which has nevertheless been rigorously observed in the lab—now seems capable of appearing on the distant side of a supposedly impassable divide.

”This paper raises more questions than it answers,” admitted coauthor Graeme Smith of IBM, whose work appears in Thursday’s online edition of Science Express and will appear in a forthcoming issue of the journal Science. ”One interpretation seems to be that there are different kinds of communication—or different kinds of quantum information that these two channels might be transmitting.”

Seen in such light, quantum information would be like green-colored water. Pipes that can’t carry green water but can carry blue and yellow water could be harnessed to carry one of each color. Then the spooky transmission of quantum information would be akin to combining the blue and yellow streams at the other end of the pipe.

The problem with this reading is that physicists had previously thought quantum information contained no component parts—no blues and yellows, as it were—but rather was as indivisible as an electron.

Patrick Hayden, professor of computer science at McGill University, in Montreal, said Smith and his coauthor, Jon Yard, appear to have opened a Pandora’s box with their finding. ”It violates my intuition so violently that I need to spend some time” mulling it over, he said.

Hayden said he nevertheless does already see room for applications down the road. ”The most striking thing about this phenomenon is you take two useless things and they become useful,” he said.

A kind of quantum synergistic effect may emerge, he said, that Smith and Yard have barely begun to address. Whether one considers quantum cryptography or computation—in which entanglement is used to encode secret communications or speed up number crunching, respectively—Hayden said the new finding could mean that a host of novel techniques to cope with quantum noise are now possible.

Smith said he’s already investigating whether his proof of principle could be used to enhance the productivity of quantum cryptography. ”These capacity [calculations] are very nice,” he said. ”But from a practical point of view, the motivation is to try to actually build something.”

About the Author

Mark Anderson is a freelance science and technology writer based in Northampton, Mass.

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