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GCN : November 2013
[BrieFing] If one of the 35 million books in the Library of Congress has a big red X on the inside cover, how long would it take a man to nd it? Opening each book might take the searcher hundreds of years, unless he was very lucky. But what if the searcher could replicate himself into 35 million different people, each one existing in a parallel universe? All 35 million people would head to the library and look inside a different book. Any one of them who didn't nd the X would simply disappear, until the last man standing would be holding the correct book. That's one way to illustrate the difference between traditional and quantum computing, and it explains why the government is so interest- ed in machines that can virtually try all possible solutions at once and nd the best answer more quickly. It's why NASA, Google and the Universities Space Research As- sociation have formed the Quantum Arti cial Intelligence Lab, to explore quantum computers' potential. There are, in fact, some problems that can never be solved by tradi- tional computers, according to Eric Ladizinsky, co-founder and chief sci- entist for D-Wave Systems, which built a quantum computer for NASA and is working on even more powerful versions. With traditional computers, the cir- cuits are either on or off, and the binary code is represented by ones and zeros. Adding more processors increases the computer's power linearly. By contrast, a quantum computer uses quantum bits, or qubits, the quantum equivalent of a traditional bit. A qubit's circuits exist in all possible states at the same time -- a one, a zero and whatever is in between -- and this superposition vastly increases the potential process- ing power. The National Science Foundation recently posted an animation in which theoretical physicists John Preskill and Spiros Michalakis explain the principles of quantum computing. Superposition becomes useful when quantum bits work together, multiplying the ways they can be correlated. The correlations are richer, and that richness increases markedly as even a few hundred qubits are added --- so much so that these correlations couldn't be described with classical bits. "You'd have to write down more numbers than the number of atoms in the visible universe," the scientists said. But that random richness requires that the calculations be run in a stable environment completely isolated from the outside world because observa- tion would destroy the delicate random superpositions. That decoherence, or what the scientists call "the big enemy," would destroy the quan- tum calculations as well. That's where D-Wave comes in. The D-Wave quantum computer takes a ring of metal and cools it down close to absolute zero. Then other factors are eliminated to combat the decoherence that can destroy the quantum calculations. Light is removed by sitting the ma- chine inside a black box. Radiation is shielded, and sound is reduced as much as possible. All air is also removed from the enclosure. The result is that when a current is applied to the ring, scientists can measure the superposition -- 100 percent of the current is going clockwise at the same time that 100 percent of the current is going counterclockwise. That dual state is harnessed to solve problems. The secret to D-Wave's approach is that it has been able to achieve the quantum phenomenon using con- crete parts, so it can build its computers in a more traditional way, as opposed to trying to work with atoms and electrons directly. "In a sense, we are harvesting the parallel worlds to solve problems in this one," Ladizinsky said. • Quantum computing and the power of alternate worlds BY JOHN BREEDEN II D-Wave Vesuvius processor inside the quantum computer is cooled to 20 millikelvin, near absolute zero. D-WAVE SYSTEMS GCN NOVEMBER 2013 • GCN.COM 5