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Quantum Computing Makes Solid Progress

Computers that could harness the fuzziness of the quantum world could breeze in minutes through computations that would take today's supercomputers billions of years to crunch. So far, quantum computing researchers have managed to carry out a few simple logic operations in the quantum regime, but that has typically required roomfuls of lasers, magnets, or other control equipment. In this week's Nature, however, a team of researchers describe a simple, solid-state device that functions as the key component of a quantum computer.

Like all quantum computing schemes, this one makes use of the fact that a quantum system--such as the magnetic orientation of an atomic nucleus, or the location of an electron--can exist as a superposition of all its possible states at once. Unlike the classical bits of data in a computer, which are decidedly either a zero or a one, qubits hover in an indecisive fog somewhere between these two values. When this fuzzy two-state bit is plugged into a logical operation, the computer in essence computes both outcomes simultaneously.

To create a qubit in a solid-state device, physicist Yasunobu Nakamura of the NEC Fundamental Research Laboratory in Tsukuba, Japan, and his colleagues resorted to superconductors, whose electrons all share the same quantum state and travel together in pairs. They designed a set of tiny superconducting electrical components that enabled single electron pairs to jump between a tiny bar-shaped metal island and a nearby metal reservoir. Applying a brief voltage pulse to a control electrode allows the superconducting electron pairs to oscillate back and forth between the two locations, representing the one and zero of a digital system. To read out the electrons' location, they apply a continuous voltage to a so-called DC electrode. This boosts the energy level of electron pairs on the island, causing them to break their superconducting bond to one another and hop to a nearby probe, which then channels them to a detector.

"It's an extremely significant success," says David Awschalom, a physicist at the University of California, Santa Barbara. "It opens the door to building a solid-state quantum computer that's scalable. In computing, that's the name of the game." Still, Awschalom and others caution that making complex solid-state quantum computers is still many years away, and researchers must first learn how to keep their quantum data from decaying almost the instant they're made.