QUIBIT CHIP Four quibits are symmetrically coupled via a capacitive island, the cross in the center.
By JOHN MARKOFF
In 1981 the physicist Richard Feynman speculated about the possibility of “tiny computers obeying quantum mechanical laws.” He suggested that such a quantum computer might be the best way to simulate real-world quantum systems, a challenge that today is largely beyond the calculating power of even the fastest supercomputers.
Since then there has been sporadic progress in building this kind of computer. The experiments to date, however, have largely yielded only systems that seek to demonstrate that the principle is sound. They offer a tantalizing peek at the possibility of future supercomputing power, but only the slimmest results.
Recent progress, however, has renewed enthusiasm for finding avenues to build significantly more powerful quantum computers. Laboratory efforts in the United States and in Europe are under way using a number of technologies.
Significantly, I.B.M. has reconstituted what had recently been a relatively low-level research effort in quantum computing. I.B.M. is responding to advances made in the past year at Yale University and the University of California, Santa Barbara, that suggest the possibility of quantum computing based on standard microelectronics manufacturing technologies. Both groups layer a superconducting material, either rhenium or niobium, on a semiconductor surface, which when cooled to near absolute zero exhibits quantum behavior.
The company has assembled a large research group at its Thomas J. Watson Research Center in Yorktown Heights, N.Y., that includes alumni from the Santa Barbara and Yale laboratories and has now begun a five-year research project.
“I.B.M. is quite interested in taking up the physics which these other groups have been pioneering,” said David DiVincenzo, an I.B.M physicist and research manager.
Researchers at Santa Barbara and Yale also said that they expect to make further incremental progress in 2011 and in the next several years. At the most basic level, quantum computers are composed of quantum bits, or qubits, rather than the traditional bits that are the basic unit of digital computers. Classic computers are built with transistors that can be in either an “on” or an “off” state, representing either a 1 or a 0. A qubit, which can be constructed in different ways, can represent 1 and 0 states simultaneously. This quality is called superposition.
The potential power of quantum computing comes from the possibility of performing a mathematical operation on both states simultaneously. In a two-qubit system it would be possible to compute on four values at once, in a three-qubit system on eight at once, in a four-qubit system on 16, and so on. As the number of qubits increases, potential processing power increases exponentially.
There is, of course, a catch. The mere act of measuring or observing a qubit can strip it of its computing potential. So researchers have used quantum entanglement — in which particles are linked so that measuring a property of one instantly reveals information about the other, no matter how far apart the two particles are — to extract information. But creating and maintaining qubits in entangled states has been tremendously challenging.
“We’re at the stage of trying to develop these qubits in a way that would be like the integrated circuit that would allow you to make many of them at once,” said Rob Schoelkopf, a physicist who is leader of the Yale group. “In the next few years you’ll see operations on more qubits, but only a handful.”
The good news, he said, is that while the number of qubits is increasing only slowly, the precision with which the researchers are able to control quantum interactions has increased a thousandfold.
The Santa Barbara researchers said they believe they will essentially double the computational power of their quantum computers next year.
John Martinis, a physicist who is a member of the team, said, “We are currently designing a device with four qubits, and five resonators,” the standard microelectronic components that are used to force quantum entanglement. “If all goes well, we hope to increase this to eight qubits and nine resonators in a year or so.”
Two competing technological approaches are also being pursued. One approach involves building qubits from ions, or charged atomic particles, trapped in electromagnetic fields. Lasers are used to entangle the ions. To date, systems as large as eight qubits have been created using this method, and researchers believe that they have design ideas that will make much larger systems possible. Currently more than 20 university and corporate research laboratories are pursuing this design.
In June, researchers at Toshiba Research Europe and Cambridge University reported in Nature that they had fabricated light-emitting diodes coupled with a custom-formed quantum dot, which functioned as a light source for entangled photons. The researchers are now building more complex systems and say they can see a path to useful quantum computers.
A fourth technology has been developed by D-Wave Systems, a Canadian computer maker. D-Wave has built a system with more than 50 quantum bits, but it has been greeted skeptically by many researchers who believe that it has not proved true entanglement. Nevertheless, Hartmut Neven, an artificial-intelligence researcher atGoogle, said the company had received a proposal from D-Wave and NASA’s Jet Propulsion Laboratory to develop a quantum computing facility for Google next year based on the D-Wave technology.
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