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Practical quantum computers are at least a decade away, and some researchers are betting that they will never be built.
This is because controlling individual particles like atoms, electrons and photons is extraordinarily challenging. Information carried in particles always comes in shades of gray and can be corrupted or wiped out by the slightest wisp of energy from the environment.
A pair of experiments has brightened prospects for quantum computing, however, by making it more likely that a practical means of reading electron-based quantum bits, or qubits, can be developed. Research teams from the University of California at Los Angeles and from Delft University of Technology in the Netherlands have developed electronic methods of detecting the spins of individual electrons.
Spin is a property of electrons that is akin to the rotation of a top. The two spin directions, spin up and spin down, are magnetically opposite, like the two poles of a kitchen magnet. The spins can represent the 1s and 0s and digital information.
Particles that are isolated from their environment are in the weird quantum state of superposition, meaning they are in some mix of the two spin directions. This means a qubit can be in some mix of 1 and 0, which allows a string of qubits to represent every binary number at once.
This gives a quantum computer the ability to check every possible answer to a problem with a single set of operations, promising speedy solutions to problems that classical computers have to churn through one answer at a time. These include factoring large numbers, a problem whose difficulty is the foundation of most of today's security codes.
The UCLA team's method of electron spin detection uses devices that are already mass-produced. The researchers flipped a single electron spin in a commercial transistor chip, and detected the spin flip by measuring changes in current flowing through the device.
Several proposed quantum computer architectures call for circuits that can be manufactured using today's chipmaking techniques. "The transistor structure used for our experiment [closely] resembles some proposed spin-based qubit architectures," said Hong-Wen Jiang, a professor of physics at the University of California at Los Angeles. "We believe that our read-out scheme can be readily adapted in a scalable quantum information processor," he said.
Electrons travel through a transistor via a semiconductor channel that is electrically insulated. The transistor is controlled by a gate electrode, which produces an electric field that penetrates the insulator and increases the conductivity of the channel, allowing electrons to flow. Occasionally defects occur, producing one or more spots in the insulator that can draw individual electrons from the channel and trap them.
The researchers sought out transistors that contained single defect traps, set the gate voltage so that the trap had an equal chance of attracting an electron or not, and applied a large magnetic field to the trap.
The researchers next step is to to use pulsed microwaves to control the exact quantum superposition of the spin, said Elzerman. They then plan to entangle two spins. "When this is done, all the basic ingredients for a quantum computer are in place," he said.
Coupling many spins and controlling their interactions accurately enough to perform a quantum algorithm is a matter of improving control over the fabrication process, said Elzerman. "We need cleaner and purer materials and more reproducible electron beam lithography so that all dots on a single chip are really identical," he said.
Jiang's research colleagues were Ming Xiao and Eli Yablonovitch of UCLA, and Ivar Martin of Los Alamos National Laboratory. They published the research in the July 22, 2004 issue of Nature. The research was funded by the Defense Advanced Research Projects Agency (DARPA) and the Defense Microelectronics Activity (DMEA).