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Lighting the Way to a Quantum Computer

29 June 2001 7:00 pm
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Spin control. Laser pulses make electrons spin in unison around the axis of a magnetic field, tip the spins, and measure the result.

For researchers working to build a quantum computer, speed is of the essence. That's because quantum data bits last just billionths of a second, too short for scientists to do meaningful computations. But in today's Science, researchers report a new, ultrafast way to manipulate quantum data. The discovery is a small but important step toward creating machines that can, in seconds, complete calculations that would take eons on current supercomputers.

In both conventional and quantum computers, data are represented by bits that reside in one of two states, a 0 or 1. But quantum computers have an extra trick. They can take advantage of a fuzzy quantum mechanical notion known as "superposition of state." It says that a quantum system, such as the orientation of an electron's spin, simultaneously exists in all of its possible states until it is measured or observed. For instance, instead of being a simple 1 or 0, a quantum bit, or qubit, could be one-third 1 and two-thirds 0, or any other combination. When this fuzzy qubit is plugged into a logical operation, the computer essentially computes all possible outcomes simultaneously. The trouble is, qubits in solid state components--the type favored by researchers hoping to make a full-scale quantum computer--fall apart too quickly.

To manipulate fleeting quantum information faster, physicist David Awschalom and colleagues at the University of California, Santa Barbara, and Pennsylvania State University, University Park, started with a semiconducting material called zinc cadmium selenide (ZnCdSe) and a laser setup designed to jockey electrons around in the material. Like all electrons, those in ZnCdSe have spin, a quantum mechanical property associated with magnetism. Normally, the spins of ZnCdSe electrons tend to make them wander their own way. But by blasting the semiconductor with a 100-femtosecond pulse of circularly polarized blue light, the researchers got a group of electrons to carry the same spin.

To manipulate these spins, the researchers then fired a second 100-femtosecond pulse, this one containing photons of blue-green light. Individually, these lower-energy photons are too weak to be absorbed by the electrons in the semiconductor. But as they passed through the semiconductor, Awschalom explains, their collective presence effectively created a brief magnetic field, which tapped the electron spins into a new orientation. In a final step, the group used a third 100-femtosecond pulse to spot the electron spins in their new state.

Theoretically, the techniques would allow researchers to carry out about 1 million such manipulations before the quantum information falls apart, although the team hasn't yet demonstrated any computation power. Nevertheless, the ability to manipulate quantum information so quickly "is a very important milestone," says Stuart Wolf, a quantum mechanics expert at the Pentagon's Defense Advanced Research Projects Agency in Arlington, Virginia.

Related sites

David Awschalom's group
Quantum computers, at How Stuff Works

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