Like restless kindergartners posing for a class picture, electron spins in semiconductors, which can point up or down, are hard to keep facing the same way for very long. In the language of physics, they have a short coherence time. But because electron spins offer one of the most promising models for quantum bitsphysical states that can store far more information than conventional computer bitsscientists have sought ways around the coherence problem. The hope was that they could find a way to keep electron spins aligned a little longer and thus have enough time to actually perform calculations on them. But in today's issue of Science, researchers from the University of California at Santa Barbara and the Pennsylvania State University suggest an alternative to lengthening coherencenamely, speeding up quantum computations.
UCSB's David Awschalom and his longtime collaborator at Penn State, Nitin Samarth, devised a new technique for manipulating electron spins within ultrafast timescales. Using pulses of laser light lasting a mere 10 to 13 femtoseconds, the scientists were able to "tip," or rotate, the spin axes of a group of aligned electrons. The ultrafast laser pulses are some 100,000 times faster than other methods used to tweak electron spins and fall well under the coherence limit of about a microsecond or less. "These rotations are made possible by an effective magnetic field that arises when very intense light of a certain energy interacts with the electron spins in a semiconductor," Awschalom explains. "Although the degree of rotation is currently about half of what is needed to perform a full operation, many avenues exist for further optimization."
To make the experiment possible, the Penn State group first needed to engineer a custom semiconductor. "The goal of developing a quantum computer provides an exciting opportunity for combining cutting-edge materials synthesis with sophisticated physical measurements," Samarth says. "Tailoring the material to this experiment was a real challenge. In this case we had to meet several constraints imposed by both the physics and the technology of the experimental measurements. Since molecular beam epitaxy (MBE) allows us to act as atomic-scale architects, we met the tight conditions needed for the measurements by building 'digital' quantum structures, one atomic layer at a time."