Light Is Key to Quantum Computing

Hank Hogan

Researchers at the University of California and Pennsylvania State University in University Park have discovered that taking a light touch may be the right way to build a quantum computer. Using femtosecond pulses of blue-green laser light, they have created, tipped and probed the electron spins in a semiconductor, zinc cadmium selenide. The result could be a machine capable of solving problems that stump today's supercomputers.

Electron spins are promising candidates for qubits, or quantum bits. Unlike binary digits, qubits can represent a 1, a 0 or a mixture of the two. Because computation by qubits essentially produces all of the possible outcomes simultaneously, quantum computers are expected to be able to solve problems that are beyond today's computers.

Unfortunately, electron spins decohere and revert to pointing in random directions in a microsecond or less. The new method, however, may offer researchers the ability to perform rapid-fire optical aligning, tipping and probing operations that could solve that problem.

"The fact that one can manipulate these states so rapidly compared to the dephasing time of electron spins means that millions of operations can be performed on the state before it decoheres," said David D. Awschalom, a physics professor at the University of California and leader of the team.

The researchers employed a pair of optical parametric amplifiers to generate nominally 150-fs-wide pump and probe laser pulses. The wavelength was tunable from the blue, below 500 nm, to blue-green, above 500 nm. By circularly polarizing the blue pulses and shining them on a slab of the semiconductor, the researchers induced the electron spins in the semiconductor to align. A second pulse at the slightly longer blue-green wavelength tipped the spins, which a third pulse measured.

Not on shelves

But don't rush out to buy a quantum computer yet. There are still problems to be solved and fundamental research to be done.
"The next step is to entangle quantum states," Awschalom explained. "That is, to bring them together, let them interact with one another and then separate them. This is a significant challenge."

A report of the researchers' work appeared in the June 29 issue of Science.