UC Berkeley and College of New York researchers are using lasers to control the fundamental nuclear spin properties of semiconductor materials.

Today’s computers are reaching the limits of what simple miniaturization can achieve.

Their laser techniques promise to speed the creation of “spintronic” devices that use the spin state of electrons to control the memory and logic circuits in chips.

“We can use these laser techniques to manipulate spin states for a radically new type of computing,” said UC Berkeley’s Jeff Reimer. “For now, quantum computing relies on expensive, exotic materials or on temperatures very close to absolute zero.

“Our laser techniques can allow quantum computing to become far more practical and inexpensive. With lasers, this research can be conducted with standard semiconductor materials.”

“Spin is the characteristic of the neutrons, protons and electrons that make up all matter. It is spin that allows us to get images inside the human body using magnetic resonance imaging (MRI).”

Circularly polarized laser beams

Reimer and colleagues have discovered that they can use circularly polarized laser beams to control spin states in the semiconductor material gallium arsenide.

“Circularly polarized light moves forward either clockwise or counterclockwise, like the threads on a bolt, or a corkscrew. By tuning the laser to just the right intensity and frequency, and by picking the isotopes of gallium and arsenic we use for the semiconductor material, we can control the spins in the semiconductor by using polarized laser light.

“Best of all, we don’t have to build any miniature electromagnetic devices onto the chip. For our research we use off-the-shelf lasers designed for light shows.”

With spintronics, the spin state of electrons is used to control how the electrons shuttle through the logic gates that make up the billions of transistors on a computer chip. Spintronics can allow computer chips to operate more quickly and with lower power, but ultimately, the nature of the computer remains conventional.

What if the spin states themselves could be used for computing operations?

The problem that Reimer and co-authors are trying to overcome is noise. “At the atomic scale,” he says, “the world of qubits is extremely noisy. We might be trying to conduct logic operations by flipping the spins on a handful of electrons. Meanwhile, we are surrounded by millions of nuclei whose spins are flipping up and down in a very random way, obscuring the signal.”

There are ways to quiet the spins and make them more orderly, but existing techniques are very difficult and expensive. One is to cool down the semiconductor to very close to absolute zero, where the random spins settle down into more orderly patterns. Another technique is to synthesize material that is naturally “spin quiet.” The leading candidate for this is artificial diamond made from materials that contain only the spinless isotope, carbon-12.

Says Reimer, “Both these options are so expensive and difficult that the amount of quantum computing research that can be conducted will be very limited. Using circularly polarized laser light may prove to be a cost-effective alternative.

“With our laser technique, we have been able to create regions in semiconductor material where the spins stay aligned over micrometer-scale regions for minutes to hours. In effect, we have used lasers to create a ‘spin freezer’ in a tiny and specified region in space. We hope our spin freezing technique serves as the basis for new developments in spintronics and quantum computing.”

Adds co-author Jonathan King, a Ph.D. candidate in the Reimer lab, “We envision using the intrinsic magnetic field of the nuclei to gain three-dimensional control of electron spins on microscopic length scales. Instead of being a source of unwanted noise, the nuclei can be organized into a valuable tool for spintronic devices.”