Researchers at the University of Chicago’s Institute for Molecular Engineering have taken a crucial step toward nuclear spintronic technologies that use the “spin” — or magnetization — of atomic nuclei to store and process information. The new technologies could be used for ultra-sensitive magnetic resonance imaging, advanced gyroscopes, and quantum computers.

The researchers used infrared light to make nuclear spins line themselves up in a consistent, controllable way, using a high-performance semiconductor that is practical, convenient, and inexpensive.

The research was featured as the cover article of the June 17 issue of Physical Review Letters.

No cryogenic temperatures and high magnetic fields

Nuclear spins tend to be randomly oriented. Aligning them in a controllable fashion is usually a complicated and only marginally successful proposition. The reason, explains Paul Klimov, a co-author of the paper, is that “the magnetic moment of each nucleus is tiny, roughly 1,000 times smaller than that of an electron.”

This small magnetic moment means that little thermal kicks from surrounding atoms or electrons can easily randomize the direction of the nuclear spins. Extreme experimental conditions such as high magnetic fields and cryogenic temperatures (-238 degrees Fahrenehit and below) are usually required to get even a small number of spins to line up. In magnetic resonance imaging, for example, only one to 10 out of a million nuclear spins can be aligned and seen in the image, even with a high magnetic field applied.

Using their new technique, David Awschalom, the Liew Family Professor in Spintronics and Quantum Information, and his associates aligned more than 99 percent of spins in certain nuclei in silicon carbide. Equally important, the technique works at room temperature — no cryogenics or intense magnetic fields needed. Instead, the research team used light to “cool” the nuclei.

While nuclei do not interact with light themselves, certain imperfections, or “color-centers,” in the silicon carbide crystals do. The electron spins in these color centers can be readily optically cooled and aligned, and this alignment can be transferred to nearby nuclei.

Getting spins to align in room-temperature silicon carbide brings practical spintronic devices a significant step closer, said Awschalom. The material is already an important semiconductor in the high-power electronics and opto-electronics industries. Sophisticated growth and processing capabilities are already mature. So prototypes of nuclear spintronic devices that exploit the IME researchers’ technique may be developed in the near future.

“Wafer-scale quantum technologies that harness nuclear spins as subatomic elements may appear more quickly than we anticipated,” Awschalom said.

Abstract of Optical Polarization of Nuclear Spins in Silicon Carbide

We demonstrate optically pumped dynamic nuclear polarization of Si29 nuclear spins that are strongly coupled to paramagnetic color centers in 4H- and 6H-SiC. The 99%±1% degree of polarization that we observe at room temperature corresponds to an effective nuclear temperature of 5 μK. By combining ab initio theory with the experimental identification of the color centers’ optically excited states, we quantitatively model how the polarization derives from hyperfine-mediated level anticrossings. These results lay a foundation for SiC-based quantum memories, nuclear gyroscopes, and hyperpolarized probes for magnetic resonance imaging.