In a computer laptop, the binary ones and zeroes are transistors either running at low or high voltage. On a compact disc, the one is a spot where a tiny indented pit turns to a flat land or vice versa, while a zero is when there’s no change. Historically, the size of the object making the “ones” and “zeroes” has put a limit on the size of the storage device. But now, University of Chicago Pritzker School of Molecular Engineering (UChicago PME) researchers have explored a technique to make ones and zeroes out of crystal defects, each the size of an individual atom for classical computer memory applications.

“Each memory cell is a single missing atom – a single defect,” said Tian Zhong. “Now you can pack terabytes of bits within a small cube of material that's only a millimeter in size.” The innovation is a true example of UChicago PME’s interdisciplinary research, using quantum techniques to revolutionize classical, non-quantum computers and turning research on radiation dosimeters into groundbreaking microelectronic memory storage. “We found a way to integrate solid-state physics applied to radiation dosimetry with a research group that works strongly in quantum, although our work is not exactly quantum,” said Leonardo França, a postdoctoral researcher in Zhong’s lab. “There is a demand for people who are doing research on quantum systems, but at the same time, there is a demand for improving the storage capacity of classical non-volatile memories. And it's on this interface between quantum and optical data storage where our work is grounded.”

The research got its start during França’s PhD research at the University of São Paulo in Brazil. He was studying radiation dosimeters, the devices that passively monitor how much radiation workers in hospitals, synchrotrons and other radiation facilities receive on the job. “In the hospitals and in particle accelerators, for instance, it’s needed to monitor how much of a radiation dose people are exposed to,” said França. “There are some materials that have this ability to absorb radiation and store that information for a certain amount of time.” He soon became fascinated about how through optical techniques he could manipulate and read that information.

“When the crystal absorbs sufficient energy, it releases electrons and holes. And these charges are captured by the defects,” França said. “We can read that information. You can release the electrons, and we can read the information by optical means.” França soon saw the potential for memory storage. He brought this non-quantum work into Zhong’s quantum laboratory to create an interdisciplinary innovation using quantum techniques to build classical memories. “We're creating a new type of microelectronic device, a quantum-inspired technology,” Zhong said.

To create the new memory storage technique, the team added ions of rare earth elements to a crystal. Specifically, they used Praseodymium and an Yttrium oxide crystal, but the process they reported could be used with a variety of materials, taking advantage of rare earths’ powerful, flexible optical properties. “It’s well known that rare earths present specific electronic transitions that allows you to choose specific laser excitation wavelengths for optical control, from UV up to near-infrared regimes,” França said.

Unlike with dosimeters, which are typically activated by X-rays or gamma rays, here the storage device is activated by a simple ultraviolet laser. The laser stimulates the lanthanides, which in turn release electrons. The electrons are trapped by some of the oxide crystal’s defects, for instance the individual gaps in the structure where a single oxygen atom should be, but isn’t. “It’s impossible to find crystals – in nature or artificial crystals – that don't have defects,” França said. “So what we are doing is we are taking advantage of these defects.”

While these crystal defects are often used in quantum research, entangled to create qubits in gems from stretched diamond to spinel, the team found another use. They were able to guide when defects were charged and which weren’t. By designating a charged gap as “one” and an uncharged gap as “zero,” they were able to turn the crystal into a powerful memory storage device on a scale unseen in classical computing. “Within that millimeter cube, we demonstrated there are about at least a billion of these memories – classical memories, traditional memories – based on atoms,” Zhong said. (Source: U. Chicago)

Reference: L. V. S. França et al.: All-optical control of charge-trapping defects in rare-earth doped oxides, Nanophot., online 14 February 2025; DOI: 10.1515/nanoph-2024-0635

Link: Zhong Quantum Lab, Pritzker School of Molecular Engineering, University of Chicago, Chicago, USA