Terabytes of data in a millimeter crystal

(Press-News.org) From punch card-operated looms in the 1800s to modern cellphones, if an object has an “on” and an “off” state, it can be used to store information.

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.

Their research was published today in Nanophotonics.

“Each memory cell is a single missing atom – a single defect,” said UChicago PME Asst. Prof. 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 – most commonly known as the devices that store how much radiation hospital workers absorb from X-ray machines – 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 first author 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.”

From radiation dosimetry to optical storage

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 – shining a light – 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.

Rare earth

To create the new memory storage technique, the team added ions of “rare earth,” a group of elements also known as lanthanides, to a crystal.

Specifically, they used a rare-earth element called 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 UChicago PME 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.

Citation: “All-optical control of charge-trapping defects in rare-earth doped oxides,” França et al, Nanophotonics, February 14, 2025. DOI: 10.1515/nanoph-2024-0635

Funding: This work was supported by the U.S. Department of Energy, Office of Science, for support of microelectronics research, under Contract No. DE-AC0206CH11357

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