Stoichiometric crystal shows promise in quantum memory

For over two decades, physicists have been working toward implementing quantum light storage—also known as quantum memory—in various matter systems. These techniques allow for the controlled and reversible mapping of light particles called photons onto long-lived states of matter. But storing light for long periods without compromising its retrieval efficiency is a difficult task.

In recent years, rare earth atoms in solid materials at cryogenic temperatures have shown to be promising for quantum memory. As part of this inquiry, researchers from The Grainger College of Engineering at the University of Illinois Urbana-Champaign have identified favorable properties in a stoichiometric europium material with a layered structure. Their observations, published in Physical Review Letters, report the growth and characterization of NaEu(IO₃)₄, a rare earth material that may have future implications in quantum memory.

Although classical memory—like the type used to store text messages between users—is relatively simple to facilitate, quantum information cannot be stored, copied, or retained in this way.

“If I want to send someone quantum information or a quantum bit, I can't make a copy of it or hold onto it locally,” said Elizabeth Goldschmidt, a professor of physics and a supporting author of the paper. “Once I send it, it’s gone. If it gets lost on the way, it’s lost forever.”

One way around this challenge is using rare earth elements such as europium, which can be used to store quantum information long-term. Photons tend to degrade when stored for long periods, but europium can both absorb and preserve photons. Scientists can dope these rare earth species into crystals to facilitate quantum light storage. However, stoichiometric — or undoped — crystals tend to have fewer defects due to the lack of intentional doping.

Leaning into this property, Illinois Grainger engineers sought to pack a crystal with as much europium as possible by pivoting to stoichiometric crystals where the europium is part of the structure, rather than a randomly distributed dopant.

By combing the literature for crystals with attractive properties and evaluating their efficacy in photonic integration, the research team zeroed in on NaEu(IO₃)₄, a layered stoichiometric europium-containing crystal that is environmentally stable with strong bonds that produce 2D layers. If a single layer of NaEu(IO₃)₄ were to be isolated, it could be integrated with a photonic chip—an important step towards building a good quantum memory. However, the behavior of europium atoms can change when they are positioned too closely together, which required careful characterization of those properties in this new and novel material. An additional benefit of the material was enhanced storage time.

“Millisecond or longer quantum memory allows us to store a quantum state for the time it takes to communicate to anyone anywhere else on earth,” Goldschmidt said. “The maximum time it takes to communicate elsewhere is some tens of microseconds or milliseconds: up to a satellite and back down, or via an optical fiber circling the earth. That’s the time scale that we’re talking about.”

So far, Illinois researchers have demonstrated storage times of up to 800 nanoseconds. Going forward, they aim to demonstrate longer storage times and isolate a single layer of their stoichiometric material in hopes of one day building a quantum memory.

“Today’s computing hardware got its start the same way: as shards and plates of crystals from the past century,” said Daniel Shoemaker, a professor of materials science and engineering whose lab grew the crystals for this research. “We don’t know what the next century will hold for quantum memory, but we know they will rely on the actions of electrons on individual atoms and ions, like the europium in our materials here. Right now, we are exploring the first few materials, so it’s an exciting time.”