This tiny gold crystal could bring quantum technology out of the deep freeze

Quantum materials could reshape fields as varied as high-performance computing, secure communication, sensitive detection, and energy production. Yet one persistent limitation has kept most of them from becoming practical technologies.

Almost every quantum material discovered so far works only when cooled to temperatures near absolute zero. At ordinary temperatures, heat causes atoms to vibrate constantly. Those vibrations disrupt the fragile quantum effects researchers want to control.

Suppressing this motion typically requires large cryogenic refrigeration systems. As a result, quantum materials can perform remarkable tasks in carefully controlled laboratories, but they are difficult and expensive to use in real-world devices.

A Quantum Material That Works at Room Temperature

LSU physicists have now created the first room-temperature quantum material able to identify and transport distinct quantum states of light. The advance, reported in Nature, addresses one of the most significant barriers in quantum materials research.

The study was led by Associate Professor of Physics Omar S. Magaña-Loaiza. Beyond demonstrating a single new material, the researchers established a broader design strategy that could be used to create an entirely new family of quantum materials.

Such materials could eventually support quantum computers, highly secure communication systems, advanced sensors, and new energy technologies.

For Chenglong You, a former postdoctoral researcher who is now a professor at the University of Electronic Science and Technology of China, a particularly satisfying moment came when the unusual design performed exactly as the theory had predicted.

"One of the most exciting parts of this project was realizing that we could build a material that does something nature doesn't provide on its own. Seeing it work exactly as we predicted was incredibly rewarding," said You.

Building an Artificial Quantum Crystal

Instead of searching nature for a substance with the needed properties, the LSU team chose to engineer one from the ground up.

The researchers began by placing a thin layer of gold on a glass chip. They then used focused ion beams to cut hundreds of extremely small slits into the metal. Each slit functions like an artificial atom, or meta-atom.

Together, these meta-atoms create a crystal unlike anything found naturally. The completed structure is also thinner than a human hair.

When light reaches the chip, it moves across the gold surface and interacts with the engineered meta-atoms. By precisely adjusting the size, shape, and spacing of those structures, the researchers gained control over how the material responds to light.

The result was a form of light manipulation that had never previously been achieved at room temperature.

"By engineering the distribution of meta-atoms in the plasmonic metacrystal, we can systematically dictate which quantum statistics are allowed to pass through the structure. So, our crystal essentially acts as a statistical filter on quantum states," said Riley B. Dawkins, who recently completed his Ph.D. and is now joining the National Institute of Standards and Technology (NIST) as an NRC Postdoctoral Research Associate.

Magaña-Loaiza's Quantum Photonics Group at LSU completed every part of the research, including the initial theory, material design, nanofabrication, and experimental testing.

The importance of the work, however, goes beyond the construction of an unusual new material. Its greatest value lies in what the metacrystal can do.

Sorting Different Quantum States of Light

Light does not always behave in the same way.

Sunlight, laser light, and fluorescent light are all made of photons. However, the photons in each type of light fluctuate and interact in different ways. These small variations influence how the light behaves at the quantum level.

Until now, identifying those differences has generally required complex equipment, detectors cooled to extremely low temperatures, and millions of individual measurements.

The new metacrystal performs the sorting process on its own. Rather than reacting only to familiar properties such as color or intensity, it detects subtle quantum distinctions in the incoming light.

It then directs different quantum states along separate routes through the crystal.

These routes also allow certain quantum states to move through the material while undergoing fewer changes to their statistics. Those statistics are the defining characteristics that distinguish one quantum state from another.

"We call this robust transport," Magaña-Loaiza said. "These quantum states carry information. Our crystal can distinguish them and move them from one point to another in a robust way without requiring cryogenic cooling. That's what opens the door to practical quantum technologies."

Physicists refer to this shared quantum behavior as quantum coherence. Maintaining coherence is one of the most difficult problems in quantum information science because interactions with the surrounding environment can quickly destroy it.

In the Nature study, the team describes the metacrystal as the first room-temperature quantum material inherently sensitive to the quantum coherence of many-body systems.

An Entirely New Class of Quantum Material

The material differs so greatly from conventional quantum materials that the researchers created a new term for it: the quantum statistical plasmonic metacrystal.

"For me, this wasn't just a project -- it was a collective effort built around the idea of creating something completely new in quantum technology," said Jannatul Ferdous, a graduate student in Magaña-Loaiza's group. "What made it truly exciting was that we were not only creating a new class of room-temperature quantum material but also developing the theory to understand and control its behavior. Seeing this idea become an experimental reality was incredibly rewarding."

The team also found that the metacrystal naturally produces structures they call quantum statistical bands.

These bands are similar in concept to the electronic band structures that control how electricity travels through semiconductors. In the new material, however, the bands govern the movement and statistical behavior of quantum states of light.

By changing how the meta-atoms are arranged, researchers can select which quantum states pass through the material without being altered and which states undergo statistical changes.

This level of control represents a major shift in how quantum materials can be developed. Scientists no longer have to depend entirely on finding naturally occurring substances with useful properties. They can instead design materials that guide quantum states in deliberate and predictable ways.

The findings therefore provide a general blueprint for building many future quantum materials, rather than offering only one isolated discovery.

Potential Uses in Computing and Communication

Operating at room temperature makes the metacrystal relevant to technologies well beyond basic physics research.

Similar materials might one day carry fragile quantum information inside quantum computers without requiring enormous cooling systems. Removing or reducing the need for cryogenic refrigeration could make quantum devices smaller, less costly, and easier to deploy.

The same design principles could also contribute to more practical quantum communication networks, highly sensitive sensors, and other developing quantum technologies.

A Possible Boost for Solar Energy

The material's ability to guide light with fewer losses may also have important applications in renewable energy.

Modern solar cells do not convert all incoming sunlight into electricity. Some of the light becomes trapped inside the material and eventually turns into heat, lowering the amount of usable energy the cell can produce.

A metacrystal that directs light along more stable pathways could help prevent some of that energy from being lost. More of the incoming light might remain available for conversion into electricity.

Testing that possibility is the team's next goal.

The researchers plan to incorporate the metacrystal into solar cells and determine whether it can increase the share of sunlight transformed into usable electrical energy.

Success would show how a breakthrough originating in fundamental quantum physics could lead directly to improvements in next-generation solar energy technology.

The team acknowledges funding from the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under Award DE-SC0021069.

Reporting from ScienceDaily contributed to this story.