Scientists bring crystal clarity to diamond quantum signals

They say you can miss the forest for the trees. But it is often worth taking a closer look at the trees to understand the dense and strong whole. That’s what a Stanford University group did to tackle a thorny problem of quantum information in diamond.

A stellar material for storing quantum information, however, diamond presents a challenge: Signals from bits of quantum information embedded in diamond are often erratic and unstable. Scientists have offered explanations for the discrepancy, but they needed a way to examine the diamond’s constituent parts to expose the culprit.

The Stanford group, led by Jennifer Dionne, did just that, using a powerful microscope to zoom in on the atomic-level composition of diamond. In a paper published in PNASthe team demonstrated that the heterogeneous interior of diamond largely explained the disordered signals from the quantum particles embedded inside.

“There was no good way to relate the structure of the qubit — the quantum bit — to the emitted signal, yet researchers would observe considerable heterogeneity in the emission,” said Dionne, Q-NEXT’s deputy director and a Stanford professor. materials science and, courtesy, radiology. “We tackled the problem by linking atomic-scale structure to quantum properties.”

Vacancy of silicon

The group worked with a type of qubit called a silicon vacancy center. Two carbon atoms are removed from diamond and replaced with a silicon atom. Because one atom occupies the second position, a gap sits on either side of the silicon atom—a half-filled hole.

Silicon gap centers are promising for quantum sensors, which can achieve precision many times higher than today’s best tools, as well as for quantum communication networks, which, by their quantum nature, are practically protected from eavesdropping.

Dionne’s group tested free silicon centers in diamond nanoparticles, tiny pieces of diamond that are several hundred nanometers across. Typically, a few vacancies are scattered throughout the sample like holes in a sponge.

The signal from a vacant center takes the form of a photon – a particle of light. In a perfect world, a vacancy in the diamond acts as a reliable photon factory, reliably producing the same type of photon every time it comes off the assembly line—same color, same brightness.

“We want indistinguishable photons,” said Daniel Angell, the paper’s first author, who conducted the research while a graduate student at Stanford.

But scientists were seeing a variety of colors and brightnesses of photons emanating from their diamond sources. This led the Dionne group to dig deeper.

The many facets of the diamond

A diamond is a patchwork thing. Like most crystals, a diamond is made up of areas that stick together like irregularly shaped Lego bricks. Regions – or domains – are differentiated by their atomic “grains”, like wood grain. A domain with diagonally aligned atoms can attach to another in a front-to-back orientation.

The team used a scanning transmission electron microscope to examine the domains one by one, measuring the photon emission from each—an extremely precise task that would have been virtually impossible with a less powerful tool. They began to notice a pattern.

“We kept looking at these diamonds, and eventually we could start to see these really cool, very distinct regions of photon emission—the photon profile would change from region to region,” Angell said.

The bottom line was crystal clear: Domains make the difference.

The grain of each domain shapes the void within it, stretching or squeezing it. While a vacancy in one domain may be squeezed in one way, the vacancy in the neighboring site may be strained another way.

The group found that the way the gap is strained affects the properties of the emitted photon, as does its location within the grain structure.

The scientists had measured ambiguous or unstable signals from the diamond because they had treated the sample as a single source, a single photon emitter. But a diamond sample comprises multiple tightly packed domains, each housing its own photon emitter. The researchers had measured the signal from the forest, not the trees.

“The position of the vacancy within the crystal matters,” Dionne said. “The different facets of the diamond crystal and the particular orientation of the crystal can have a significant impact on both the brightness and the color of the emission.”

Even vacancies that are a small distance apart can generate significantly different photon emissions.

“We saw a completely distinct jump in the emission signal when two vacancies were just 5 nanometers apart,” Angell said. “Seeing this almost perfect line of separation between emissions at the nanoscale—a clear difference in emission—is something I’ve never seen before. It’s really compelling data to see.”

Crystal clarity

Angell correlated the different types of grain strain with their corresponding photon profiles, providing the researchers with a high-resolution map of strain and emission to better understand their findings.

While grain variety is not the only factor contributing to fuzzy photon signals, the Dionne group showed that it plays an important role.

“We emphasize how important it is to know exactly the grain structure of the crystal particle being studied. If you’re collecting emission from the whole particle and you have fuzzy emission, it’s probably because there’s some kind of grain boundary in there. You’re collecting places of job vacancies with different signatures and you don’t know,” said Angell.

Their work has a broader reach, too, applying to other members of the holiday center qubit family.

“The door has been opened to a large number of studies that enable precise structure-function correlation in quantum systems and ultimately improve quantum communication, quantum networking and quantum sensing,” said Dionne.