Scientists bring crystal clarity to diamond’s quantum signals

It is said that the forest can be missing because of the trees. But it’s often worth looking at the trees more closely to make sense of the dense, brambly whole. This is what a group from Stanford University did to tackle a thorny problem of quantum information in diamonds.

A star material for hosting quantum information, diamond nevertheless presents a challenge: the signals from the bits of quantum information embedded in diamond are often disordered and incoherent. Scientists proposed explanations for this inconsistency, but they needed a way to examine the diamond’s constituent parts to identify the culprit.

That’s exactly what the Stanford group, led by Jennifer Dionne, did, using a powerful microscope to zoom in on the diamond’s atomic composition. In an article published in PNASthe team demonstrated that the diamond’s varied interior largely explained the erratic signals from the quantum bits embedded inside.

“There was no effective way to correlate the structure of the qubit – the quantum bit – with the emitted signal, but researchers would observe considerable heterogeneity in the emission,” said Dionne, deputy director of Q-NEXT and professor of Stanford. materials science and, by courtesy, radiology. “We solved the problem by relating atomic-scale structure to quantum properties.”

Silicon vacancy

The group worked with a type of qubit called a silicon vacancy center. Two carbon atoms are removed from the diamond and replaced with one silicon atom. Because one atom replaces two, there is a space on each side of the silicon atom: a half-filled hole.

Silicon research centers show promise for quantum sensors, which can achieve accuracy several times higher than today’s best tools, as well as for quantum communications networks, which are, by their quantum nature, virtually at eavesdropping test.

The Dionne group tested silicon vacancy centers in diamond nanoparticles, tiny pieces of diamond measuring a few hundred nanometers in diameter. Typically, several vacancies are scattered throughout the sample like holes in a sponge.

The signal coming from a holiday center takes the form of a photon, a particle of light. In a perfect world, a diamond vacancy acts as a reliable photon factory, reliably producing the same type of photons every time a diamond rolls off the assembly line: same color, same brightness.

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

But scientists were seeing a variety of colors and brightnesses of photons emitted by their diamond sources. This caused the Dionne group to dig deeper.

The many facets of the diamond

A diamond is a motley thing. Like most crystals, a diamond is made up of regions that touch each other like irregularly shaped Lego bricks. Regions – or domains – are differentiated by their atomic “grains”, like the grain of wood. A domain with diagonally aligned atoms can adjoin another with 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 ultra-precise task that would be virtually impossible with a less powerful tool. They began to notice a trend.

“We continued to look at these diamonds and were finally able to start seeing these really interesting, very distinct regions of photon emission – the photon profile differed from region to region,” Angell said.

The conclusion could not be clearer: domains make the difference.

The grain of each area shapes the gap within, stretching or squeezing it. While a vacancy in one area may be filled one way, and the vacancy next to it may be stretched differently.

The group discovered that the way the vacancy is constrained affects the properties of the emitted photon, as does its location in the grain structure.

The scientists were measuring the diamond’s fuzzy or incoherent signals because they had treated the sample as a single source, a single emitter of photons. But a diamond sample includes several tightly packed domains, each housing its own photon emitter. The researchers measured the signal coming from the forest and not the trees.

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

Even vacancies that are very far apart can generate significantly different photon emissions.

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

Crystal clear

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

Although grain variety is not the only factor contributing to fuzzy photonic signals, the Dionne group has shown that it plays an important role.

“We emphasize how important it is to know exactly the underlying grain structure of the crystal particles being studied. If you collect the emission from the entire particle and get a fuzzy emission, it’s probably because there is kind of a grain line in there, you put together different vacancies with different signatures, and you don’t know it,” Angell said.

Their work also has a broader scope, applying to other members of the vacancy center qubit family.

“The door has been opened to a large number of studies enabling precise correlation between structure and function in quantum systems and, ultimately, improvement in quantum communications, quantum networks and quantum sensing,” Dionne said.