Deterministic positioning accuracy of B centers in hBN by electron irradiation

In the quest to build the next generation of quantum computers and secure communication networks, scientists are searching for tiny, reliable sources of light that can carry information one photon at a time. Imagine a single particle of light that can be switched on and off with perfect precision, acting as a carrier of quantum data. To make these systems work, researchers need to place these light sources exactly where they are needed, often inside microscopic structures that guide and amplify their signal. The material of choice for this task is hexagonal boron nitride, a crystal that is as thin as a sheet of paper but incredibly strong and stable. Within this crystal, scientists can create specific defects—tiny imperfections in the atomic structure—that glow with a unique, bright light. These defects are the single-photon emitters, the essential building blocks for future quantum technologies. However, creating them is not enough; they must be placed with extreme accuracy. If a light source is even slightly off-center in a tiny optical device, the device may fail to work, much like a key that is slightly bent and cannot turn in a lock.

A team of researchers at Université Paris-Saclay in France has taken a significant step toward solving this placement problem. They focused on a specific type of defect in boron nitride known as a "B center," which emits a distinct blue light. These defects can be created on demand by firing a beam of electrons at the crystal, a process that transforms existing, invisible imperfections into the desired light sources. The challenge lies in knowing exactly where the electron beam is when it creates the light. The researchers set out to measure how precisely they could position these new light sources relative to where they intended to put them. By creating grids of these emitters and measuring the distance between the intended target and the actual location of the light, they found that the process is highly accurate, with the light sources appearing within a range of about 40 nanometers from their target. This level of precision is sufficient for many current applications, but the team went further to understand exactly what limits this accuracy and how it might be improved.

To achieve this, the scientists used a sophisticated setup inside a scanning electron microscope, a powerful instrument that uses a focused beam of electrons to image and modify materials. They placed a thin slice of boron nitride crystal under the beam and began creating arrays of light sources. As the electron beam scanned across the crystal, the researchers watched a monitor in real time, looking for the moment a new light source appeared. The moment the specific blue glow of a B center was detected, the beam would immediately stop and move to the next spot on the grid. This method allowed them to create precise patterns of individual light sources. Once the creation process was complete, they used a different type of microscope to map the exact locations of these new sources. By comparing the map of where the light actually appeared against the grid of where the researchers intended to place them, they could calculate the error in the positioning.

The analysis revealed that the total error in placing these light sources was a standard deviation of roughly 40 nanometers. To understand why the light sources did not land exactly on the target, the researchers broke down the error into four distinct causes. First, there is the inherent precision of the machine itself, which controls where the electron beam points; this contributed about 18 nanometers of uncertainty. Second, there is the difficulty in pinpointing the exact center of the light source once it is created, which added about 8 nanometers of uncertainty. The third and most significant factor was the physical interaction between the electron beam and the crystal. When the beam hits the material, it does not just affect a single point; the energy spreads out slightly as it travels through the crystal, creating a small zone where the transformation can happen. This spread meant the light source could be created anywhere within a small area around the beam's center, contributing about 27 nanometers to the total error.

The fourth factor was a matter of chance related to the ingredients available inside the crystal. To create a B center, the electron beam must find a specific type of pre-existing defect, a pair of carbon atoms, to transform. If these carbon pairs are sparse, the beam might have to travel a bit further or the energy might have to spread out more to find one to convert, adding another layer of randomness. The researchers calculated that this scarcity of ingredients added about 23 nanometers to the positioning error. When they combined all these factors mathematically, the predicted total error matched their actual measurements almost perfectly, confirming that they had correctly identified the sources of the inaccuracy.

The study concludes that while the current method is already quite precise, there is a clear path to making it even better. The researchers suggest that by making the crystal thinner, they could reduce the spread of the electron beam's energy, and by increasing the number of carbon pairs in the crystal through intentional doping during its growth, they could ensure that a target is always found immediately. If these improvements are made, the team estimates that the positioning accuracy could be pushed down to less than 10 nanometers. This level of precision would be a major leap forward, allowing for the creation of complex, fully integrated quantum photonic chips where every light source is placed exactly where it needs to be, unlocking the full potential of these materials for future quantum technologies.