Ultraviolet Laser Pulses Create New Diamond Defects While Leaving Quantum Qubit Centers Intact

Diamonds are prized for perfection, but for quantum technology the interesting part is the flaws. A new study shows that short ultraviolet laser pulses can create one kind of flaw in a diamond crystal while leaving another, more valuable kind largely untouched. The work, by Emmanuel E. Umukoro and colleagues, appears in the journal Diamond and Related Materials, and Umukoro described it in a Phys.org Science X Dialog piece published October 10.

The flaw that matters most is the nitrogen-vacancy center, or NV center. It forms when a nitrogen atom sits next to an empty spot in the diamond lattice. NV centers can serve as quantum bits, or qubits, and as highly sensitive sensors of magnetic and electric fields. The difficulty, Umukoro wrote, is that creating or changing one type of defect can also affect the others nearby. To build useful devices, researchers need ways to control individual defect populations more selectively.

Umukoro used a single-crystal diamond grown by chemical vapor deposition and irradiated small regions with 266-nanometer ultraviolet pulses lasting only a few nanoseconds. The idea was to deliver energy to a tiny area without heating the whole crystal. Before using the laser, he characterized the untouched diamond with confocal photoluminescence, ultraviolet-visible absorption and Fourier-transform infrared spectroscopy. That step mattered, he said, because it is difficult to claim a laser created a defect if it was already there. The measurements showed the low-nitrogen diamond held very little substitutional nitrogen compared with nitrogen-rich diamond.

After irradiation, the laser-exposed regions showed a light emission near 563 nanometers that was absent before, along with a second feature near 579 nanometers. Earlier studies have associated these emissions with defect configurations related to carbon self-interstitials, carbon atoms sitting in positions different from their normal lattice sites. The diamond already contained NV centers, and their optical signals stayed largely unchanged in the treated regions compared with untreated ones. In Umukoro's words, the laser produced a measurable change in one part of the defect landscape without a comparable change in the existing NV population.

He was careful about what the result does not show. The 563- and 579-nanometer centers are not newly demonstrated qubits, he wrote. The significance is the ability to manipulate one defect population while preserving a quantum-relevant one. The process also is not a simple one-way build-up: under continued irradiation, the 563-nanometer signal could rise to a maximum and then fall, which suggests the laser both creates defects and later modifies them.

The exact atomic-scale mechanism remains an open question. The results fit a picture in which ultraviolet photons below the diamond's band gap interact with electronic states tied to existing defects, producing localized excitation and passing energy to the surrounding lattice, which could rearrange nearby carbon atoms. Confirming that will take more experiments, and Umukoro noted that optical signatures alone do not always reveal a defect's full atomic structure.

What the team has shown, he wrote, is a materials-engineering concept: nanosecond ultraviolet pulses can selectively alter the optical defect structure of diamond while the background NV population stays mostly intact. The next steps are to understand the defect transformations in more detail and to learn how reproducibly they can be controlled. If laser settings can be tied to specific transformations, diamond makers might one day design the defect landscape on purpose, treating the flaws inside a diamond as parts that can be addressed separately rather than as one inseparable collection.