Quantum technologies are anticipated to transform computing, communication, and sensing by harnessing the unusual behavior of matter at the atomic scale. Turning that promise into practical devices requires finding physical systems that have desirable quantum properties and can also be easily manufactured. Silicon, the material behind today’s computer chips, is highly attractive as a platform because it plays to the strengths of the trillion-dollar semiconductor industry that has already been built. Identifying quantum building blocks — qubits —in silicon is therefore an important frontier research area. In a new study, researchers in UC Santa Barbara materials professor Chris Van de Walle’s Computational Materials group identified a robust new qubit in silicon, called the CN center; the work is published as a Letter in the February 10 issue of the journal Physical Review B.

Qubits can be based on atomic-scale defects in a crystal; a prototype example is the NV center, which consists of a nitrogen (N) atom sitting next to a vacancy (V, a missing carbon atom) in a diamond crystal. These defects can interact with both electrons and light, allowing them to emit single photons (quanta of light) that can transmit quantum information or be processed in quantum networks.

Recent work has focused on a silicon defect called the T center, which can store quantum information for long periods of time — comparable to the duration of storage in an NV center —and emits light in the telecom band, which is the range of wavelengths that can be transmitted with low loss through optical fibers. The T center, however, is made up of carbon and hydrogen atoms, and the presence of hydrogen renders it fragile and sensitive to fabrication conditions. Hydrogen can easily move within the crystal and is difficult to control during processing, creating challenges in terms of achieving reproducible and reliable device manufacturing.

In their new study, researchers in the Van de Walle group identified a promising alternative — the CN center, which consists of carbon and nitrogen atoms. “Unlike the T center, this defect does not contain hydrogen, and will, therefore, be more robust and easier to realize in actual devices,” says Kevin Nangoi, a postdoctoral scholar in the Van de Walle group who led the project.

The team used advanced first-principles computer simulations to model the defect at the atomic level. Because such simulations allow researchers to predict material properties of new systems that have not yet been realized experimentally, they can guide future efforts in engineering and fabricating novel devices. “Our results show that the CN center reproduces the key electronic and optical properties that render the T center attractive for quantum applications; in particular, the center is structurally stable and produces light in the telecom range,” says Mark Turiansky, a group alumnus and now a postdoctoral researcher at the U.S. Naval Research Laboratory who was involved in the project.

Identifying a hydrogen-free telecom-wavelength quantum-light emitter in silicon is an important step that helps to bridge the gap between quantum science and scalable technology. Looking ahead, Van de Walle notes, “If confirmed experimentally, the CN center could serve as a practical new building block for quantum devices, potentially accelerating the development of advanced quantum technologies [while] using the same silicon material that powers today’s electronics.”

Funding for this research was provided by the Department of Energy Office of Science, Office of Basic Energy Sciences, through the Co-design Center for Quantum Advantage (C2QA); the computations were performed at the National Energy Research Scientific Computing Center.