A Carbon-Nitrogen Defect in Silicon Could Replace the Fragile Hydrogen-Based Qubit Holding Up Quantum Networking

Silicon has an obvious appeal for quantum computing and quantum communication: the semiconductor industry has spent 70 years learning to manufacture it at extraordinary precision and scale. If qubits - the quantum equivalents of classical bits - could be built into silicon using existing chipmaking infrastructure, the path from laboratory demonstration to commercial deployment would be far shorter than for exotic materials like diamond. The challenge is finding silicon defects with the right properties.

A study published in Physical Review B from UC Santa Barbara's Computational Materials Group identifies a new candidate: the CN center, a defect in silicon formed by a carbon atom paired with a nitrogen atom. Using first-principles quantum mechanical simulations, researchers led by postdoctoral scholar Kevin Nangoi found that the CN center reproduces the key properties of a previously studied silicon qubit while eliminating that qubit's main practical weakness.

The Problem With the T Center

The quantum information field has been closely watching the T center, a silicon defect composed of carbon and hydrogen atoms. Its attractions are significant: it stores quantum information for periods comparable to the NV center in diamond - one of the best-characterized qubits known - and it emits light in the telecom band. That second property matters enormously. Telecom-band photons travel through standard optical fiber with low loss, meaning quantum information encoded in these photons could potentially be transmitted through existing communications infrastructure.

The hydrogen is the problem. Hydrogen atoms move easily within silicon crystals and are difficult to control during the high-temperature processing steps used in device fabrication. This makes T center devices sensitive to manufacturing conditions and difficult to reproduce reliably across different facilities or batches. For a quantum technology to scale, reliable manufacturing is not optional.

What the CN Center Offers

The CN center substitutes nitrogen for hydrogen. Nitrogen does not have the same tendency to migrate through silicon during processing. A defect without hydrogen should be more structurally stable and less sensitive to the thermal and chemical steps involved in device fabrication.

The computational simulations confirmed that the CN center possesses the electronic and optical properties needed for quantum applications. It is structurally stable - the nitrogen stays in place. It emits light in the telecom band - the same wavelength range the T center uses and that fiber optic networks are designed to carry. "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," said Mark Turiansky, a former member of the Van de Walle group now at the U.S. Naval Research Laboratory.

Computationally Predicted, Not Yet Experimentally Demonstrated

A critical caveat: the CN center has been identified through computer simulation, not through laboratory fabrication and characterization. First-principles simulations are powerful predictive tools - they model material behavior from quantum mechanical fundamentals without empirical fitting parameters - but a prediction is not a demonstration. The next step is for experimental groups to attempt to create the CN center in silicon, characterize its actual properties, and determine whether it performs as the simulations suggest.

"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 using the same silicon material that powers today's electronics," said Chris Van de Walle, professor of materials at UC Santa Barbara and head of the Computational Materials Group.

The theoretical prediction is nonetheless useful: it gives experimentalists a specific target to attempt to realize, with quantitative predictions against which their measurements can be checked. The simulation work was performed at the National Energy Research Scientific Computing Center, with funding from the Department of Energy's Co-design Center for Quantum Advantage (C2QA).