Researchers at the University of California, Santa Barbara have identified a carbon-nitrogen defect in silicon that emits light squarely in the telecom S-band, offering a hydrogen-free alternative to existing qubit designs that struggle with manufacturing stability. The finding, published in Physical Review B and detailed in a companion preprint, provides first-principles evidence that this CN complex resists decomposition and could slot directly into silicon photonic chips already used by the telecommunications industry. If experimental validation follows the theoretical predictions, the discovery could simplify one of the hardest problems in quantum networking: building qubits that survive the fabrication process.
Why the T Center Falls Short at Scale
The leading candidate for a silicon-based telecom qubit has been the T center, a defect made of carbon, hydrogen, and silicon atoms. Experimental work has shown that the T center can function as a spin-photon interface integrated with silicon photonics, and a team demonstrated a three-qubit register of electron and nuclear spins with electron spin echo coherence of approximately 0.41 milliseconds. Those numbers are promising for quantum memory and entanglement operations. But the T center carries a built-in liability: hydrogen.
Hydrogen serves as an intrinsic nuclear memory qubit inside the T center, which is useful for storing quantum information. At the same time, hydrogen is a well-documented fabrication complication because it migrates unpredictably during the high-temperature steps common in semiconductor manufacturing. Controlling hydrogen placement at the atomic level adds cost and complexity. Earlier theoretical work estimated the T center’s radiative lifetime at microsecond scales, consistent with experiments, yet also flagged concerns about stability against decomposition under processing conditions. The practical result is that scaling T centers from single-device demonstrations to chip-scale arrays remains difficult, especially in commercial foundries where thermal budgets and process flows are optimized for classical electronics, not fragile quantum defects.
A Hydrogen-Free Path to Telecom Qubits
The CN complex sidesteps this problem entirely. According to the UCSB team’s first-principles study, the proposed defect is a carbon-nitrogen complex that contains no hydrogen. Their calculations predict a zero-phonon line at 828 meV, placing its optical emission in the telecom S-band, the same wavelength window used by fiber-optic networks worldwide. The theory also indicates that the CN complex is stable against decomposition, meaning it should survive the thermal budgets of standard silicon chip processing without breaking apart or diffusing away from its intended location. In practice, that kind of robustness can determine whether a qubit design remains a laboratory curiosity or becomes a viable building block for large-scale quantum networks.
The absence of hydrogen is not just a minor convenience; it removes an entire category of fabrication uncertainty. As the UC Santa Barbara team explained, the CN center is expected to be more practical for fabrication precisely because it eliminates the need to manage hydrogen diffusion. Carbon and nitrogen are both standard dopants in silicon manufacturing, with well-understood implantation and annealing behavior. That familiarity could shorten the timeline between theoretical prediction and working devices, allowing researchers to plug the CN defect into existing process flows rather than inventing bespoke recipes that only work in specialized academic cleanrooms.
How the CN Complex Fits the Telecom Puzzle
Quantum networks need qubits that emit photons at wavelengths compatible with existing fiber infrastructure. Most solid-state qubits either emit at the wrong wavelength or require frequency conversion, which adds loss and noise and complicates system engineering. The CN complex’s predicted 828 meV zero-phonon line falls within the telecom S-band, so photons from this defect could travel through standard fiber without conversion. That direct compatibility is rare among silicon-based defects and gives the CN approach a structural advantage over alternatives that require additional optical engineering, such as nonlinear frequency converters or specialized waveguides tuned to non-telecom wavelengths.
Competing approaches exist outside silicon as well. A recent peer-reviewed study in Nature Communications demonstrated erbium-based interfaces with long optical and spin coherence in an epitaxial thin-film platform. Erbium ions emit naturally at telecom wavelengths and have shown strong coherence properties, making them attractive for quantum repeaters and memory nodes. But erbium systems lack the manufacturing ecosystem that silicon enjoys. Silicon foundries already produce billions of chips annually, and any qubit that can be fabricated using those same tools gains an enormous deployment advantage. The CN complex, if it performs as predicted, would inherit that entire industrial base, enabling hybrid chips where classical control electronics, photonic routing, and quantum defects coexist on the same wafer.
What Still Needs to Happen
The CN complex research remains at the theoretical stage. The preprint provides first-principles evidence, not experimental measurements of coherence times, spin properties, or photon emission rates. Those numbers will determine whether the defect can actually function as a qubit rather than just a telecom-wavelength light source. No published data yet confirms how the CN center’s spin coherence compares to the T center’s 0.41-millisecond benchmark, and until that gap is filled, the CN complex is a prediction rather than a proven device. Experimentalists will need to identify the defect in real silicon samples, isolate single emitters, and characterize their optical and spin dynamics under realistic operating conditions.
Funding for this line of research comes from the DOE Office of Science, Office of Basic Energy Sciences, channeled through the C2QA program. That federal backing signals institutional confidence in silicon-defect approaches to quantum networking and aligns with broader efforts to leverage existing semiconductor infrastructure for quantum technologies. Parallel work across the field, including experiments at UNSW on electron-mediated entanglement between atomic nuclei in silicon and molecular qubit research at the University of Chicago focused on telecom-frequency communication, underscores that multiple groups are converging on the same goal: qubits that speak the language of existing fiber networks and can be integrated into scalable hardware platforms.
The Real Stakes for Quantum Networking
The gap between laboratory qubit demonstrations and deployable quantum networks has always been a manufacturing problem as much as a physics problem. Individual qubits can be exquisitely controlled in isolated experiments, but replicating that control across thousands of identical devices on a single chip demands materials that behave predictably during fabrication. The T center’s hydrogen sensitivity illustrates this tension perfectly: excellent quantum properties in the lab, but a processing headache at scale. The CN complex, by removing hydrogen from the equation, targets the manufacturing bottleneck directly, aiming to deliver a defect that can be created, positioned, and preserved using the same high-temperature steps and implantation tools already standard in industry.
If the CN defect lives up to its theoretical promise, it would not merely add another candidate to the crowded field of qubit platforms. It would provide a concrete pathway toward quantum network nodes that can be produced in volume, coupled efficiently to existing fiber, and co-integrated with classical control circuitry. That combination (telecom compatibility, silicon process readiness, and qubit-grade coherence) defines the real stakes of the UCSB work. The next few years of experimental validation will determine whether the CN complex becomes a cornerstone of practical quantum networking or a stepping stone that informs the search for an even more robust silicon-based telecom qubit.
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*This article was researched with the help of AI, with human editors creating the final content.
