New method models the complete optical cycle of quantum spin defects

University of Chicago framework resolves a long-standing theory-experiment gap in silicon carbide

By LineZotpaper

Published23 October 2026

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Researchers at the University of Chicago have developed a computational framework that models the complete optical cycle of spin defects in crystals such as diamond and silicon carbide, including internal conversion, the energy-loss pathway that has long resisted accurate simulation. The work, published in Physical Review Letters, could improve the design of materials for quantum sensors and computers.

The defects at the centre of the work are atomic-scale imperfections in crystals. When light strikes one, it can transfer energy to electrons and push them into an excited state. Because these spin defects can act as stable qubits whose states can be read, they are seen as promising building blocks for quantum sensors and future quantum computers.

A defect returns to its lower-energy state through several pathways. It may emit a photon, transition between states with different spins, or release energy as vibrations of the surrounding atoms. That final pathway, called internal conversion, has been especially hard to model. Stefano Villani, a postdoctoral researcher at UChicago PME and the study's first author, said the difficulty lies in the complex interactions between electrons and surrounding atoms in realistic solids modelled with hundreds or thousands of atoms.

The team, led by Giulia Galli, the Liew Family Professor of Electronic Structure and Simulations at UChicago PME and the Department of Chemistry, combined computational techniques developed in her group over many years. Using many-body wavefunctions and exact mathematical derivations, they worked out how strongly each excited state interacts with atomic vibrations, allowing them to calculate connections to all possible vibrational configurations without analysing each one separately.

"In the past, a clear mathematical derivation and efficient algorithms were not available," said Yu Jin, a former graduate student in Galli's group who is now at the Flatiron Institute in New York. "The new derivation came together after years of work on the optical cycle of spin defects."

The framework is now part of WEST, an open-source simulation program optimised for graphics processing units that Galli's group has developed through the Midwest Integrated Center for Computational Materials, with funding from the US Department of Energy's Office of Basic Energy Sciences since 2015.

Tests on a nitrogen-vacancy centre in diamond matched recent spectroscopy measurements, with internal conversion contributing little to energy loss in triplet states but serving as the primary mechanism in singlet states, as expected. The surprise came with a divacancy defect in silicon carbide, where internal conversion has a much greater influence on the lifetime of the excited triplet state than earlier models accounted for, helping explain a long-standing disagreement between theoretical predictions and experiments.

"We were expecting to see an effect, but of a smaller magnitude," Galli said. "What we found is that this internal conversion process turns out to be more important than we thought."

Galli said the work has relevance beyond quantum technologies, since internal conversion is also related to processes in energy conversion and storage. The team is now planning to study spin defects in silicon and energy-related materials, and is exploring the use of machine learning to reduce the considerable computing power the simulations still require. "We provided the missing piece of the puzzle needed to completely describe the optical cycle of spin defects," Galli said.

Analysis

Why This Matters

Spin defects are candidate qubits for quantum sensors and computers, and accurate models are needed to select and design materials for those devices.
The framework resolves a long-standing mismatch between theory and experiment for a silicon carbide defect, a leading quantum platform.
Because internal conversion also governs energy loss in some energy conversion materials, the method has applications beyond quantum technology.

Background

Spin defects are atomic-scale imperfections in crystals such as diamond and silicon carbide, formed when an atom is missing from the lattice, sometimes with an impurity atom nearby. When they absorb light, electrons in the defect are pushed into excited states, and the way the defect returns to its ground state determines how it can be used as a qubit or sensor. One relaxation pathway, internal conversion, dissipates energy as vibrations of the surrounding lattice. Modelling it has been difficult because it requires tracking the interaction between excited electrons and hundreds of surrounding atoms, even though the underlying equations of quantum mechanics are known.

Key Perspectives

The UChicago PME researchers: The framework provides the missing piece needed to fully describe the optical cycle of spin defects, allowing more accurate predictions for many materials of interest in quantum technology. Quantum technology developers: Better predictions of defect lifetimes and behaviour could help engineers choose or engineer materials with more reliable qubits and more sensitive magnetic or electric field sensors. Sceptics and practical limits: The simulations remain computationally demanding, which limits who can use them. The team is working on machine learning techniques to improve efficiency and widen access.

What to Watch

Adoption of the method by other groups now that it is available in the open-source WEST program.
The team's planned extension to spin defects in silicon, a material of interest for industrial-scale quantum devices.
Whether machine learning integration can cut the computational cost, and whether the framework improves models of energy conversion and storage materials.

Sources

A tiny defect in a crystal reveals a big quantum surprise · Latest Science News -- ScienceDaily

Checked

11 claims in this story checked against 1 source: 8 supported.

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SupportedThe method models the complete optical cycle of spin defects.Latest Science News -- ScienceDaily: “Their approach can simulate the entire optical cycle, including a particularly difficult process known as internal conversion.”
Partly supportedThe University of Chicago framework resolves the long-standing theory-experiment gap in silicon carbide.Latest Science News -- ScienceDaily: “This previously underestimated contribution helped explain a long-standing disagreement between theoretical predictions and experimental observations.”
Partly supportedThe findings could improve the design of materials for quantum sensors and computers.Latest Science News -- ScienceDaily: “The findings, published in Physical Review Letters , could help scientists design better materials for quantum technologies.”
SupportedYu Jin said, “In the past, a clear mathematical derivation and efficient algorithms were not available.”Latest Science News -- ScienceDaily: “"In the past, a clear mathematical derivation and efficient algorithms were not available," explained Yu Jin.”
SupportedGiulia Galli said, “We were expecting to see an effect, but of a smaller magnitude. What we found is that this internal conversion process turns out to be more important than we thought.”Latest Science News -- ScienceDaily: “"We were expecting to see an effect, but of a smaller magnitude," Galli said. "What we found is that this internal conversion process turns out to be more important than we thought."”
SupportedGiulia Galli said, “We provided the missing piece of the puzzle needed to completely describe the optical cycle of spin defects.”Latest Science News -- ScienceDaily: “"We provided the missing piece of the puzzle needed to completely describe the optical cycle of spin defects," Galli said.”
SupportedThe new derivation came together after years of work on the optical cycle of spin defects, Yu Jin said.Latest Science News -- ScienceDaily: “"The new derivation came together after years of work on the optical cycle of spin defects."”
Partly supportedThe research team was led by Giulia Galli.Latest Science News -- ScienceDaily: “"This internal conversion process is not only relevant to spin defects -- it's also related to processes of interest to energy technologies," said Giulia Galli, the Liew Family Professor of Electronic Structure and Simulations in UChicago PME and the Department of Chemistry and the senior author of the study.”
SupportedTests on a nitrogen-vacancy centre in diamond matched recent spectroscopy measurements; internal conversion contributed relatively little to triplet-state energy loss and was the primary mechanism in singlet states.Latest Science News -- ScienceDaily: “Their calculations closely matched recent spectroscopy measurements. For triplet states, internal conversion contributed relatively little to energy loss. In singlet states, however, it was the primary mechanism responsible for releasing energy.”
SupportedFor a silicon-carbide divacancy, internal conversion had a greater influence on the excited triplet state’s lifetime than earlier models accounted for, helping explain disagreement between theory and experiment.Latest Science News -- ScienceDaily: “Their calculations revealed that internal conversion has a much greater influence on the lifetime of the defect's excited triplet state than earlier models had accounted for. This previously underestimated contribution helped explain a long-standing disagreement between theoretical predictions and experimental observations.”
SupportedWEST has received funding from the US Department of Energy’s Office of Basic Energy Sciences since 2015.Latest Science News -- ScienceDaily: “Galli's group has spent the past decade developing WEST through the Midwest Integrated Center for Computational Materials (MICCoM), which has received funding from the U.S. Department of Energy's Office of Basic Energy Sciences since 2015.”