New quantum simulations expose hidden energy loss in spin defects

Somewhere inside a diamond, a single missing atom and a stray nitrogen are quietly performing one of the most delicate dances in modern physics. When laser light strikes this tiny imperfection, known as a spin defect, it kicks the defect’s electrons into an excited state, and the way that energy subsequently escapes, whether as a flash of light, a flip of spin, or a silent shudder of the surrounding atomic lattice, determines whether the defect can ever serve as a reliable qubit. For years, physicists could model the first two escape routes with confidence, but the third, a process called internal conversion, remained stubbornly beyond the reach of accurate prediction. Now a team at the University of Chicago Pritzker School of Molecular Engineering has closed that gap with a first-principles computational framework that captures the entire optical cycle of spin defects, and in doing so has uncovered a previously overlooked energy drain in one of quantum technology’s most promising host materials.

The research, published in Physical Review Letters, was led by Giulia Galli, the Liew Family Professor of Electronic Structure and Simulations at UChicago PME and the Department of Chemistry, with postdoctoral researcher Stefano Villani as first author and Yu Jin, a former graduate student in the Galli group now at the Flatiron Institute in New York, as a key contributor. Their work addresses a deceptively simple question: when a spin defect absorbs light, where exactly does the energy go, and how quickly? The answer matters enormously, because the excited-state lifetime of a defect governs how fast it can be read out, how faithfully it retains quantum information, and ultimately whether it can anchor practical technologies such as quantum sensors, quantum communication devices, and quantum computers.

Spin defects owe their appeal to a remarkable combination of properties. Embedded in wide-bandgap crystals like diamond or silicon carbide, they can hold a quantum state for long periods at room temperature, and their optical transitions allow that state to be initialized and measured with ordinary lasers. The canonical example is the nitrogen-vacancy center in diamond, in which a nitrogen atom sits next to a vacant lattice site. When excited by light, such a defect can relax back to its ground state along several competing pathways. It can emit a photon, it can make non-radiative jumps between states of different spin character, or it can shed its energy through internal conversion, in which the electronic excitation is dissipated as vibrations of the surrounding atoms rather than as light.

Internal conversion is the awkward sibling of these processes. The fundamental quantum-mechanical equations describing it have long been known, but solving them accurately for a realistic solid is a computational nightmare. The process hinges on the coupling between electronic states and the motion of hundreds or thousands of neighboring atoms, and capturing those interactions from first principles, without empirical fitting parameters, demands an enormous amount of calculation. Villani noted that although the equations are known, describing the process accurately is difficult because one must account for complex interactions between the electrons and the surrounding atoms in realistic solids represented by models with hundreds or thousands of atoms. Simplified models could estimate the effect, but they left researchers unable to make reliable quantitative predictions, and discrepancies between theory and experiment lingered.

The Galli group’s breakthrough came from coupling several computational methods the team had developed over the years into a new formulation for internal conversion. Instead of treating the defect’s excited state with a single-electron picture, the researchers described it with many-body wavefunctions, which account for the correlated behavior of multiple electrons simultaneously. They then derived, through exact mathematical steps, a way to calculate how strongly each excited state couples to atomic motion. Crucially, the formulation allows them to compute how each of a defect’s states couples to all of the vibrational configurations of hundreds of nearby atoms at once, rather than evaluating each coupling one at a time. Jin explained that in the past a clear mathematical derivation and efficient algorithms were not available, and that the new derivation came together after years of work on the optical cycle of spin defects.

The framework was implemented in WEST, an open-source, GPU-optimized simulation code that the Galli group has developed over the past decade within the Midwest Integrated Center for Computational Materials, or MICCoM, a center funded by the U.S. Department of Energy’s Office of Basic Energy Sciences since 2015. Because the new capability lives inside an established, publicly available code, other research groups can begin applying it to their own materials almost immediately, though Galli cautioned that the calculations remain demanding and require substantial computational resources. The team is already working on ways to integrate machine learning into the excited-state calculations to improve the method’s efficiency, according to Villani.

The first test case was the nitrogen-vacancy center in diamond, arguably the most studied spin defect in the world. The new predictions matched recent spectroscopy measurements closely. When triplet states are involved, the calculations showed that internal conversion plays little role in the defect’s energy loss, while for singlet states it is the dominant relaxation pathway. Both findings were consistent with what researchers had long assumed but had never been able to derive from first principles with this level of rigor. In effect, the framework converted a set of plausible assumptions into verified, quantitative predictions, giving the diamond quantum community a firmer theoretical foundation for interpreting experiments and engineering better devices.

The real surprise came when the team turned the same machinery on a divacancy in silicon carbide, a defect formed by adjacent missing silicon and carbon atoms that is prized for its compatibility with semiconductor manufacturing. Here the calculations revealed that internal conversion makes a substantial contribution to the defect’s triplet excited-state lifetime, a contribution that had been entirely overlooked until now. The result resolves a long-standing discrepancy between theoretical predictions and experimental measurements of the lifetime. Galli admitted that the team was expecting to see an effect, but of a smaller magnitude, and that internal conversion turned out to be more important than anyone had thought. For engineers hoping to build qubits from silicon carbide divacancies, the finding means that models of defect performance must now account for a relaxation channel that was previously absent from the ledger, with direct implications for how readout fidelity and coherence are optimized.

The implications reach well beyond quantum information science. Internal conversion is a central process in many energy technologies, governing how efficiently materials convert absorbed light into useful energy or store it without losing it to heat. Galli emphasized that the internal conversion process is not only relevant to spin defects but is also related to processes of interest to energy technologies, and that this is one of the reasons the work is so exciting. The team is now extending the framework to spin defects in silicon and to energy materials where internal conversion plays an important role, potentially informing the design of better materials for renewable energy conversion and storage, from photovoltaic absorbers to photocatalysts.

In a field where a single misplaced atom can make or break a quantum device, having a complete, predictive theory of how defects lose energy is a genuinely transformative tool. As Galli put it, the team has provided the missing piece of the puzzle needed to completely describe the optical cycle of spin defects, and Jin added that the work can now be used to make more accurate predictions about many materials of interest for quantum technologies. What began as an attempt to tame a computationally intractable equation has ended up rewriting the energy budget of some of the most important defects in quantum materials, and the ripple effects are likely to be felt everywhere from diamond-based quantum sensors to the next generation of solar energy materials.

Subject of Research: First-principles calculations of internal conversion processes in spin defects in diamond and silicon carbide

Article Title: Quantum calculations reveal how quantum defects lose energy

Article References: Quantum calculations reveal how quantum defects lose energy. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: spin defects, quantum computing, qubits, internal conversion, nitrogen-vacancy center, silicon carbide, first-principles calculations, excited-state dynamics, WEST code, quantum sensing, energy materials, computational materials science