What if a nearly invisible particle in a wafer-thin magnet ended up reshaping the future of quantum computers? That is exactly the kind of possibility scientists are exploring as they learn how strange quantum effects arise and behave in ultrathin magnetic crystals.
Some areas of basic science are so abstract and mathematically dense that experiments alone cannot reveal the full story of what is going on inside a material. Researchers often measure signals in the lab but still need powerful theoretical tools to translate those signals into a clear picture of the hidden quantum processes underneath. In condensed matter physics—where the focus is on the subtle motions of electrons and other tiny particles inside solids—many of the most interesting effects are either extremely hard to detect or very tricky to interpret with current instruments.
Since the 1990s, physicists have built many different models to explain these quantum phenomena, but experts still debate which assumptions in those models are valid and which might be misleading. To reduce this uncertainty, modern studies increasingly combine detailed experiments with rigorous theory based on first-principles quantum calculations, rather than relying only on simplified models. In this work, theorists like NREL’s Mark van Schilfgaarde and Swagata Acharya play a crucial role by turning raw experimental data into a consistent quantum-level description of what is really happening inside the material.
“The hardest part is not always doing the measurement—it’s understanding what the measurement actually means in such a complex system,” van Schilfgaarde explained in essence. Theory makes it possible to explore quantities that are extremely difficult—or even impossible—to access directly in the lab, such as the precise nature of electron interactions or subtle magnetic responses. This approach lets scientists describe optical and magnetic properties without leaning on oversimplified models that might completely miss important quantum effects because they are built on shaky assumptions.
New insights into excitons in CrSBr
This challenge of explaining puzzling optical and magnetic behavior motivated a 2025 study published in Nature Materials, supported by the U.S. Department of Energy’s Office of Basic Energy Sciences. A large international team of experimentalists and theorists from Columbia University, the Technical University of Munich, Dresden University of Technology, King’s College London, Radboud University, the University of Chemistry and Technology Prague, the University of Regensburg, and NREL investigated an unusual type of particle-like excitation called an exciton in ultrathin magnetic films made from the compound chromium sulfide bromide (CrSBr).
To probe how excitons form and behave in these layered CrSBr films, the team combined carefully designed optical experiments with high-precision quantum-mechanical calculations using NREL’s Questaal electronic structure software. This tight integration of theory and experiment allowed them to explain features in the optical spectra that had previously been mysterious and to map them onto specific types of excitons inside the material. The work delivers foundational knowledge about a niche but very promising class of materials and could eventually help engineers deliberately create, stabilize, and tune excitons for use in quantum computing, photonic circuits, spin-based electronics (spintronics), and other advanced devices.
Why excitons matter for quantum tech
Traditional computers use bits to carry information, and each bit can only be in one of two states: 0 or 1, much like a basic light switch that is either fully off or fully on. Quantum computers instead rely on quantum bits, or qubits, which can be in state 0, state 1, or a quantum superposition of both at the same time—similar to a dimmer switch that can smoothly take on many levels rather than only two extremes. Because of this superposition, a system of qubits can, in principle, process certain kinds of information far more efficiently than even the fastest conventional supercomputer.
Scientists have proposed a variety of physical systems to implement qubits, including superconducting circuits, trapped ions, and defects in crystals. Recently, another intriguing candidate has gained attention: excitons that form in carefully engineered materials and can be controlled with light and magnetic fields. This idea is still emerging—but it is already raising big questions about how best to design materials so that excitons behave in a stable, controllable, and reproducible way. Could exciton-based qubits ultimately compete with or even outperform today’s leading quantum platforms?
An exciton forms when a photon (a particle of light) gives an electron enough energy to jump to a higher energy level, leaving behind a positively charged “hole” where the electron used to be. The negatively charged electron and the positively charged hole attract each other and bind together through their opposite charges, much like a tiny, electrically neutral molecule held together by Coulomb forces. Even though the pair is made of two charged objects, the bound electron–hole duo overall behaves as a single neutral quasiparticle: the exciton.
Physicists first introduced the concept of excitons in the early 1930s, but only in the last several years have researchers been able to probe their behavior in ultra-thin, two-dimensional magnetic materials with enough precision to reveal exotic effects. In such systems, some theorists have suggested that applying and then removing a magnetic field could cause excitons to split into distinct energy states and later relax back to equilibrium. This splitting would divide what looked like a single excitonic feature into two or more energy levels, altering both the optical response and the electronic properties of the material.
If this splitting is small but well-defined, it can effectively create a two-level quantum system: one state slightly higher in energy than the other. In principle, that two-level structure could serve as the basis for a qubit in a quantum computer, where the two excitonic states represent the logical 0 and 1, and quantum superpositions of those states perform computations. But here’s where it gets controversial: is it really practical to build qubits from such delicate quasiparticles, or will they be too fragile to use at scale?
Surface vs. bulk excitons: more “knobs to turn”
Turning excitons into robust qubits requires far more than just showing that they exist. Engineers would need a deep, quantitative understanding of how excitons form, how long they survive, how they respond to light and magnetic fields, and how consistently they behave from one device to the next. Only then could excitons be reliably used to encode quantum information or to generate and manipulate spin currents in cutting-edge quantum technologies.
Over the past several years, Acharya, van Schilfgaarde, and collaborators have focused on CrSBr as a candidate platform for such exciton-based applications, publishing multiple studies that examine its electronic, magnetic, and optical properties in detail. Earlier work explored phenomena such as hyperbolic exciton polaritons in this van der Waals magnet, the paramagnetic electronic structure of CrSBr using advanced GW calculations compared with angle-resolved spectroscopy, and large exchange splitting effects in this two-dimensional antiferromagnet, among others. These studies collectively laid the groundwork needed to tackle a more specific and puzzling question about CrSBr’s exciton spectrum.
In the new study, the team zoomed in on two closely spaced exciton features observed in CrSBr at energies of 1.34 electron volts (eV) and 1.36 eV, detected through optical spectroscopy. Some researchers had speculated that the material’s magnetic ordering might be responsible for the two peaks, but there was no complete theoretical explanation that showed precisely how the magnetic structure and the excitons were linked or where within the material each exciton originated. This ambiguity left plenty of room for debate—and for misinterpretation.
To resolve this, Acharya and van Schilfgaarde used the Questaal software suite to carry out detailed electronic-structure and many-body calculations that capture the relevant quantum mechanics at a microscopic level. Experimentalists at Columbia University performed highly sensitive reflectance spectroscopy and photoluminescence measurements, which served as an independent check on the theoretical predictions. When the theory and the measurements were compared, a consistent picture finally emerged.
The analysis showed that CrSBr’s antiferromagnetic ordering causes two distinct types of excitons to appear: one localized near the surface of the crystal at 1.34 eV and another residing within the inner “bulk” layers at 1.36 eV. In simple terms, the lower-energy feature comes from excitons that live at the outermost layer of the material, while the slightly higher-energy feature corresponds to excitons that sit between layers inside the crystal stack. This explanation not only accounts for the two peaks but also ties them directly to the material’s layered structure and magnetic properties.
Acharya described the situation this way in substance: surface excitons effectively interact with just a single layer of the material, while bulk excitons are positioned between two layers. At the surface, the electron cloud extends into open space, so the electron–hole pair experiences less screening from surrounding electrons, leading to a stronger Coulomb attraction between them. Stronger attraction means more tightly bound excitons, which translates into a different binding energy compared with excitons in the interior. This discrepancy in binding energy between surface and bulk excitons is not imposed from the outside but arises naturally from the material’s internal structure and antiferromagnetic ordering.
What surprised the researchers even more was how these excitons behave as additional layers of CrSBr are stacked. The energies associated with both the surface excitons and the bulk excitons remain essentially unchanged as the film thickness increases, which is not what happens in many other layered semiconductors. The brightness, or optical intensity, of the surface exciton stays nearly constant with added layers, while the bulk exciton becomes brighter as the crystal grows thicker. That means one type of exciton is remarkably insensitive to thickness, and the other becomes increasingly prominent—an unusual and potentially useful combination.
This behavior sharply contrasts with materials such as molybdenum disulfide (MoS₂) or black phosphorus, where exciton energies typically shift downward (lower in energy) as the number of layers increases. In CrSBr, even when the material is grown to thicknesses that would be realistic for commercial device fabrication, the excitons maintain relatively high energies and do not “soften” in the same way. On top of that, surface and bulk excitons respond to light in a largely independent manner rather than strongly mixing with each other—a counterintuitive outcome that the theoretical calculations helped explain. And this is the part most people miss: this independence effectively gives device designers two separate excitonic channels to work with in a single material.
Why CrSBr might be a game-changer
CrSBr is not just another layered semiconductor; it is also a magnet. As Acharya pointed out, its magnetic nature introduces spin degrees of freedom on top of the two types of excitons already present. When spin and excitons interact, the system can host many possible quantum states that lie between the classical 0 and 1, potentially supporting rich forms of quantum superposition. This combination—magnetism, distinct surface and bulk excitons, and robust excitonic behavior over practical thickness ranges—gives engineers many more “knobs to turn” when imagining new quantum or optoelectronic devices.
Still, the field is at an early stage, and the ultimate impact of these findings on real-world quantum computers or next-generation optoelectronic components remains uncertain. There is a reasonable debate to be had about whether complex, exciton-based schemes will ever beat more mature qubit technologies, or whether they will instead carve out specialized niches where their unique properties shine. What is already clear, though, is that CrSBr combines strong excitonic effects, the ability to tune properties through magnetism, and stability in air—all attractive traits for practical applications that need materials to survive outside ultra-controlled lab environments.
For the moment, simply being able to parse and explain these excitonic features counts as a major step. By combining advanced theory with sophisticated optical experiments, van Schilfgaarde, Acharya, and their collaborators have demonstrated that it is possible to systematically understand how surface and bulk excitons arise and behave in CrSBr. The natural next question is whether similar theoretical frameworks can reliably predict quantum effects in other, perhaps entirely new, materials.
In a world where nations and companies are racing to build faster, more efficient, and more secure technologies, such predictive power could be a decisive advantage. Being able to screen candidate materials on a computer before ever making them in the lab would save time, money, and effort, and might reveal unexpected options that human intuition alone would miss. So here’s the question to you: if excitons in magnetic crystals like CrSBr really can be tamed for quantum computing, should research funding shift more heavily toward these exotic materials, or should we double down on today’s more established quantum platforms instead? Share where you stand—and why—in the comments.
