Gold “super atoms” could unlock scalable, customizable quantum computing. (Artist’s concept.) Credit: SciTechDaily.com

Researchers found that gold “super atoms” can behave like the atoms in top-tier quantum systems—only far easier to scale.

These tiny clusters can be customized at the molecular level, offering a powerful, tunable foundation for the next generation of quantum devices.

Gold Clusters as Scalable Quantum Building Blocks

Quantum computers, sensors, and other advanced technologies depend heavily on the behavior of electrons, especially the way they spin. One of the most precise approaches for high-performance quantum systems uses the spin characteristics of electrons in atoms held within a gas. These gaseous setups offer exceptional accuracy but are extremely difficult to scale into larger quantum devices, including full quantum computers. A research team from Penn State and Colorado State has now shown that a gold cluster can imitate the behavior of these trapped gas-phase atoms, making it possible to access similar spin properties in a format that can be expanded far more easily.

“For the first time, we show that gold nanoclusters have the same key spin properties as the current state-of-the-art methods for quantum information systems,” said Ken Knappenberger, department head and professor of chemistry in the Penn State Eberly College of Science and leader of the research team. “Excitingly, we can also manipulate an important property called spin polarization in these clusters, which is usually fixed in a material. These clusters can be easily synthesized in relatively large quantities, making this work a promising proof-of-concept that gold clusters could be used to support a variety of quantum applications.”

The work, described in two papers published in ACS Central Science and The Journal of Physical Chemistry Letters, confirms the spin behavior of the gold clusters in detail.

How Electron Spin Shapes Quantum Performance

“An electron’s spin not only influences important chemical reactions, but also quantum applications like computation and sensing,” said Nate Smith, graduate student in chemistry in the Penn State Eberly College of Science and first author of one of the papers. “The direction an electron spins and its alignment with respect to other electrons in the system can directly impact the accuracy and longevity of quantum information systems.”

An electron spins around its axis in a way that can be compared to Earth spinning on its axis, which is tilted relative to the sun. However, electrons can spin either clockwise or counterclockwise. When many electrons in a material spin in the same direction and their tilts match, they become correlated. A material with a strong level of this alignment has high spin polarization.

“Materials with electrons that are highly correlated, with a high degree of spin polarization, can maintain this correlation for a much longer time, and thus remain accurate for much longer,” Smith said.

Limitations of Trapped Ions and the Need for New Solutions

The leading method for achieving extremely low error rates in quantum information systems involves trapped atomic ions, which are atoms with an electric charge kept in a gaseous environment. In these setups, electrons can be excited into Rydberg states, which offer long-lasting and precisely defined spin polarizations. These systems also allow electrons to exist in superposition, meaning they can occupy multiple states at the same time until measured. Superposition is fundamental to quantum computing.

“These trapped gaseous ions are by nature dilute, which makes them very difficult to scale up,” Knappenberger said. “The condensed phase required for a solid material, by definition, packs atoms together, losing that dilute nature. So, scaling up provides all the right electronic ingredients, but these systems become very sensitive to interference from the environment. The environment basically scrambles all the information that you encoded into the system, so the rate of error becomes very high. In this study, we found that gold clusters can mimic all the best properties of the trapped gaseous ions with the benefit of scalability.”

Gold Nanoclusters and Their Quantum Potential

Gold nanostructures have long been studied for applications in optics, sensing, therapeutics and catalysis, but their magnetic and spin-related behaviors have received far less attention. In the new research, the team focused on monolayer-protected clusters. These consist of a gold core surrounded by molecules known as ligands. The structure of these clusters can be precisely adjusted, and they can be produced in relatively large amounts.

“These clusters are referred to as super atoms, because their electronic character is like that of an atom, and now we know their spin properties are also similar,” Smith said. “We identified 19 distinguishable and unique Rydberg-like spin-polarized states that mimic the super-positions that we could do in the trapped, gas-phase dilute ions. This means the clusters have the key properties needed to carry out spin-based operations.”

Tuning Spin Polarization Through Chemical Design

The scientists measured spin polarization in the gold clusters using an approach similar to techniques used for individual atoms. One type of cluster showed 7 percent spin polarization, while another cluster with a different ligand reached nearly 40 percent. Knappenberger noted that this higher value is comparable to that of some leading two-dimensional quantum materials.

“This tells us that the spin properties of the electron are intimately related to the vibrations of the ligands,” Knappenberger said. “Traditionally, quantum materials have a fixed value of spin polarization that cannot be significantly changed, but our results suggest we can modify the ligand of these gold clusters to tune this property widely.”

The team now plans to investigate how altering specific features within the ligands affects spin polarization and how these changes might be used to fine tune quantum behavior.

“The quantum field is generally dominated by researchers in physics and materials science, and here we see the opportunity for chemists to use our synthesis skills to design materials with tunable results,” Knappenberger said. “This is a new frontier in quantum information science.”

References:

“Diverse Superatomic Magnetic and Spin Properties of Au144(SC8H9)60 Clusters” by Juniper Foxley, Marcus Tofanelli, Jane A. Knappenberger, Christopher J. Ackerson and Kenneth L. Knappenberger, Jr., 29 May 2025, ACS Central Science. DOI: 10.1021/acscentsci.5c00139

“The Influence of Passivating Ligand Identity on Au25(SR)18 Spin-Polarized Emission” by Nathanael L. Smith, Patrick J. Herbert, Marcus A. Tofanelli, Jane A. Knappenberger, Christopher J. Ackerson and Kenneth L. Knappenberger, Jr., 15 May 2025, The Journal of Physical Chemistry Letters. DOI: 10.1021/acs.jpclett.5c00723

In addition to Smith and Knappenberger, the research team includes Juniper Foxley, graduate student in chemistry at Penn State; Patrick Herbert, who earned a doctoral degree in chemistry at Penn State in 2019; Jane Knappenberger, researcher in the Penn State Eberly College of Science; as well as Marcus Tofanelli and Christopher Ackerson at Colorado State

Funding from the Air Force Office of Scientific Research and the U.S. National Science Foundation supported this research.

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