Building a Quantum Model with Nanoscale Carbon Molecules

Empa researchers have experimentally recreated a fundamental theoretical model from quantum physics, originally proposed by Nobel Prize winner Werner Heisenberg. The experiment was based on "quantum Lego," composed of tiny carbon molecules known as nanographenes. This synthetic bottom-up approach enables flexible experimental studies of quantum technologies, potentially advancing the field in the future.

In 2024, Empa researchers and their collaborators successfully realized a one-dimensional alternating Heisenberg model using a synthetic material. This theoretical quantum-physical model, which has been in use for nearly a century, describes a linear chain of spins—a form of quantum magnetism. The researchers, led by Roman Fasel, Head of Empa's nanotech@surfaces lab, recreated this model in the lab.

The alternating model involves a pattern of strong and weak couplings between spins, while the new model has equally coupled spins. Despite this minor difference, the properties of the models are significantly distinct. The homogeneous chain has no energy difference between the ground and excited states, and the spins are highly entangled with long-range correlations.

In contrast, the alternating chain forms an energy gap, with spins tending to form strong pairwise connections, and correlations rapidly decreasing exponentially.

The researchers validated these theoretical predictions using nanographene spin chains. Nanographenes, small pieces of the two-dimensional carbon material graphene, were used to create both models. By adjusting the shape of these pieces, the researchers can modify their (quantum) physical characteristics, aiming to develop a material platform for the experimental study of various quantum models and effects, similar to "quantum Lego."

Moving Quantum Technologies a (Tiny) Step Closer to Practical Applications

The two Heisenberg experiments demonstrate this method. For the alternating spin chain model, the researchers used hourglass-shaped nanographene molecules called Clar's goblets, which consist of eleven carbon rings. For the homogeneous Heisenberg chain, they used Olympicene, a nanographene with five rings, named for its resemblance to the Olympic rings.

We have now demonstrated for the second time that theoretical models of quantum physics can be realized with nanographenes, making their predictions experimentally testable.

Roman Fasel, Head, nanotech@surfaces Lab, Empa

The scientists plan to develop and study ferrimagnetic spin chains using nanographenes. In these chains, the magnetic moments do not fully cancel out but align antiparallel. Two-dimensional spin lattices are also of significant interest, as they exhibit a broader range of phases than spin chains, such as quantum spin liquids, exotic critical phenomena, and topological states. This is particularly relevant for both basic research and practical applications.

Reproducing quantum models from textbooks serves a practical purpose beyond academic interest. It is expected to lead to advancements in communication, processing power, measuring technologies, and more areas of technology.

However, research into practical applications is challenging due to the fragility of quantum states and the difficulty in understanding their effects. Researchers at Empa aim to better understand these quantum effects and enable the development of practical quantum technologies using their nanographene-based "quantum Lego."

Journal Reference:

Zhao, C., et al. (2025) Spin excitations in nanographene-based antiferromagnetic spin-1/2 Heisenberg chains. Nature Materials.doi.org/10.1038/s41563-025-02166-1