An international team of researchers from Vienna University of Technology, Rice University in Texas, the University of Toronto, Rutgers University, and several Neutron Scattering Facilities has identified the first strong candidate for a true three-dimensional quantum spin liquid. This research was published in the journal Nature Physics.
Diana Kirschbaum in Silke Bühler-Paschen's lab at TU Wien. Image Credit: Angelika Bosak/Vienna University of Technology
Since the 1970s, scientists have theorized about the existence of materials with a particularly specific type of magnetic disorder, known as quantum spin liquids.
Such materials are quite interesting for a variety of reasons. They may hold the key to the development of new forms of superconductors, as well as unique possibilities in quantum computing and associated technologies. However, true quantum spin liquids have been exceedingly difficult to obtain.
Many experiments have been conducted, particularly with two-dimensional materials. Even though intriguing elements of a QSL have been discovered, there has never been complete agreement between experiment and theory.
Key signals anticipated for such a state, including so-called emergent photons, were discovered during experiments on cerium zirconate (Ce₂Zr₂O₇). These are magnetic excitations in the material that behave somewhat like photons, but they are not real photons.
Ordered Magnets, Disordered Liquid
The spins, or quantum mechanical angular momentum of particles, align into regular patterns in conventional magnets. Below a certain temperature, for instance, all of the spins in a ferromagnet point in the same direction.
However, certain materials have spins that will not settle into any static state, even at absolute zero. Instead, they continue to exist in a state of quantum fluctuation that is continuous.
They behave like a liquid form of magnetism without any fixed ordering.
Silke Bühler-Paschen, Professor, Institute of Solid State Physics, Vienna University of Technology
Therefore, a quantum spin liquid is a solid crystal rather than a liquid in the conventional sense. The spin system is said to be “liquid” if there is no magnetic order.
In such a system, the individual spins are still quantum mechanically entangled, but they still point in different directions and remain disordered. Although their paths seem arbitrary, they are inextricably linked: One spin’s measurement might have an impact on other spins' states. Quantum spin liquids are a very promising platform for upcoming quantum technology because of this entanglement.
Emergent Photons – Light That isn’t Light
It is quite challenging to prove that a material actually forms a quantum spin liquid.
Bühler-Paschen added, “That is exactly why a real breakthrough in this area has remained elusive for decades. We studied cerium zirconate, which forms a three-dimensional network of spins and shows no magnetic ordering even at temperatures as low as 20 millikelvin. For the first time, we were able to detect signals that strongly indicate a three-dimensional quantum spin liquid particularly the presence of so-called emergent photons.”
Waves can propagate through the spin system of a quantum spin liquid, just as they can travel through liquid water when it is disturbed. Although these waves are collective excitations of many spins rather than electromagnetic in origin, they behave rather similarly to light.
They do, however, obey the same electrodynamic equations as genuine photons. The measured signals match theoretical expectations for energy, momentum, and polarization.
Bühler-Paschen concluded, “The discovery of these emergent photons in cerium zirconate is a very strong indication that we have indeed found a quantum spin liquid. We plan to conduct further experiments, but from our perspective, cerium zirconate is currently the most convincing candidate for a quantum spin liquid.”
Additional high-resolution investigations and studies into related materials are also planned.
Journal Reference:
Gao, B., et al. (2025) Neutron scattering and thermodynamic evidence for emergent photons and fractionalization in a pyrochlore spin ice. Nature Physics. doi.org/10.1038/s41567-025-02922-9.