Physicists at University College Cork have cracked open a scientific mystery that has eluded researchers for decades: they have detected the presence of spinons—exotic quantum particles that exist only in the most bizarre state of matter—by treating impurity atoms as eavesdroppers.
The discovery represents a watershed moment in the hunt for quantum spin liquids, states of matter so unusual that their atoms remain in constant quantum flux even at temperatures approaching absolute zero. Unlike ordinary liquids that freeze solid when cooled, or superfluids like helium that flow without friction, quantum spin liquids represent something far stranger: a magnetic system that never settles, held in perpetual motion by universal quantum entanglement. Every spin becomes entangled with every other spin—not in some carefully controlled laboratory experiment, but naturally, in minerals you might find lying on the ground.
The breakthrough centers on Herbertsmithite, a mineral named after British mineralogist George Frederick Herbert Smith and first synthesized in 2004. It stands as the leading candidate for hosting a genuine quantum spin liquid. But for years, researchers struggled with a stubborn obstacle: magnetic impurity atoms within the crystal scattered their signals like static on a radio, drowning out the very phenomenon they were trying to measure. Previous attempts to subtract these impurities from the data failed at low temperatures, where the unwanted signals overwhelmed everything else.
The team—led by Prof. Seamus Davis at UCC, with theoretical guidance from Dr. Felix Flicker of the University of Bristol—reimagined the problem entirely. Rather than treating impurities as noise to be eliminated, they reconceived them as quantum bits, or qubits, that could serve as "witnesses" to the quantum spin liquid's internal structure. "By introducing the quantum witness technique we provide a completely new perspective on the physics of quantum spin liquids and access their internal quantum excitations or 'spinons' directly for the first time at UCC," Davis explained.
The experimental setup was ingenious. Experimentalists Jack Murphy at UCC and Hiroto Takahashi (now at Princeton University) deployed a superconducting quantum interference device—a SQUID—to measure the spontaneous magnetic fluctuations generated by Herbertsmithite crystals. A SQUID is among the most sensitive detectors of magnetic fields ever built, capable of detecting changes roughly a billion times smaller than Earth's magnetic field. The device proved sensitive to the same frequency range of magnetic fluctuations as the human ear is to sound.
When Murphy and Takahashi analyzed the magnetic noise they detected, they found something striking: the signal exhibited a precise form of pink noise, the same kind of statistical pattern found in music and natural processes throughout the world. This specificity was crucial. Different types of noise reveal different underlying physics, and the pink noise pattern allowed the team to identify how the witness spins were interacting with one another through the quantum spin liquid itself—and therefore to detect the spinons mediating those interactions.
The work, published in Nature Physics, opens a new frontier in understanding quantum matter. Because quantum spin liquids represent a fundamentally different phase of matter, understanding their properties could unlock pathways toward exotic quantum technologies. The breakthrough in Herbertsmithite brings researchers closer to identifying what Davis and colleagues call "quantum silicon"—a material that could serve as the foundation for quantum computers, just as conventional silicon powers today's devices.