At first glance the physics seems obvious. When we cool water, it ends up turning into ice. If we continue to cool most substances, their atoms lose mobility and end up organizing themselves into increasingly rigid structures. Is one of the most basic rules of nature: cold freezes. But quantum mechanics has been suggesting for decades that there are exceptions.
Some materials could harbor an extraordinary state known as “quantum spin liquid,” an exotic form of matter that, despite being found extremely low temperatures, it never freezes magnetically. For more than fifty years, physicists have tried to conclusively prove its existence.
Now, an international team led by researchers from University College Cork (UCC) in Ireland claims to have found some of the strongest evidence yet obtained thanks to a new experimental technique capable of directly observing the internal excitations of this mysterious quantum state. The results have been published in Nature.
“By introducing the quantum witness technique we provide a completely new perspective on the physics of quantum spin liquids and we directly access their internal excitations or ‘spinones’ for the first time,” explains study leader Seamus Davis.
To understand the finding, it is worth starting with a simple question: What exactly does it mean for something to be a liquid? In a conventional liquid, such as water, the molecules can move freely relative to each other. When the temperature drops enough, that movement disappears and the material solidifies. Something similar happens with magnets.
Each atom has a quantum property called “spin”, which can be thought of as a small microscopic compass. In most magnetic materials, when cooled, those spins end up lining up in a stable pattern, just like soldiers in an army forming ranks. But some materials seem to refuse to obey that rule.
In them, spins continue to fluctuate even at temperatures close to absolute zero. They are never completely ordered. They remain in a kind of permanent liquid state governed by quantum entanglement.
“Normally, when we think about quantum entanglement we imagine a carefully prepared experiment with two or three particles – adds Felix Flicker, co-author of the study -. But in a quantum spin liquid each spin is entangled with all the others. And this “It happens naturally: you can find these crystals lying on the ground.”
The protagonist of the study is a mineral called herbertsmithite, first synthesized in 2004 and since then considered the best known candidate to host a quantum spin liquid. However, proving this has proven extremely difficult. The problem is that the mineral contains small magnetic impurities that interfere with measurements. For years, scientists tried to mathematically eliminate these signals in order to observe the material’s actual behavior. But Davis’ team opted for a completely different strategy. Instead of considering impurities as a problem, they decided to use them as a tool. The authors of the study treated these magnetic impurities as if they were small natural qubits, capable of acting as “witnesses” of the internal behavior of the crystal.
“The witness spins interact with each other through the quantum spin liquid,” says Ion Wood-Thanan, co-author of the study. By measuring the dynamics of these witnesses we can deduce the properties of the quantum liquid.Imagine that a friend calls you from the other end of a swimming pool. You can hear it because the vibrations travel through the water. Now imagine the same situation at sea. You will hear their voice sooner because salt water transmits sound more quickly. By observing how the signal arrives you can deduce properties of the medium through which it has traveled.”
Davis’ team did something similar to sending signals at sea, but instead of hearing sound, they heard extraordinarily weak magnetic fluctuations produced by the crystal. For this they developed a technique called “witness spin spectroscopy.” The instrument used, known as SQUID (superconducting quantum interference device), is one of the most sensitive magnetic detectors ever built.
The signal they managed to measure was approximately a billion times weaker than the Earth’s natural magnetic field. At first glance it seemed like random noise. But upon careful analysis they discovered something unexpected: the noise followed a very specific pattern known as “pink noise,” a statistical signature present in numerous natural systems. This fingerprint allowed us to reconstruct how the witness spins interacted and revealed the presence of emerging particles called spinons.
Spinons are not fundamental particles like electrons or photons. They are emergent particles: collective entities that appear only when many particles interact in a specific way. A useful analogy is to think of a wave inside a stadium. No individual spectator is the wave, but when thousands of people coordinate their movements a new phenomenon emerges with its own properties. Spinons would be something similar, although in the quantum world.
And the important thing is that its existence constitutes one of the distinctive characteristics of quantum spin liquids. That is why its detection represents one of the strongest evidence obtained so far that herbertsmithite actually harbors this exotic state of matter. Beyond fundamental physics, the finding could have important technological implications, for example as a basis for future error-resistant quantum computers.
Although Herbertsmithite doesn’t yet have exactly the properties needed to build a quantum computer, Davis’ team compares it to the history of silicon: for decades it was simply one more mineral until it became the foundation of the entire information revolution. Quantum spin liquids have the potential to follow a similar path.