The 50-year quest to create a quantum spin liquid may finally be over

For minerologists, a mine is an invitation. The earth has been broken open, its veins laid bare – and those who enter hope to find unknown wonders. In the 1970s, Kali Kafi mine near the small Iranian town of Anarak fulfilled that hope. There, among the dusty desert rocks, Joachim Otteman and Darius Adib saw a bluish-green glow.

They took samples of the glassy mineral and analysed its structure back in the lab. What they knew was that this geological species had never been catalogued – they named it anarakite, and it lay forgotten for decades. What they didn’t know was that the emerald glow they unearthed may have been hiding a remarkable quantum secret.

This isn’t any ordinary rock. Anarakite – later renamed herbertsmithite – could be a rare type of matter known as a quantum spin liquid (QSL). Whether these occur naturally is hotly debated, but if the physicists who think they can are right, nature could be creating highly entangled states. Physicists know how to create entanglement, too, but only in limited ways, such as entangling particles of light or ultracold atoms. Entangling particles within a chunk of stuff has so far eluded them.

Minerals like herbertsmithite suggest quantum entanglement might not be something we must make, but something that just exists naturally, which we could potentially use to push quantum computers into a new realm of usefulness.

“The whole machine is sort of like an entangled network, sort of like a hive mind. And this is what a spin liquid is,” says Michael Norman at Argonne National Laboratory in Illinois. “If nature does it better than us, that would be great.”

For those who weren’t in that Iranian mine, the story of quantum spin liquids starts in 1973 with condensed matter theorist Philip W. Anderson. He wasn’t concerned with shiny minerals. He was at his desk, pen and paper in hand, mathematically investigating quantum spin, a property of quantum particles that makes them behave like tiny bar magnets. Large magnets, for example, stick to your fridge because all their atomic spins are locked into a specific pattern. Anderson thought he had concocted a situation in which quantum spins could never get locked in like that, where even when they were made extremely cold and devoid of energy, quantum effects would keep them jiggling between different orientations. He was clear-eyed about this new state of matter, writing: “A disclaimer is in order: we really know very little about it.” Anderson didn’t quite land on the right mathematical recipe for this, but the forever-jiggling magnet he dreamed up was, in fact, a quantum spin liquid.

Harnessing natural entanglement

What makes QSLs unique is the way their spins are connected through quantum entanglement. Two entangled particles share a quantum state sensitive to what happens to either of them. Maximising entanglement can push a material into a different phase – a magnet is transmuted into a QSL when all its spins become inextricable from each other. Finding so much entanglement in a naturally occurring mineral would be nothing short of striking quantum gold.

That’s because it could solve one of the biggest problems with quantum computers. Right now, every quantum computer is stymied by errors that cascade through it. We could combat that by spreading the information it processes across many of its constituent parts. This typically means adding components, then working out how to entangle them with other parts of the device. But if we could start with a quantum spin liquid, we wouldn’t have to engineer entanglement; we could harness it.

There are plenty of practical details to work out for how to do that, but the very promise of it has already inspired theoretical studies. However, an even bigger issue remains: whether QSLs truly exist in nature has been the subject of contentious disagreement among physicists. One man thinks herbertsmithite is a crucial part of the proof that they do.

Young Lee at SLAC National Accelerator Laboratory in California has spent his career studying herbertsmithite and its mineral cousins. Now, he thinks his team has solid evidence of the minerals’ long-sought quantumness.

Herbertsmithite contains flat layers of magnetic copper atoms separated by non-magnetic zinc. The copper atoms form a “Kagome pattern”, rows of interlinked six-pointed stars. Ordinarily, copper atoms would flip a neighbouring atom’s spin, but the Kagome pattern makes it impossible for a flipped spin to stabilise – spins can’t lock each other into a static arrangement. Mathematical models of spins in Kagome patterns show that some groups of spin could move around without needing any extra energy, which sounds an awful lot like a QSL’s relentless jiggling.

At first glance, then, the chances that herbertsmithite is a naturally occurring quantum spin liquid, roiling with entanglement underneath its glassy surface, seem rather good. But definitively showing that it is – and Lee has spent years trying – proved remarkably difficult.

From the earliest days, mathematically proving that the Kagome pattern always leads to a QSL was challenging, and various approximate proofs caused disagreements among theorists, says Hitesh Changlani at Florida State University. The only way to really settle it was in the lab.

Making a quantum mineral

That’s where Lee found himself in 2007, when a chemistry breakthrough allowed him and a colleague to synthesise herbertsmithite from scratch. The mineral is rare in nature, and being able to make it gave them more control over its purity. Changlani says the first reports of these experiments caused a splash among physicists. Finally, they had something real with which to compare their theories, says Zi Yang Meng at the University of Hong Kong.

But a new kind of trouble reared its head: what exactly do you measure to prove a mineral contains the right kind of quantum weirdness to be a quantum spin liquid?

For one, physicists don’t know how to directly measure quantum entanglement in a chunk of a material, so it is currently impossible to observe one of the defining properties of a QSL. Experiments that probe the orientation of spins are a good bet, but to be definitive, they would have to be performed incredibly close to absolute zero, which is possible but might not give unimpeachable results. We can get things pretty cold, but there’s a chance the spins could lock into some unexpected arrangement if they were just a little colder.

Theoretical models posit that quantum spin liquids should support two kinds of emergent particles, spinons and visons. If we could capture or image one of them, that could also prove a material’s QSL status. Unfortunately, this too seems to elude the capabilities of existing experiments. Spinons and visons have no electric charge, so typical electronic techniques are blind to their existence. For all these reasons, many experimental tests of suspected QSLs hit a wall.

But Lee and his colleagues zeroed in on one line of investigation that could prove decisive: inelastic neutron scattering. Here, researchers shoot neutrons at mineral samples and, from the energy and momentum of particles that bounce off of them, they reconstruct whether the sample contained any spinons. In 2025, Lee and his colleagues used this technique on herbertsmithite and a closely related mineral called zinc barlowite, another shiny, greenish crystal with a Kagome structure. What they found convinced Lee that the quest for quantum spin liquids ought to be over.

Making this work was an experimental odyssey. Synthesising herbertsmithite in the lab takes months. Once they mixed and heated the necessary chemicals in quartz test tubes, they had to wait up to 10 months for chemical reactions to slowly “grow” the mineral. More than half of the test tubes failed to produce it, and those that did yielded tiny amounts. Growing zinc barlowite was similarly time-consuming, with the added difficulty that the initial chemical mixture eats the walls of test tubes, and the researchers had to figure out how to line them with Teflon.

The final crystals could all still fit on the tip of your finger. “We don’t know if the quality is good until we stop the growth, break the quartz tube and collect the pieces,” says Lee. “We grow a whole bunch of smaller pieces that could be tens of milligrams or less, then we align many of them like a puzzle, putting them all together to make one bigger crystal.”

They took it to Oak Ridge National Laboratory in Tennessee, home of the world’s best neutron-scattering facility, to see what they had made. Locked in a metal chamber just 2 kelvin above absolute zero, herbertsmithite and zinc barlowite were subjected to a barrage of neutrons travelling at hundreds of metres per second.

For Lee and his team, it was all worth it. They analysed their data and arrived at a verdict: both minerals seemed to be quantum spin liquids. “I certainly have my own biases, but I think reasonable minds should already be convinced,” he says.

Not quite sold

However, disagreements persist. “To announce that you have found, for sure, the first example of a QSL, you want to have evidence that’s airtight,” says Steven Kivelson at Stanford University in California. “For a first, we rightly want to use extremely high standards.”

What makes the case of herbertsmithite and zinc barlowite less than airtight is the issue of material imperfections, says Norman. Both contain copper atoms, sometimes called “orphan spins”, that don’t belong to any neat, patterned plane and become bits of unwanted magnetic dirt within each mineral. A neutron interacting with one of these orphans can produce a signal that looks troublingly similar to a neutron interacting with a pair of spinons, mixing up the telltale signs of a quantum spin liquid with the much less exciting possibility that all you’ve got is a messy magnet. “People will say, ‘Is it the impurity that’s causing everything that you’re seeing?’ This is a tough problem,” says Norman.

Much of Lee’s work has, for this reason, been to create less disordered minerals and to understand the details of disorder within them better. “Young Lee is a hero in this. He’s had a focus and persistence that is incredibly admirable,” says Kivelson.

Because zinc barlowite has a slightly different structure than herbertsmithite, its disordered atoms fall differently throughout the Kagome pattern. Both Kivelson and Norman say that the fact that the two minerals show similar QSL-like features despite this difference strengthens the case that those features aren’t just mirages created by disorder. “I think that’s a really good argument. One is increasingly confident that Lee’s seeing a QSL,” says Kivelson.

This slowly cresting wave of optimism doesn’t rest purely on the backs of the two emerald materials. Meng was part of a team that found very sharp signatures of spinons in a material made from yttrium, copper and bromine, which also has a Kagome pattern. He says it’s not impossible that a future experiment could contradict his team’s findings, but he is optimistic he’s seen signs of a quantum spin liquid. “For me, I think [today’s] evidence is good enough,” he says.

Changlani and his colleagues have also seen hints. They tackled the problem of QSLs in materials that can’t be divided into flat planes, but where spins are arranged in a pattern that uses all three spatial dimensions. For one such lab-grown material made from cerium, zirconium and oxygen, they looked for signatures of QSL behaviour in neutron-scattering data and in how the entanglement and spin jiggling affect its response to heat. Observations from these experiments match a theoretical model of a 3D quantum spin liquid well enough to convince them that QSLs exist in nature and across many different material structures. “There is now growing evidence that these things, QSLs, are real,” says Changlani.

“There’s this popular notion of the smoking gun experiment, one experiment that establishes everything. But often what happens is that you end up with a web of evidence, and it gets to the point that the evidence supporting something comes from so many sides and is so strong that even if one branch of the web turns out to be flawed, it doesn’t change your understanding of what is true,” says Kivelson. In his view, researchers have made real progress in weaving this web for quantum spin liquids.

Norman would like to see that smoking gun before the question of naturally occurring QSLs can be settled. Primarily, he wants physicists to detect and manipulate spinons or visons more directly. Spinons can carry heat, which ought to be detectable. Another option could be to force two particles within a potential quantum spin liquid to encircle each other, which would change their properties in ways we can precisely predict based on mathematical models. “That kind of experiment, which would be a tour de force, is what would be needed to be done to prove that a QSL really exists in nature,” says Norman.

Controlling particle motion in that way would also be crucial for any QSL-based quantum computers, as many computational steps would consist of spinons and visons dancing around each other.

Quantum computers might even come in handy for building their own future QSL-based replacements. For example, instead of messing with copper atoms in a chemistry lab, each spin could be emulated by a qubit – the basic building block of a quantum computer – creating a synthetic QSL within the device. Norman says existing quantum computers aren’t yet big and reliable enough for this task, but they could get there in the near term. The number of error-free qubits he estimates would do the trick is in line with plans that a few quantum computing companies have for the next five years.

For his part, Lee is grounded in the material science and chemistry of the now. There is a lot less glamour and hype here, but he has clarity about what his team ought to keep doing: “The wish list is to convince the community that we have at least QSLs in herbertsmithite and zinc barlowite.”

If they win over the materials physics community, could that lead to a run on the mines for these minerals across the globe? Even if it can be gathered, Lee is wary of having to wait for Earth to produce a perfect crystal, especially as they are often too small and have unpredictable impurities.

Norman, on the other hand, says herbertsmithite has several naturally occurring mineral relatives that may still be promising if they have fewer imperfections.

In the aftermath of the Anarak discovery, herbertsmithite turned up in some mines in Chile, and Norman says he’s recently heard of physicists visiting to collect new samples, hoping to confirm that quantum entanglement is waiting for us to dig it up.