Lauren Fuge
After 50 years of hunting, physicists have finally observed a new state of matter known as a quantum spin liquid.
“It is a very special moment in the field,” says physicist Mikhail Lukin, co-director of the Harvard Quantum Initiative (HQI) and a senior author on the study in Science.
“You can really touch, poke, and prod at this exotic state and manipulate it to understand its properties… It’s a new state of matter that people have never been able to observe.”
Quantum spin liquids were first theorised by Nobel Prize-winning physicist Philip W. Anderson in 1973, and have been hotly sought-after because of their potential applications in quantum computing and high-temperature superconductivity.
Now, researchers led by Harvard University have finally experimentally documented this new state of matter.
They set out to find it using a programmable quantum simulator at the HQI lab. This is a special kind of quantum computer that can create shapes like squares, honeycombs or triangular lattices, which in turn can engineer various interactions between ultracold atoms. It allows researchers to reproduce physics on a quantum scale, study the complex processes that arise – and control them.
“You can move the atoms apart as far as you want, you can change the frequency of the laser light, you can really change the parameters of nature in a way that you couldn’t in the material where these things are studied earlier,” explains co-author Subir Sachdev, also from Harvard University.
“Here, you can look at each atom and see what it’s doing.”
The team used this simulator to create a quantum spin liquid.
What exactly is a quantum spin liquid?
This state of matter actually has nothing to do with liquids at all – it revolves around magnetism, and (weirdly enough) how magnets freeze.
When a regular magnet gets cold enough, its electrons stabilise to form a solid piece of matter with magnetic properties. But a quantum spin liquid has magnetic properties even though its electrons don’t stabilise and it doesn’t form into a solid; instead, its atoms become entangled and the material fluctuates and changes.
To understand why, let’s step back and understand how magnets in general work.
Magnetism arises because of a property of electrons called spin, which makes each individual electron act like a tiny compass needle. All the millions of electron spins in a material interact with each other in a range of ways and can stabilise into different magnetic states, giving the material magnetic properties.
In an ordinary magnet – like one on your fridge – all the electron spins align as the material is cooled, into large-scale patterns like stripes of checkerboards. These patterns are kind of like the crystal structures formed by many solids.
But a quantum spin liquid doesn’t have that same order. Instead of electron spins pairing up and helping each other align like in a fridge magnet, there’s a third spin added, creating a triangular pattern or lattice.
This prevents the spins stabilising in any particular direction, even when the material gets incredibly cold – even at absolute zero. This is called a “frustrated” magnet, because it can’t settle: the three electrons constantly force one another to switch their spin direction.
How is a quantum spin liquid created?
The conditions for a quantum spin liquid to arise are often found in nature, like in the magnetic layers of copper ions of the mineral Herbertsmithite. But synthetically creating this state of matter on-demand in the lab is crucial for fully understanding its properties, and it’s eluded scientists until now.
Now, the researchers have used the quantum simulator to create a lattice pattern, then placed atoms in it and watched them interact and entangle. Observing the “strings” that connected the entangled structure signified that a quantum liquid spin state of matter had emerged.
The properties of a quantum spin liquid, according to the researchers, could be key to creating qubits – the building blocks of a quantum computer – that aren’t affected by noise or interference.
“That is a dream in quantum computation,” says Giulia Semeghini, lead author of the study from Harvard. “Learning how to create and use such topological qubits would represent a major step toward the realisation of reliable quantum computers.”
And it might be possible with their simulator – qubits could be created by placing quantum spin liquids within a particular geometrical array.
“We show the very first steps on how to create this topological qubit, but we still need to demonstrate how you can actually encode it and manipulate it,” Semeghini said. “There’s now a lot more to explore.”