A quantum spin liquid is an enigmatic kind of substance that isn’t a liquid and doesn’t spin — but it is quantum. Quantum spin liquids, predicted about 50 years ago, have long defied unequivocal detection in the laboratory. Researchers report in Science that a lattice of ultracold atoms kept in place by lasers has displayed signs of the long-sought form of matter.
In the newly created substance, quantum entanglement swings into overdrive. Entanglement, or quantum linkages, exist between atoms on opposite sides of the lattice, implying that the properties of distant atoms are associated with one another. “It’s extremely, very entangled,” says Harvard University physicist Giulia Semeghini, a coauthor of the new work.
“If you pick any two points of your system, they are connected to each other through this huge entanglement.” This strong, long-range entanglement could prove useful for building quantum computers, the researchers say.
The novel material meets predictions for a quantum spin liquid, however its composition differs somewhat from what is expected. While the traditional concept of a quantum spin liquid is based on the quantum property of spin, which provides magnetic fields to atoms, the novel substance is based on distinct atomic quirks.
The quantum spin liquid effect was validated by the scientists by analyzing the behavior of atoms that fell along loops drawn through the material. According to quantum mechanics, such atoms should have displayed features peculiar to quantum spin liquids.
Christopher Laumann
A normal quantum spin liquid should form among atoms with opposing spins. Atoms become small magnets as a result of spin. At low temperatures, such atoms would normally align their magnetic poles in a predictable fashion. For instance, if one atom points up, its neighbors will point down. However, if atoms are arranged in a triangle, each atom has two neighbors who point in opposite directions. That arrangement leaves the third one with nowhere to turn — it can’t oppose both of its neighbors at once.
As a result, atoms in quantum spin liquids refuse to make a choice. Instead, the atoms end up in a superposition, a quantum mixture of spin up and spin down, with each atom’s state related to that of its neighbors. The atoms are constantly fluctuating and never settle into an orderly arrangement of spins, comparable to how atoms in a typical liquid are dispersed rather than arranged in a regularly repeating pattern, hence the name.
In solid materials, conclusive proof of quantum spin liquids has proven difficult to come by. The researchers in the new study adopted a different approach: They constructed an artificial substance made of 219 trapped rubidium atoms cooled to roughly 10 microkelvins (about –273.15° Celsius). The array of atoms, known as a programmable quantum simulator, allows scientists to fine-tune how atoms interact to investigate exotic forms of quantum matter.
Instead of the atoms’ spins being in conflict, a different feature caused disagreement in the current experiment. The researchers utilized lasers to induce Rydberg states in the atoms, which means that one of the atom’s electrons is bumped to an extremely high energy level. If one atom is in a Rydberg state, its neighbors would prefer not to be there as well. This configuration results in a Rydberg-or-not conflict, which is akin to the spin-up and spin-down battle in a classical quantum spin liquid.
The quantum spin liquid effect was validated by the scientists by analyzing the behavior of atoms that fell along loops drawn through the material. According to quantum mechanics, such atoms should have displayed features peculiar to quantum spin liquids.
Notably, the entanglement of the material is topological. That is, it is characterized by a field of mathematics known as topology, in which an item is defined by geometrical qualities such as the number of holes it has. Topology can preserve information from being destroyed: for example, if a bagel falls off the counter, it will still contain exactly one hole. This information-preserving feature could be beneficial to quantum computers, which must deal with fragile, easily destroyed quantum information, which makes calculations prone to errors.
According to theoretical physicist Christopher Laumann of Boston University, who was not engaged in the study, whether the substance genuinely qualifies as a quantum spin liquid, despite not being based on spin, depends on your choice of words. Because it has the same mathematics as atomic spins that can point either up or down, some physicists use the term “spin” to describe other systems with two conceivable outcomes. “Words have significance until they don’t,” he says wryly. It’s all in how you spin them.
