An elusive form of matter called a quantum spinning liquid is not a liquid and does not rotate, but it is definitely quantum. Predicted nearly 50 years ago, quantum spin liquids have long eluded definitive detection in the laboratory. But now, a lattice of ultracold atoms held in place with lasers showed the hallmarks of the long-sought form of matter, the researchers report on Science of December 3.
Quantum entanglement goes into overdrive in the new material. The atoms on opposite sides of the lattice also share entanglement, or quantum connections, which means that the properties of distant atoms are related to each other.
“It’s very, very intricate”, says physicist Giulia Semeghini of Harvard University, co-author of the new study. “If you choose any two points of your system, they are connected to each other through this huge intertwining”. This strong long-range entanglement could come in handy for building quantum computers, say the researchers.
The new material matches predictions for a liquid with quantum spin, although its composition deviates somewhat from conventional expectations. While the traditional idea of a liquid with quantum spin is based on the quantum property of spin, which provides atoms with magnetic fields, the new material is based on several atomic quirks. A standard quantum spin liquid should arise between atoms whose spins are in conflict.
Quantum spin
Spin causes atoms to act like tiny magnets. Normally, at low temperatures, those atoms would align their magnetic poles in a regular pattern. For example, if an atom points up, its neighbors point down. But if the atoms are arranged in a triangle, for example, each atom has two neighbors pointing in opposite directions. This arrangement leaves the third party with no one to turn to: he cannot oppose both of his neighbors at the same time.
So the atoms in quantum spin liquids refuse to choose. Instead, the atoms run out in an overlay, a quantum combination of spin up and down, and the state of each atom is linked to that of its compatriots. Atoms are constantly fluctuating and never settle in an ordered spin arrangement, similar to how atoms in a normal liquid are scattered rather than arranged in a regularly repeating pattern, hence the name.
It has been difficult to find conclusive evidence of quantum spin liquids in solid materials. In the new study, the researchers adopted a different strategy: they created an artificial material composed of 219 trapped rubidium atoms cooled to a temperature of around -273.15 ° Celsius. The array of atoms, known as a programmable quantum simulator, allows scientists to fine-tune how atoms interact to study exotic forms of quantum matter.
In the new experiment, rather than the spins of opposing atoms, a different property created disagreement. The researchers used lasers to put atoms into Rydberg states, which means that one of an atom’s electrons is raised to a very high energy level. If an atom is in a Rydberg state, its neighbors prefer not to be. This configuration generates a Rydberg discord or not, analogous to the spin-up and-down battle in a traditional quantum spin liquid.
Scientists confirmed the liquid effect of quantum spin by studying the properties of atoms that fell along circuits traced through the material. According to quantum mathematics, those atoms should have exhibited some properties unique to liquids with quantum spin. The results met expectations for a quantum spin liquid and revealed that long-range entanglement was present.
In particular, the entanglement of the material is topological. This means that it is described by a branch of mathematics called topology, where an object is defined by certain geometric properties, such as the number of holes.
Topology can protect information from destruction – a bagel that falls off the counter will still have exactly one hole, for example. This information retention feature could be a boon to quantum computers, which have to contend with fragile and easily destroyed quantum information. which make calculations prone to errors.
Whether the material truly qualifies as a quantum spin liquid, despite not being spin-based, depends on the choice of language, says theoretical physicist Christopher Laumann of Boston University, who was not involved in the study. Some physicists use the term “Spin” to describe other systems with two possible options, because it has the same math as atomic spins that can point up or down. “Words have meaning, until they do”he jokes. It all depends on how you turn them.
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