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| Droplets of silicon, used to illustrate movements similar to those of quantum particles. Aleks Labuda |
A solid is made of atoms that are, more or less, locked in an ordered
structure. A liquid, on the other hand, is made of atoms that can flow
freely around and past each other. But imagine atoms that stay unfrozen,
like those in a liquid–but which are in a constantly changing magnetic mess.
What you have then is a never-before-seen state of matter, a state of
quantum weirdness called a quantum spin liquid. Now, by carefully
manipulating atoms, researchers have managed to create this state in the
laboratory. The researchers
published their work
in the journal Science on December 2.
Scientists had discussed theories about spin liquids for years. “But we
really got very interested in this when these theorists, here at Harvard,
finally found a way to actually generate the quantum spin liquids,” says
Giulia Semeghini, a physicist and postdoc at Harvard University, who
coordinated the research project and was one of the paper authors.
Under extreme conditions not typically found on Earth, the rules of quantum
mechanics can twist atoms into all sorts of exotica. Take, for instance,
degenerate matter, found in the hearts of dead stars like white dwarfs or
neutron stars, where extreme pressures cook atoms into slurries of subatomic
particles. Or, for another, the Bose-Einstein condensate, in which multiple
atoms at very low temperatures sort of merge together to act as one (its
creation won the
2001 Nobel Prize in Physics).
The quantum spin liquid is the latest entry in that bestiary of cryptid
states. Its atoms don’t freeze into any sort of ordered state, and they’re
constantly in flux.
The “spin” in the name refers to a property inherent to each particle–either
up or down–which gives rise to magnetic fields. In a normal magnet, all the
spins point up or down in a careful order. In a quantum spin liquid, on the
other hand, there’s a third spin in the picture. This prevents coherent
magnetic fields from forming.
This, combined with the esoteric rules of quantum mechanics, means that the
spins are constantly in different positions at once. If you look at just a
few particles, it’s hard to tell whether you have a quantum liquid or, if
you do, what properties it has.
Quantum spin liquids were first theorized in 1973 by a physicist named
Philip W. Anderson, and physicists have been trying to get their hands on
this matter ever since. “Many different experiments…tried to create and
observe this type of state. But this has actually turned out to be very
challenging,” says Mikhail Lukin, a physicist at Harvard University and one
of the paper authors.
The researchers at Harvard had a new tool in their arsenal: what they call a
“programmable quantum simulator.” Essentially, it’s a machine that allows them to play with individual
atoms. Using specifically focused laser beams, researchers can shuffle atoms
around a two-dimensional grid like magnets on a whiteboard.
“We can control the position of each atom individually,” says Semeghini. “We
can position them individually in any shape or form that we want.”
Moreover, to actually determine if they had successfully created a quantum
spin liquid, the researchers took advantage of something called quantum
entanglement. They energized the atoms, which began to interact: changes in
the property of one atom would reflect in another. By looking at those
connections, the scientists found the confirmation they needed.
All this might seem like creating abstract matter for abstract matter’s
sake–but that’s part of the appeal. “We can kind of touch it, poke, play
with it, even in some ways talk to this state, manipulate it, and make it do
what we want,” says Lukin. “That’s what’s really exciting.”
But scientists do think quantum spin liquids have valuable applications,
too. Just venture into the realms of quantum computers.
Quantum computers have the potential to far outstrip their traditional
counterparts. Compared with computers today, quantum computers could create
better simulations of systems such as molecules and far more quickly
complete certain calculations.
But what scientists use as the building blocks of quantum computers can
leave something to be desired. Those blocks, called qubits, are often things
like individual particles or atomic nuclei–which are sensitive to the
slightest bit of noise or temperature fluctuations. Quantum spin liquids,
with information stored in how they’re arranged, could be less finicky
qubits.
If researchers were able to demonstrate that a quantum spin liquid could be
used as a qubit, says Semeghini, it could lead to an entirely new sort of
quantum computer.
Reference:
G. Semeghini et al. Probing topological spin liquids on a programmable
quantum simulator.
DOI: 10.1126/science.abi8794
Tags:
Physics