When materials undergo major changes, such as water boiling or a magnet losing its magnetism, very different systems can sometimes start behaving according to the same mathematical rules. "Physicists call this trait universality -- the messy, microscopic details wash out and only a few essential features survive," explains Jason Alicea, William K. Davis Professor of Theoretical Physics.
Physicists often describe this universal behavior using a mathematical framework known as conformal field theory. Quantum simulators test fundamental physics In a study published in Nature, researchers carried out the first experiments of their kind on two different conformal field theories using quantum simulators. These specialized systems are simpler than general purpose quantum computers and are designed to reproduce particular quantum behaviors.
The collaboration brought together the experimental group of Caltech's Manuel Endres, professor of physics, Alicea's theory group, and theorists at Université Paris-Saclay and the Technical University of Munich. Using newly developed quantum simulator technology, the researchers made the first direct measurements of energy levels in synthetic quantum matter predicted by the Ising and tricritical Ising conformal field theories. (Ising refers to Ernst Ising, a physicist who, in the 1920s, solved an early model of magnetism.) Both theories describe universal behavior that can appear when a quantum system with unusual properties such as entanglement and superposition reaches a critical point between two states, with one state being more ordered than the other. A quantum tipping point near absolute zero Unlike an ordinary phase transition such as water becoming steam, this transition is not caused by temperature.
Instead, it arises entirely from quantum effects at temperatures close to absolute zero. At this critical point, lasers can excite the system into a sequence of specific energy states. The researchers compare those levels to the rungs of a ladder.
"The energy levels predicted by these theories are important because they encode profound information about the theories themselves," Alicea says. For about 40 years, physicists have used conformal field theories to calculate how far apart those energy rungs should be. The levels are expected to appear in precise ratios, but until this experiment, those predictions had never been directly measured.
"Our new tools borrow from quantum computing platforms," says Xiangkai Sun, a co-lead author of the new study and a graduate student working in the Endres lab. "Over the past 10 years, people have been learning to control these systems, and now we are at the point where we can use them to do fundamental physics research." The experiment builds on technology that the Endres lab also uses for developing quantum computers. The system relies on arrays of neutral atoms held in place by tightly focused lasers known as optical tweezers.
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