Physicists used advanced quantum technologies to test predictions from a decades-old theory. At the point where matter changes from one phase to another, very different materials can suddenly begin following the same mathematical rules. Water approaching a boil and a magnet losing its magnetism are familiar examples of phase transitions. “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 describe much of this universal behavior using a mathematical framework known as conformal field theory. In work reported in the journal Nature, the experimental group of Caltech professor of physics Manuel Endres and Alicea’s theory group joined researchers at Université Paris-Saclay and the Technical University of Munich to experimentally investigate two different conformal field theories with quantum simulators, specialized systems related to quantum computers that are designed for particular tasks. Using technology developed for these simulators, the researchers directly measured energy levels in synthetic quantum matter predicted by the Ising and tricritical Ising conformal field theories for the first time. (Ising refers to Ernst Ising, a physicist who, in the 1920s, solved an early model of magnetism.) Both theories describe universal behavior that appears when a quantum system, with properties such as entanglement and superposition, reaches a critical point between two states, with one more ordered than the other. Unlike an everyday transition such as water becoming steam, this change is produced entirely by quantum effects rather than temperature, although it occurs near absolute zero. At this critical point, lasers can excite the system into a sequence of distinct energies resembling 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. Quantum simulators put old predictions to