Caltech and partners find universality in quantum matter

Caltech researchers have achieved the first direct measurement of energy levels predicted by the Ising and tricritical Ising conformal field theories, validating calculations made decades ago. The team, led by Manuel Endres and Jason Alicea, used quantum simulators, simplified quantum computers, to observe these universal patterns in synthetic quantum matter at temperatures near absolute zero.

“Physicists call this trait universality—the messy, microscopic details wash out and only a few essential features survive,” explains Alicea, William K. Davis Professor of Theoretical Physics. This work connects Ernst Ising’s early 20th-century model of magnetism to modern quantum simulation techniques.

Ising and Tricritical Ising Theories Tested with Quantum Simulators

These measurements validate predictions stemming from work Ernst Ising completed in the 1920s, establishing a clear link between early 20th-century magnetism models and modern quantum simulation techniques. Unlike typical phase transitions observed in everyday phenomena, these experiments occurred at temperatures nearing absolute zero, driven by quantum effects rather than thermal changes.

Researchers utilized arrays of neutral strontium atoms trapped by lasers, a technology initially developed for building quantum computers, to construct the quantum system. They excited the atoms into Rydberg states, inducing strong interactions between neighboring atoms and allowing the chain to behave as a unified entity.

A novel technique, many-body modulation spectroscopy, was then employed to map the energy ladder of the system; this involved gently “shaking” the atomic chain with lasers and measuring the resulting response at various frequencies, similar to inducing resonance in a wine glass. Xiangkai Sun, a co-lead author of the study, explained that they repeated the experiment on chains of up to 35 atoms, and the spectra collapsed onto a single universal curve once rescaled for size. The team’s ability to individually address each atom within the array enabled further investigation, revealing a secondary set of energy rungs previously hidden from view.

By manipulating the atoms at the chain’s ends, they demonstrated patterns predicted by the tricritical Ising theory, confirming its validity. Jason Alicea said, “Even though we believed these theories to be true, it’s important to have an experimental realization, something you can poke and prod.” He added, “To see those predictions borne out is a beautiful thing.” The researchers now plan to expand their system to two dimensions, exploring conformal field theories in a more complex setting, and ultimately aim to apply this technique to systems beyond the reach of classical computers.

Many-Body Modulation Spectroscopy Maps Synthetic Quantum Matter Energies

The ability to directly measure energy levels within synthetic quantum matter has been achieved using a novel technique called many-body modulation spectroscopy. This new spectroscopic approach bypasses the need for prior knowledge of the system’s behavior, allowing for direct comparison with theoretical predictions. Manuel Endres, professor of physics at Caltech, expressed excitement that the technique doesn’t require knowing the answer in advance, allowing them to check their measurements against exact predictions.

Co-lead author Xiangkai Sun, a graduate student in the Endres lab, explained that they then tuned to the tricritical point and measured the lowest levels of its distinct spectrum, which came out in the ratios theory predicts. Jason Alicea, William K. Davis Professor of Theoretical Physics, added that the energy levels predicted by these theories are important because they encode profound information about the theories themselves.

Optical Tweezer Arrays Enable Control of Strontium Atom Chains

These optical tweezers, initially developed for building quantum computers, were repurposed to investigate fundamental physics, specifically the behavior of matter at its quantum tipping point. The team’s work builds on technology that recently trapped 6,100 atoms in a single array, demonstrating a significant leap in the ability to manipulate individual atoms. By exciting the strontium atoms into Rydberg states, the team created a chain where each atom strongly interacted with its neighbors, effectively behaving as a single quantum entity.