Caltech Physicists Finally Measure a Quantum 'Energy Ladder' Predicted 40 Years Ago
Using laser-trapped strontium atoms as a quantum simulator, the team directly measured energy levels in two theories describing matter at a tipping point.
Physicists at Caltech have measured an energy pattern that theorists predicted about 40 years ago but nobody had been able to observe directly. By arranging laser-trapped atoms into a quantum simulator, the team recreated two different quantum tipping points and watched the atoms settle into the exact energy ratios that theory demands. The results appear in the journal Nature.
The patterns come from conformal field theories, mathematical frameworks that describe what happens to a material right at a phase transition, the critical point where it changes from one state to another. At that point the system looks the same at every scale, and the theories predict a ladder of energy levels whose spacing encodes deep information about the system.
The experiment tested two such theories, known as the Ising and tricritical Ising conformal field theories. It was run by the group of Manuel Endres at Caltech, working with theorists at Caltech, Université Paris-Saclay and the Technical University of Munich. The team used strontium atoms held in place by optical tweezers, tightly focused laser beams that can grab and arrange individual atoms.
To read out the energy rungs, the researchers developed a technique they call many-body modulation spectroscopy. It involves gently shaking the system and watching how it responds, which reveals the energy levels in the same way that tapping a bell reveals its pitch. The measured levels matched the predicted ratios.
"The energy levels predicted by these theories are important because they encode profound information about the theories themselves," said Jason Alicea, the lead theorist on the project. Until now such levels had been computed on paper and on classical computers, but never measured in a controlled synthetic quantum system.
Endres said the payoff is what comes next. Researchers can now "point this at systems where nobody knows the response of the system quantitatively -- including regimes that classical computers can't reach." That is the central promise of quantum simulators: to answer questions that are too hard for conventional computing.
The group plans to move beyond the one-dimensional atom chains used here to two-dimensional grids. Conformal field theories in two spatial dimensions are far less well understood, and no exact solutions exist for many of the cases that matter in real materials.
The result also serves as a validation. When a simulator reproduces a long-standing prediction this precisely, it builds confidence that the same machine can be trusted on problems where the answer is unknown, a step quantum simulation needs to take before it becomes a standard research tool.
