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'String breaking' observed in 2D quantum simulator

An international team led by Innsbruck quantum physicist Peter Zoller, together with the US company QuEra Computing, has directly observed a gauge field theory similar to models from particle physics in a two-dimensional analog quantum simulator for the first time. The study, in Nature, opens up new possibilities for research into fundamental physical phenomena.

String breaking occurs when the string between two strongly bound particles, such as a quark-antiquark pair, breaks and new particles are created. This concept is central to understanding the strong interactions that occur in quantum chromodynamics (QCD), the theory that describes the binding of quarks in protons and neutrons.

String breaking is extremely difficult to observe experimentally, as it only occurs in nature under extreme conditions. The recent work by scientists from the Universities of Innsbruck and Harvard, the ÖAW-Institute for Quantum Optics and Quantum Information (IQOQI) and the quantum computer company QuEra shows for the first time how this phenomenon can be reproduced in an analog quantum simulator.

Simulating elementary particle physics with atoms

Based on a proposal by Zoller's theory team, the researchers arranged up to several dozen rubidium atoms in optical traps with Kagome geometry—similar to a traditional Japanese braiding pattern—using QuEra's programmable Aquila neutral atom platform.

"We considered theoretically what would be the minimum setup in which this phenomenon could be observed. And we took advantage of the progress made in the experimental control of neutral atom simulators," says Torsten Zache from Zoller's team.

This allowed a theory reminiscent of the strong interaction to be simulated on the quantum simulator. "The van der Waals interactions between the Rydberg atoms used here imply that two atoms cannot be excited at the same time if they come very close to each other. They block each other," explains Zache. "This effect reflects the restriction that elementary particles such as gluons or quarks face due to the strong interaction."

High-energy physics in a quantum simulator

In the experiment, the physicists were able to follow the dynamics that lead to string breaking in real time. "Seeing string breaking in a controlled 2D environment marks a critical step toward using quantum simulators to explore high-energy physics," says Daniel González-Cuadra, the first author of the study.

Back in 2016, a team led by Rainer Blatt and Zoller demonstrated the one-dimensional simulation of a gauge field theory for the first time. "Gauge theories govern much of modern physics. Demonstrating them in two dimensions—where strings can bend and fluctuate—sets the stage for exploring even richer phenomena, including non-Abelian gauge fields and topological matter," says Zoller, one of the founding fathers of modern quantum simulation.

"Our collaboration underscores the value of open, programmable neutral-atom hardware for fundamental research," adds Alexei Bylinskii from QuEra Computing.

The study was published today together with a on a digital quantum simulation of a gauge theory in the journal Nature.