Physicists have successfully simulated a fundamental particle-physics process called string breaking on a trapped-ion quantum computer, an advance that could help refine the theory of the strong nuclear force. The research, led by quantum-computing pioneer Christopher Monroe at Duke University in North Carolina, reproduced key features of the process and revealed that existing models may need slight adjustments.
String breaking is the mechanism by which the strong interaction, the force that binds quarks inside protons and neutrons, creates new matter. Unlike gravity or electromagnetism, the strong force does not weaken with distance; instead, the energy stored in the gluon field between two quarks grows as they are pulled apart. Eventually, that energy becomes large enough to spawn a new quark-antiquark pair, which binds to the original particles and leaves two separate hadrons. Free quarks are never observed in isolation.
The theory of this process was first outlined in 1951 by Julian Schwinger, but its details remain poorly understood because the equations of quantum chromodynamics are notoriously difficult to solve. Quantum computers offer a way to simulate such processes by mapping them onto controllable quantum systems.
Monroe's team used trapped ions to represent the quarks, encoding their states in the ions' spins. They created «domain walls» — regions where adjacent ions have opposite spins — to model the quarks and antiquarks. By applying an external magnetic field, they simulated the increasing energy in the gluon field as the quarks were separated. «You initialize the system with a certain number of domain walls, and then you apply the Hamiltonian — which means you do this QCD process — and you see how many domain walls you end up with. We show that the number actually increases,» explained Arinjoy De, a former PhD student of Monroe who now works at QuEra.
The team found that once the magnetic field exceeded a certain threshold, additional domain walls — representing new particles — appeared. More importantly, the simulation showed that string breaking begins at the edges of the string, where coupling to the quantum vacuum lowers the energy cost of creating new particles, and then propagates inward. This contradicts Schwinger's original prediction of uniform breaking and suggests that canonical theories need modification.
The work, published in Nature Physics, follows similar efforts in 2025 by researchers at Google using superconducting qubits and by a team at the University of Innsbruck with neutral atoms. All three approaches have now produced consistent results, reinforcing confidence in the findings.
Boris Blinov, a quantum computing researcher at the University of Washington in Seattle, called the work «significant in its own right as a demonstration of the potential of a quantum simulator in the future to do things faster than any classical analogue of it.» He noted that the current system is too small to outperform supercomputers, but that with more qubits the technology could become important for quantum chromodynamics, gauge theories, and fundamental physics.
Monroe and his colleagues are now planning to use more qubits and greater circuit depth to study domain walls farther apart. This would allow them to investigate the inverse problem of string formation when particles collide, a phenomenon relevant to high-energy experiments. Theoretical colleagues Alexey Gorshkov and Zohreh Davoudi at the University of Maryland are already thinking about meson scattering experiments that could be simulated with the improved system.
The research demonstrates the growing power of quantum simulators to tackle problems in fundamental physics that are inaccessible to classical computers. As the technology scales up, it may provide new insights into the strong force and the behavior of matter at the most basic level.
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