The particle-physics process of string breaking has been modelled on a trapped ion quantum computer by researchers in the US. The results are similar to previous findings obtained independently using neutral atom quantum simulators and superconducting quantum computers. The result suggests that traditional models of string breaking need to be modified slightly.

The strong interaction is the force that binds quarks together inside hadrons such as protons and neutrons. Unlike gravity, electromagnetism and the weak interaction, which all diminish with distance – its effect becomes stronger as particles are pulled apart. In quark–gluon plasma, where quarks and antiquarks swim in a sea of gluons, they are effectively free. However, when an isolated quark is pulled away from an isolated antiquark (or a proton, for example), the energy in the gluon field increases indefinitely. At some point, this energy grows large enough to create a new quark and antiquark. These bind to the original particles, leaving two hadrons. Free quarks are therefore never observed.

This process, in which the potential energy in the gluon field grows and grows until it is released with the creation of new matter, is known as string breaking. The theory of this mechanism was first outlined in 1951 by the future Nobel laureate Julian Schwinger. The process often underpins the formation of new particles in colliders, but its fundamental details remain poorly understood.

Superconducting and atomic qubits

The notion of simulating experimentally inaccessible quantum processes using quantum processors has been discussed – and implemented for some systems – in the past few years. In 2025 scientists at Google Research in California and colleagues performed simplified calculations of the dynamics of string breaking using their Sycamore processor, which uses superconducting qubits, obtaining new insights into the process.

Also in 2025, researchers at the University of Innsbruck in Austria, together with colleagues at Harvard and the commercial company QuEra, took a slightly different tack. Instead of breaking the underlying quantum chromodynamic equations down into a series of discrete time steps and calculating their evolution, they used lasers to initialize the states of rubidium atoms such that their interaction would be analogous to quarks. This allowed them access to the dynamics of the string breaking process that would otherwise remain hidden.

Now, quantum-computing pioneer Christopher Monroe at Duke University in North Carolina and colleagues in the US and Belgium have achieved similar results to the QuEra group using trapped ions instead of neutral atoms. Monroe’s team modelled the behaviour of quarks using “domain walls” – regions where adjacent ions have opposite spins.

“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.” Explains Arinjoy De, who was Monroe’s PhD student and now works at QuEra. To simulate the increasing energy in the gluon field as a quark is pulled away, the researchers used an external magnetic field: “We show that once you go beyond a certain threshold of the magnetic field, you go from two domain walls – or two particles – to create more particles,” says De.

Modifications needed

The results suggest some modifications to the canonical theories of string breaking. Notably, they found that, contrary to the predictions of Schwinger, string breaking is initiated at the edges, where coupling to the quantum vacuum reduces the energetic cost of new particle creation, before propagating inwards to the bulk.

The researchers are now hoping that, with more qubits and greater circuit depth, they will be able to look at domain walls farther apart. This would allow them to study the inverse problem of string formation when particles collide: “Our theory colleagues Alexey Gorshkov and Zohreh Davoudi at the University of Maryland are really thinking a lot about these meson scattering experiments,” says Monroe.

Quantum computing researcher Boris Blinov of the University of Washington in Seattle says that the work is “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 says the current system is too small to outperform what can be achieved with a supercomputer but “with more qubits and this technology, this way of controlling qubits would be of significance at the very least for quantum chromodynamics, gauge theories and fundamental physics.”

Monroe and colleagues describe their work in Nature Physics.

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While modern Japan has had a good record in winning Nobel prizes, the country had to wait 48 years since the inception of the awards in 1901 to receive its first Nobel prize.

The accolade went to particle physicist Hideki Yukawa, who published his theory of mesons in the 1930s that explained the interaction between protons and neutrons.

Yukawa was born in  Tokyo on 23 January 1907, graduating from Kyoto University in 1929. He then moved to Osaka University in 1933 for a PhD, which is where he did his Nobel-prize-winning work.

As positively charged protons repel each other through electromagnetic force, an issue at the time in nuclear physics was that this should cause the atomic nucleus to fly apart.

In 1935 Yukawa proposed a short-range, highly attractive nuclear force that overcomes this repulsion and suggested this force is created by the exchange of massive subatomic particles between protons and neutrons, which he called mesons.

Yukawa calculated that the mass of these exchange particles should be roughly between an electron and a proton.

Building a legacy

In 1937 three separate research groups conducted cloud-chamber experiments and discovered a new particle with a mass consistent with Yukawa’s prediction, but it was later identified as a muon.

In 1938 Yukawa was awarded a PhD and a year later moved back to Kyoto University but it took a few more years before Yukawa’s particle was finally discovered.

The first evidence of a meson came in 1947 with the discovery of the pi meson (pion) by a research group led by Cecil Powell at the University of Bristol. The team measured the particle by exposing photographic plates to cosmic rays at high altitudes (Powell won the 1950 Nobel Prize in Physics for the work).

Yukawa was then awarded the 1949 Nobel Prize in Physics “for his prediction of the existence of mesons on the basis of theoretical work on nuclear forces”.

The news was met with tremendous joy in Japan and encouraged scientists to continue their research in spite of the economic condition and the challenges of doing experimental science in postwar Japan.

In Yukawa’s honour, the Japanese government built a three-storey building named Yukawa Hall in 1952. Its focus was on developing theoretical physics in Japan and featured lecture halls, seminar rooms, offices and a library.

In 1953 the hall became home to the Research Institute for Fundamental Physics (now the Yukawa Institute for Theoretical Physics), of which Yukawa was the first director – a position he held until retiring in 1970.

The institute invited research scholars from all over Japan and held symposia and seminars on particle physics. It also published the monthly English-language journal Progress of Theoretical Physics (now known as Progress of Theoretical and Experimental Physics), together with the Physical Society of Japan (JPS).

The journal was founded by Yukawa in 1946 to publish results that had remained unpublished because of JPS’s financial difficulties.

As well as the Nobel prize, Yukawa also won many other international prizes and he was also vocal about issues in physics and society. In 1955 Yukawa joined 10 other scientists in signing the Russell–Einstein Manifesto that called for nuclear disarmament.

Yukawa died on 8 September 1981 and since he was awarded the Nobel Prize in Physics, 11 other Japanese-born physicists have gone on to win the physics Nobel – making Japan one of the most successful countries in the world.

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Physicists at CERN have recreated the collisions that trigger cosmic-ray air showers in Earth’s atmosphere. Results from the ATLAS experiment on the Large Hadron Collider (LHC) show that none of the computer models used to simulate showers gets every detail right. Their measurements of particle production in proton–oxygen collisions are more than ten times more precise than the differences between the models – which could be improved by these latest results. This will be a step toward understanding the most energetic particles in the universe.

Cosmic rays are energetic particles (mostly protons) from outer space. They strike Earth’s atmosphere at nearly the speed of light, setting off cascades of secondary particles.

“Cosmic-ray air showers are sprays of particles raining down from high in the sky,” explains Jesse Liu of New York University, the lead author of a paper that describes the study. He adds that cosmic rays come from “extreme corners of our universe,” with exploding stars and supermassive black holes in distant galaxies as likely sources.

Still, the origin of the most energetic cosmic rays remains “one of the biggest mysteries in astrophysics,” says Karl-Heinz Kampert of the University of Wuppertal in Germany, who was not involved in the study. Such particles carry a hundred billion-billion electron volts or more – compared to the million-million electron volts achieved by the LHC. At the highest energies, only about one such particle hits each square kilometer of Earth’s surface per century. “The only way to measure them in sufficient numbers is to detect the extensive air showers they produce,” he says.

Difficult calculations

Interpreting these showers relies on computer simulations. But, the role of the strong nuclear force, which governs the underlying collisions, is notoriously difficult to calculate. The simulations therefore use models informed by accelerator data, yet “existing models widely disagree with one another on how these showers form,” says Liu.

The most energetic cosmic rays collide at energies far beyond the reach of the LHC, so “we must extrapolate the known properties of particle interactions at man-made accelerators to those occurring in the upper atmosphere,” Kampert explains. Moreover, the LHC’s beams had previously consisted only of protons or heavy nuclei such as lead, rather than the light nitrogen and oxygen nuclei found in air. “For more than a decade, the cosmic-ray community has been working to convince the CERN and particle physics communities of the importance of such measurements,” he says.

That changed in July 2025, when the LHC was reconfigured “to make protons acting as cosmic rays collide with oxygen nuclei playing the role of Earth’s atmosphere,” says Liu. The ATLAS detector recorded the charged particles these collisions produced.

Key parameter

ATLAS physicists studied “how many particles are created during collisions of protons and oxygen nuclei, alongside what energies and angles these particles fly out at,” Liu explains. They also determined the cross section, which “tells us how often these collisions occur,” he adds. Kampert calls it “a key parameter,” because it determines the average depth at which cosmic rays first interact in the atmosphere. This depth affects “essentially all air-shower measurements that are needed to infer the mass” of the incoming particle.

The measured cross section lies at the low end of model predictions, agreeing with only two of the seven models tested. The team also used it to infer the cross section for protons colliding with air, a quantity previously measured at such high energies only by cosmic-ray observatories. The result agrees with earlier air-shower measurements at similar energies.

“Our measurements are more than ten times more precise than the differences between the existing computer models,” says Liu. “Our data show that no model correctly describes the number of particles created in these collisions.” Some models misjudge the frequency of rare, particle-rich collisions by a factor of ten. A model called Angantyr best reproduces the particles’ energies and directions, but no model describes all the results consistently.

Blind spots and next steps

Kampert points out that the measurement has a blind spot because multipurpose detectors like CERN’s ATLAS and CMS are optimized to track particles flying out at relatively large angles to the beam. He explains, “most of the collision energy remains unseen by ATLAS and CMS and escapes close to the beam pipe, at very small angles. This unseen region most strongly influences the features of the air showers.” Specialized experiments, such as LHCf at CERN, are designed to fill this gap.

The next step, says Liu, is “to apply our data to improve the computer models of cosmic-ray showers.” Physicists will then check whether this eases the “muon puzzle” — facilities such as the Pierre Auger Observatory in Argentina see more muons, heavier cousins of electrons, in air showers than simulations predict. “Our proton–oxygen data are an important step toward resolving these puzzles and help decipher cosmic mysteries, namely what high-energy cosmic rays are made of and where they come from,” Liu adds.

“Model builders have been eagerly awaiting this data and have started improving their models based on it and earlier accelerator data,” says Kampert. Because Auger results suggest that the most energetic cosmic rays are mostly heavier nuclei such as carbon, nitrogen, oxygen and silicon, he calls data from oxygen–oxygen or nitrogen–nitrogen collisions “another important step forward.”

Earlier this year, he and colleagues outlined in Nature Reviews Physics how such data could feed into a common tuning of the models, an undertaking he calls “a great community effort involving researchers from different fields of science.”

“I hope [our work] strengthens the cross-disciplinary connections between particle physics and high-energy astrophysics, with much to learn from each other,” says Liu.

The research is described in Physical Review Letters.

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