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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The oldest known solids in the solar system contain evidence that the protoplanetary disc that went on to form the planets around our Sun experienced a magnetic field stronger than the field on Earth today, new research suggests. Study of inclusions in a primordial meteorite called a carbonaceous chondrite indicates that magnetism needs to be considered in simulations of planet formation, and may even be the dominant factor.

When planetary systems condense, they initially form high-temperature molecular nebulae. These later cool and accrete into planets surrounding central stars – which, at some point, begin to undergo nuclear fusion. Even before this happens, however, the gas is highly ionized. “A good portion of the ionization is coming from the molecular cloud itself,” explains planetary scientist Cauê Borlina of Purdue University. “It’s a combination of radiation from space and high temperature.”

In any ionized system there is the potential for magnetic fields, but simulations of disc formation often take no account of them, partly as they remain poorly constrained: “It’s a lot easier to just crank up the gravity and use that as a way to match the accretion rate,” says Borlina.

The new study from Borlina and colleagues in the US, China and the UK provides an unprecedented peek at fields in the terrestrial planet-forming region in the first 500,000 years of the Solar System’s existence – before the Earth had formed – using calcium-aluminium-rich inclusions. These are the oldest known solar system solids, comprising minerals with extremely high melting points that are inclined to condense early. “It is very unlikely to get them at later stages,” says Borlina. Previous researchers have confirmed their ages using isotopic dating.

Millions of years after these inclusions formed, they agglomerated with other protoplanetary material into planetesimals, some of which went on to form planets. Planetary scientists think the remaining material accreted to form the chondrites – which astronomers distinguish into several different types depending on their chemical composition. When they fall to Earth as chondrite meteorites, therefore, they provide a valuable glimpse into the solar system’s raw material.

Carbonaceous chondrites hold particular interest as they are thought to have the most primitive chemical compositions of all meteorites. The researchers studied calcium-aluminium inclusions in the 667 g Dominion Range 08006 carbonaceous chondrite discovered in Antarctica in 2008. Such inclusions had, until around 2010, been thought to contain no magnetic minerals. Borlina and colleagues, however, found that some – although not all – of the inclusions contained nanometre-scale amounts of ferromagnetic iron-nickel.

The researchers measured the thermoremanent magnetization of these iron-nickel grains (the field strength they experienced at the time when they cooled down through their “Curie temperature”, thereby fixing their own magnetism). In theory, this could be achieved by heating the sample above the Curie temperature, which for iron-nickel is about 1050 K, and measuring the field required to demagnetize it. Unfortunately, says Borlina, heating meteorites up in the laboratory tends to oxidize the materials extremely quickly, making reliably calibrated experiments very difficult.

Instead, the researchers used an established method called anhysteric remanent magnetization, concluding that the inclusions were exposed to fields of 150–600 µT when they cooled through the Curie temperature (the field on Earth today is around 30–60 µT). “This is good evidence that…you can’t just ignore magnetic fields,” says Borlina. “They need to be present in your simulations.”

The researchers are now studying other carbonaceous chondrites and other types of chondrite meteorites – some of which are thought to have formed in different regions of the protoplanetary disc – hoping to gain a fuller picture of how magnetism shapes disc evolution. “The idea was to put it out there that there’s this new inclusion that might hold magnetic carriers, and there’s a lot of work to be done,” says Borlina.

Meenakshi Wadhwa of University of California, San Diego – who formerly directed the Center for Meteorite Studies at Arizona State University – describes the work as “genuinely significant”. First, she says, it provides the first definitive paleomagnetic signal from a carbonaceous chondrite. Second, the estimated field is multiple times previous estimates.

“Yes, the sample size is small (five inclusions from a single chondrite), but I think the authors make a reasonably solid argument for their conclusions,” she says. The next step, she concludes, “is to measure inclusions from other chondrite groups to see if the signal is reproducible, and to test whether it’s a real nebular record versus something very localized, specific to DOM 08006 or the carbonaceous chondrite reservoir. Beyond that…pinning down when and where inclusions acquired their magnetization will also be important.”

The research is published in Proceedings of the National Academy of Sciences.

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Water droplets sliding across solid surfaces can acquire electric potentials of thousands of volts and thereby corrode non-conductive surfaces on metals when they discharge. That is the conclusion of researchers in Germany who say that the poorly-understood phenomenon warrants further investigation to help prevent corrosion of vulnerable outdoor equipment and cultural heritage sites.

Metal corrosion by water droplets is a serious economic and safety issue, and prevention begins with the proper understanding of the relevant processes. Conventional wisdom is that most corrosion is caused by a combination of physical abrasion by the motion of the droplets and chemical degradation from natural acids and anthropogenic pollutants in the water. The notion that electrochemistry might play a significant role in corrosion had not been seriously considered, says physical chemist Hans-Jürgen Butt of the Max Planck Institute for Polymer Research in Mainz.

Part of the reason, he believes, is that, until relatively recently, it was thought that the triboelectric effect (electrically charging objects by rubbing them together) did not work in liquids. “If you have a solid and you rub another insulating solid against it you get electron transfer – I think this has been known for more than 2500 years” he says. However, this relies on atomic-scale roughness of the surface to concentrate charge at specific points where bonds can be broken – and fluids cannot, by definition, be rough.

“If you take a water droplet and let it slide there is some mechanical force from surface tension, but that’s much too low,” says Butt. “There is no way you could generate enough energy locally to pick up an electron or to put an electron or an ion from the water onto the surface.” In the past 10 years, however, it has become increasingly clear that sliding droplets do become highly charged.

Sloping surfaces

In their present work Butt and colleagues deposited water droplets onto copper surfaces covered with 60 nm of Teflon.  When they dropped droplets straight onto the surface, they observed no corrosion. They then deposited droplets first onto a variety of other sloped insulating surfaces such as plant leaves, PVC construction boards and the perfluorooctadecyltrichlorosilane (PFOTS) hydrophobic coating often used on window glass. The droplets ran down these surfaces and then fell onto the Teflon-coated copper. After around 3000 droplet impacts, atomic force microscopy and confocal microscopy revealed evidence that the droplets had corroded both the coating and the underlying copper.

The researchers believe that the droplets become positively charged as they slide down the sloping surface. As they fall onto the coating, the potential difference between the copper and the underlying copper can exceed 1 kV. This is greater than the dielectric breakdown strength of the coating, causing the droplet to discharge. This damages the coating and leaves the underlying metal vulnerable to further oxidation.

To test their hypothesis, the researchers measured charge movement within the copper surface, showing that negative charge flowed towards droplets dripping off insulating surfaces. They also used high-speed cameras to show that, whereas a drop deposited directly retained its spherical shape before impact, a dripping drop was drawn into a cone shape, producing a tip of positive charge that would increase its corrosive capacity.

Uncertain consequences

The practical implications of this are unclear. “Technical coatings on cars, ships etc. are typically 100 microns and thicker, so the process we describe is probably not of direct relevance,”says Butt. However, the phenomenon could be involved in the degradation of monuments and other outdoor historical objects. “We have evidence that surfaces change their properties when you slide charges over them, but the real consequences are not yet known,” explains Butt.

He says his own group is most clearly focused on understanding the fundamental physics involved. “We still don’t know why the heck there is such a charge separation: it’s energetically unfavourable, it should not happen, but it happens.” he says. “The one effect we describe here is, in a way, pretty trivial – everyone knows that if you have a high potential and it’s somehow grounded there is a breakthrough – but what happens at the surface with this deposited charge is not clear.”

Materials scientist Zhong Lin Wang of Georgia Institute of Technology in Atlanta was one of the first researchers to discover the triboelectric effect in liquids, and helped to develop the triboelectric nanogenerator used to quantify it. “The transferred electrons can be kicked back to the water solution, resulting in chemical reaction at the vicinity surface. This process, called contact-electro-catalysis, was first proposed in 2022, and is now an active field of research in chemistry, materials and environmental science.” He says. “[Butt and colleagues] show that the electrons transferred from the water droplet can ‘break’ the surface coating layer and lead to local oxidation. This is an interesting discovery.”

The research is described in Nature.

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Particles such as protons and neutrons are more accurately represented as “baryon junctions” than as simple collections of three quarks, say physicists. This finding, which is based on studies of heavy-ion collisions, is at odds with textbook descriptions, but it confirms a longstanding belief among many nuclear physicists that these descriptions are over-simplified.

Like charge and energy, baryon number is conserved in all experimentally observed reactions. This conservation law was first articulated in 1938, long before physicists developed the theory of quantum chromodynamics (QCD) to explain the behaviour of nuclei. It also predates the coinage of the term “quark” by Murray Gell-Mann in 1963.

Initially, theorists working on QCD felt that the simplest way to incorporate baryon number conservation was to assign a baryon number of 1/3 to every quark and -1/3 to every antiquark. As bound states of three quarks, protons and other baryons therefore have a baryon number of +1, antibaryons are -1, and quark-antiquark pairs, or mesons, are 0. However, this association of the baryon number with the quark – known as the valence quark model – is not fundamental to QCD, and as early as the 1970s theorists were already considering alternatives. In 1996, for example, Dmitri Kharzeev of Stony Brook University in New York, US proposed a model in which the baryon number was assigned not to the quarks themselves but to the Y-shaped junction in the gluon field that mediates their interactions.

At high-energy accelerators such as the LHC, collisions are usually modelled in the high-energy “perturbative QCD” regime. Here, quarks can be treated as isolated particles, and the two models are effectively equivalent. Inside a proton or in a quark-gluon plasma, however, quarks are more deeply immersed in the gluon field. This makes perturbative calculations such as those used in the theory of quantum electrodynamics (QED) impossible. “You can’t do these Feynman-like calculations anymore,” explains Anselm Vossen, an experimental nuclear physicist at Duke University in North Carolina, US. “Every diagram you write out has the same importance.” Within this so-called “lattice QCD” regime, the two models’ predictions can differ.

Discrepancies between models

In the new work, the STAR collaboration – which studies particles produced in quark-gluon plasmas generated at Brookhaven National Laboratory’s Relativistic Heavy Ion Collider (RHIC) in New York, US – homed in on these differences. By comparing predictions from the baryon junction model with those from the valence quark model in various scenarios, and matching them against experimental results, they hoped to find evidence to support one model over another.

First, they studied head-on collisions of gold nuclei, plus near-misses that led to the exchange of a virtual photon and the break-up of one nucleus. They found that the proportion of particles scattered varied sharply with the energy of the incoming nuclei, but not as the valence quark model predicted.

“The net baryons scattered decreases as the beam energy increases – that is expected – because the higher the energy the more things will just move straight through without being scattered to a large angle,” explains Chun Yuen Tsang, a STAR project leader who is now at Argonne National Laboratory in Illinois. However, he adds that the rate of decline “is actually slower than the existing valence quark model would have you believe”. A baryon junction should scatter more easily than a valence quark, so the fact that a simulation designed using the latter model underpredicts scattering at high energies points towards the baryon junction model.

A charged result

The researchers also compared fragments produced in collisions with zirconium-96 and rubidium-96 nuclei. Since both particles contain the same number of nucleons, they have the same baryon number. However, the ruthenium-96 nucleus has 44 protons, whereas the zirconium-96 nucleus has only 40, giving them different electric charges.

In a nuclear collision, both charge and baryon number – while being conserved overall – are rearranged among the fragments. The charge is localized to the quarks, so, according to the valence quark model, charge and baryon number should move approximately the same amount. Because the colliding nuclear pairs in this experiment differed only in their charge, the outcomes of collisions could be predicted relatively well across a range of energies. However, these predictions were strongly violated, with baryon number moving much more easily than electric charge. “This is evidence for quarks not being the carriers of both baryon number and electric charge,” Tsang says.

The researchers now wish to study collisions from other experiments. They also hope that the Electron-Ion Collider currently planned to replace RHIC will provide a more precise baryon probe and yield a definitive answer.

Vossen, who was not involved in the research, says that in some sense, its conclusions are “not that surprising”. His own research has examined the so-called “proton spin crisis” precipitated by the discovery that the proton’s spin cannot be explained by the spin of its constituent quarks, while the proton’s mass is much greater than the sum of the quark masses. “It’s part of this general picture that shows that a proton is much more complex than three valence quarks,” he says. Nevertheless, he says the STAR findings are not trivial: “It’s still a very interesting result,” he concludes.

The work is published in Science.

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The first direct glimpse of vacuum birefringence has been claimed by astronomers who have studied X-ray and radio emissions of a neutron star that is both a magnetar and a pulsar. A central, but unconfirmed prediction of quantum electrodynamics (QED), vacuum birefringence involves the polarization of the quantum vacuum by powerful magnetic fields.

Made by a US-led international team, the observation could lead to the use of magnetars as natural laboratories to study other extreme-field phenomena of the quantum vacuum. However, another group headed by researchers in Italy remains unconvinced that the data constitute a “smoking gun” for vacuum birefringence. They believe that alternative explanations remain viable.

Birefringence occurs when the refractive index of a medium depends on the polarization of the radiation passing through it. When refraction occurs at such a medium, there is an angular separation of the two differently-polarized waves. This occurs with certain crystals such as calcite and lithium niobate, where the anisotropic lattice structure interacts differently with oppositely polarized photons. Birefringence can also arise from the interaction of photons with magnetic fields in plasma.

Virtual pairs

A key prediction of QED – first made in 1935 by Werner Heisenberg and Hans Euler – is that sufficiently strong magnetic fields can make a vacuum birefringent by polarizing the virtual electron–positron pairs that fluctuate in and out of existence. Ninety years on,  however, vacuum birefringence had not been observed because the huge fields required cannot be generated in the laboratory.

“Having a natural lab already existing that could look into some of these effects is something that would be very beneficial,” says astronomer Rachel Stewart of George Washington University in the US. “That’s something people have probably been looking into for decades.”

Stewart and colleagues’ natural lab is a magnetar. These objects comprise a rare subtype of neutron star that have extreme magnetic fields of up to 10¹¹ T. This makes magnetars the most magnetic objects observed in the universe and bright sources of X-rays. Data from NASA’s Imaging X-ray Polarimetry Explorer (IXPE) telescope have shown that this radiation is often polarized.

This is not surprising because the surface of a magnetar is likely to be surrounded by magnetized, birefringent plasma. The plasma’s field, however, should be tangled and variable, so the net polarization imprinted on light emitted into the far field should be relatively small.

Magnetic axis

If the magnetar’s intense magnetic field polarizes the vacuum of empty space, then the birefringence polarization imprinted onto the X-rays should follow the orientation of the magnetic poles into the far field as the magnetar rotates. Disentangling the two effects is difficult, however, because it is not generally possible to ascertain the orientation of the magnetic poles with respect to the direction of our observation.

A very few magnetars, however, are also pulsars that emit narrow beams of radio waves from their magnetic poles. The magnetic and rotational poles of a pulsar are misaligned, which causes the radio beam to sweep around like a lighthouse beam. If Earth happens to be in the path of the beam, we observe radio pulses.

In the new research, Stewart and colleagues observed the magnetar 1E 1547.0−5408, which is unique among observed objects for having persistent, bright radio emission alongside its X-ray emission. They combined observations from two space-based X-ray telescopes – IXPE and the Neutron Star Interior Composition Explorer (NICER) – with radio observations from Australia’s Murriyang telescope.

The radio observations allowed the team to the work out the angle between the magnetar’s magnetic and rotational poles and the angle between the rotational pole and the direction of our observation. This gave them the information needed to disentangle the two birefringence effects.

Large polarization

The researchers found a high degree of polarization in the detected X-rays – up to 80% at photon energies 2–3 keV in energy. They point out that this high degree of polarization is consistent with vacuum birefringence being driven by the magnetar’s powerful field.

The X-ray emission is closely aligned with the radio emission, which the researchers believe is naturally explained by the magnetic poles of the star being closely aligned with the rotational poles – and difficult to reconcile with alternative explanations.

However, another recent paper challenges this interpretation. Roberto Taverna of the University of Padova in Italy, who led the work, says that pairing the radio emission data with the X-ray observations was “very smart”, but he says that “it is not obvious to me whether the two observations – the radio and the X-rays – are compatible or not.” He says that, if the X-ray emission all emerged from one small region such as a hotspot slightly away from the radio polarization axis, the X-ray radiation would be polarized in the absence of vacuum birefringence. As a result he does not believe the observation is a “smoking gun”.

George Younes of NASA’s Goddard Space Flight Center in Maryland – part of the US group –  argues that Taverna and colleagues are “ignoring about 60 years of radio pulsar science…They are hiding behind the fact that magnetars are different from pulsars, so we don’t know exactly where the radio emission comes from and all results are on the table,” he says, “but there are fundamentals that we should not ignore.”

The US-led team is now investigating further to see whether they can find more evidence for vacuum birefringence. Nuclear astrophysicist Hoa Dinh Thi of Rice University in Texas, who led the theoretical effort, is now modelling QED effects of magnetism on radiation in the plasma. “In the far future we also plan to use machine learning in the model so we can explore the different sources together and understand better the properties of neutron stars and magnetars.”

Stewart and colleagues describe their work in Nature. Taverna’s team has published in The Astrophysical Journal. 

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