Page 32 – Physics World

Home » Page 32

Defects as topological sensors

Topological features are important because they give rise to states that cannot easily be destroyed. These robust states give materials special behaviours that enable new kinds of electronics and photonics. Some topological systems can even host exotic quasiparticles such as Majorana modes, which are their own antiparticles and are promising building blocks for quantum computing.

However, existing methods for detecting topological states in materials have significant limitations. They typically probe only the surface, even though topology is a bulk property. Topological transitions can be subtle and difficult to identify, and topological signatures can be obscured by impurities or disorder. In some cases, topological features have no easily measurable experimental signature at all.

In this work, the researchers made a striking discovery: ordinary crystal defects can be used to detect topological behaviour inside a material. When a defect is placed in a material with a topological electronic structure, it produces an additional energy state inside the band gap, known as a mid‑gap mode. This effect is universal and appears for the most common types of imperfections: vacancies, Schottky defects, substitutions, and interstitials.

The defect acts as a marker for topology because a mid‑gap mode forms only when the surrounding material is topological. In a trivial material, no such state appears. This happens because the wavefunctions in a topological material have a global structure that cannot be altered locally; introducing a defect forces the system to compensate by creating a mid‑gap state.

This topological probe is not symmetry‑dependent and works in any spatial dimension, making it relevant to 2D materials, 3D crystals, superconductors, photonic and acoustic lattices, and even non‑Hermitian systems. The researchers confirmed this experimentally by building acoustic Chern lattices, where sound waves behave mathematically like electrons. By introducing controlled defects, they observed mid‑gap states exactly where the theory predicted.

This work shows that ordinary crystal defects can serve as reliable, built‑in indicators of a material’s hidden topological character, offering a simple and universal way to detect topology directly within the bulk.

Read the full article

Ordinary lattice defects as probes of topology

Aiden J Mains et al 2026 Rep. Prog. Phys. 89 048002

Do you want to learn more about this topic?

Topological order, emergent gauge fields, and Fermi surface reconstruction by Subir Sachdev (2018)

Cosmic crisis averted as supernovae put dark energy back on track

Through fresh analysis of supernovae data, a team of astronomers has discredited a recent study that suggested that dark energy has weakened since the early universe.

Led by Phil Wiseman at the UK’s University of Southampton, the team showed that the apparent weakening almost disappears after accounting for a known correlation between supernova brightness and galaxy mass. This latest work confirms that the universe’s expansion is accelerating – a discovery that bagged three astronomers the 2011 Nobel Prize for Physics.

Type 1a supernovae occur when a white dwarf accretes mass from a binary companion star. Eventually, it reaches a critical mass where the crushing weight of its gravity can no longer be supported by electron degeneracy pressure, triggering an explosion that can briefly outshine entire galaxies.

These events are especially valuable to cosmologists because white dwarfs always explode at the same critical mass – meaning type 1a supernovae always reach around the same peak brightness. As a result they are “standard candles”, whereby their distances from Earth is inferred by their apparently brightness. The rate at which a type 1A is moving away from us is given by the redshift of its light. By measuring the distance and motion of these objects, the rate of the universe’s expansion can be determined as a function of time.

Driving acceleration

Since the 1990s, researchers have observed thousands of type 1a supernovae. Contrary to expectations for a matter-dominated universe, these measurements revealed that the universe’s expansion is speeding up. This lead to the concept of dark energy – a hypothetical entity that is driving this acceleration.

But in 2025, a team of astronomers in South Korea, led by Junhyuk Son at Yonsei University, Seoul, made an observation that appeared to turn this long-standing picture on its head. “They claimed that a subtle correlation between the brightness of the supernova and the age of the galaxy it exploded in could, if left uncorrected, means that the overall measurements actually now indicate that the expansion is now slowing,” Wiseman explains. “In other words, that dark energy was strong billions of years in the past, but has subsequently weakened.”

In their study, Wiseman and his colleagues revisited the data used by Son’s team – accounting for a correction that the previous astronomers hadn’t applied. Regardless of redshift, the peak brightness of type 1a supernovae in lower-mass galaxies is known to be slightly fainter than those in higher-mass galaxies. Although the reasons for this correlation remain unclear, astronomers have routinely been correcting for it over the past 15 years.

Wiseman’s team applied the same correction – ensuring their calculated distances were unaffected by differences in galaxy mass. “We also used simulations, where we traced how the age of the exploding stars and the galaxies they explode in evolve with cosmic time,” Wiseman says.

Similar ages

With this correction applied, the claimed correlation between supernova brightness and host galaxy age became almost undetectable. “Even if we account for the prediction that the relationship between the supernova and the galaxy should change as we look further back in time, we end up with almost identical measurements of dark energy,” Wiseman continues. Their simulations also showed that supernova progenitor stars have exploded at consistently similar ages throughout cosmic history.

DESI delivers a cosmological bombshell

The results will likely come as a relief to many cosmologists, averting the dark energy crisis some had feared. All the same, the team says Son’s study provides a valuable test of existing approaches to measuring the universe’s expansion rate, given the limitations of current observational capabilities.

“In an ideal world, we think a correction based on the age of the galaxy would be slightly more accurate than using the galaxy mass, but these measurements are currently not available for more than a handful of supernova host galaxies in the distant universe,” Wiseman says. “Until then, a mass correction is adequate and the implication is that the universe is still accelerating due to dark energy.”

The research is described in Monthly Notices of the Royal Astronomical Society.

Belgian Nobel-prize-winning theoretical physicist François Englert dies aged 93

The Nobel-prize-winning Belgian theoretical physicist François Englert, whose work led to the discovery of the Higgs boson, died on 18 June at the age of 93.

Englert’s studies, which he carried out in the early 1960s with his colleague Robert Brout, demonstrated that fundamental particles can acquire mass by interacting with a field that permeates the universe. The work set in motion a decades-long hunt for the Higgs boson, which was finally discovered at the CERN particle-physics lab near Geneva in 2012.

For the work, Englert shared half of the 2013 Nobel Prize for Physics together with the British theoretical physicist Peter Higgs, who died in 2024.

Born in Brussels, Belgium, on 6 November 1932, Englert studied electrical engineering at the Free University of Brussels (ULB) before switching to physics and completing a PhD at the university in 1959.

For two years he then worked at Cornell University before returning to ULB in 1961, where he remained for the rest of his career.

New particle

“Spontaneous symmetry breaking” is a well-known concept in condensed-matter physics where it had been used to explain how, for example, unordered regions of a magnetic material could suddenly align themselves in a specific direction.

In the early 1960s, the Japanese–American particle physicist Yoichiro Nambu adapted the phenomenon into quantum field theory.

Working at the ULB, Englert and Brout took inspiration from Nambu’s work and in 1964 the pair proposed that the weak and electromagnetic interactions could be united by spontaneous symmetry breaking.

Higgs, who independently came to the same conclusion as Brout and Englert, showed that the particles that carried the weak force acquired their mass through interactions with an all-pervasive field, now known as the Higgs field with the interactions occurring via the Higgs boson.

In 2012 at CERN’s Large Hadron Collider, physicists working on the LHC’s giant ATLAS and CMS detectors discovered the Higgs boson with a mass of about 125 GeV.

The following year, Englert shared half of the 2013 Nobel Prize in Physics with Higgs “for the theoretical discovery of a mechanism that contributes to our understanding of the origin of mass of subatomic particles, and which recently was confirmed through the discovery of the predicted fundamental particle, by the ATLAS and CMS experiments”.

Brout died in 2011 so was not given the Nobel prize as it is not awarded posthumously.

As well as the Nobel prize, Englert won many other awards including the Francqui Prize, awarded by the Francqui Foundation in 1982, the Wolf Prize in Physics (2004) and the American Physical Society J J Sakurai Prize (2010). In 2013, he was ennobled a baron by King Albert II of Belgium.

Particle physicist John Ellis from King’s College London, who has spent most of his career at CERN, told Physics World that he will remember Englert for his “his modest and polite demeanour, despite his evident and justified pride in his ground-breaking work”.

“His memory lives on as one of the founders of the Standard Model and an originator of one of its greatest puzzles, the Brout-Englert-Higgs mechanism for breaking symmetries and generating particle masses,” adds Ellis. “How long will it be before we understand this remarkable, yet crucial, aspect of Nature?”

An open access future is only possible by addressing its problems

At the turn of the 21st century the open access (OA) movement proposed an inspiring idea: to make scientific knowledge freely available to everyone, everywhere. In what became known as the Budapest Open Access Initiative, the movement aimed to challenge scientific publishers who continued to raise subscription prices, sometimes without justification. This, OA proponents suggested, was increasingly undermining poorer countries and institutions, who often could not afford the subscriptions.

A quarter of a century on, numerous institutions – both national and international – have strongly promoted the transition from the subscription-based model to OA. But to what extent has this transition actually taken place?

In a recent post on his Journalology blog, publishing consultant James Butcher showed that while open-access publishing increased rapidly from 2000 to 2023, by 2024 it had stagnated if not reversed, with papers in subscription-based journals increasing. Gold OA – where an article processing charge (APC) is paid by the author to make the paper immediately freely available to read – showed a decline compared to hybrid OA, in which a traditional, subscription-based journal makes individual articles freely available to the public.

Butcher offers China as a possible explanation for the trend. The country now has the fastest-growing scientific output, accounting for roughly 25% of all research articles published annually – up from about 13% a decade ago. This rapid expansion of Chinese research activity is influencing the pace of the global OA transition. Data indicate that Chinese researchers rarely publish OA in hybrid journals, tending instead to publish either in fully OA journals or, more commonly, in subscription-based journals.

The data clearly show that the global transition to OA is only feasible once Chinese researchers significantly change their publishing practices. However, this shift entails a major financial challenge for China.

Historically, China has paid considerably less in institutional subscription fees than Europe, Japan or the US. As China’s research output continues to grow, publishing a greater share of papers as OA would impose increasingly high costs, making traditional subscription journals a financially attractive alternative for many researchers.

The reassessment of the open-access model is not limited to China. In 2018 cOAlition S – a consortium of national research agencies and funders from 12 European countries – launched Plan S, which proposes that publicly funded research is made immediately available in full OA without embargo. The consortium has now released a new strategy for 2026–2030 that adopts a more flexible approach, supporting alternatives to paywalled journals without explicitly aiming to replace them.

If these trends consolidate, the OA model may face additional, often overlooked obstacles: coexistence of different publication traditions and the unequal distribution of publication costs across countries and institutions. Both factors hinder progress toward a fully OA system.

Access all areas

OA is a noble concept but, unfortunately, what works in theory does not always withstand the complexity of the real world. APCs today can exceed $10,000 and are paid by the authors’ own research funds. It is worth emphasizing that a good journal has far more readers than authors, so distributing costs among readers is much more equitable than placing them on authors.

We are also in a paradoxical situation where publishers could be collecting both APC revenues and subscription fees. Many publishers are now signing Transformative Agreements with institutions, designed to eliminate this “double dipping” by bundling reading fees and open-access publishing costs into a single contract.

In practice, we have moved from a system of “free to publish, pay to read” to one of “free to read, pay to publish” where the burden of cost has shifted from institutions to individuals. This radical transformation has not eliminated inequality. On the contrary, it paradoxically risks deepening it, producing effects that run counter to the original goals of the OA movement.

OA has also inadvertently promoted the rise of predatory journals, which continue to prosper, while, in the meantime, an entirely different major challenge has emerged: artificial intelligence (AI). Generative AI systems are trained on massive volumes of text, including OA scientific articles. In other words, the scientific community is providing free data to large tech companies, which use them to develop proprietary models that generate large profits.

OA articles are often published under Creative Commons licenses that allow reuse, even for commercial purposes. Yet AI models often do not cite their sources or attribute authorship. Thus, publicly funded knowledge is exploited for private profit, but academic institutions, researchers and publishers often do not receive any recognition in return, not even symbolic.

If AI tools can be properly regulated, however, they can help accelerate scientific progress by using open, high-quality data. Still, the question of whether the benefits of OA outweigh the drawbacks remains largely unanswered.

Taking responsibility

OA has not been a total failure, but neither has it lived up to the high hopes it once inspired. With OA we have gained broader access, but at the cost of new imbalances. We have often sacrificed quality for quantity. And above all, we have largely forgotten that science is not just about sharing, but also about recognition, responsibility and fairness.

OA remains an idea with remarkable potential but only if we resist its most rigid and dogmatic interpretations, which consider it as an unquestionable good. Initially, only the advantages were acknowledged but now, after a quarter of a century, we are beginning to recognize its downsides.

Identifying problems is the only way to develop solutions: without a proper diagnosis, there is no effective cure. Resolving the persistent and emerging challenges of OA is now urgent, especially as the expanding role of Chinese research introduces new and unexpected dynamics into global publishing given their fondness for the subscription model.

There is no magic solution. Improving OA in scientific publishing requires shared responsibility across the research community. Researchers should engage responsibly in publishing and peer review, develop a better understanding of the complexity of the editorial process, and reject unethical practices.

Universities must reform evaluation systems by prioritizing research quality over publication volume, remove perverse financial incentives, and strengthen training in publishing ethics at the earliest stages of scientific careers.

Publishers should ensure fair and institutionally supported publication costs, enhance editorial and peer review standards, and actively counter predatory publishing.

At the policy level, funding agencies and governing bodies can foster greater accountability by supporting ethics training, and develop traceability and compensation mechanisms for AI use of OA content. Agencies could also invest in sustainable community-led models such as diamond OA platforms, in which research articles are published free to read and free to publish, with publishing costs subsidized by universities or government grants.

No-one wants to go back. But if we truly aspire to a science that is open, fair and sustainable, scientists must acknowledge the problem and take responsibility for addressing it.

Quiz of the week: what’s the secret to the Venus flytrap’s swift closure?

Fancy some more? Check out our puzzles page.

New class of magnons live a hundred times longer

From left to right: University of Vienna researchers Rostyslav Serha, Andrii Chumak, David Schmoll and Sebastian Knauer standing in front of a cryostat. (Courtesy: Ian Ehm)

Bosonic quasiparticles such as magnons show much promise for enabling on-chip quantum information technologies that can be scaled down to the nanoscale, but their too-short lifetime has held back such applications until now. An international team of physicists says it has now increased this lifetime by a hundredfold, thanks to the discovery of a new class of magnons known as short-wavelength dipole-exchange magnons.

Magnons are the collective oscillations of coupled magnetic spins in a material. They are particularly promising for next-generation computing because they could allow for information processing based on the wave-like dynamics of these spins rather than the flow of electrons, thereby significantly reducing energy losses. They can also naturally couple to many other fundamental quasiparticles, such as phonons and photons, which makes them ideal as the building blocks for hybrid quantum systems.

The main drawback of magnons, however, is that they die too fast, explains Andrii Chumak of the University of Vienna, who led this latest study. “Indeed, their lifetimes are typically limited to a few hundred nanoseconds, something that puts a hard ceiling on what we can do with them in a quantum architecture.”

A lifetime of up to 18 µs

In this work, which is detailed in Science Advances, the researchers discovered short-wavelength dipole-exchange magnons in highly pure, single-crystal yttrium iron garnet (YIG) spheres held at millikelvin temperatures. These magnons last for up to 18 µs, which is nearly two orders of magnitude longer than that previously observed for these quasiparticles.

As often in science, the discovery was accidental, says Chumak. “We had prepared three YIG spheres with varying degrees of purity for planned quantum experiments with superconducting qubits, and we simply wanted to characterize these spheres and identify the one that hosted the longest magnon lifetime,” he recalls.

“An 18 microsecond lifetime places magnon coherence on a par with that of typical transmon superconducting qubits used in today’s quantum processors,” he tells Physics World. “This reshapes the role magnons can play in hybrid quantum architectures – from lossy intermediaries to robust quantum memories and low-loss links able to mediate interactions between distant qubits on a chip.”

A programmable on-chip “quantum bus”

And that’s not all: long-lived magnons coupled to superconducting circuits could act as a programmable on-chip “quantum bus”, entangling many distant quantum bits (qubits) made from these hybrid structures along a common waveguide, rather than only nearest neighbours, he adds. The results also point to a clear materials-science route to push magnon lifetimes even further, in particular by continuing to reduce rare-earth impurity concentrations in YIG.

There is a way to go before real-world applications see the light of day, however. For one, the researchers still have some physics to understand, but that is what makes the project so intriguing for them, they say.

Chumak says that the most immediate future step in his team’s work will be an echo-type experiment to measure the lifetime of these magnons directly, rather than inferring it from the so-called parametric instability threshold, as was done in this work. “In parallel, we will need to learn how to excite and detect these short-wavelength magnons efficiently using nanoscale transducers, which is essential before they can be integrated with superconducting qubits.”

“The field of quantum magnonics is still very young compared to other quantum platforms,” Chumak notes, “so it will take some time before we reach actual quantum computing applications – but the path is now much more concrete than it was a year ago.”

Ken’ichi Nomoto and Stanford Woosley share the 2026 Shaw Prize in Astronomy

The 2026 Shaw Prize in Astronomy has been awarded to Ken’ichi Nomoto and Stanford Woosley. They share the prize “for their studies of stellar explosions and the origin of the elements”.

In this episode of the Physics World Weekly podcast, Woosley and Nomoto talk about the astrophysics of supernovae – exploding stars – and what these spectacular events tell us about the elemental composition of the universe and how it has expanded since the Big Bang.

Nomoto is emeritus professor and visiting senior scientist at the Kavli Institute for the Physics and Mathematics of the Universe, at Japan’s University of Tokyo. Woosley is professor of astronomy and astrophysics at the University of California, Santa Cruz in the US.

This podcast is sponsored by the Shaw Prize Foundation.

The Shaw Prize is an international prize based in Hong Kong. Currently, it consists of three annual awards. The Shaw Prize in Astronomy; The Shaw Prize in Life Science & Medicine; and The Shaw Prize in Mathematical Sciences. In 2027 a fourth prize will be awarded for work in computer science. Each prize carries a monetary award of $1.2m. The prize was established by Runrun Shaw, a media mogul and philanthropist. It was first awarded in 2004 and since then 121 prizes have been given individuals from across the world.

Word wave puzzle no.5

Here’s how the game works:

1. Enter a word guess – in this game the word has six letters.
2. After submitting your guess, each letter in the guessed word is coloured to provide feedback:
  - Green: The letter is correct and is in the correct position in the target word.
  - Yellow: The letter is correct but is in the wrong position in the target word.
  - Grey: The letter is not in the target word at all.
3. Using this colour feedback, refine your next guess.
4. Continue guessing until you correctly identify the hidden word(s) or run out of attempts.

If you want the answer, read the article here.

Fancy some more? Check out our puzzles page.

AI data centres’ electricity use nears country-scale levels, finds report

Data centres that house AI infrastructure are on track by the end of the decade to consume nearly three times as much electricity annually as Pakistan, Bangladesh and Nigeria combined. That is according to a report published by the United Nations University Institute for Water, Environment and Health (UNU-INWEH), which also discovers that the water footprint of such centres could match the annual domestic needs of all 1.3 billion people in Sub-Saharan Africa by 2030.

For hundreds of millions of people, generative AI is now woven into their daily lives. ChatGPT processes 2.5 billion prompts each day and, by integrating AI into its search engine, Google processes billions of AI-assisted interactions.

Behind every virtual compute, however, stands sprawling data centres that operate thousands of high-performance processors. Data centres consume vast amounts of electricity and water to operate, cool servers and transmit data.

Day-to-day search queries account for 80-90% of data centres’ energy costs. An average text-based prompt consumes about 0.42 Watt-hours (Wh) of electricity. ChatGPT interactions alone result in 383 GWh per year — enough to cover the domestic electricity needs of nearly three million people in Sub-Saharan Africa.

As of late 2025, only 32 countries had AI-specialized data centres, with nearly half of global data centres located in the US. More than 150 countries, including most of Africa and South America, lack infrastructure to participate in the AI economy.

Those who benefit least from AI, however, often bear a disproportionate share of its cost. Vulnerable communities near data centres face air pollution, drought conditions, and land shortage.

Even regions in the Global South that are devoid of data centres are impacted by mineral extractions to build AI hardware. Such extractions are energy-intensive, deplete water, and pollute the environment.

A responsible ecosystem

The report finds that the land footprint of data centres could span an area roughly 10 times the size of Mexico City by the end of the decade. Additionally, it projects that 2.5 million metric tonnes of e-waste could be produced annually from AI hardware — equivalent to scrapping 250 Eiffel Towers each year.

UNU-INWEH director Kaveh Madani says that the report is not “a case against artificial intelligence” adding that the technology is improving the lives of billions of people worldwide. “[The report] is a call for using it responsibly and addressing its unintended impacts proactively to make it sustainable and equitable,” adds Madani.

To build a responsible AI ecosystem, the report calls for a multilateral approach. AI developers should improve efficiency of design and transparently report their environmental footprint. International and national governance should engage local communities in siting decisions and enforce standardized environmental disclosure while investors should consider environmental impact in their due diligence.

For individuals, the report encourages users to avoid unnecessary use, choose the smallest model capable of the task and write concise prompts.

Laser-based rotation of floating samples enables high-resolution 3D microscopy

A new contact-free way of rotating delicate samples, such as biological cells, undergoing three-dimensional (3D) optical microscopy increases the image resolution and could reveal more details about microscale structures and processes.

Current optical microscopes provide extremely good image resolution in a single plane, but since depth information is often lacking, samples are routinely rotated and imaged at multiple different angles to create a 3D picture. Because rotating via mechanical microtools such as tiny grippers, needles or pipettes can damage samples, contact-free approaches such as optical tweezers, magnetic manipulation or optothermal forces are often used instead. However, these methods rely on the sample being birefringent, magnetic, light-absorbing or specially shaped.

By contrast, the laser-based technique developed by a team led by Moritz Kreysing and Fan Nan from the Institute of Biological and Chemical Systems at the Karlsruher Institut für Technologie (KIT) in Germany can rotate and spin floating microscopic objects no matter what their shape or material properties.

Their method involves focusing a 1455 nm infrared laser beam on a small region of a highly viscous liquid – initially at 25°C – in which the sample will be suspended. This gently heats that region of the liquid by a few degrees. But rather than focusing on one spot, an acousto-optic deflector steers the beam rapidly along a carefully designed two-dimensional (2D) pattern.

“Because the viscosity of the liquid changes with temperature, this moving heat pattern generates tiny fluid flows, known as thermoviscous flows. By choosing the scan geometry and timing correctly, these flows become three-dimensional and helical, like microscopic corkscrew-shaped currents,” explain Kreysing and Nan. Once a small object is floated freely in these 3D helical flows, it can be rotated, spun, transported or stabilized solely via software-defined scan patterns, with different symmetries controlling the movement of the surrounding liquid.

The KIT technique allows stacks of high-resolution images focused at different depths to be acquired along multiple different angles by re-orienting the sample, a feature that “can help overcome the anisotropic resolution of conventional 3D microscopy”, explain the researchers. Using a high-viscosity liquid to suspend the samples not only enables kinematic stop-and-go actuation, it also suppresses Brownian motion (the jiggling around of tiny particles in fluids), which otherwise decreases image resolution and makes the study of cellular transport processes tricky.

Since previous methods of laser-driving fluid flows could only create motion in one plane, the researchers found it “particularly exciting” when they first observed their laser scanning in a 2D plane generating 3D flow fields. “Another key moment was the observation of opto-hydrodynamic focusing. The particles did not just rotate; their spiral motion converged toward a stable height and position. This was significant because it meant that the optofluid could stabilize the sample, rather than requiring mechanical confinement or complex feedback control.”

As detailed in their recent Light: Science & Applications paper, the KIT team demonstrated rotation and spinning of various samples including stained biological cells, nano-printed micro-tiles and perfectly spherical beads. “We see immediate potential in advanced optical microscopy, especially multi-view imaging of suspended cells, cell clusters, organelles, microstructures or soft materials,” Kreysing and Nan tell Physics World.

Since the basic hardware components of their system will be familiar to many optics and microscopy labs, they are hoping for early adoption by specialized research groups. But to optimize the technique for biological applications, they will now investigate the use of different liquids and operating conditions “that better preserve the viability of live mammalian cells while still providing sufficient stability for precise rotation and imaging”.

The researchers suggest that future areas of application could include microfluidics, microrobotics, colloidal assembly, materials science and 3D microfabrication. As such, they also want to develop more advanced scan patterns that can generate curved, programmable or multi-axis helical flows. “This could allow more complex 3D manipulation, such as controlled orientation of irregular objects, coordinated actuation of multiple particles, or integration with microfabrication workflows,” they explain.