A new way of creating arrays of ultracold neutral atoms could make it possible to build quantum computers with more than 100,000 quantum bits (qubits) – two orders of magnitude higher than today’s best machines. The approach, which was demonstrated by physicists at Columbia University in the US, uses optical metasurfaces to generate the forces required to trap and manipulate the atoms. According to its developers, this method is much more scalable than traditional techniques for generating arrays of atomic qubits.

“Neutral atom arrays have become a leading quantum technology, notably for quantum computing, where single atoms serve as qubits,” explains atomic physicist Sebastian Will, who co-led the study with his Columbia colleague Nanfang Yu. “However, the technology available so far to make these arrays limits array sizes to about 10,000 traps, which corresponds to a maximum of 10,000 atomic qubits.”

Building on a well-established technique

In common with standard ways of constructing atomic qubit arrays, the new method relies on a well-established technique known as optical tweezing. The principle of optical tweezing is that highly focused laser beams generate forces at their focal points that are strong enough to trap individual objects – in this case, atoms.

To create many such trapping sites while maintaining tight control of the laser’s light field, scientists typically use devices called spatial light modulators (SLMs) and acousto-optic deflectors (AODs) to split a single moderately intense laser beam into many lower-intensity ones. Such arrays have previously been used to trap thousands of atoms at once. In 2025, for example, researchers at the California Institute of Technology in the US created arrays containing up to 6100 trapped atoms – a feat that Will describes as “an amazing achievement”.

A superposition of tens of thousands of flat lenses

In the new work, which is detailed in Nature, Will, Yu and colleagues replaced these SLMs and AODs with flat optical surfaces made up of two-dimensional arrays of nanometre-sized “pixels”. These so-called metasurfaces can be thought of as a superposition of tens of thousands of flat lenses. When a laser beam hits them, it produces tens of thousands of focal points in a unique pattern. And because the pixels in the Columbia team’s metasurfaces are smaller than the wavelength of light they are manipulating (300 nm compared to 520 nm), Yu explains that they can use these metasurfaces to generate tweezer arrays directly, without the need for additional bulky and expensive equipment.

The Columbia researchers demonstrated this by trapping atoms in several highly uniform two-dimensional (2D) patterns, including a square lattice with 1024 trapping sites; patterns shaped like quasicrystals and the Statue of Liberty with hundreds of sites; and a circle made up of atoms spaced less than 1.5 microns apart. They also created a 3.5 mm diameter metasurface that contains more than 100 million pixels and used it to generate a 600 × 600 array of trapping sites. “This is two orders of magnitude beyond the capabilities of current technologies,” Yu says.

Another advantage of using metasurfaces, Will adds, is that they are “extremely resilient” to high laser intensities. “This is what is needed to trap hundreds of thousands of neutral atom qubits,” he explains. “Metasurfaces’ laser power handling capabilities go several orders of magnitude beyond the state of the art with SLMs and AODs.”

Laying the groundwork

For arrays of up to 1000 focal points, the researchers showed that their metasurface-generated arrays can trap single atoms with a high level of control and precision and with high single-atom detection fidelity. This is essential, they say, because it demonstrates that the arrays’ quality is high enough to be useful for quantum computing.

While they are not there yet, Will says that the metasurface atomic tweezer arrays they developed “lay the critical groundwork for realizing neutral-atom quantum computers that operate with more than 100,000 qubits”. These high numbers, he adds, will be essential for realizing quantum computers that can achieve “quantum advantage” by outperforming classical computers. “The large number of qubits also allows for more ‘redundancy’ in the system to realize highly-efficient quantum error correction codes, which can make quantum computing – which is usually fragile – more resilient,” he says.

The Columbia team is now working on further improving the quality of their metasurfaces. “On the atomic arrays side, we will now try to actually fill such arrays with more than 100 000 atoms,” Will tells Physics World. “Doing this will require a much more powerful laser than we currently have, but it’s in a realistic range.”

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Classical mechanics describes our everyday world of macroscopic objects very well. Quantum mechanics is similarly good at describing physics on the atomic scale. The boundary between these two regimes, however, is still poorly understood. Where, exactly, does the quantum world stop and the classical world begin?

Researchers in Austria and Germany have now pushed the line further towards the macroscopic regime by showing that metal nanoparticles made up of thousands of atoms clustered together continue to obey the rules of quantum mechanics in a double-slit-type experiment. At over 170 000 atomic mass units, these nanoparticles are heavier than some viroids and proteins – a fact that study leader Sebastian Pedalino, a PhD student at the University of Vienna, says demonstrates that quantum mechanics remains valid at this scale and alternative models are not required.

Multiscale cluster interference

According to the rules of quantum mechanics, even large objects behave as delocalized waves. However, we do not observe this behaviour in our daily lives because the characteristic length over which this behaviour extends – the de Broglie wavelength λ_dB = h/mv, where h is Planck’s constant, m is the object’s mass and v is its velocity – is generally much smaller than the object itself.

In the new work, a team led by Vienna’s Markus Arndt and Stefan Gerlich, in collaboration with Klaus Hornberger at the University of Duisburg-Essen, created clusters of sodium atoms in a helium-argon mixture at 77 K in an ultrahigh vacuum. The clusters each contained between 5000 and 1000 atoms and travelled at velocities of around 160 m s⁻¹, giving them de Broglie wavelengths between 10‒22 femtometres (1 fm = 10^-15 m).

To observe matter-wave interference in objects with such ultra-short de Broglie wavelengths, the team used an interferometer containing three diffraction gratings constructed with deep ultraviolet laser beams in a so-called Talbot–Lau configuration. The first grating channels the clusters through narrow gaps, from which their wave function expands. This wave is then modulated by the second grating, resulting in interference that produces a measurable striped pattern at the third grating.

This result implies that the clusters’ location is not fixed as it propagates through the apparatus. Instead, its wave function is spread over a span dozens of times larger than an individual cluster, meaning that it is in a superposition of locations rather than occupying a fixed position in space. This is known as a Schrödinger cat state, in reference to the famous thought experiment by physicist Erwin Schrödinger in which he imagined a cat sitting in a sealed box to be both dead and alive at once.

Pushing the boundaries for quantum experiments

The Vienna-Duisburg-Essen researchers characterized their experiment by calculating a quantity known as macroscopicity that combines the duration of the quantum state (its coherence time), the mass of the object in that state and the degree of separation between states. In this work, which they detail in Nature, the macroscopicity reached a value of 15.5 – an order of magnitude higher than the best known previous reported measurement of this kind.

Arndt explains that this milestone was reached thanks to a long-term research programme that aims to push quantum experiments to ever higher masses and complexity. “The motivation is simply that we do not yet know if quantum mechanics is the ultimate theory or if it requires any modification at some mass limit,” he tells Physics World. While several speculative theories predict some degree of modification, he says, “as experimentalists our task is to be agnostic and see what happens”.

Arndt notes that the team’s machine is very sensitive to small forces, which can generate notable deflections of the interference fringes. In the future, he thinks this effect could be exploited to characterize the properties of materials. In the longer term, this force-sensing capability could even be used to search for new particles.

Interpretations and adventures

While Arndt says he is “impressed” that these mesoscopic objects – which are in principle easy to see and even to localize under a scattering microscope – can be delocalized on a scale more than 10 times their size if they are isolated and non-interacting, he is not entirely surprised. The challenge, he says, lies in understanding what it means. “The interpretation of this phenomenon, the duality between this delocalization and the apparently local nature in the act of measurement, is still an open conundrum,” he says.

Looking ahead, the researchers say they would now like to extend their research to higher mass objects, longer coherence times, higher force sensitivity and different materials, including nanobiological materials as well as other metals and dielectrics. “We still have a lot of work to do on sources, beam splitters, detectors, vibration isolation and cooling,” says Arndt. “This is a big experimental adventure for us.”

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Hints of non-gravitational interactions between dark matter and “relic” neutrinos in the early universe have emerged in a study of astronomical data from different periods of cosmic history. The study was carried out by cosmologists in Poland, the UK and China, and team leader Sebastian Trojanowski of Poland’s NCBJ and NCAC PAS notes that future telescope observations could verify or disprove these hints of a deep connection between dark matter and neutrinos.

Dark matter and neutrinos play major roles in the evolution of cosmic structures, but they are among the universe’s least-understood components. Dark matter is thought to make up over 25% of the universe’s mass, but it has never been detected directly; instead, its existence is inferred from its gravitational interactions. Neutrinos, for their part, are fundamental subatomic particles that have a very low mass and interact only rarely with normal matter.

Analysing data from different epochs

According to the standard (ΛCDM) model of cosmology, dark matter and neutrinos do not interact with each other. The work of Trojanowski and colleagues challenges this model by proposing that dark matter and neutrinos may have interacted in the past, when the universe was younger and contained many more neutrinos than it does today.

This proposal, they say, was partly inspired by a longstanding cosmic conundrum. Measurements of the early universe suggest that structures such as galaxies should have grown more rapidly than ΛCDM predicts. At the same time, observations of today’s universe indicate that matter is slightly less densely packed than expected. This suggests a slight mismatch between early and late measurements.

To explore the impact that dark matter-neutrino interactions (νDM) would have on this mismatch, a team led by Trojanowski’s colleague Lei Zu analysed data from different epochs of the universe’s evolution. Data from the young (high redshift) universe came from two instruments – the ground-based Atacama Cosmology Telescope and the space-based Planck Telescope, which the European Space Agency operated from 2009 to 2013 – that were designed to study the afterglow of the Big Bang, which is known as the cosmic microwave background (CMB). Data from the older (low-redshift, or z< 3.5) universe, meanwhile, came from a variety of sources, including galaxy maps from the Sloan Digital Sky Survey and weak gravitational lensing data from the Dark Energy Survey (DES) conducted with the Dark Energy Camera on the Victor M Blanco Telescope in Chile.

“New insight into how structure formed in the universe”

Drawing on these data, the team calculated that an interaction strength u ≈10⁻⁴ between dark matter and neutrinos would be enough to resolve the discrepancy. The statistical significance of this result is nearly 3σ, which team member Sming Tsai Yue-Lin of the Purple Mountain Observatory in Nanjing, China says was “largely achieved by incorporating the high-precision weak lensing data from the DES with the weak lensing component”.​

While this is not high enough to definitively disprove the ΛCDM model, the researchers say it does show that the model is incomplete and requires further investigation. “Our study shows that interactions between dark matter and neutrinos could help explain this difference, offering new insight into how structure formed in the universe,” explains team member Eleonora Di Valentino, a senior research fellow at Sheffield University, UK.

Trojanowski adds that the ΛCDM has been under growing pressure in recent years, while the Standard Model of particle physics cannot explain the nature of dark matter. “These two theories need to be extended to resolve these problems and studying dark matter-neutrino interactions are a promising way to achieve this goal,” he says.

The team’s result, he continues, adds to the “massive amount of data” suggesting that we are reaching the limits of the standard cosmological model and may be at the dawn of understanding physics beyond it. “We illustrate that we likely need to bridge cosmological data and fundamental particle physics to describe the universe across different scales and so resolve current anomalies,” he says.

Two worlds

One of the challenges of doing this, Trojanowski adds, is that the two fields involved – cosmological data analysis and theoretical astroparticle physics – are very different. “Each field has its own approach to problem-solving and even its own jargon,” he says. “Fortunately, we had a great team and working together was really fun.”

The researchers say that data from future telescope observations, such as those from the Simonyi Survey Telescope at the Vera C Rubin Observatory (formerly known as the Large Synoptic Survey Telescope, LSST) and the China Space Station Telescope (CSST), could place more stringent tests on their hypothesis. Data from CMB experiments and weak lensing surveys, which map the distribution of mass in the universe by analysing how distant galaxies distort light, could also come in useful.

They detail their present research in Nature Astronomy.

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Our usual picture of Uranus and Neptune as “ice giant” planets may not be entirely correct. According to new work by scientists at the University of Zürich (UZH), Switzerland, the outermost planets in our solar system may in fact be rock-rich worlds with complex internal structures – something that could have major implications for our understanding of how these planets formed and evolved.

Within our solar system, planets fall into three categories based on their internal composition. Mercury, Venus, Earth and Mars are deemed terrestrial rocky planets; Jupiter and Saturn are gas giants; and Uranus and Neptune are ice giants.

An agnostic approach

The new work, which was led by PhD student Luca Morf in UZH’s astrophysics department, challenges this last categorization by numerically simulating the two planets’ interiors as a mixture of rock, water, hydrogen and helium. Morf explains that this modelling framework is initially “agnostic” – meaning unbiased – about what the density profiles of the planets’ interiors should be. “We then calculate the gravitational fields of the planets so that they match with observational measurements to infer a possible composition,” he says.

This process, Morf continues, is then repeated and refined to ensure that each model satisfies several criteria. The first criteria is that the planet should be in hydrostatic equilibrium, meaning that its internal pressure is enough to counteract its gravity and keep it stable. The second is that the planet should have the gravitational moments observed in spacecraft data. These moments describe the gravitational field of a planet, which is complex because planets are not perfect spheres.

The final criteria is that the modelled planets need to be thermodynamically and compositionally consistent with known physics. “For example, a simulation of the planets’ interiors must obey equations of state, which dictate how materials behave under given pressure and temperature conditions,” Morf explains.

After each iteration, the researchers adjust the density profile of each planet and test it to ensure that the model continues to adhere to the three criteria. “We wanted to bridge the gap between existing physics-based models that are overly constrained and empirical approaches that are too simplified,” Morf explains. Avoiding strict initial assumptions about composition, he says, “lets the physics and data guide the solution [and] allows us to probe a larger parameter space.”

A wide range of possible structures

Based on their models, the UZH astrophysicists concluded that the interiors of Uranus and Neptune could have a wide range of possible structures, encompassing both water-rich and rock-rich configurations. More specifically, their calculations yield rock-to-water ratios of between 0.04-3.92 for Uranus and 0.20-1.78 for Neptune.

The models, which are detailed in Astronomy and Astrophysics, also contain convective regions with ionic water pockets. The presence of such pockets could explain the fact that Uranus and Neptune, unlike Earth, have more than two magnetic poles, as the pockets would generate their own local magnetic dynamos.

Traditional “ice giant” label may be too simple

Overall, the new findings suggest that the traditional “ice giant” label may oversimplify the true nature of Uranus of Neptune, Morf tells Physics World. Instead, these planets could have complex internal structures with compositional gradients and different heat transport mechanisms. Though much uncertainty remains, Morf stresses that Uranus and Neptune – and, by extension, similar intermediate-class planets that may exist in other solar systems – are so poorly understood that any new information about their internal structure is valuable.

A dedicated space mission to these outer planets would yield more accurate measurements of the planets’ gravitational and magnetic fields, enabling scientists to refine the limited existing observational data. In the meantime, the UZH researchers are looking for more solutions for the possible interiors of Uranus and Neptune and improving their models to account for additional constraints, such as atmospheric conditions. “Our work will also guide laboratory and theoretical studies on the way materials behave in general at high temperatures and pressures,” Morf says.

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A new sensor detects helium leaks by monitoring how sound waves propagate through a topological material – no chemical reactions required. Developed by acoustic scientists at Nanjing University, China, the innovative, physics-based device is compact, stable, accurate and capable of operating at very low temperatures.

Helium is employed in a wide range of fields, including aerospace, semiconductor manufacturing and medical applications as well as physics research. Because it is odourless, colourless, and inert, it is essentially invisible to traditional leak-detection equipment such as adsorption-based sensors. Specialist helium detectors are available, but they are bulky, expensive and highly sensitive to operating conditions.

A two-dimensional acoustic topological material

The new device created by Li Fan and colleagues at Nanjing consists of nine cylinders arranged in three sub-triangles with tubes in between the cylinders. The corners of the sub-triangles touch and the tubes allow air to enter the device. The resulting two-dimensional system has a so-called “kagome” structure and is an example of a topological material – that is, one that contains special, topologically protected, states that remain stable even if the bulk structure contains minor imperfections or defects. In this system, the protected states are the corners.

To test their setup, the researchers placed speakers under the corners that send sound waves into the structure and make the gas within it vibrate at a certain frequency (the resonance frequency). When they replaced the air in the device with helium, the sound waves travelled faster, changing the vibration frequency. Measuring this shift in frequency enabled the researchers to calculate the concentration of helium in the device.

Many advantages over traditional gas sensors

Fan explains that the device works because the interface/corner states are impacted by the properties of the gas within it. This mechanism has many advantages over traditional gas sensors. First, it does not rely on chemical reactions, making it ideal for detecting inert gases like helium. Second, the sensor is not affected by external conditions and can therefore work at extremely low temperatures – something that is challenging for conventional sensors that contain sensitive materials. Third, its sensitivity to the presence of helium does not change, meaning it does not need to be recalibrated during operation. Finally, it detects frequency changes quickly and rapidly returns to its baseline once helium levels decrease.

As well as detecting helium, Fan says the device can also pinpoint the direction a gas leak is coming from. This is because when helium begins to fill the device, the corner closest to the source of the gas is impacted first. Each corner thus acts as an independent sensing point, giving the device a spatial sensing capability that most traditional detectors lack.

Other gases could be detected

Detecting helium leaks is important in fields such as semiconductor manufacturing, where the gas is used for cooling, and in medical imaging systems that operate at liquid helium temperatures. “We think our work opens an avenue for inert gas detection using a simple device and is an example of a practical application for two-dimensional acoustic topological materials,” says Fan.

While the new sensor was fabricated to detect helium, the same mechanism could also be employed to detect other gases such as hydrogen, he adds.

Spurred on by these promising preliminary results, which they report in Applied Physics Letters, the researchers plan to extend their fabrication technique to create three-dimensional acoustic topological structures. “These could be used to orientate the corner points so that helium can be detected in 3D space,” says Fan. “Ultimately, we are trying to integrate our system into a portable structure that can be deployed in real-world environments without complex supporting equipment.,” he tells Physics World.

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Researchers in Switzerland have found an unexpected new use for an optical technique commonly used in silicon chip manufacturing. By shining a focused laser beam onto a sample of material, a team at the Paul Scherrer Institute (PSI) and ETH Zürich showed that it was possible to change the material’s magnetic properties on a scale of nanometres – essentially “writing” these magnetic properties into the sample in the same way as photolithography etches patterns onto wafers. The discovery could have applications for novel forms of computer memory as well as fundamental research.

In standard photolithography – the workhorse of the modern chip manufacturing industry – a light beam passes through a transmission mask and projects an image of the mask’s light-absorption pattern onto a (usually silicon) wafer. The wafer itself is covered with a photosensitive polymer called a resist. Changing the intensity of the light leads to different exposure levels in the resist-covered material, making it possible to create finely detailed structures.

In the new work, Laura Heyderman and colleagues in PSI-ETH Zürich’s joint Mesoscopic System group began by placing a thin film of a magnetic material in a standard photolithography machine, but without a photoresist. They then scanned a focused laser beam over the surface of the sample while modulating the beam’s wavelength of 405 nm to deliver varying intensities of light. This process is known as direct write laser annealing (DWLA), and it makes it possible to heat areas of the sample that measure just 150 nm across.

In each heated area, thermal energy from the laser is deposited at the surface and partially absorbed by the film down to a depth of around 100 nm). The remainder dissipates through a silicon substrate coated in 300-nm-thick silicon oxide. However, the thermal conductivity of this substrate is low, which maximizes the temperature increase in the film for a given laser fluence. The researchers also sought to keep the temperature increase as uniform as possible by using thin-film heterostructures with a total thickness of less than 20 nm.

Crystallization and interdiffusion effects

Members of the PSI-ETH Zürich team applied this technique to several technologically important magnetic thin-film systems, including ferromagnetic CoFeB/MgO, ferrimagnetic CoGd and synthetic antiferromagnets composed of Co/Cr, Co/Ta or CoFeB/Pt/Ru. They found that DWLA induces both crystallization and interdiffusion effects in these materials. During crystallization, the orientation of the sample’s magnetic moments gradually changes, while interdiffusion alters the magnetic exchange coupling between the layers of the structures.

The researchers say that both phenomena could have interesting applications. The magnetized regions in the structures could be used in data storage, for example, with the direction of the magnetization (“up” or “down”) corresponding to the “1” or “0” of a bit of data. In conventional data-storage systems, these bits are switched with a magnetic field, but team member Jeffrey Brock explains that the new technique allows electric currents to be used instead. This is advantageous because electric currents are easier to produce than magnetic fields, while data storage devices switched with electricity are both faster and capable of packing more data into a given space.

Team member Lauren Riddiford says the new work builds on previous studies by members of the same group, which showed it was possible to make devices suitable for computer memory by locally patterning magnetic properties. “The trick we used here was to locally oxidize the topmost layer in a magnetic multilayer,” she explains. “However, we found that this works only in a few systems and only produces abrupt changes in the material properties. We were therefore brainstorming possible alternative methods to create gradual, smooth gradients in material properties, which would open possibilities to even more exciting applications and realized that we could perform local annealing with a laser originally made for patterning polymer resist layers for photolithography.”

Riddiford adds that the method proved so fast and simple to implement that the team’s main challenge was to investigate all the material changes it produced. Physical characterization methods for ultrathin films can be slow and difficult, she tells Physics World.

The researchers, who describe their technique in Nature Communications, now hope to use it to develop structures that are compatible with current chip-manufacturing technology. “Beyond magnetism, our approach can be used to locally modify the properties of any material that undergoes changes when heated, so we hope researchers using thin films for many different devices – electronic, superconducting, optical, microfluidic and so on – could use this technique to design desired functionalities,” Riddiford says. “We are looking forward to seeing where this method will be implemented next, whether in magnetic or non-magnetic materials, and what kind of applications it might bring.”

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By studying how light from eight distant quasars is gravitationally lensed as it propagates towards Earth, astronomers have calculated a new value for the Hubble constant – a parameter that describes the rate at which the universe is expanding. The result agrees more closely with previous “late-universe” probes of this constant than it does with calculations based on observations of the cosmic microwave background (CMB) in the early universe, strengthening the notion that we may be misunderstanding something fundamental about how the universe works.

The universe has been expanding ever since the Big Bang nearly 14 billion years ago. We know this, in part, because of observations made in the 1920s by the American astronomer Edwin Hubble. By measuring the redshift of various galaxies, Hubble discovered that galaxies further away from Earth are moving away faster than galaxies that are closer to us. The relationship between this speed and the galaxies’ distance is known as the Hubble constant, H₀.

Astronomers have developed several techniques for measuring H₀. The problem is that different techniques deliver different values. According to measurements made by the European Space Agency’s Planck satellite of CMB radiation “left over” from the Big Bang, the value of H₀ is about 67 kilometres per second per megaparsec (km/s/Mpc), where one Mpc is 3.3 million light years. In contrast, “distance-ladder” measurements such as those made by the SH0ES collaboration those involving observations of type Ia supernovae yield a value of about 73 km/s/Mpc. This discrepancy is known as the Hubble tension.

Time-delay cosmography

In the latest work, the TDCOSMO collaboration, which includes astronomers Kenneth Wong and Eric Paic of the University of Tokyo, Japan, measured H₀ using a technique called time-delay cosmography. This well-established method dates back to 1964 and uses the fact that massive galaxies can act as lenses, deflecting the light from objects behind them so that from our perspective, these objects appear distorted.

“This is called gravitational lensing, and if the circumstances are right, we’ll actually see multiple distorted images, each of which will have taken a slightly different pathway to get to us, taking different amounts of time,” Wong explains.

By looking for changes in these images that are identical, but slightly out of sync, astronomers can measure the time differences required for the light from the objects to reach Earth. Then, by combining these data with estimates of the distribution of the mass of the distorting galactic lens, they can calculate H₀.

A real tension, not a measurement artefact

Wong and colleagues measured the light from eight strongly lensed quasars using various telescopes, including the James Webb Space Telescope (JWST), the Keck Telescopes and the Very Large Telescope (VLT). They also made use of observations from the Sloan Lens ACS (SLACS) sample with Keck and the Legacy Survey (SL2S) sample.

Based on these measurements, they obtained a H₀ value of roughly 71.6 km s⁻¹ Mpc⁻¹, which is more consistent with current-day observations (such as that from SH0ES) than early-universe ones (such as that from Planck). Wong explains that this discrepancy supports the idea that the Hubble tension arises from real physics, not just some unknown error in the various methods. “Our measurement is completely independent of other methods, both early- and late-universe, so if there are any systematic uncertainties in those, we should not be affected by them,” he says.

The astronomers say that the SLACS and SL2S sample data are in excellent agreement with the new TDCOSMO-2025 sample, while the new measurements improve the precision of H₀ to 4.6%. However, Paic notes that nailing down the value of H₀ to a level that would “definitely confirm” the Hubble tension will require a precision of 1-2%. “This could be possible by increasing the number of objects observed as well as ruling out any systematic errors as yet unaccounted for,” he says.

Wong adds that while the TDCOSMO-2025 dataset contains its own uncertainties, multiple independent measurements should, in principle, strengthen the result. “One of the largest sources of uncertainty is the fact that we don’t know exactly how the mass in the lens galaxies is distributed,” he explains. “It is usually assumed that the mass follows some simple profile that is consistent with observations, but it is hard to be sure and this uncertainty can directly influence the values we calculate.”

The biggest hurdle, Wang adds, will “probably be addressing potential sources of systematic uncertainty, making sure we have thought of all the possible ways that our result could be wrong or biased and figuring out how to handle those uncertainties.”

The study is detailed in Astronomy and Astrophysics.

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Physicists at the University of Stuttgart, Germany have teleported a quantum state between photons generated by two different semiconductor quantum dot light sources located several metres apart. Though the distance involved in this proof-of-principle “quantum repeater” experiment is small, members of the team describe the feat as a prerequisite for future long-distance quantum communications networks.

“Our result is particularly exciting because such a quantum Internet will encompass these types of distant quantum nodes and will require quantum states that are transmitted among these different nodes,” explains Tim Strobel, a PhD student at Stuttgart’s Institute of Semiconductor Optics and Functional Interfaces (IHFG) and the lead author of a paper describing the research. “It is therefore an important step in showing that remote sources can be effectively interfaced in this way in quantum teleportation experiments.”

In the Stuttgart study, one of the quantum dots generates a single photon while the other produces a pair of photons that are entangled – meaning that the quantum state of one photon is closely linked to the state of the other, no matter how far apart they are. One of the photons in the entangled pair then travels to the other quantum dot and interferes with the photon there. This process produces a superposition that allows the information encapsulated in the single photon to be transferred to the distant “partner” photon from the pair.

Quantum frequency converters

Strobel says the most challenging part of the experiment was making photons from two remote quantum dots interfere with each other. Such interference is only possible if the two particles are indistinguishable, meaning they must be similar in every regard, be it in their temporal shape, spatial shape or wavelength. In contrast, each quantum dot is unique, especially in terms of its spectral properties, and each one emits photons at slightly different wavelengths.

To close the gap, the team used devices called quantum frequency converters to precisely tune the wavelength of the photons and match them spectrally. The researchers also used the converters to shift the original wavelengths of the photons emitted from the quantum dots (around 780 nm) to a wavelength commonly used in telecommunications (1515 nm) without altering the quantum state of the photons. This offers further advantages: “Being at telecommunication wavelengths makes the technology compatible with the existing global optical fibre network, an important step towards real-life applications,” Strobel tells Physics World.

Proof-of-principle experiment

In this work, the quantum dots were separated by an optical fibre just 10 m in length. However, the researchers aim to push this to considerably greater distances in the future. Strobel notes that the Stuttgart study was published in Nature Communications back-to-back with an independent work carried out by researchers led by Rinaldo Trotta of Sapienza University in Rome, Italy. The Rome-based group demonstrated quantum state teleportation across the Sapienza University campus at shorter wavelengths, enabled by the brightness of their quantum-dot source.

“These two papers that we published independently strengthen the measurement outcomes, demonstrating the maturity of quantum dot light sources in this domain,” Strobel says. Semiconducting quantum dots are particularly attractive for this application, he adds, because as well as producing both single and entangled photons on demand, they are also compatible with other semiconductor technologies.

Fundamental research pays off

Simone Luca Portalupi, who leads the quantum optics group at IHFG, notes that “several years of fundamental research and semiconductor technology are converging into these quantum teleportation experiments”. For Peter Michler, who led the study team, the next step is to leverage these advances to bring quantum-dot-based teleportation technology out of a controlled laboratory environment and into the real world.

Strobel points out that there is already some precedent for this, as one of the group’s previous studies showed that they could maintain photon entanglement across a 36-km fibre link deployed across the city of Stuttgart. “The natural next step would be to show that we can teleport the state of a photon across this deployed fibre link,” he says. “Our results will stimulate us to improve each building block of the experiment, from the sample to the setup.”

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A new microscope that can simultaneously measure both forward- and backward-scattered light from a sample could allow researchers to image both micro- and nanoscale objects at the same time. The device could be used to observe structures as small as individual proteins, as well as the environment in which they move, say the researchers at the University of Tokyo who developed it.

“Our technique could help us link cell structures with the motion of tiny particles inside and outside cells,” explains Kohki Horie of the University of Tokyo’s department of physics, who led this research effort. “Because it is label-free, it is gentler on cells and better for long observations. In the future, it could help quantify cell states, holding potential for drug testing and quality checks in the biotechnology and pharmaceutical industries.”

Detecting forward and backward scattered light at the same time

The new device combines two powerful imaging techniques routinely employed in biomedical applications: quantitative phase microscopy (QPM) and interferometric scattering (iSCAT).

QPM measures forward-scattered (FS) light – that is, light waves that travel in the same direction as before they were scattered. This technique is excellent at imaging structures in the Mie scattering region (greater than 100 nm, referred to as microscale in this study). This makes it ideal for visualizing complex structures such as biological cells. It falls short, however, when it comes to imaging structures in the Rayleigh scattering region (smaller than 100 nm, referred to as nanoscale in this study).

The second technique, iSCAT, detects backward-scattered (BS) light. This is light that’s reflected back towards the direction from which it came and which predominantly contains Rayleigh scattering. As such, iSCAT exhibits high sensitivity for detecting nanoscale objects. Indeed, the technique has recently been used to image single proteins, intracellular vesicles and viruses. It cannot, however, image microscale structures because of its limited ability to detect in the Mie scattering region.

The team’s new bidirectional quantitative scattering microscope (BiQSM) is able to detect both FS and BS light at the same time, thereby overcoming these previous limitations.

Cleanly separating the signals from FS and BS

The BiQSM system illuminates a sample through an objective lens from two opposite directions and detects both the FS and BS light using a single image sensor. The researchers use the spatial-frequency multiplexing method of off-axis digital holography to capture both images simultaneously. The biggest challenge, says Horie, was to cleanly separate the signals from FS and BS light in the images while keeping noise low and avoiding mixing between them.

Horie and colleagues, Keiichiro Toda, Takuma Nakamura and team leader Takuro Ideguchi, tested their technique by imaging live cells. They were able to visualize micron-sized cell structures, including the nucleus, nucleoli and lipid droplets, as well as nanoscale particles. They compared the FS and BS results using the scattering-field amplitude (SA), defined as the amplitude ratios between the scattered wave and the incident illumination wave.

“SA characterizes the light scattered in both the forward and backward directions within a unified framework,” says Horie, “so allowing for a direct comparison between FS and BS light images.”

Spurred on by their findings, which are detailed in Nature Communications, the researchers say they now plan to study even smaller particles such as exosomes and viruses.

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The Compact Muon Solenoid (CMS) Collaboration has made the first measurements of the quantum properties of a family of three “all-charm” tetraquarks that was recently discovered at the Large Hadron Collider (LHC) at CERN. The findings could help shed more light on the properties of the strong nuclear force, which holds protons and neutrons together in nuclei. The result could help us better understand how ordinary matter forms.

In recent years, the LHC has discovered tens of massive particles called hadrons, which are made of quarks bound together by the strong force. Quarks come in six types: up, down, charm, strange, top and bottom. Most observed hadrons comprise two or three quarks (called mesons and baryons, respectively). Physicists have also observed exotic hadrons that comprise four or five quarks. These are the tetraquarks and pentaquarks respectively. Those seen so far usually contain a charm quark and its antimatter counterpart (a charm antiquark), with the remaining two or three quarks being up, down or strange quarks, or their antiquarks.

Identifying and studying tetraquarks and pentaquarks helps physicists to better understand how the strong force binds quarks together. This force also binds protons and neutrons in atomic nuclei.

Physicists are still divided as to the nature of these exotic hadrons. Some models suggest that their quarks are tightly bound via the strong force, so making these hadrons compact objects. Others say that the quarks are only loosely bound. To confuse things further, there is evidence that in some exotic hadrons, the quarks might be both tightly and loosely bound at the same time.

Now, new findings from the CMS Collaboration suggest that tetraquarks are tightly bound, but they do not completely rule out other models.

Measuring quantum numbers

In their work, which is detailed in Nature, CMS physicists studied all-charm tetraquarks. These comprise two charm quarks and two charm antiquarks and were produced by colliding protons at high energies at the LHC. Three states of this tetraquark have been identified at the LHC. These are: X(6900); X(6600); and X(7100), where the numbers denote their approximate mass in millions of electron volts. The team measured the fundamental properties of these tetraquarks, including their quantum numbers: parity (P); charge conjugation (C); angular momentum, and spin (J). P determines whether a particle has the same properties as its spatial mirror image; C whether it has the same properties as its antiparticle; and J, the total angular momentum of the hadron. These numbers provide information on the internal structure of a tetraquark.

The researchers used a version of a well-known technique called angular analysis, which is similar to the technique used to characterize the Higgs boson. This approach focuses on the angles at which the decay products of the all-charm tetraquarks are scattered.

“We call this technique quantum state tomography,” explains CMS team member Chiara Mariotti of the INFN Torino inItaly. “Here, we deduce the quantum state of an exotic state X from the analysis of its decay products. In particular, the angular distributions in the decay X → J/ψJ/ψ, followed by J/ψ decays into two muons, serve as analysers of polarization of two J/ψ particles,” she explains.

The researchers analysed all-charm tetraquarks produced at the CMS experiment between 2016 and 2018. They calculated that J is likely to be 2 and that P and C are both +1. This combination of properties is expressed as 2++.

Result favours tightly-bound quarks

“This result favours models in which all four quarks are tightly bound,” says particle physicist Timothy Gershon of the UK’s University of Warwick, who was not involved in this study. “However, the question is not completely put to bed. The sample size in the CMS analysis is not sufficient to exclude fully other possibilities, and additionally certain assumptions are made that will require further testing in future.”

Gershon adds, “These include assumptions that all three states have the same quantum numbers, and that all correspond to tetraquark decays to two J/ψ mesons with no additional particles not included in the reconstruction (for example there could be missing photons that have been radiated in the decay).”

Further studies with larger data samples are warranted, he adds. “Fortunately, CMS as well as both the LHCb and the ATLAS collaborations [at CERN] already have larger samples in hand, so we should not have to wait too long for updates.”

Indeed, the CMS Collaboration is now gathering more data and exploring additional decay modes of these exotic tetraquarks. “This will ultimately improve our understanding how this matter forms, which, in turn, could help refine our theories of how ordinary matter comes into being,” Mariotti tells Physics World.

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Sound waves can make small objects hover in the air, but applying this acoustic levitation technique to an array of objects is difficult because the objects tend to clump together. Physicists at the Institute of Science and Technology Austria (ISTA) have now overcome this problem thanks to hybrid structures that emerge from the interplay between attractive acoustic forces and repulsive electrostatic ones. By proving that it is possible to levitate many particles while keeping them separated, the finding could pave the way for advances in acoustic-levitation-assisted 3D printing, mid-air chemical synthesis and micro-robotics.

In acoustic levitation, particles ranging in size from tens of microns to millimetres are drawn up into the air and confined by an acoustic force. The origins of this force lie in the momentum that the applied acoustic field transfers to a particle as sound waves scatter off its surface. While the technique works well for single particles, multiple particles tend to aggregate into a single dense object in mid-air because the acoustic forces they scatter can, collectively, create an attractive interaction between them.

Keeping particles separated

Led by Scott Waitukaitis, the ISTA researchers found a way to avoid this so-called “acoustic collapse” by using a tuneable repulsive electrostatic force to counteract the attractive acoustic one. They began by levitating a single silver-coated poly(methyl methacrylate) (PMMA) microsphere 250‒300 µm in diameter above a reflector plate coated with a transparent and conductive layer of indium tin oxide (ITO). They then imbued the particle with a precisely controlled amount of electrical charge by letting it rest on the ITO plate with the acoustic field off, but with a high-voltage DC potential applied between the plate and a transducer. This produces a capacitive build-up of charge on the particle, and the amount of charge can be estimated from Maxwell’s solutions for two contacting conductive spheres (assuming, in the calculations, that the lower plate acts like a sphere with infinite radius).

The next step in the process is to switch on the acoustic field and, after just 10 ms, add the electric field to it. During the short period in which both fields are on, and provided the electric field is strong enough, either field is capable of launching the particle towards the centre of the levitation setup. The electric fields is then switched off. A few seconds later, the particle levitates stably in the trap, with a charge given, in principle, by Maxwell’s approximations.

A visually mesmerizing dance of particles

This charging method works equally well for multiple particles, allowing the researchers to load particles into the trap with high efficiency and virtually any charge they want, limited only by the breakdown voltage of the surrounding air. Indeed, the physicists found they could tune the charge to levitate particles separately or collapse them into a single, dense object. They could even create hybrid states that mix separated and collapsed particles.

And that wasn’t all. According to team member Sue Shi, a PhD student at ISTA and the lead author of a paper in PNAS about the research, the most exciting moment came when they saw the compact parts of the hybrid structures spontaneously begin to rotate, while the expanded parts remained in one place while oscillating in response to the rotation. The result was “a visually mesmerizing dance,” Shi says, adding that “this is the first time that such acoustically and electrostatically coupled interactions have been observed in an acoustically levitated system.”

As well as having applications in areas such as materials science and micro-robotics, Shi says the technique developed in this work could be used to study non-reciprocal effects that lead to the particles rotating or oscillating. “This would pave the way for understanding more elusive and complex non-reciprocal forces and many-body interactions that likely influence the behaviours of our system,” Shi tells Physics World.

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The boundary between a substance’s liquid and solid phases may not be as clear-cut as previously believed. A new state of matter that is a hybrid of both has emerged in research by scientists at the University of Nottingham, UK and the University of Ulm, Germany, and they say the discovery could have applications in catalysis and other thermally-activated processes.

In liquids, atoms move rapidly, sliding over and around each other in a random fashion. In solids, they are fixed in place. The transition between the two states, solidification, occurs when random atomic motion transitions to an ordered crystalline structure.

At least, that’s what we thought. Thanks to a specialist microscopy technique, researchers led by Nottingham’s Andrei Khlobystov found that this simple picture isn’t entirely accurate. In fact, liquid metal nanoparticles can contain stationary atoms – and as the liquid cools, their number and position play a significant role in solidification.

Some atoms remain stationary

The team used a method called spherical and chromatic aberration-corrected high-resolution transmission electron microscopy (Cc/Cs-corrected HRTEM) at the low-voltage SALVE instrument at Ulm to study melted metal nanoparticles (such as platinum, gold and palladium) deposited on an atomically thin layer of graphene. This carbon-based material acted a sort of “hob” for heating the particles, says team member Christopher Leist, who was in charge of the HRTEM experiments. “As they melted, the atoms in the nanoparticles began to move rapidly, as expected,” Leist says. “To our surprise, however, we found that some atoms remained stationary.”

At high temperatures, these static atoms bind strongly to point defects in the graphene support. When the researchers used the electron beam from the transmission microscope to increase the number of these defects, the number of stationary atoms within the liquid increased, too. Khlobystov says that this had a knock-on effect on how the liquid solidified: when the stationary atoms are few in number, a crystal forms directly from the liquid and continues to grow until the entire particle has solidified. When their numbers increase, the crystallization process cannot take place and no crystals form.

“The effect is particularly striking when stationary atoms create a ring (corral) that surrounds and confines the liquid,” he says. “In this unique state, the atoms within the liquid droplet are in motion, while the atoms forming the corral remain motionless, even at temperatures well below the freezing point of the liquid.”

Unprecedented level of detail

The researchers chose to use Cc/Cs-corrected HRTEM in their study because minimizing spherical and chromatic aberrations through specialized hardware installed on the microscope enabled them to resolve single atoms in their images.

“Additionally, we can control both the energy of the electron beam and the sample temperature (the latter using MEMS-heated chip technology),” Khlobystov explains. “As a result, we can study metal samples at temperatures of up to 800 °C, even in a molten state, without sacrificing atomic resolution. We can therefore observe atomic behaviour during crystallization while actively manipulating the environment around the metal particles using the electron beam or by cooling the particles. This level of detail under such extreme conditions is unprecedented.”

Effect could be harnessed for catalysis

The Nottingham-Ulm researchers, who report their work in ACS Nano, say they obtained their results by chance while working on an EPSRC-funded project on 1-2 nm metal particles for catalysis applications. “Our approach involves assembling catalysts from individual metal atoms, utilizing on-surface phenomena to control their assembly and dynamics,” explains Khlobystov. “To gain this control, we needed to investigate the behaviour of metal atoms at varying temperatures and within different local environments on a support material.

“We suspected that the interplay between vacancy defects in the support and the sample temperature creates a powerful mechanism for controlling the size and structure of the metal particles,” he tells Physics World. “Indeed, this study revealed the fundamental mechanisms behind this process with atomic precision.”

The experiments were far from easy, he recalls, with one of the key challenges being to identify a thin, robust and thermally conductive support material for the metal. Happily, graphene meets all these criteria.

“Another significant hurdle to overcome was to be able to control the number of defect sites surrounding each particle,” he adds. “We successfully accomplished this by using the TEM’s electron beam not just as an imaging tool, but also as a means to modify the environment around the particles by creating defects.”

The researchers say they would now like to explore whether the effect can be harnessed for catalysis. To do this, Khlobystov says it will be essential to improve control over defect production and its scale. “We also want to image the corralled particles in a gas environment to understand how the phenomenon is influenced by reaction conditions, since our present measurements were conducted in a vacuum,” he adds.

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Physicists in the UK have succeeded in routing and teleporting entangled states of light between two four-user quantum networks – an important milestone in the development of scalable quantum communications. Led by Mehul Malik and Natalia Herrera Valencia of Heriot-Watt University in Edinburgh, Scotland, the team achieved this milestone thanks to a new method that uses light-scattering processes in an ordinary optical fibre to program a circuit. This approach, which is radically different from conventional methods based on photonic chips, allows the circuit to function as a programmable entanglement router that can implement several different network configurations on demand.

The team performed the experiments using commercially-available optical fibres, which are multi-mode structures that scatter light via random linear optical processes. In simple terms, Herrera Valencia explains that this means the light tends to ricochet chaotically through the fibres along hundreds of internal pathways. While this effect can scramble entanglement, researchers at the Institut Langevin in Paris, France had previously found that the scrambling can be calculated by analysing how the fibre transmits light. What is more, the light-scattering processes in such a medium can be harnessed to make programmable optical circuits – which is exactly what Malik, Herrera Valencia and colleagues did.

“Top-down” approach

The researchers explain that this “top-down” approach simplifies the circuit’s architecture because it separates the layer where the light is controlled from the layer in which it is mixed. Using waveguides for transporting and manipulating the quantum states of light also reduces optical losses. The result is a reconfigurable multi-port device that can distribute quantum entanglement between many users simultaneously in multiple patterns, switching between different channels (local connections, global connections or both) as required.

A further benefit is that the channels can be multiplexed, allowing many quantum processors to access the system at the same time. The researchers say this is similar to multiplexing in classical telecommunications networks, which makes it possible to send huge amounts of data through a single optical fibre using different wavelengths of light.

Access to a large number of modes

Although controlling and distributing entangled states of light is key for quantum networks, Malik says it comes with several challenges. One of these is that conventional methods based on photonics chips cannot be scaled up easily. They are also very sensitive to imperfections in how they’re made. In contrast, the waveguide-based approach developed by the Heriot-Watt team “opens up access to a large number of modes, providing significant improvements in terms of achievable circuit size, quality and loss,” Malik tells Physics World, adding that the approach also fits naturally with existing optical fibre infrastructures.

Gaining control over the complex scattering process inside a waveguide was not easy, though. “The main challenge was the learning curve and understanding how to control quantum states of light inside such a complex medium,” Herrera Valencia recalls. “It took time and iteration, but we now have the precise and reconfigurable control required for reliable entanglement distribution, and even more so for entanglement swapping, which is essential for scalable networks.”

While the Heriot-Watt team used the technique to demonstrate flexible quantum networking, Malik and Herrera Valencia say it might also be used for implementing large-scale photonic circuits. Such circuits could have many applications, ranging from machine learning to quantum computing and networking, they add.

Looking ahead, the researchers, who report their work in Nature Photonics, say they are now aiming to explore larger-scale circuits that can operate on more photons and light modes. “We would also like to take some of our network technology out of the laboratory and into the real world,” says Malik, adding that Herrera Valencia is leading a commercialization effort in that direction.

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An international team of researchers has discovered a new mechanism that can trigger the formation of bubbles in magma – a major driver of volcanic eruptions. The finding could improve our understanding of volcanic hazards by improving models of magma flow through conduits beneath Earth’s surface.

Volcanic eruptions are thought to occur when magma deep within the Earth’s crust decompresses. This decompression allows volatile chemicals dissolved in the magma to escape in gaseous form, producing bubbles. The more bubbles there are in the viscous magma, the faster it will rise, until eventually it tears itself apart.

“This process can be likened to a bottle of sparkling water containing dissolved volatiles that exolve when the bottle is opened and the pressure is released,” explains Olivier Roche, a member of the volcanology team at the Magmas and Volcanoes Laboratory (LMV) at the Université Clermont Auvergne (UCA) in France and lead author of the study.

Magma shearing forces could induce bubble nucleation

The new work, however, suggests that this explanation is incomplete. In their study, Roche and colleagues at UCA, the French National Research Institute for Sustainable Development (IRD), Brown University in the US and ETH Zurich in Switzerland began with the assumption that the mechanical energy in magma comes from the pressure gradient between the nucleus of a gas bubble and the ambient liquid. “However, mechanical energy may also be provided by shear stress in the magma when it is in motion,” Roche notes. “We therefore hypothesized that magma shearing forces could induce bubble nucleation too.”

To test their theory, the researchers reproduced the internal movements of magma in liquid polyethylene oxide saturated with carbon dioxide at 80°C. They then set up a device to observe bubble nucleation in situ while the material was experiencing shear stress. They found that the energy provided by viscous shear is large enough to trigger bubble formation – even if decompression isn’t present.

The effect, which the team calls shear-induced bubble nucleation, depends on the magma’s viscosity and on the amount of gas it contains. According to Roche, the presence of this effect could help researchers determine whether an eruption is likely to be explosive or effusive. “Understanding which mechanism is at play is fundamental for hazard assessment,” he says. “If many gas bubbles grow deep in the volcano conduit in a volatile-rich magma, for example, they can combine with each other and form larger bubbles that then open up degassing conduits connected to the surface.

“This process will lead to effusive eruptions, which is counterintuitive (but supported by some earlier observations),” he tells Physics World. “It calls for the development of new conduit flow models to predict eruptive style for given initial conditions (essentially volatile content) in the magma chamber.”

Enhanced predictive power

By integrating this mechanism into future predictive models, the researchers aim to develop tools that anticipate the intensity of eruptions better, allowing scientists and local authorities to improve the way they manage volcanic hazards.

Looking ahead, they are planning new shear experiments on liquids that contain solid particles, mimicking crystals that form in magma and are believed to facilitate bubble nucleation. In the longer term, they plan to study combinations of shear and compression, though Roche acknowledges that this “will be challenging technically”.

They report their present work in Science.

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