Page 361 – Physics World

Home » Page 361

Citations in science are biased towards a handful of nations – and the gap is growing

Scientists from a handful of “core” countries – including China, the US and the UK – tend to be cited much more than those working in “periphery” nations. That’s the conclusion of an analysis by sociologists in the US, who find that this citation gap is largest in the physical sciences – and that it is growing across all scientific fields. This inequality and lack of diversity in geographical spread of science could impact the spread of knowledge and the growth of new ideas, the researchers warn.

The study has been conducted by a team led by Charles Gomez from the City University of New York, who examined around 20 million academic papers across nearly 150 fields published between 1980 and 2012. They analysed the article’s text and citations to see how the number of references a paper receives deviates from what would be expected based on comparisons with other academic publications on similar topics. Known as “citational lensing”, this approach reveals how citations vary between authors in different countries and over time.

The team found that researchers in a small group of highly active countries – which includes Australia, China, Germany, Japan, the UK and the US – are more likely to be cited than scientists in other nations around the world. These “over-cited” scientists end up receiving more citations than academics in other countries who work on similar topics. This citation gap is biggest in the physical and mathematical sciences, followed by engineering and computational sciences.

Some of the greatest breakthroughs have happened by accident, so it is dangerous to potentially be excluding other voices
Charles Gomez

The results show that over the 30 years examined, academics in the core nations became more and more over-cited, while researchers in periphery countries – such as Brazil, Mexico and Turkey – are increasingly cited less than would be expected. Gomez told Physics World that the citation gap is “growing quite rapidly” and adds that much of the citation bias is occurring because scientists within the core countries over-recognize each other. He describes this as being similar to a “rich get richer” effect, explaining that once you are at the top the inequality grows.

Growth of China

The other major trend uncovered in the study is the rise of China as a research superpower. In the 1980s and 1990s China was an under-cited country but by the 2000s its academics were more likely to be cited than those from other countries. This was particularly the case in physics and mathematics as well as in engineering and computational sciences, with citations in these fields from China overtaking western European countries.

China overtakes the US in terms of research quality, finds study

In other fields, scientists in China have caught up with their contemporaries in the core countries, though over the period studied, the US still dominates across all scientific fields. “It is important that global science is equitable and inclusive,” says Gomez, who adds that this is not just because science should be welcoming, but also because science works by serendipity. “Some of the greatest breakthroughs have happened by accident, so it is dangerous to potentially be excluding other voices,” he explains. “It is detrimental to science’s own progress.”

Mirror world of dark particles could explain cosmic anomaly

A long-standing disparity between different measurements of the cosmic expansion rate might be explained, at least in part, by the existence of a “mirror world” containing copies of all known particles. That is the conclusion of three physicists in the US, who have shown how to reconcile contrasting values of the Hubble constant obtained from observations of the nearby and distant universe. Their scheme involves recalibrating the size of the universe without altering other cosmic parameters, but they caution that more research is needed to understand exactly how light and matter interacted at the dawn of time.

The Hubble constant is one of the most important parameters defining cosmic evolution. It tells us how quickly galaxies fly apart from one another – and therefore how fast the universe is expanding. For years, however, its value has been in dispute. Measurements of distances and speeds of objects in the local universe consistently yield a higher value than those that infer the expansion rate from cosmological data. Most notably the cosmological value inferred from fluctuations in radiation known as the cosmic microwave background (CMB), which was generated when electrons and protons combined to form neutral hydrogen atoms a few hundred thousand years after the Big Bang.

That disparity has now reached a statistical significance of 5σ, which means that the disagreement is unlikely to be a chance fluke. Recently, Adam Riess at the Space Telescope Science Institute in Baltimore, US, and colleagues have established a Hubble-constant value of 73.04 ± 1.04 km/s per megaparsec (Mpc) via local observations. Meanwhile, data from the CMB, courtesy of the European Space Agency’s Planck satellite, instead yield 67.49 ± 0.53 km/s/Mpc.

This disagreement could potentially be explained away by the existence of systematic errors in the measurements, but no such errors have been identified. As a result, many researchers are looking for holes in cosmological theory and new physics.

Rescaling parameters

In the latest work, Francis-Yan Cyr-Racine of the University of New Mexico and Fei Ge and Lloyd Knox of the University of California, Davis propose a new strategy for revising the standard cosmological model while still retaining consistency with the numerous planks of observational evidence supporting it. Their idea is to rescale certain parameters to bring the data from the CMB in line with that from local observations of the Hubble constant, but to do so while preserving cosmological observables’ internal consistency or “symmetry”. These observables include all distance ratios, CMB temperature anisotropies and CMB polarization.

As the trio points out, a similar idea had already been put forward by Oliver Zahn and Matias Zaldarriaga back in 2003. But that work involved rescaling just one parameter – Newton’s gravitational constant – and was unable to preserve symmetry in certain cases such as small angular scales. The new research relies on rescaling at all times both the cosmic expansion rate and the rate of photon scattering (crucial for the generation of the CMB), which in turn rescales the lengths and timescales needed to shift the Hubble constant. In this way, say Cyr-Racine and colleagues, symmetry breaking is zero under the ideal conditions of equilibrium recombination and zero neutrino mass while it is “mild” in real-world conditions.

One straightforward way of rescaling the universe’s expansion rate would be to increase the energy density of all particles in the cosmos, both matter particles and force carriers. Doing so, however, would also affect the CMB’s mean energy density, which has been established very precisely using data from NASA’s COBE satellite. Instead, the researchers propose that the universe contains what is known as a mirror world dark sector.

Dark particles

Already extensively studied by particle physicists looking to understand why gravity appears so much weaker than the other known forces, the mirror world would contain copies of all existing fundamental particles. These would interact with each other through “mirror” versions of the known force particles, albeit having different masses and coupling strengths. The existence of both “dark baryons” and “dark photons” would allow for a higher matter density while simultaneously preserving the well-measured baryon-to-photon ratio and remaining consistent with the COBE data (which would otherwise be contradicted by too many visible photons). The presence of “dark neutrinos” would in turn conserve the existing proportions of free-streaming to tightly-coupled particles.

Doing the sums, the trio concludes that this mirror world could deliver the re-scaling needed to eradicate the Hubble constant inconsistency. However, they have not so far enjoyed similar success when it comes to rescaling the rate of photon scattering. They say that this could in principle be done by altering the ratio of helium to hydrogen but have found that the revised number conflicts with the amount of helium and deuterium that is thought to have existed in the early universe.

They point to some possible solutions to this problem. Boosting the photon scattering rate, they say, could perhaps be achieved either by altering the spectrum of photons in the high-energy tail of the CMB or by stipulating that electrons’ mass varies in time. The abundances of light elements, on the other hand, could conceivably be modified by introducing new kinds of interaction between the light and dark sectors.

But the researchers do not intend to try and overcome this obstacle on their own. As they put it, they have “provided clear model-building targets for the community to explore”.

Radek Wojtak, a cosmologist at the University of Copenhagen in Denmark, says that the new proposal bears a “quite intriguing” resemblance to conformal transformations – the idea that the laws of physics remain fixed even when units are changed. But he argues that introducing a new transformation and a new species of particle to solve just a single anomaly is “quite a big investment”. He is also concerned about the absence of any new perspective on dark energy, which, he says, “is currently the most challenging part of the standard cosmological model”.

The research is described in Physical Review Letters.

Can machine learning deliver one-minute brain MRI scans?

With a few enhancements, including machine learning, a quantitative technique called MR fingerprinting could make a one-minute clinical brain MRI scan a reality, according to a talk presented at the recent International Society for Magnetic Resonance in Medicine (ISMRM) meeting in London.

Researchers from Stanford University developed an MR fingerprinting acquisition and reconstruction framework for quantitative and multicontrast imaging that requires a scanning time of approximately one minute and a reconstruction time of as little as five minutes.

With help from a machine-learning algorithm for image synthesis, the method can provide five high-quality images with common clinical contrasts at 1-mm isotropic resolution, as well as quantitative T1, T2 and proton density maps, according to presenter Sophie Schauman and colleagues.

The ISMRM meeting was held in conjunction with the European Society for Magnetic Resonance in Medicine and Biology and the International Society for MR Radiographers and Technologists.

Room for improvement

There’s certainly room to speed up MRI. Traditional MRI operates on k-space data, thus enabling quick reconstruction using standard parallel imaging methods, according to Schauman.

However, “scan times are long, and thick slices are often acquired to overcome this,” she said. “Most clinical MRI scans are T1- or T2-weighted. Thus, the image contrast is qualitative and not quantitative.”

Modern, highly undersampled acquisition methods can reduce scan times drastically, as well as encode tissue properties in a quantitative manner. However, these faster acquisition times often come at the cost of a longer reconstruction time, rendering these techniques impractical in clinical settings, according to Schauman.

“In order to translate modern MRI into clinically useful tools, we need fast acquisition, we need faster reconstruction, and we need flexibility in acquiring both [the] contrasts that are useful for clinicians as well as quantitative imaging that can be used, for example, in longitudinal studies,” she said.

MR fingerprinting

The researchers turned to MR fingerprinting to pursue this goal. MRI fingerprinting is a quantitative technique that allows simultaneous measurement of multiple tissue properties in a single data acquisition.

In their project, the Stanford researchers used a tiny golden-angle shuffling multi-axis spiral projection MR fingerprinting sequence. This method yields 1-mm isotropic resolution for the whole brain, yet this isn’t currently feasible for use due to its need for more than four hours of reconstruction time.

In an effort to make MR fingerprinting an even more promising method for clinical settings, the researchers sought to incorporate a fast reconstruction method, Schauman said. They used a subspace reconstruction technique that takes approximately seven minutes to perform and involves three subspace components – instead of the usual five that are used – and three coils.

Speed it up: Timeline analysing the MR fingerprinting acquisition and reconstruction protocol, which takes approximately 6 min (of which the patient is required to be still for 1 min), compared to the time required to acquire conventional contrasts (approximately 28 min). The image shows five synthesized contrasts in the test subject using the proposed method (row 1), a much slower reconstruction (row 2), a simple model-based synthesis (row 3) and the reference scans (row 4). (Courtesy: Sophie Schauman and ISMRM)

The researchers then used machine learning-based synthesis to further improve scan quality and speed. To train the algorithm, they used data contributed from 14 healthy volunteers. Of the 14 subjects, 10 were used for training, two were used for validation, and two were used to test the model – a previously proposed generative adversarial network.

“To improve robustness of the pipeline in the clinic, a 30-second large field-of-view prescan was included,” Schauman said. “In future work, we intend to use the prescan for B0 and B1 estimation, but for now, we use it to optimize our coil compression to suppress signal outside the field of view using a method called [region-optimized virtual (ROVir) coils] and also automatically apply shifts to the data [to] ensure that the brain was centred in the field of view.”

Compared to images reconstructed using the traditional technique that takes four hours, the fast reconstruction method has more undersampling artefacts, more blurring and more noise, Schauman said.

“However, if this information can be recovered in the synthesis network, all of this doesn’t matter at all,” she said.

In the two test subjects, the synthesized T1-weighted magnetization-prepared rapid acquisition gradient-echo (MP-RAGE), T2-weighted, T2 fluid-attenuated inversion recovery (FLAIR) and double inversion recovery (DIR) images had highly similar slicewise structural similarity indexes compared with synthesized images produced from the reference reconstruction technique.

“Future directions of the project include continued clinical data collection, with the aim to include patients in the training data set using semisupervised methods and improved robustness of the pipeline regarding patient positioning in the field of view,” Schauman said. “We also aim to further optimize the time/quality trade-off by acquiring faster B0 and B1 maps for calibration of quantitative imaging.”

This article was originally published on AuntMinnieEurope.com ©2022 by AuntMinnieEurope.com. Any copying, republication or redistribution of AuntMinnieEurope.com content is expressly prohibited without the prior written consent of AuntMinnieEurope.com.

Superposition and entanglement flee the quantum nest

When a forest loses its trees, it also loses its identity as a forest (as well as creating an ecological disaster). Similarly, quantum theory would be a different beast altogether without the defining effects of superposition and entanglement. However, the converse is not necessarily true. The two physical effects can be observed independently of the theoretical framework used to explain them, and an international team of researchers has now shown that the connection between them does not hinge on quantum theory’s mathematical formalism either. In practical terms, the result opens the door to realizing quantum cryptography even if quantum theory proves incorrect.

Quantum theory is one of the most successful theories in physics to date. Despite its non-intuitive nature, its predictions have consistently agreed with empirical observations. Over time, emerging applications such as quantum computing and quantum cryptography have received considerable interest from industry. However, it remains a challenge to reconcile it with gravity, which suggests that quantum theory may be superseded or overthrown by another more complete theory in the future. This is a natural process in scientific evolution, where falsified theories are discarded and the paradigm shifts as a result.

Diagram showing a pair of electron spins (representing entanglement), a double slit pattern (representing superposition) and a lock with a key (representing quantum cryptography), each connected to the other via thick ropes — Researchers have found a new connection between entanglement (top), superposition (left) and quantum encryption (right). (Courtesy: Ludovico Lami)

Although entanglement and superposition are widely observed in the lab, it was the mathematical formalism provided by quantum theory that previously explained how the two concepts were related. This left their exact relationship prone to future corrections.

General probabilistic theories

In the latest work, which is described in Physical Review Letters, the researchers proved that in any physical theory, entanglement can exist between different systems if and only if superposition can exist in each of them. The result yields an equivalency between the two concepts that extends beyond the quantum realm.

To show that the connection between superposition and entanglement holds in any physical theory, the researchers used a framework called general probabilistic theories. This framework provides a general mathematical description for the key requirements of a physical theory: physical states, their transformations and measurements. The framework encompasses both classical and quantum theory. More importantly, it also includes more exotic theories that exhibit typically quantum features such as superposition and entanglement. For example, in one such non-classical theory, known as “Boxworld”, the non-local correlations that characterize entanglement can be so strong that they surpass what is allowed by quantum theory.

Quantum cryptography

The link between this result and quantum cryptography is that the latter uses features of quantum theory to provide secure information transfer between two parties. A notable example is the textbook protocol BB84, which uses entanglement to ensure that any hacker can be detected and excluded from the communication. If this protocol were to become widely adopted, only for its unhackability to be overturned by a new theory, catastrophic consequences could ensue. For instance, vast swathes of sensitive data, such as credit card details, could be compromised by evil actors.

The novel theory-independent connection implies that this apocalyptic depiction is unlikely to become reality, as the researchers show that the protocol can be realized according to any non-classical theory. In a press release coinciding with the publication of the research, Ludovico Lami, a co-author of the paper and a physicist at the University of Ulm, Germany, stated: “It is somehow reassuring to know that cryptography is really a feature of all non-classical theories, and not just a quantum oddity, since many of us believe that the ultimate theory of nature will likely be non-classical.”

Rare X-rays from white-dwarf explosion spotted by chance

Astronomers have made the first observations of X-rays being emitted from a white-dwarf star that has burst into life while stripping material from a companion star. The process caused a thermonuclear runaway that resulted in a massive explosion or nova.

Ole König a PhD student at the Dr Karl Remeis-Observatory and Erlangen Centre for Astroparticle Physics at Friedrich-Alexander-University, Germany and colleagues used the eROSITA telescope to observe the tell-tale X-rays that signal the onset of the violent process. The X-rays were emitted by the white dwarf YZ Reticuli 11 h, which is located over 8800 light-years from Earth. Remarkably, the observation was made by chance – but previous research allowed the scientists to understand what they had seen.

“We were quite lucky that eROSITA was pointed to the right patch of the sky at the right moment,” König, lead author of a Nature paper describing the discovery, tells Physics World. “For over 30 years it has been postulated that novae should show X-ray flashes. These flashes are a direct consequence of the fusion process, a so-called thermonuclear runaway, that occurs on the surface of the white dwarf.”

Brief emission

König explains that X-rays from such an event should be emitted briefly, just before the white dwarf brightens with visible light. The spectral distribution of these X-rays is expected to resemble that of a black body. The team found that there was no X-ray source detected by eROSITA both four hours before and four hours after the event meaning the bright flash must have lasted less than six hours. This is in good agreement with theoretical models, and this told the team they had indeed spotted an X-ray flash from the nova explosion for the first time.

König explains why this kind of X-ray emission is only just being glimpsed for the first time. “It is very difficult to find such a source because there are only around 40 to 50 novae per year in our galaxy and most of them are from sources that are not known before they are seen in the optical wavelengths,” he says. “At this time the X-ray flash is, however, already over and this is the reason why it took so long to discover them.”

Despite fitting well with theoretical predictions, the astronomers’ fortuitous discovery still yielded some surprises. “The most surprising thing was that the source was so bright. Too bright actually,” König says. “It overexposed the central part of the detector because so many X-ray photons arrived in such a short time.”

This meant the researchers had to come up with a clever way to analyse the overexposed data , which was done using a simulator called SIXTE, designed specifically to study bright sources with eROSITA.

Kick-started fusion

White dwarfs are the remnants of stars like the Sun. These objects have burned all the hydrogen in their cores, but they do not have mass needed to kick-start the fusion of heavier elements like carbon and nitrogen. As a result, isolated white dwarfs spend the rest of their existence as a gradually cooling inert exposed core of mostly carbon.

However, white dwarfs can spring back into life, albeit briefly, when in a close-proximity binary system with a star that donates matter to the white dwarf. This material, mostly hydrogen, is stripped from the donor and accreted onto the white dwarf’s surface. As this happens immense pressure builds beneath this donated envelope of material. Because the gas is forced into a state in which quantum properties set the pressure, the accreted hydrogen cannot expand despite the temperature increasing.

As the temperature rises in this region, it triggers thermonuclear reactions. Fusion rates increase rapidly as the temperature builds and the reactions run out of control. The white dwarf then blows off much of the accreted layer —which is estimated to have a temperature of several million degrees — reliving the pressure and expanding its radius.

‘Fireball’ created

“The X-rays are emitted as a consequence of the thermonuclear runaway. The fusion process creates a lot of energy that diffuses through the envelope of mostly hydrogen,” König says. “When it reaches the top of the envelope the nova is a very hot glowing black body or a ‘fireball’.”

“This black body emits radiation and it peaks in the soft X-rays. The explosion is a very, very dynamic situation. The envelope quickly expands and the temperature drops. Thus, the source becomes visible in the optical a few hours after the X-ray flash.”

The explosion that results is often called a “recurrent nova” because the white dwarf is left intact, unlike in “classic” supernova explosions, and can continue to accrete material from its donor star companion allowing the process to occur over again.

“There are certainly still questions left to answer about these novae. For example, how the X-ray flash decays or how fast the envelope expands,” König explains. “It would be great to go through the eROSITA dataset or propose another satellite with higher spatial resolution and sensitivity to find more of these flashes.”

Microcavity could make a platform for quantum sensing

The motional fluctuations of nanoparticles trapped between mirrors in optical cavities could form the basis for a new type of high-precision quantum sensor, according to calculations by researchers at the Austrian Academy of Sciences, the University of Innsbruck in Austria and ETH Zurich in Switzerland. The team’s theoretical work shows that the dynamical instabilities of a quantum system can be exploited as a resource rather than being considered as a problem to be avoided, as is usually the case.

Optical cavities, or resonators, are structures in which light is reflected back and forth between two or more mirrors. This light interacts with nanoparticles trapped in the cavity, creating dynamical instabilities that are usually considered undesirable for sensing applications.

In the new work, researchers led by Romain Quidant of ETH Zurich and Oriol Romero-Isart of the Institute of Quantum Optics and Quantum Information (IQOQI) and the University of Innsbruck’s department of theoretical physics showed that by properly controlling these instabilities, the unstable dynamics within the optical resonator can be used to quickly and strongly “squeeze” the motion of nanoparticles levitating in the cavity. This squeezing significantly reduces the motional fluctuations of the nanoparticles trapped in the optical resonator to below their so-called “zero-point” motion.

While optical cavities are routinely used to cool the motion of levitated particles via successive momentum “kicks” from reflected photons, these structures need to be relatively large to keep a photon inside the cavity for longer than the oscillation period of a nanoparticle. However, Romero-Isart explains that if the goal is not to cool the nanoparticles’ motion, but to squeeze it, the requirement is different. Here, much like a balloon, the nanoparticles are squeezed in one direction in position-momentum space, which creates a bulge in the other direction. This generates stronger light-nanoparticle coupling, which can be achieved using smaller cavities.

The researchers applied their theoretical approach to a model system consisting of levitated silica nanoparticles coupled to a microcavity. Squeezing the motion of particles is helpful, Romero-Isart says, because it makes them more sensitive to external signals – something that can be exploited for inertial and force sensing. Optical resonators therefore offer a new platform for the design of quantum sensors that could be used, for example, in satellite missions, self-driving cars and in seismology, to name but three examples.

According to the researchers, the study is timely because other research groups have recently demonstrated ground-state cooling and quantum control of optically-levitated nanoparticles in free space in laboratory experiments. The team is now looking into how to prepare macroscopic quantum superpositions of a nanoparticle. “This means preparing it in a state in which its position is delocalized over length scales comparable to the size of the nanoparticle,” Romero-Isart tells Physics World.

The present research is detailed in Physical Review Letters.

Artificial intelligence beats humans at crossword solving and assesses the health of ‘singing’ coral reefs

Many people around the world enjoy doing crosswords, taking a few moments out of their day to test their knowledge and wordplay by answering clues to find words that complete the crossword grid. Crosswords are a puzzle that humans particularly excel at, so much so that we can outperform machines — or so we thought.

One of the most prestigious crossword competitions is the annual American Crossword Puzzle Tournament and the best that a machine could do in 2017 was 11th place thanks to a programme called Dr Fill. But machines are now making a comeback and in 2021 the Berkeley Crossword Solver – built by a team at the University of California, Berkeley – managed, for the first time, to outscore every human player.

The programme, which is outlined in a paper on arXiv, is based on “neural question answering, structured decoding, and local search,” which involves finding solutions to questions and then using databases and natural language artificial intelligence (AI) to refine the answers. And despite such progress, there is still room for improvement. The Berkeley Crossword Solver has an “exact puzzle accuracy” of 82% for crossword puzzles set by The New York Times – compared to 57% in previous incarnations. Crossworders beware.

Singing coral reefs

Another novel application for AI that we came across this week is using the technology to assess the health of coral reefs. An international team led by researchers at the UK’s University of Exeter and Lancaster University have used AI to analyse the sounds emitted by reefs. These “songs” are made by the rich variety of fish and other creatures that live on the reef – and the presence or absence of these organisms is related to the health of the ecosystem.

The team trained their AI system using audio recordings made at the Mars Coral Reef Restoration Project in Indonesia. The recordings were from both healthy and damaged coral reefs and once trained, the system was able to identify the health of a reef 92% of the time.

“Coral reefs are facing multiple threats including climate change, so monitoring their health and the success of conservation projects is vital,” says Exeter’s Ben Williams. He adds that assessing the health of a reef using conventional visual and audio techniques can be labour intensive.

“Our findings show that a computer can pick up patterns that are undetectable to the human ear. It can tell us faster, and more accurately, how the reef is doing,” adds Williams.

Lancaster’s Tim Lamont adds, “In many cases it’s easier and cheaper to deploy an underwater hydrophone on a reef and leave it there than to have expert divers visiting the reef repeatedly to survey it – especially in remote locations”.

The research is described in Ecological Indicators.

Desktop air curtain could block viral spread in hospitals

Medical procedures such as collection of blood samples or intubation, for example, require doctors and other healthcare staff to work in close proximity to the patient. To protect them from exposure to infections in such scenarios, a team at Nagoya University in Japan has developed a desktop air curtain system (DACS) that blocks emitted aerosol particles and prevents potential spread of viruses such as SARS-CoV-2.

The DACS contains a generator at the top that produces a steady airflow, which is then guided to a suction port at the bottom of the device, effectively creating a smooth curtain of air. As this integrated system contains both a discharge and a suction port, it can be installed at any location, and is portable enough to be placed on a desk. A high-efficiency particulate air (HEPA) filter inside the suction port can provide air purification.

“We envisage this system will be effective as an indirect barrier for use in blood-testing labs, hospital wards, and other situations where sufficient physical distance cannot be maintained, such as at a reception counter,” says first author Kotaro Takamure in a press statement.

To assess the potential of using the DACS in a medical environment, Takamure and colleagues performed a series of experiments using a set-up replicating a blood-collection booth. First, they used particle image velocimetry (PIV) and a hot-wire anemometer to evaluate the velocity field of the air curtain. The measurements confirmed that the flow rate of the air curtain generated by the DACS is maintained from the discharge port up to the suction port.

Next, the team used an air compressor connected to a mannequin to simulate human exhalation. A tube at the mouth of the mannequin blew out air containing aerosol particles (2–3 µm-diameter particles of the solvent dioctyl sebacate) towards the air curtain at a flow rate of 52 l/min. The distance from the air outlet to the centre of the DACS was 250 mm.

With the DACS switched off, PIV measurements showed that the emitted aerosol particles diffused as they moved forward and travelled straight through the gate of the DACS to the other side. The particles had maximum velocity immediately after ejection from the mannequin’s mouth and then gradually slowed.

When the DACS is operational, the researchers observed similar initial behaviour. However, when the aerosol particles approached the gate, they were bent abruptly downward along with the air curtain flow and were eventually sucked into the suction port, with none passing through the gate.

Blocking aerosol particles: Average velocities of aerosol particles with the DACS switched off (left), switched on (centre) and operating with an arm placed on the gate (right). (Courtesy: CC BY 4.0/*AIP Advances* 10.1063/5.0086659)

The researchers then investigated a scenario mimicking the use of the DACS during blood collection, with the arm of the mannequin resting on the gate. They saw that arm disrupted the curtain’s airflow, creating turbulent flow nearby. The aerosol blocking performance, however, was unaffected. Statistical evaluations revealed that even with the arm on the gate, no aerosol particles reached the other side of the air curtain, demonstrating effective particle blocking even in the presence of turbulence.

The team is now also integrating a virus inactivation system into the DACS, using UV LEDs connected to the suction port. The UV irradiation destroys the outer coat of virus particles; the sanitized air can then be recirculated to maintain airflow of the air curtain. Laboratory tests revealed that the combination of the air curtain with UV irradiation inactivated 99.9% of SARS-CoV-2 particles.

“Although acrylic sheets are currently widely used as partitions, our air curtain not only blocks, but also deactivates, viruses,” says co-author Tomomi Uchiyama. “Therefore, we expect this device to render acrylic partitions obsolete and become widely used.”

Takamure says that the group’s future goal is to develop a compact and lightweight virus inactivation device. “If we can achieve miniaturization without compromising virus inactivation performance, we expect the device to be more versatile,” he tells Physics World.

The DACS is described in AIP Advances.

Ultrathin nanowires could be a boon for error-resistant quantum computing

Researchers have fabricated ultrathin semiconductor-superconductor hybrid nanowires measuring less than 20 nm across. Such wires are thinner than those grown previously and are predicted to host phenomena known as Majorana zero modes – the core ingredient of so-called topological quantum bits (qubits), which could form the basis of a stable and error-resistant quantum computer.

Originally, Majorana zero modes (MZMs) were simply a mathematical construction that allowed an electron to be described theoretically as being composed of two halves. From a quantum computing perspective, they are attractive because if an electron can be “split” in two, the quantum information it encodes will be protected from local perturbations as long as the “half-electrons” can be stored far away from each other. According to theory, these entities should appear in a set-up consisting of a semiconducting nanowire wrapped in a shell made from a superconducting material and placed in a magnetic field.

In theory, the simplest type of nanowire in which MZMs should appear is a one-dimensional electron system – that is, one in which electrons occupy a single electronic sub-band in the semiconductor. In experiments, however, multiple sub-bands are occupied.

The ultrathin semiconductor-superconductor hybrid nanowires and transport characteristic measurements. (Courtesy: H Zhang)

Diameter of less than 20 nm

In a new study, researchers led by Jianhua Zhao and Dong Pan of the State Key Laboratory of Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, grew ultrathin nanowires of the semiconductor indium arsenide (InAs) covered with an in situ epitaxial superconducting aluminium (Al) film using a technique called molecular-beam epitaxy (MBE). They used a silver (Ag) catalyst to grow the wires – a technique routinely employed in this type of experiment. The new nanowires have a diameter of less than 20 nm, which is five times smaller than semiconductor nanowires previously grown using this approach.

The diameter of the of the wires depends on the diameter of the Ag catalyst, and Zhao explains that very small Ag catalysts (ranging from 5 to 40 nm) can be prepared using the team’s MBE system. The crystal quality of the wires also depends on their diameter and the wires grown in the new study are of high quality.

New avenue for future MZMs searches

“When combined with Al superconducting films, these ultrathin wires offer a possible way of reaching the fewer sub-band regime (and ultimately the single sub-band regime),” Hao Zhang of Tsinghua University, who led the electron transport measurements in the work, tells Physics World. “These wires therefore open a new avenue for exploring fewer sub-band regimes for future MZMs searches.”

Thanks to basic transport characteristic measurements, the researchers have already discovered two phenomena in their system: a “hard” superconducting gap in tunnelling spectroscopy measurements; and a “parity preserving Coulomb blockade” in so-called hybrid island devices. Both phenomena are crucial ingredients for future Majorana searches, Zhang explains.

The team says it is now looking for stronger evidence for MZMs by measuring the quantum transport properties of its ultrathin InAs-Al nanowire structures.

The work is detailed in Chinese Physics Letters.

Experiments with quantum cause and effect reveal hidden nonclassicality

Cause-and-effect explanations like “catnip causes cats to be happy”, “jokes cause laughter” and “exciting research causes Physics World articles” are a useful way to organize knowledge about the world. The mathematics of cause-and-effect underpin everything from epidemiology to quantum physics. In the quantum world, however, the link between cause and effect is not so straightforward. An international team of physicists has now used quantum violations of classical causality to better understand the nature of cause-and-effect. In the process, the team uncovered quantum behaviour in a situation where standard methods indicate that the system ought to be classical – a result that could have applications in quantum cryptography.

In quantum physics, a result known as Bell’s theorem states that no theory that incorporates local “hidden” variables can ever reproduce the correlations between measurement outcomes that quantum mechanics predicts. A similar result occurs in the theory of causal inference, where quantum systems likewise defy the rules of classical causal reasoning. The idea behind the causal inference approach is that while a statistical correlation between two variables can arise due to a direct causal relationship between them, the correlation may also contain the contribution of a hidden common cause. In some cases, this hidden contribution can be quantified, and this can be used to show that quantum correlations do exist even when Bell’s theorem cannot be violated.

Inferring causal structure achieves direct control over cause and effect

In the latest work, a team led by experimental physicist Davide Poderini and colleagues in Brazil, Germany, Italy and Poland combine theory and experiment to show quantum phenomena in a system that would otherwise appear classical. The researchers explore the notion of cause and effect by considering whether correlations between two variables, A and B, imply that one is the cause of the other, or whether some other (potentially unobserved) variable may be the source of the correlations.

In their investigation, the researchers use a causal model (see image) in which the statistics of variable A influence those of variable B, either directly or by the action of a common source (called Λ) that connects the outcome of both variables even without the presence of a causal link between them. To distinguish between these two scenarios, the researchers perform an intervention on variable A that erases any external influences. This leaves the variable A under the experimenter’s complete control, making it possible to estimate the direct causal link between A and B.

A directed acyclic graph of various cause-and-effect scenarios — **How it works:** A directed acyclic graph of the instrumental scenario. Circular nodes represent observable variables, while triangular nodes represent hidden ones. a) Direct causal influence can be difficult to estimate between variables A and B in the presence of a hidden factor or common cause Λ affecting both of them. b) An instrumental variable X is therefore introduced to bound variable A as a direct influence on B. c) Variable A is left under the experimenter’s control, removing the influence of the common factor Λ, and making it possible to estimate the direct causal link to B. (Courtesy: Davide Poderini/Sapienza University of Rome)

Alternatively, by introducing an additional variable X that is independent of B and Λ, any observed correlations between variables A and B can be decomposed into conditional probabilities. These conditional probabilities place a lower bound on the degree of causal effect between the variables, making it possible to estimate the level of influence between A and B.

The researchers call this lower bound an instrumental inequality, and it is a classical constraint that (similar to the inequality that arises from Bell’s theorem) stems from imposing this causal structure on an experiment. As a result, the degree of quantum causal influence between variables A and B will be less than the minimum required for a classical system, allowing nonclassicality to be observed through an intervention even when no Bell inequality is violated.

Experimental intervention reveals quantum effects

To observe the instrumental causal process, the researchers generated pairs of photons with entangled polarizations and measured them in different representations of state space, or bases. Thanks to photons’ entangled nature, the choice of basis for one is determined by the measurement on the other, producing a “feed-forward” mechanism that implements a direct causal link between the two variables. As a result of this feed-forward process, the researchers experimentally observe violations of the classical lower bounds for causal influence between two variables by producing several quantum states characterized by different degrees of entanglement.

Like Bell’s inequality, violation of this classical lower bound represents a signature of quantum correlations. Furthermore, it yields statistical data that can act as the foundation of any basic quantum cryptographic protocol. While current cryptographic protocols rely on Bell’s theorem, inferring causal structure from instrumental intervention represents a more general compatibility between classical causality and quantum theory. Poderini and his colleagues seek to experiment with different causal scenarios to explore complex networks with richer correlations, which can be exploited to develop novel quantum technologies. The researchers believe their experimental techniques could lead to quantum advantages in cryptographic protocols, making it possible to realize more resilient and less technologically demanding cryptographic tools.

Browse articles by content type