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Connecting the dots to artificially restore vision

A team of researchers from the Ecole Polytechnique Federale de Lausanne has developed a retinal implant that transposes images acquired by camera-equipped smart glasses into a simplified, black and white image made from 10,500 pixels. Although it has not been approved for human trial yet, the team has tested the implant in both a mouse model and a dedicated virtual reality programme, reporting the findings in Communications Materials.

For many patients suffering from retinitis pigmentosa – an inherited disease where progressive loss of retinal photoreceptors eventually leads to blindness – current retinal implants do not provide clear benefits. In fact, three years after surgery, most patients have stopped using them.

Two limiting parameters are often cited as the reason for the interruption: a small field vision angle (usually limited to 20°) and coarse visual resolution (less than 100 pixels in the most commonly used implant). These require the patient to constantly scan their environment to recreate a mental map of their surroundings, which is impractical and cognitively exhausting.

One electrode: one pixel

To tackle these limitations, Diego Ghezzi and his team developed POLYRETINA, a wide-field high-density epiretinal prosthesis that can be implanted at the back of the retina, close to the optic nerve. The implant contains 10,498 photovoltaic pixels (80-µm diameter, 120-µm pitch) distributed in a tiled fashion over a 13 mm-diameter active area, and provides a 43° vision angle.

A camera embedded in the smart glasses captures images in the wearer’s field-of-vision and sends the data to a microcomputer placed in one of the glasses’ end-pieces. The data are then turned into light signals that are transmitted to the 10,498 electrodes of the retinal implants, creating a star-spangled-sky-like version of the image.

The team conducted a battery of tests to ensure that the implant was fit for purpose. Combining conjugated polymers and less rigid substrates, for example, allowed for a wider coverage of the retinal surface. However, the main question was how many electrodes the prosthesis should contain: a small number would not significantly improve resolution compared with existing implants; a large number increases risks of crosstalk with neighbouring pixels.

By firing combinations of pixels of increasing pattern complexity, the researchers confirmed that even when using 10,498 electrodes, the voltage generated by each pixel is sharply discriminated from its neighbouring pixels and does not show a voltage summation effect. This was observed even in the most extremes cases where a central pixel is off while the surrounding eighteen pixels are on.

Retinal prosthesis
The prosthesis contains 10,498 photoreceptors situated 120 µm apart from each other. (Credit: CC BY 4.0/Commun. Mater. 10.1038/s43246-021-00133-2)

Virtual reality while waiting for human trials

The researchers performed further experiments ex vivo on a mouse model of retinitis pigmentosa and showed that each electrode could reliably produce a dot of light in the retina.

“We wanted to make sure that two electrodes don’t stimulate the same part of the retina. So we carried out electrophysiological tests that involved recording the activity of retinal ganglion cells [a type of neuron at the inner surface of the retina]. And the results confirmed that each electrode does indeed activate a different part of the retina,” explains Ghezzi.

Currently, the team is awaiting approval to test their prosthesis in humans. Meanwhile, to continue testing the implant, they have developed a virtual reality programme that recreates what the patient would see using their prosthetic. The simulations confirmed the ability of the current setup to generate perceptible images and the implant’s readiness for clinical trials.

Quantum dot array could make ultralow-energy switches

Interactions between matter and light in microcavities made of mirrors are fundamentally important for many modern technologies, including lasers. Researchers at the University of Michigan, Ann Arbor, US, have now gained tighter control of these interactions by exploiting a nonlinear effect that occurs in a new kind of hybrid semiconductor made from bilayers of two-dimensional materials. These semiconducting sheets form an egg-carton-like array in which the “pockets” are quantum dots that can be controlled using light, and they could be used to make ultralow-energy switches.

Led by Hui Deng, the researchers made their hybrid semiconductor from flakes of tungsten disulphide (WS2) and molybdenum diselenide (MoSe2) just a few atoms thick. In their bulk form, these transition-metal dichalcogenides (TMDCs) act as indirect band-gap semiconductors. When scaled down to a monolayer thickness, however, they behave as direct band-gap semiconductors, capable of efficiently absorbing and emitting light.

When laid on top of one another, the electronic structures of TMDCs can form a larger electron lattice (known as a moiré lattice) thanks to the slight mismatch of the materials’ lattice constants. The period of this lattice can be tuned by twisting the monolayers with respect to each other at different angles. In the WS2 and MoSe2 bilayer studied in this work, this angle is about 56.5° and the moiré lattice produced contains “pockets” measuring around 10 atoms across. These pockets, explains study lead author Long Zhang, are the quantum dots – tiny pieces of semiconducting materials that can isolate individual quantum particles such as electrons.

Quantum dots confine excitons

In Deng and colleagues’ experiments, the “particles” thus isolated are excitons: particle-like excitations (quasiparticles) created when an electron in a semiconductor’s valence band is excited by a photon to the conduction band. A positively charged “hole” is then left behind in the valence band in the place of the electron. Because the electron remains strongly attracted to the hole, the two “pair up” and behave like a single entity – the exciton.

In conventional, linear, devices, excitons can travel freely throughout a device, so they hardly interact with each other. If the exciton is confined to a quantum dot, however, as in this new work, it is impossible to add a second identical exciton to the same quantum dot, explains Deng. To do this, a higher energy photon is required. “This is known as quantum blockade and causes the nonlinearity we have seen in our experiments,” she adds.

Since quantum dots are only a few atoms across, they are too small for practical applications. Deng and colleagues therefore created an array of quantum dots that they describe as contributing to the nonlinearity “all at once”.

Mirrored microcavity

To control the arrays of dots as a group using light inside the 2D semiconductors, the researchers built a resonator by embedding the 2D hybrid semiconductor between two mirrors that form a microcavity. When they excited the structure with red laser light, they found that it resonated within the cavity and formed another quasiparticle, called a polariton, which is a hybrid of an exciton and light. This observation, they explain, confirms that all the quantum dots are interacting with light in concert.

When the researchers then introduced a few excitons into the material lattice, they observed a measurable change of the polariton’s energy. This implies that the system is showing nonlinear behaviour due to quantum blockade, Deng says.

“Engineers can use that nonlinearity to discern the energy deposited into the system, potentially down to that of a single photon, which makes the system promising as an ultralow-energy switch,” she explains.

The researchers say their work, which they report in Nature, might be extended to achieve polariton blockade similar to the exciton blockade seen in their experiments. “We also plan to increase the nonlinearity we have observed by varying the moiré lattice and reducing the size of the cavity, and look for ways to create quantum states of light from the system,” Deng tells Physics World.

Certifiable quantum random number generation picks up the pace

Researchers at Japan’s Nippon Telegraph and Telephone Corporation (NTT) have built a quantum random number generator (QRNG) that delivers random bits periodically with high speed and is robust against noise that would otherwise compromise the bits’ security. Where previous QRNGs needed to run for a long time before they could generate random bits at high average rates, Yanbao Zhang and colleagues devised a way to do away with this so-called “latency” and fight against imperfections in their QRNG device. These innovations made it possible to certify random bits in less time. Their QRNG could find application in computation and communication networks, where low-latency random number generation is necessary for high-speed encryption.

Randomness is key to many applications, including numerical simulations, statistical sampling, and cryptography. Simulations and sampling require high-speed, high-rate random number generation, while cryptography prizes secure (certifiable) random bits.

Since quantum measurement is inherently probabilistic, quantum mechanics naturally lends itself to random number generation. The distinguishing feature of QRNGs lies in the fact that output random bits are certifiable based only on measurement observations with verifiable physical conditions. “One can certify that the random bits generated by a QRNG are pretty close to the ideal random bits that are completely unknown by an external adversary who may hold additional information about the QRNG device,” Zhang explains.

Image showing the setup of the QRNG, in which a pulse from a quantum light source passes through a Mach-Zehnder interferometer and is detected using a pair of single-photon detectors.
Random source: The QRNG developed by Zhang and his colleagues. (Courtesy: Hsin-Pin Lo and Takuya Ikuta)

Low latency despite adversarial attack

To reduce the latency of their device, the NTT team developed an efficient method for certifying quantum randomness against both classical and quantum adversaries. A quantum adversary is defined as someone who has access to quantum resources, including quantum memories that store an arbitrary state entangled with the state prepared in the experiment. A classical adversary, in contrast, can only store a classical description of measurement results. Zhang and colleagues demonstrated that their device could certify a block of 8,192 random bits every 0.1 seconds with high security against all quantum adversaries, or a block of 2 x 8,912 random bits against all classical adversaries.

Besides reducing latency, the new method has a further advantage: it requires neither the source of the random numbers nor the measurement apparatus to be characterized in full. Therefore, practical security with realistic devices is guaranteed. In contrast, previous methods for certifying randomness against quantum adversaries addressed imperfections in either the source or measurement, but not both.

Now that they have realized a high-speed, high-security QRNG, Zhang and colleagues want to reduce the size of their QRNG so that it can be used in mobile phone technology. They also suggest that the QRNG they developed could be used to build high-speed randomness servers (beacons) that periodically produce fixed blocks of certifiable and public random bits, which would be a boon to communication networks.

Scientists refuse to be cowed by the livestock methane problem

Cows and other livestock emit a surprisingly large quantity of methane, a powerful greenhouse gas. Globally, the livestock sector accounts for the equivalent of seven gigatonnes of carbon dioxide emissions every year. To put that in perspective, that is 15% of all emissions linked with human activities, and it is comparable to the amount emitted by cars.

This video looks at how scientists are involved in both quantifying the problem and providing solutions. To find out more, read the article ‘Battling bovine belching: measuring methane emissions from cows’ by science writer Michael Allen, originally published in the April 2021 issue of Physics World.

The muon’s theory-defying magnetism is confirmed by new experiment

A long-standing discrepancy between the predicted and measured values of the muon’s magnetic moment has been confirmed by new measurements from an experiment at Fermilab in the US. The 200-strong Muon g–2 collaboration has published a result consistent with data collected two decades ago by an experiment bearing the same name at the Brookhaven National Laboratory, also in the US. This pushes the disparity between the experimental value and that predicted by the Standard Model of particle physics up to 4.2σ, suggesting that physicists could be close to discovering new fundamental forces or particles.

The muon, like its lighter and longer-lived cousin the electron, has a magnetic moment due to its intrinsic angular momentum or spin. According to basic quantum theory, a quantity known as the “g-factor” that links the magnetic moment with the spin should be equal to 2. But corrections added to more advanced theory owing to the effects of short-lived virtual particles increase g by about 0.1%. It is this small difference – expressed as the “anomalous g-factor”, a = (g – 2)/2 – that is of interest because it is sensitive to virtual particles both known and unknown.

In 1997–2001, the Brookhaven collaboration measured this quantity using a 15 m-diameter storage ring fitted with superconducting magnets that provide a vertical 1.45 T magnetic field. The researchers injected muons into the ring with their spins polarized so that initially the spin axes aligned with the particles’ forward direction. Detectors positioned around the ring then measured the energy and direction of the positrons generated by the muons’ decay.

Spin precession

Were there no anomalous moment, the magnetic field would cause the muon spins to precess such that their axes remain continuously aligned along the muons’ direction of travel. But the anomaly causes the rate of precession to slightly outstrip the muons’ orbital motion so that for every 29 trips around the ring the spin axes undergo about 30 complete rotations. Because the positrons have more energy on average when the spin aligns in a forward direction, the intensity of the most energetic positrons registered by the detectors varies cyclically – dropping to a minimum after about 14.5 revolutions and then rising back up to a maximum. It is this frequency – the number of such cycles per second – that reveals the precise value of a.

When the Brookhaven collaboration announced its final set of results in 2006, it reported a value of a = 0.00116592080 and an error of 0.54 parts per million (ppm) – putting it at odds with theory by between 2.2–2.7σ. That discrepancy then rose as theorists refined their Standard Model predictions, so that it currently stands at about 3.7σ. The latest measurements extend the disparity still further.

The recent measurements were made using the same storage ring as in the earlier work – the 700 tonne apparatus was transported in 2013 over 5000 km (via land, sea and river) from Brookhaven near New York City to Fermilab on the outskirts of Chicago. But while the core of the device remains unchanged, the uniformity of the magnetic field that it produces has been increased by a faxtor of 2.5  and the muon beams that feeds it are purer and more intense.

Avoiding human bias

The international collaboration at Fermilab has so far analyzed the results from one experimental run, carried out in 2018. It has gone to great lengths to try and avoid any sources of human bias, having even made its experimental clock deliberately out-of-synch to mask the muons’ true precession rate until the group’s analysis was complete.

Describing its results in Physical Review Letters, alongside more technical details in three other journals, the collaboration reports a new value for a of 0.00116592040 and an uncertainty of 0.46 ppm. On its own, this is 3.3σ above the current value from the Standard Model and slightly lower than the Brookhaven result, but consistent with it. Together, the results from the two labs yield a weighted average of 0.00116592061, an uncertainty of 0.35 ppm and a deviation from theory – thanks to the smaller error bars – of 4.2σ. That is still a little short of the 5σ that physicists normally consider the threshold for discovery.

Tamaki Yoshioka of Kyushu University in Japan praises Fermilab Muon g–2 for its “really exciting result”, which, he says, indicates the possibility of physics beyond the Standard Model. But he argues that it is still too early to completely rule out systematic errors as the cause of the disparity, given that the experiments at both labs have used the same muon storage ring. This, he maintains, raises the importance of a rival g–2 experiment under construction at the Japan Proton Accelerator Research Complex in Tokai. Expected to come online in 2025, this experiment will have quite different sources of systematic error.

Alternative theory

Indeed, if a group of theorists going by the name of the Budapest-Marseille-Wuppertal Collaboration is correct, there may be no disparity between experiment and theory at all. In a new study in Nature, it shows how lattice-QCD simulations can boost the contribution of known virtual hadrons so that the predicted value of the muon’s anomalous moment gets much closer to the experimental ones. Collaboration member Zoltan Fodor of Pennsylvania State University in the US says that the disparity between the group’s calculation and the newly combined experimental result stands at just 1.6σ.

The Fermilab collaboration continues to collect data and plans to release results from at least four more runs. Those, it says, will benefit from a more stable temperature in the experimental hall and a better-centred beam. “These changes, amongst others,” it writes, “will lead to higher precision in future publications.”

Inverse planning with Leksell Gamma Knife Lightning: Clinical plan quality and efficiency

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Leksell Gamma Knife® (LGK) Icon™ is the most recent LGK system that enables high-precision frameless stereotactic radiosurgery. The treatment planning system tailored for the LGK, the Leksell GammaPlan®, can be used either by manual forward planning or inverse planning. Leksell Gamma Knife® Lightning is the new inverse planning software that provides an optimizer that calculates inverse plans based on a set of constraints defined by the user.

Dr Florian Stieler and Manon Spaniol from the Department of Radiation Oncology and Medical faculty, Mannheim, will present their work on comparing manual forward treatment planning for LGK SRS to inverse planning using a pre-release of the Leksell Gamma Knife® Lightning.

This webinar will give great insight into the clinical value of the new inverse planning tool with improved plan quality measures and reduced Beam-on-Time compared to manual forward planning.

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Florian Stieler, PhD, is the senior medical physicist at the University Medical Center Mannheim, Department of Radiation Oncology and Medical Faculty Mannheim, University of Heidelberg, Germany.

 

 

 

Manon Spaniol is a medical physicist trainee and PhD student at the University Medical Center Mannheim, Department of Radiation Oncology, University of Heidelberg, Germany.

 

 

 

 

E2E test for spine stereotactic radiosurgery on MR-Linac using RTsafe Spine phantom

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This webinar will share the results of end-to-end dose measurements for spine stereotactic radiosurgery quality assurance test in both MR-Linac and a conventional linac. Treatment plans were generated for lumbar and thoracic spines. Target and spinal cord doses were measured directly with two ion chambers inserted into the RTsafe Spine phantom. The RTsafe anthropomorphic Spine phantom was used to examine the feasibility of spine SBRT with the MRL.

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Eun Young Han is an associate professor of radiation oncology at the University of Texas MD Anderson Cancer Center at Houston, USA. She works as a clinical medical physicist. She earned her PhD at the University of Florida focusing on Monte Carlo-based internal dosimetry using pediatric and adult anthropometric phantoms. Today, her clinical and research interests are in the areas of end-to-end test and dosimetric comparison of brain stereotactic radiosurgery using Gamma Knife ICON and spine SBRT using MR-guided Linac.

Jinzhong Yang is an assistant professor of radiation physics at the University of Texas MD Anderson Cancer Center, USA. He is the physics lead of the MR-Linac programme at MD Anderson. He earned his PhD in electrical engineering from Lehigh University in 2006. Dr Yang has more than 12 years of research experience in medical image registration and image segmentation, with a focus on translating novel imaging computing technologies into clinical radiation oncology practice. His current research interest focuses on MR-guided online adaptive radiotherapy.

2D boron sheets show novel ‘half-auxetic’ effect

Researchers have discovered a two-dimensional material that expands regardless of whether it is stretched or compressed. This hitherto unobserved “half-auxetic” effect, as it has been dubbed, could find use in future nanoelectronics applications.

Most materials become thinner when stretched. If you stretch a rubber band across its length, for example, it will shrink in the other two directions (perpendicular and in-plane), becoming narrower and thinner as you pull. Auxetic materials, however, do just the opposite, expanding in both the perpendicular and in-plane directions relative to the applied strain. They also shrink when they are compressed, unlike ordinary materials that expand. In mathematical terms, conventional materials are characterized by a positive Poisson’s ratio and auxetic ones by a negative Poisson’s ratio.

One of the oldest and best-known applications of a natural material that is borderline auxetic is cork, which has a Poisson’s ratio of near zero and can be pushed into the thinner neck of a wine bottle. Other naturally-occurring examples include human tendons and cat skin.

Researchers seeking to mimic such behaviour in artificially-engineered auxetic materials have previously succeeded in making structures that are robust to indentation and tearing (shear stress). Such materials are now increasingly employed in products such as bicycle helmets or safety jackets.

Palladium-decorated borophene

An international team led by Thomas Heine from TU Dresden in Germany has now discovered “half-auxetic” behaviour in an atomically-thin version of the element boron that they made more stable to strain and stress by adding palladium (Pd) to it. The Pd-decorated borophene, as it is called, has three stable phases, one of which exhibits half-auxetic behaviour along one of its crystal axes.

Using computational modelling, Heine and colleagues showed that the material behaves like an auxetic material when strained (a negative Poisson’s ratio) but expands like an ordinary material when compressed (a positive Poisson’s ratio). Simply put, regardless of whether it is strained or compressed, the material always expands.

Novel negative Poisson’s ratio material

The researchers chose palladium as a stabilizer for their borophene studies because it is a transition metal widely employed in electronics and in catalysis and an efficient donor of electrons to boron. It also has the lowest melting point of all platinum group metals, which makes it easier to handle in experiments.

Heine and colleagues studied their palladium borides (PdBn, where n=2,3,4) theoretically using first-principles calculations combined with a “particle swarm optimization” (PSO) algorithm that enabled them to check the materials’ properties. “Poisson’s numbers are typically calculated by the ratio of strain in two directions, but for compressive and tensile strain, we found that the numbers were different in one PdBn,” Heine explains. “We therefore used the more complex (but more accurate) definition that the Poisson’s number is the derivative of one strain direction with respect to the other.”

These calculations revealed a material with a novel negative Poisson’s ratio and intriguing mechanical and electronic properties. Of the three stable phases of the PdBn they discovered, the PdB4 monolayer – a semiconductor with an indirect band gap of 1.22 eV – was the one that showed the half-auxetic behaviour.

Avoiding energetically-costly bond stretching

Describing their work in Nano Letters, the researchers say that the half-auxeticity they unearthed in PdB4 stems from the material trying to avoid an energetically-costly stretching of the Pd-B bonds when strained along its length. To overcome the significant stress it experiences during this applied strain, the sheet in effect becomes corrugated. This process pushes the neighbouring in-plane atoms away from each other, causing it to expand in both the lateral and vertical directions, like an auxetic material.

When the material is compressed, the PdB4 accommodates this stress by, again, pushing the in-plane atoms away from each other so that the material slightly expands in-plane. This is what a conventional material does.

Designing new structures

Heine says that the mechanism he and his colleagues identified might be used to design new half-auxetic structures. “These novel materials could lead to innovative applications in nanotechnology, for example in sensing or magneto-optics,” he explains. “A transfer to macroscopic materials is equally conceivable.”

Spurred on by their findings, members of the team, which includes researchers from Hebei Normal University in China and Singapore University of Technology and Design, say they now plan to find out whether the effect occurs in other classes of nanomaterials, such as metal-organic frameworks or 2D polymers and macroscopic frames produced by 3D printing. “It will also be interesting to explore if the half-auxetic effect can be found in the out-of-plane direction, that is, if the thickness of a material always expands when subject to in-plane tension or compression,” Heine tells Physics World.

Liquid-jet evolution is driven by surface tension, not gravity

Spectacular upward jets of liquid are produced when a droplet falls on a liquid surface – a phenomenon that has fascinated physicists for at least 100 years. Now, researchers led by Cees van Rijn at the University of Amsterdam have shown that surface tension plays a far larger role than gravity in slowing the upward flow and shaping the jet. The team used advanced imaging techniques to provide a clear quantitative explanation for the self-similar evolution of the jets. Their results shed new light on a widely studied area of fluid dynamics, and could lead to a better understanding of how liquids behave in microgravity.

When a raindrop hits a pool of water, the liquid it contains will rapidly move to fill in the impact crater it forms. This generates an upward-moving jet typically several centimetres in height, which rises and falls in under 100 ms. A key feature of these jets is that their shapes remain the same as they rise and fall – a phenomenon called self-similarity.

The physics of these flows has long been an active area of research, which has largely focused on how various aspects of jet evolution relate to the type of liquids involved. However, the role of one key aspect of small-scale fluid dynamics remains unclear: surface tension is crucial to understanding how droplets in jets evolve but has so far only been invoked to model the droplets that form at the tips of upward-flowing jets.

Fluorescent tracer

De Rijn’s international team explored jets in unprecedented detail using particle imaging velocimetry (PIV). This involves putting fluorescent tracer particles in fluids and illuminating them with a laser – revealing the paths and velocities of the flowing liquid.

The team’s measurements revealed that fluid elements inside the jets decelerated between 5–20 times faster than would be expected from gravity alone. Such high values could only be explained by accounting for surface tension pulling the jets downwards. This insight allowed de Rijn and colleagues to update existing theoretical models to incorporate surface tension alongside the influences of gravity and fluid inertia. Their new models held up for a variety of other liquids, including ethanol, and mixtures of water and glycerol.

These improvements allowed the team to better explain the self-similar manner in which the jets evolve. The effect of surface tension means that while the heights and widths of the jets change constantly over time, their velocity profiles and conical shapes show almost no variation. With their updated models, the researchers could match these dynamics with a particular mathematical description of self-similar systems, which they corrected for the contribution from gravity.

The insights gathered by the team provide the first fully accurate quantitative explanations for the shapes and dynamics of the jets produced by impacting droplets. In future research, they hope to repeat their experiments aboard the International Space Station, where the influence of surface tension can be studied in a low-gravity environment.

The study is described in Physical Review Fluids.

Best practice: How to choose the right detector for your water phantom

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Choosing the right detector is key for accurate dose measurements in water. Which detector characteristics have a major influence on my measurement results? Which trade-offs are acceptable for a specific measurement task? Is a diode automatically the best choice for small field dosimetry? It all depends.

In this educational webinar, Jan Würfel, research scientist and detector expert at PTW Freiburg, will provide answers to frequently asked questions on radiation detectors and give advice on how to choose the best detector for a specific application. He will go into detail about basic detector properties, address factors that need to be taken into consideration when selecting a detector for dose measurements in water, and give examples of suitable detectors.

Key topics covered in this webinar include:

  • Discussion of important detector properties, such as range of use, measurement speed, long-term stability or energy response, and their relevance.
  • Typical examples of dose detectors with the discussed properties.
  • Overview of major detector applications and their specific requirements.
  • Typical examples of detectors suitable for use in reference, relative and small field dosimetry.

We look forward to sharing our knowledge and best practices on detector selection and use with you.

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Jan Würfel studied physics at Karlsruhe Institute of Technology (KIT) and holds a PhD in molecular electronics. He currently works as a research scientist at PTW Freiburg with a key focus on improving detector performance and investigating new detector materials. In addition, Jan frequently serves as a speaker on a variety of dosimetry topics for the PTW Dosimetry School. His research and lecturing interests include small field dosimetry, detector physics and reference dosimetry.

 

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