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Quantum computer based on shuttling ions is built by Honeywell

A quantum charged coupled device – a type of trapped-ion quantum computer first proposed 20 years ago – has finally been fully realized by researchers at Honeywell in the US. Other researchers in the field believe the design, which offers notable advantages over other quantum computing platforms, could potentially enable quantum computers to scale to huge numbers of quantum bits (qubits) and fully realize their potential.

Trapped-ion qubits were used to implement the first quantum logic gates in 1995, and the proposal for a quantum charged coupled device (QCCD) – a type of quantum computer with actions controlled by shuffling the ions around – was first made in 2002 by researchers led by David Wineland of the US National Institute of Standards and Technology, who went on to win the 2012 Nobel Prize for Physics for his work.

Quantum gates have subsequently been demonstrated in multiple platforms, from Rydberg atoms to defects in diamond. The quantum computing technology first used by IT giants, however, was solid state qubits. In these, the qubits are superconducting circuits, which can be mounted directly on to a chip. These rapidly surpassed the benchmarks set by trapped ions, and are used in record-breaking machines from IBM and Google: “Working with trapped ions, I would be asked by people, ‘Why aren’t you working with superconducting qubits? Isn’t that race pretty much already settled?’,” says Winfried Hensinger of the UK’s University of Sussex.

Progress is slowing

Recently, however, the progress made using superconducting circuits appears to be slowing as quantum computers integrate more and more qubits. To interact properly, the qubits must be identical and, whereas two copies of the same ion are guaranteed by quantum mechanics to be indistinguishable, fabricating identical circuits is near-impossible. Fabrication directly onto a chip also places superconducting circuits in thermal equilibrium with the chip: “If you build a superconducting qubit-based quantum computer, you have to cool that machine all the way to millikelvin temperatures,” says Hensinger. “That works fine if you have 10, 100…maybe 1000 qubits, but it’s going to be really challenging when you go to really large numbers.”

Some large companies have recently shown interest in the trapped ion platform, among them the multinational technology conglomerate Honeywell, which formed Honeywell Quantum Systems in 2020 to focus solely on the technology.

The firm’s latest result, unveiled in Nature, is the first demonstration of a fully functional QCCD. The device uses ytterbium-171 ions as qubits, which are chilled to their quantum ground states by barium-138 ions using a process called sympathetic cooling. The setup is contained in a linear trap above a chip cooled to around 10 K in a vacuum chamber. Ions held within the trap are shuffled between positions by dynamic electric fields, while quantum logical operations on the ions are performed by laser beams.

Teleported CNOT gate

The researchers demonstrate a sufficient set of gates to perform universal quantum logic. In addition, they created a teleported CNOT gate, which allows for non-destructive mid-circuit measurement – a crucial component for quantum error correction.

Their device has only six qubits, compared to 53 superconducting qubits in Google’s Sycamore – the machine with which Google claimed quantum advantage in 2019. However, Honeywell’s computer is arguably more powerful because of the flexibility of the QCCD architecture: “These ions are fully connected,” explains team member David Hayes; “With superconducting qubits or things like them, you can’t have a qubit over here talk to a qubit over there if there’s a whole bunch of qubits in the way – you have to move that information through there, and there’s a whole bunch of errors that will accumulate along the way.”

Hensinger is impressed with the Honeywell device: “This is really a phase change now we have a complete machine built on a shuttling-based approach,” he says; “It has been demonstrated with all the key ingredients. People often ask me when we can have a million-qubit machine: obviously there are still many, many challenges to be overcome, but I think this [research] demonstrates that it is a straight engineering path.”

Chris Monroe of University of Maryland, College Park, a co-author of Wineland’s on the original 2002 paper, who now runs the spin-off company IonQ, agrees: “In this field, every single little piece has been demonstrated separately. One of the important features of this work is that it integrated lots of them in one system. I love the QCCD idea: I actually coined that phrase myself.” He cautions, however, that, “the QCCD works great with six or eight ions, but when you get to 80, 200 or 300 ions, to enjoy that full connectivity you’re going to be spending a lot of time separating chains, moving ions around, getting them into position, doing the gate and then returning them to where they were.”

Accelerometer sensitivity gets a laser boost

An accelerometer that uses laser light instead of just mechanical strain can register changes as small as tens of billionths of the acceleration due to Earth’s gravity, making it far more sensitive than commercial devices. With further improvements, the developers of the new optomechanical sensor say it might be used to orient aircraft, satellites and submarines, and could even serve as a portable reference to calibrate accelerometers already on the market.

Accelerometers – sensors that detect sudden changes in velocity – have many applications. Among other things, they help trigger the deployment of airbags in cars, keep rockets and aeroplanes on the correct flight path, provide navigation for self-driving vehicles and rotate images so that they stay the right way up on your mobile phone. In general, they work by tracking the position of a freely moving “proof” mass with respect to a fixed reference point in the device. The distance between this proof mass and the reference changes whenever the device slows down, speeds up or switches direction, producing a signal that can then be detected.

Distance change between two micromirrors

The new accelerometer developed by Jason Gorman, Thomas LeBrun, David Long and colleagues at the US National Institute of Standards and Technology (NIST) uses infrared light to measure the change in distance between two micromirrors in a configuration known as a Fabry–Perot cavity. In their set-up, the proof mass is a single crystal of silicon with a mass of between 10 and 20 mg, and it is suspended from the first mirror using a set of 1.5 μm-thick flexible silicon nitride (Si3N4) beams. Being suspended in this way allows the proof mass to move freely, with nearly ideal translational motion. The second mirror, which is concave, acts as the accelerometer’s fixed reference point and thus cannot move.

When the team directs infrared laser light into the optical cavity, most frequencies of light are entirely reflected. However, light of a certain frequency can resonate – or bounce back and forth – between the two mirrors in the cavity, increasing its intensity. When the team makes the device accelerate, the proof mass displaces relative to the concave mirror, and this displacement produces a change in the intensity of light reflected from the cavity.

The NIST researchers track this change using a stable single-frequency laser “locked” to the cavity’s resonant frequency. By continually matching the laser’s frequency to the resonant frequency of the cavity, they can determine how much the device has accelerated. The result is a device that can sense displacements of the proof mass that are smaller than a femtometre (10–15 m) and detect accelerations as low as 3.2 × 10-8 g, where g is the acceleration due to Earth’s gravity. This is better than any accelerometer on the market today of comparable size and bandwidth, the team says.

A simple spring

While the concept of an optomechanical accelerometer may sound simple, being able to accurately convert the displacement of the proof mass into an acceleration has proved challenging. In the new work, however, the proof mass and supporting beams are designed so that they behave like a simple spring (or harmonic oscillator) that vibrates at a single frequency in the operating range of the accelerometer. This approach, say the researchers, makes the setup easy to model using first-principles calculations.

Using this technique, which they report in Optica, Gorman, LeBrun, Long and colleagues extended their approach to achieve measurement uncertainties of around 1% over a wide range of resonating frequencies (from 100 Hz to 15 kHz). Their device also does not need to be calibrated before use since it uses laser light of a known frequency to measure acceleration. It might thus ultimately serve as a portable reference standard for other accelerometers on the market (all of which do need to be calibrated) and so help make them more accurate.

In the future, the NIST group plans to refine its system so that it can be deployed in the field as an accurate sensor and intrinsic standard for acceleration. “Work is also under way on advanced applications of the technology ranging from searches for new physics to medical diagnostics and satellite measurements for climate change studies,” LeBrun tells Physics World.

Winning an IOP business award could give your firm a boost

Whether you’re an academic, a student or someone who left physics many years ago, I’m sure you were drawn to the subject by a simple desire to learn about how the world works. However, as I’ve mentioned many times before, physics is also vital for our modern hi-tech economy. The subject we know and love supports jobs, generates profits and has spawned innovation in sectors ranging from oil and gas to renewable energy, medicine, photonics, IT and more.

It was to celebrate such firms that in 2011 the Institute of Physics (IOP), which publishes Physics World, launched its business awards, which remain the only awards recognizing companies in the UK and Ireland for the innovative application of physics. Over the last 10 years, they have grown from strength to strength. In 2018 a new category was added to honour small start-up firms, while last year the Lee–Lucas award was introduced for early-stage physics-based companies in the medicine and health sectors – thanks to the generous donation by Mike and Ann Lee (née Lucas).

Over the last 10 years, the Institute of Physics’ business awards have grown from strength to strength.

James McKenzie

A total of 13 firms won IOP business innovation awards last year, while more than 65 have been recognized since they were first introduced. And with the 23 April nomination deadline for the 2021 awards fast approaching, this month I’ve picked a few of the winners that caught my eye over the last 10 years. I should add that although I was involved in judging the awards between 2016 and 2020, I have no financial involvement in any of the companies listed below, which I have picked purely as examples of innovative success stories.

Eye-catching entries

An early award-winner was Zephir, which in 2013 was honoured for developing and commercializing a novel lidar anemometry system that could be used to identify where best to locate and build new wind farms. Now known as ZX Lidars, the firm is today a leading provider of wind lidar products and services, with more than 3000 lidar deployments across more than 50 countries.

Another 2013 winner was Simpleware, which was recognized for developing and commercializing a physics-based computer-modelling package that has benefited the aerospace, advanced engineering and medical technology sectors as it allows scanned real-world objects to be integrated with 3D computer-generated models. Its ScanIP software is widely used following the company’s 2016 acquisition by Synopsis – a US-based software firm with $3bn revenue.

Endomag, meanwhile, is a great example of a successful firm growing in the highly regulated medical-physics market. Founded in 2007, its surgical, magnetic-guidance technology had treated some 14,000 patients by the time it won an IOP award in 2016. Endomag’s kit now appears in 550 hospitals in 40 different countries, while its Sentimag equipment has already given 130,000 women access to more precise and less invasive breast-cancer treatment.

Another 2016 winner was Kromek, which supplies radiation-detection components and devices for the medical-imaging, nuclear-power and security and screening markets. Founded in 2003 as a spin-off from Durham University in the UK, Kromek won the award for developing a wireless neutron- and gamma-radiation detection network, which can, for example, warn of radioactive materials being used by “nuclear terrorists”. The company is now a £60m stock-market-listed business with nearly 150 staff at several sites in the UK and US (see “Living in a materials world” in Careers).

I hope my selection of award winners will inspire your company to apply.

James McKenzie

In what with hindsight seems to have been a particularly innovative year, 2016 also saw Tesla Engineering win an award for developing compact superconducting magnets. Its technology allowed a medical cyclotron to be mounted on a gantry – broadening out the use of proton-beam therapy for cancer treatment. Originally founded in 1973, the firm now has more than 250 staff and has also supplied magnets for particle accelerators, fusion systems and MRI machines.

In this first decade of the IOP’s business awards, one firm – MSquared – has won on two different occasions. In 2015 it was honoured for its titanium-sapphire laser technology platform SolsTiS, which had sales of £14m at the time. Two years later it won for its Aurora system, which can image living cells in 3D. The company, which was founded in 2006, has expanded globally using photonics to support quantum technology, biophotonics and chemical sensing.

The final company I’d like to mention is Reaction Engines, which won in 2019 for its SABRE aerospace engine. It can provide efficient air-breathing thrust from standstill to speeds above five times the speed of sound in the atmosphere – before operating in rocket mode, allowing spaceflight at up to orbital velocity. Able to “breathe” air from the atmosphere rather than carry its own oxygen, SABRE could make space launches easier and even cut the flight time from the UK to Sydney to four hours. Founded in 1989, it now has 200 staff in the UK and US, with investment from the likes of Rolls-Royce, BAE Systems and Boeing.

In it to win it

One of the challenges that all physics-based firms face is the time it takes to develop products or services and to become globally significant. There’s also the difficulty of explaining a product idea, which is often quite specialized, to potential investors who have little or no science background. That’s why the IOP’s start-up award can come in really handy: it provides small firms with evidence that their technology has had a “stamp of approval” from a judging panel with solid physics and business experience.

I hope therefore that my selection of award winners – and of course there are many that I have left out – will inspire your company to apply. And even if you don’t work in a business, remember the IOP also offers three awards (Katharine Burr Blodgett, Denis Gabor and Clifford Paterson) for individuals or teams who have done innovative physics with a commercial angle. Good luck – and remember, you have to be in it to win it.

Sweet taste of heavy water, raindrops falling on other planets

Have you ever wondered what heavy water tastes like? Indeed, you may be tempted to taste deuterium oxide and find out for yourself. That is what the chemistry Nobel laureate Harold Urey and a colleague did in 1935, reporting that the taste of heavy water was indistinguishable from normal water. This contradicted previous reports that heavy water was sweet or caused a dry and burning sensation in the mouth.

Now, an international team of chemists has repeated those taste tests in humans and mice – along with chemical and computer studies of human taste receptors. They concluded that heavy water is indeed sweet to humans, but not to mice. While deuterium and hydrogen are identical chemically, the fact that deuterium is twice as heavy means that there are small differences in the properties of molecules containing the different isotopes. For this reason, consuming large amounts of heavy water is not recommended

“Our study thus resolves an old controversy concerning the sweet taste of heavy water using state-of-the-art experimental and computer modelling approaches, demonstrating that a small nuclear quantum effect can have a pronounced influence on such a basic biological function as taste recognition,” concludes team member Pavel Jungwirth at the Institute of Organic Chemistry and Biochemistry of the Czech Academy of Sciences.

Jovian rain

Staying on the theme of idle speculation about water, have you ever wondered what raindrops would look like on other planets? It turns out that despite the huge differences between Earth and Jupiter, Jovian rain is very similar to the stuff that ruins picnics here on Earth.

That is the conclusion of Kaitlyn Loftus and Robin Wordsworth at Harvard University in the US, who have identified a Goldilocks zone for raindrop size using just three properties: drop shape, falling speed, and evaporation speed. They looked at different types of rain such as water, methane and superheated liquid iron – which would really take the fun out of picnicking on some exoplanets. They found that raindrops of roughly the same size make it from clouds to the ground despite the very different scenarios.

“The insights we gain from thinking about raindrops and clouds in diverse environments are key to understanding exoplanet habitability,” explains Wordsworth. “In the long term, they can also help us gain a deeper understanding of the climate of Earth itself.”

Fungus turns wood piezoelectric, allowing it to power LEDs

Infecting wood with wood-decay fungus can boost its piezoelectric output by 55 times, researchers in Switzerland have discovered. The material scientists found that after 10 weeks of infection, blocks of decayed wood could power LEDs. They say that floors built from fungus-treated wood could generate renewable electricity from people’s footsteps.

Decades ago, scientists discovered that wood generates an electrical charge under mechanical stress. This piezoelectric effect is caused by the displacement of crystalline cellulose when it is deformed, whereby shear stress in one plane produces an electrical polarization perpendicular to it. But the piezoelectric effect is not very strong – around twenty times smaller than that of a quartz crystal – and wood does not deform easily.

Despite this, some researchers are keen on exploiting this property by creating piezoelectric construction materials that could help make buildings more energy efficient. Globally buildings are responsible for around 40% of our energy consumption and nearly 25% of our greenhouse gas emissions. Current attempts to minimize emissions involve reducing energy consumption or fitting buildings with solar panels so they generate their own electricity. While this can be effective, it is weather dependent and does not work everywhere. Piezoelectric construction materials could offer another source of clean energy.

Dissolving lignin

The piezoelectrical performance of wood can be improved by changing its structure. Recently Ingo Burgert, at ETH Zurich, and his colleagues found that placing wood in a mixture of hydrogen peroxide and acetic acid increases its piezoelectric output. This process dissolves the lignin in the wood leaving behind a cellulose framework that is much more flexible and elastic. When squeezed, 1.5 cm cubes of this acid-treated wood generated an output of 0.69 V, which is 85 times higher than untreated wood. This performance was stable for 600 cycles and 30 connected blocks powered light-emitting diodes (LEDs) and a simple liquid-crystal display.

Keen to create the same effect, but without the harsh chemicals, Burgert and colleagues turned to a natural process that alters the structure of wood: decay by fungi. In their latest work, described in Science Advances, they infected balsa wood with the white rot fungus Ganoderma applanatum for 4–12 weeks. After 10 weeks the wood had lost 45% of its weight and the researchers found that at this point it showed the best compressibility performance, while still returning to its original shape once the stress was released.

A single 1.5 cm cube of this decayed wood produced a maximum voltage of 0.87 V under 45 kPa of stress, while uninfected balsa wood generated 0.015 V. The treated wood maintained its performance for 500 cycles. Electrical output increased with mechanical stress, rising to 1.32 V at 100 kPa. Nine of the decayed-wood blocks connected in parallel were able to power an LED, when pressed strongly.

Cellulose remains intact

Infrared spectroscopy and X-ray diffraction analysis of decayed and untreated wood showed how the fungus altered the wood. “The selected wood decay fungi secrete enzymes that enable degradation of lignin and hemicelluloses in the wood, whereas cellulose remains intact,” Burgert told Physics World. “This type of wood decay is also known as selective delignification. This process changes the structure and chemistry of the wood cell wall enhancing the natural piezoelectric properties of wood.”

The researchers say that their results indicate that the material could be used to produce large-scale wooden floors, such as those in ballrooms, that could generate electricity from human activity.

“We are currently working at the demonstrator scale with delignified wood that can be used for sensors integrated into wooden floors,” Burgert says. “For instance, these systems could be used as security systems in wooden floors for detection of any kind of applied stress. In terms of power generation, it is on the level of lighting up LED lights, and therefore, at present the application as a sensor is more suitable. However, it is a first step and we are currently optimizing towards wood-based systems better suited for energy harvesting.”

Due to its lignin composition delignification during fungal infection is much faster in balsa than other woods such as spruce, pine and fir. “The next step is to use this concept for native wood species and incorporate the generated materials in future smart buildings,” Burgert adds.

Contrast-enhanced OCT visualizes vascular leakage in the retina

Contrast-enhanced OCT
Vascular leakage signal before and after injection of Intralipid tracer (scale bars: 100 µm). The false colour map is generated by overlaying the traditional angiogram signal (red) with the ExCEL1 (green) and ExCEL2 (blue) signals. A clear increase in leakage signal (cyan) is seen after injection. (Courtesy: CC BY 4.0/Biomed. Opt. Express 10.1364/BOE.415227)

Vascular leakage is an important biomarker for assessing vision-threatening retinal diseases such as age-related macular degeneration and diabetic retinopathy. Fluorescence angiography, the imaging exam currently used to identify vascular leakage, lacks depth resolution, which can hamper the identification and precise localization of leaky blood vessels. Optical coherence tomography (OCT) is a newer clinical ophthalmic imaging technology that provides high-resolution images and rapid volumetric scanning. To date, however, OCT has been unable to visualize vascular leakage.

Researchers at the Medical University of Vienna are developing a new OCT method, exogenous contrast-enhanced leakage OCT (ExCEL-OCT), which measures the diffusion of tracer particles around leaky vasculature. Writing in Biomedical Optics Express, they describe the use of an OCT contrast agent to visualize the slow extravasation of tracer particles from leaky blood vessels in laboratory mice. In just a single scan, ExCEL-OCT provided high-resolution structural, angiographic and leakage information that was spatially and temporally co-registered, and separable.

The researchers used a custom-built OCT ophthalmoscope designed for rodent eye imaging. For the contrast agent, they selected Intralipid 20%, an emulsion of lipid particles that can dramatically improve OCT intensity and angiogram signals.

Conrad Merkle
Conrad Merkle from the Center for Medical Physics and Biomedical Engineering at the Medical University of Vienna.

Principal investigators Bernhard Baumann and Conrad Merkle, of the Center for Medical Physics and Biomedical Engineering, and colleagues imaged the eyes of mice with leaky retinal vasculature and control mice. To track the leakage of tracer particles, they performed multiple angiogram scans (using a traditional 3D angiography protocol covering a 1 x 1 mm field-of-view centred on the optic disc) before and after injection of the OCT contrast agent, using the data to generate both angiogram and leakage maps.

The researchers employed post-processing to compensate for motion and flatten the retina. They also developed novel data processing methods to highlight the scattering signal from the extravasated Intralipid particles. To discriminate between the various signals, they used selective decorrelation gates.

“The key idea is that OCT signals in static tissue will decorrelate slowly because the tissue is not moving,” the researchers explain. “Signal within vessels will decorrelate rapidly due to the relatively high speed of the red blood cells and Intralipid particles passing through the voxel. Extravasated Intralipid particles, which are driven by slower diffusion processes rather than blood flow, will have a decorrelation rate that falls somewhere between the two.” As such, they expect much stronger extravasation and diffusion signal will be observed around leaky vessels.

The researchers used long interscan times to highlight diffusing tracer particles. They created the ExCEL signal by subtracting angiogram signals of different lag times, to specifically highlight leakage of different diffusion rates and remove the intravascular signal. They also created depth profiles, leakage maps and fly-through videos showing leakage over time.

The contrast agent dramatically increased the visibility of neovascularizations (new blood vessels) growing into a retinal lesion. Vascular leakage could be tracked over time, with results demonstrating a clear increase in ExCEL signal, visible as a circular or spherical bloom of signal, following administration of the contrast. The researchers confirmed this finding by blind grading 83 leakage and control volumes.

In addition to showing leakage in 3D, the researchers also demonstrated that for the most part, the leakage signal was separable from the angiogram signal, which is not the case for traditional fluorescence methods. Colour coding and overlaying the angiogram and leakage data revealed which blood vessels the leakage surrounds.

The researchers note that the current system produces a high number of false-positive leakage signals, and that they could not exclusively distinguish diffusion from slow intravascular flow or bulk motion of static tissue. As OCT systems become faster and motion compensation software improves, higher temporal resolution for ExCEL measurements and shorter interscan times may change this.

The team describe this work as “a starting point for future in vivo 3D volumetric leakage studies”, noting that the new method can be easily implemented in conventional OCT systems across the world.

“We hope to continue to improve our methods to the point where they can be used clinically,” Merkle tells Physics World. “To do this, we need to continue to develop the methods and post-processing to improve signal quality and reduce false-positive signals, investigate alternate sources of contrast, and/or improve the contrast agent to increase sensitivity and reduce dose. We are most interested in the first two options, as nanoparticle fabrication is something that other research groups are already investigating with great results.”

“Beyond this work on vascular leakage, we are also investigating hardware development of polarization-sensitive and visible light OCT systems, longitudinal studies of small-animal disease models, and ex vivo imaging of human brain tissue,” Merkle adds. “My own projects are currently focused on improving clinical OCT through software rather than hardware. Our hope is that by working within the limitations of existing clinical systems, we can develop methods to improve diagnostics at a larger scale and lower cost compared to hardware-based solutions that require new OCT systems.”

Photonic-crystal ‘sunflower’ follows the light

An artificial “sunflower” that autonomously bends, folds and twists itself to optimize the amount of light it receives could be a key ingredient in intelligent solar cells of the future. The device is made from a biopolymer-based photonic crystal and its developers say that future versions might be able to track the Sun across the sky, just as real sunflowers do.

Photonic crystals are nanostructured materials with a refractive index that varies on a length scale similar to the wavelength of visible light. This periodic variation produces a so-called “photonic band gap” that affects how photons propagate through the material, allowing light at some frequencies to be absorbed (thereby heating the material) while other frequencies are reflected. The angle at which the lights hits the crystal also affects which frequencies are absorbed.

Double-layer structure

The photonic material designed by Fiorenzo Omenetto and colleagues at Tufts University and Northwestern University in the US consists of two layers. The top layer is made of an opal-like film of silk fibroin doped with light-absorbing gold nanoparticles (AuNPs). Underneath this is a silicon-based polymer, polydimethylsiloxane (PDMS).

The team, which also includes researchers at the University of Pavia in Italy, chose silk fibroin for its flexibility and promising optical properties as well as its negative coefficient of thermal expansion (CTE). The latter means that silk fibroin contracts when heated and expands when cooled – unlike most materials, which have a positive CTE and do the opposite. PDMS, importantly, has a high positive CTE and expands rapidly when heated. Hence, when the bilayer photonic crystal is exposed to laser light, it bends as the PDMS expands and the silk layer contracts.

Bending with the light

The team made their photonic crystal more reflective by adjusting the size of the unit cells within the crystal. Omenetto explains that they did this by constructing patterns in the silk layer using a lithography technique that involves either exposure to UV light or applying stencils to the material and then exposing them to water vapour. Thanks to these nanostructured patterns, the silk layer can either enhance or weaken the interaction between the gold nanoparticles and the laser light, depending on the angle at which the laser beam strikes it.

Together, these design features allow the material to bend, fold and twist in ways that depend on the geometry of the patterns and the wavelength of the incident laser beam. They also, crucially, enable the material to track the path and angle of a light source. The researchers demonstrated this function by fabricating a photonic “sunflower” with solar cells integrated into the silk fibroin-PDMS bilayer. The resulting device curls towards the light source as the source moves, similar to the way that a real sunflower tracks the Sun as different sides of its stem elongate at different times of the day.

Light-tracking devices

Omenetto explains that the team’s device keeps the angle between the solar cells and the laser beam nearly constant, maximizing the cells’ light-to-energy conversion efficiency as the laser moves. Such wireless, light-responsive, heliotropic (Sun-tracking) systems could be used to make improved solar cells, he says.

The researchers also made a self-folding box and a “butterfly” with wings that opened and closed in response to light. Spurred on by their results, which they detail in Nature Communications, they plan to adapt their optomechanical actuator so that it works in different parts of the electromagnetic spectrum. “We also hope to make sunlight-tracking devices that can be used outside of the laboratory,” Omenetto tells Physics World.

Graphene gives neural interfaces a boost, the amazing physics of hearing and vision

This episode of the Physics World Weekly podcast looks at how new technologies can improve our health and how we perceive our surroundings.

First up is Kostas Kostarelos of the UK’s University of Manchester, who talks about the exciting role that graphene can play in the development of medical devices that connect to the brain with minimal invasiveness. He also chats about his involvement with the Spanish company Inbrain Neuroelectronics, which is developing graphene-based technologies for treating epilepsy, Parkinson’s disease, and other brain related disorders.

Then Ben de Mayo of the University of West Georgia in the US takes over with a lively discussion about the science of sight and sound. De Mayo has just published the second edition of his book The Everyday Physics of Hearing and Vision and chats about how technologies such as cochlear and retinal implants work. He also talks about the amazing optical and acoustic capabilities of the mantis shrimp and the future of sensory augmentation.

Record-breaking gamma ray is smoking gun for Milky Way cosmic rays

The most energetic gamma ray ever seen could be the strongest evidence yet that high-energy cosmic rays are produced within our Milky Way galaxy, where they spend millions of years accumulating, forming a “cosmic-ray pool”.

The origin of cosmic rays is one of the most enduring mysteries in astrophysics. Cosmic rays are charged particles or atomic nuclei moving at relativistic speeds. While the Sun produces low-energy cosmic rays, the most powerful originate from beyond our solar system, but their source has been a matter of debate.

Cosmic rays are easily deflected by galactic magnetic fields, making it difficult to trace them back to their source. However, when cosmic rays collide with other particles in interstellar space, they result in the production of gamma rays, which are not deflected.

Now, the Tibet ASγ Collaboration, which has hundreds of detectors located on the Tibetan Plateau, has observed 23 extremely high-energy gamma rays, with energies ranging from 400 to 955.7 TeV, the latter being the most energetic gamma ray ever detected. Gamma rays formed in this manner are about an order of magnitude less energetic than their cosmic-ray parents, which means that those cosmic rays reached energies far in excess of one peta-electronvolt (1015 eV). For this reason, the sources of these cosmic rays are referred to as PeVatrons.

This discovery “proves that gamma rays with energies up to a few hundred TeV really exist”, says Jing Huang of the Chinese Academy of Sciences in Beijing, who is a member of the collaboration. It also strongly implies that PeVatrons are located inside our galaxy.

Tibet air shower array
The Tibet air shower array located 4300 m above sea level in Tibet, China. (Courtesy: Institute of High Energy Physics)

A limit to how far we can see

Astrophysicist David Hanna of McGill University, who was not involved in the work, agrees that “The fact that the arrival directions of most of the gamma rays seem to line up with the Milky Way argues for their production there.”

Hanna does not rule out the existence of extragalactic sources, however, pointing out that there will be observational bias. The higher the energy of gamma ray, the more likely they are able to collide with lower energy photons in space, and therefore space becomes opaque to those higher energies over large distances.

Consequently, “we can’t see as far at high energies as at low energies,” explains Hanna. Many of the higher energy gamma rays from extragalactic PeVatrons may just not be reaching us, although Hanna does note that a few of the gamma rays detected by the Tibet ASγ Collaboration don’t align with the Milky Way. These could be spurious background events, he says, or be truly extragalactic.

Footprints of dinosaurs

Adding to the intrigue is the fact that the distribution of the gamma rays across the Milky Way seems random. Supernovae remnants, intense star-forming regions and active black holes have all been mooted as possible PeVatrons, but none are found in the locations the gamma rays are coming from.

Instead, the team of scientists behind the Tibet ASγ Collaboration think that what they are seeing is evidence for a cosmic-ray pool in our galaxy. The idea is that cosmic rays become contained in our galaxy by the Milky Way’s powerful magnetic fields, and they circle the galaxy for millions of years before coincidentally colliding with an atom or molecule in interstellar space, releasing a gamma ray. These gamma rays could therefore be coming from the locations where a collision has taken place. And by the time they undergo this collision, the original PeVatrons that released the cosmic rays could be long dead.

“Metaphorically speaking, we found footprints of dinosaurs in the Milky Way – a lot of extinct PeVatrons in the galaxy,” says Masato Takita of the Institute for Cosmic Ray Research at the University of Tokyo.

Not all PeVatrons are extinct, however. For example, earlier this year the Tibet ASγ Collaboration detected gamma rays with energies up to 100 TeV originating from the supernova remnant G106.3+2.7, which is just 2600 light-years away.

The next step is to extend the survey of gamma rays into the southern hemisphere sky, including the direction of the galactic centre, which cannot be seen well from Tibet. The Pierre Auger Observatory in Argentina can detect gamma rays with energies of hundreds of TeV and has hunted for gamma rays even more powerful, while the Large High Altitude Air Shower Observatory in China, which has just begun observing, may be able to detect gamma rays with energies above 1 PeV.

The findings are published in Physical Review Letters.

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.

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