Page 482 – Physics World

Home » Page 482

Molecular spins show promise as quantum bits

The states of molecular spins in a crystal can be detected optically using the polarization of the photons they emit, researchers in the US have shown. This allowed the team to prepare and precisely read out spin states within molecules – a capability that could be used to engineer quantum bits (qubits) with greater flexibility and control than possible using other platforms.

So far, the most successful platforms for quantum computing have proved to be trapped-ion qubits and superconducting qubits. However, both technologies have significant drawbacks. Ion qubits require high vacuum and electromagnetic traps, whereas superconducting qubits must be made from identical quantum circuits, which are extremely difficult to produce with the required consistency. One possible alternative is the diamond nitrogen-vacancy (NV) centre, which occurs when two adjacent carbon atoms in a diamond lattice are replaced by a single nitrogen atom. Unlike the rest of the carbon lattice, an NV centre has a spin that can be readily manipulated and read out with laser light.

Unfortunately, the process for creating NV centres involves irradiating diamonds before exposing them to nitrogen gas and is difficult to execute precisely. “One of the major challenges to building scaleable solid state quantum systems is being able to place the quantum bit where you like at the atomic scale over the length of a wafer,” explains condensed matter physicist David Awschalom at the University of Chicago.

“Bottom-up” approach

Awschalom’s group has collaborated with researchers led by chemist Danna Freedman of Northwestern University to focus on a “bottom-up” approach to creating a crystal with appropriate spin centres, Instead of implanting the spin centres into an existing crystal, they take a molecule that contains spin centres and crystallize it. “You take it for granted that every molecule in a tablet of aspirin is identical,” explains Freedman. However, the technological challenge is that that nobody had successfully produced a molecular crystal containing spins that could be manipulated and read by a laser beam.

Now, Awschalom, Freedman and colleagues synthesized three different compounds comprising chromium (IV) ions coordinated to aryl ligands. The resulting samples of organometallic molecules each behaved much like NV centres — albeit at different laser frequencies. “You optically excite the qubit into a state that rapidly relaxes into a metastable state and then emits a photon,” says Awschalom, “From our perspective, the single pulse of light ultimately initializes the system into a state from which it can emit one photon whose polarization reflects the spin of the system. That’s the qubit spin-photon interface.”

This entire process effectively creates a molecular single qubit. At present, the researchers are working with coherent ensembles of qubits, but they are working to achieve single molecule sensitivity becasue single molecular quantum states would have longer coherence times.

Optical quantum gates

The team hopes that, by connecting the output of one qubit into the input of another, it may be possible to build optical quantum gates and then quantum circuits using their system. In the short term, says Awschalom, they plan to make interfaces between their qubits using integrated photonics. A more ambitious, long-term possibility may be to craft photonic highways by chemical self-assembly.

“This opens up a new frontier for what my students like to call designer qubits,” he says, “Now you can imagine so many things that would have been very challenging with other techniques, where you’re limited by the material family…But here, because you’re literally cooking [the material], you can imagine incorporating memories, systems that are designed so that they’re heavily entangled. I think the sky’s the limit here for both computing and sensing.”

Nifty noise trick makes quantum states live longer

Freedman adds, “Broadly speaking, quantum information science is a multi-decade challenge. There are a whole host of areas that will be impacted by new materials discoveries, so there’s both a whole lot of work to do and a whole lot of fun to have.”

Commenting on the work, Stephen Hill of the National High Field Magnet Laboratory at Florida State University says, “Molecular materials are very promising as potential qubits for quantum computing but that area lags behind some of the more mature areas partly because some of the key properties have not been demonstrated. This [work] demonstrates one of those key properties. I think the result itself is not a huge surprise – but it’s exciting for the field that it has taken that big jump in actually demonstrating this.”

The research is described in Science.

Building a flexible future

The most famous 2D material is graphene, but the focus of your research has shifted to other substances. Why is that?

We are still working on graphene, it’s just that we have expanded to investigate other 2D materials as well. The value of these other 2D materials and the motivation for investigating them is that they have properties that graphene does not have.

Graphene is unique in its material properties, but other 2D materials are also unique from an electronic, optical or magnetic point of view, so they are very worthy of consideration. It’s like if you have a sports car. It has value if you want to speed down a fast road. But if you want to carry cargo during transportation, you need a truck. These other 2D materials complement what graphene has to offer in a unique way.

A few years ago you used one of those other 2D materials, molybdenum disulphide (MoS₂), to make a flexible transistor. What did that project involve?

That was a very multidisciplinary project, which was challenging for us at that time. I’m an electrical engineer and an applied physicist, but that project also involved quite a bit of materials science in terms of materials synthesis. We had to do things from the bottom up, starting with growing the materials ourselves, then putting it on a flexible substrate like plastic kapton tape and creating devices on this non-electronic substance. And then, after fabricating the devices, we had to characterize them and do measurements to ensure they were of good quality. So it was a very complex project, spanning materials science, materials chemistry, applied physics and electronic measurements.

What are some applications of flexible electronic devices?

I’ll give you some examples that we have worked on and that are still a matter of very active investigation. One of the applications is transparent conductive films like the ones used in mobile phone screens. Because these materials are thinner than what is currently used in most phones, they are much more transparent. That translates into screens that are brighter or more efficient in their battery use.

Flexible electronics can also be used to make whole devices. For example, we’ve demonstrated a flexible radio, where you have a piece of plastic, you have a speaker on that piece of plastic, and that piece of plastic can receive signals – let’s say music signals – wirelessly and play them. So you can imagine having a foldable, flexible radio that you can take with you and unfold when you go to the beach.

Another application is graphene tattoos, where you put the tattoo on the skin, use it to record your vital signals, and then convey that information to an app on a mobile phone. A system like that can record your pulse, your heartbeat, your blood pressure and other information for health. We’re working on a system for continuous monitoring of blood pressure, which is very important for many diagnostic reasons in medicine.

You mentioned making a radio from flexible electronic components. More recently, you’ve been developing flexible memory components called atomristors. Could you explain what they are?

Of all the vast array of electronic components available today, memory or information storage components are the most important. These are the components that store information in binary code – ones and zeros – and those ones and zeros can represent any kind of data, from videos to images and so on. That’s why, when you buy a smartphone, if you get 512 gigabytes instead of 256 gigabytes of memory, it’s going to cost you significantly more money.

Because memory is so important, we knew that if we wanted to make a complete system of flexible electronics, we needed to have a flexible memory, too. And in order to make a flexible memory, you need materials that are themselves flexible, which implies that they must be very thin.

In this sense, 2D materials are the best class of flexible materials because they are the thinnest materials that exist in nature. Our atomristors are atomically thin, less than one nanometre thick, and you can make them from molybdenum disulfide, which is a common 2D material. And that can lead to flexible memories. We made that discovery a few years ago and we continue to advance it today.

Your most recent work is an extension of that project, but it involves a different 2D material, hexagonal boron nitride (hBN). Can you tell us more about that?

We were challenged by some of our sponsors to consider what other applications, in addition to information storage, could be served by our new memory technology. That led us to explore hexagonal boron nitride, which has the distinction of being the thinnest insulator on earth. We found out that, not only is it good as a memory, but it can also be used as a radio-frequency (RF) switch.

At the most basic level, a switch is a component that toggles between states, such as the different bands of communication we have on our mobile networks. For example, for 5G there are many, many bands: LTE, 4G, 3G, 2G, Bluetooth and so on. That means that RF switches need to be able to toggle rapidly between these bands, and ideally, they need to do that without consuming significant energy.

Our present switching technologies are based on transistors, but transistor switches consume a lot of energy. Even when they’re not toggling or switching, the standby energy to keep them operational is substantial. A memory switch like the kind we have been working on and developing is different, because its standby consumption of energy is zero. That would extend the battery life on your device significantly, and switches of this type are also much faster – they can handle not only 5G data but even 6G, which is 100 GB per second. For streaming applications, those speeds are unheard-of with current technologies.

Imagine having a foldable, flexible radio that you can take with you and unfold when you go to the beach

There was a lot of hype about graphene when it was first isolated. Why has it taken so long to exploit its electronic properties in a commercial device?

If you look at the history of technology, normally it takes 20 to 30 years to take an idea all the way from concept to product. Graphene has really only been in development for 10 years, and 2D materials, in general, a few years less than that. The technology is still at a nascent stage.

Nonetheless, several applications (including some that we have worked on) are on the market today. I mentioned graphene-based transparent conductive films earlier, and these are a real technology – you can buy them on certain smartphones in Asia, and the manufacturers have sold those phones in a reasonable volume. Some of our esteemed colleagues have also commercialized graphene sensor technology; I know there’s a start-up company in San Diego, California, called Cardea Bio that sells monolayer graphene biosensors.

There are also many other applications for graphene composites. An example is the packaging of smartphones, where composites help spread the heat around the back of the phone. And there are some consumer applications too, like using graphene to reinforce sportswear and the frames of bicycles so they are lighter and stronger.

What do you think the future holds for electronic devices made from 2D materials?

I think the future is brighter than it was a few years ago. The reason I say that is because several major semiconductor firms – including TSMC, Intel and some European companies – have invested a huge amount in terms of infrastructure, talent and R&D funds to try to integrate these materials into silicon-based semiconductor technology. Based on these trends from well-known, leading companies, I think we’ll see more 2D materials in electronic devices in the future.

However, as an addendum to that, when you consider the 2D materials that we’ve been working on, and many others across the world that have enjoyed billions of dollars in R&D funding, there’s a huge number of possibilities out there. It’s not one or two or five or a dozen materials. We’re talking about a thousand materials, and there’s a lot in this landscape that is undiscovered from a basic physics and science point of view. So even though there’s a lot of pressure for development and applications, there’s still a lot of basic science that we don’t know because there’s just so many materials to investigate.

Physics and LEGO: an enduring love affair

An unlimited world of structures built from precision-engineered unit parts – it is easy to see why LEGO appeals to many physicists. But in addition to the pure enjoyment, this plastic construction toy is also a great teaching tool, and it has even featured in serious science experiments. In the November episode of Physics World Stories, Andrew Glester meets physicists who have used LEGO in fun and creative ways to communicate physics.

The first guest is Lewis Matherson aka @LegoPhysicsGuy, a former physics teacher who now makes physics videos aimed at students and teachers. These videos regularly incorporate LEGO to illustrate core physics concepts in GCSEs and A levels – exams typically sat by 16- and 18-year-olds in the UK.

Next up is Joshua Chawner, a low-temperature-physics researcher at Lancaster University, UK. Chawner captured the imagination by subjecting LEGO pieces to the coldest temperatures on Earth, placing them inside his group’s dilution refrigerator, as documented in an award-winning video (below). Despite reaching 1.6 millidegrees above absolute zero (2000 times colder than deep space) the bricks proved to be extremely good thermal insulators, a surprising result described in Scientific Reports.

Last but not least is Maria Parappilly, a physics education expert at Flinders University, Australia. One of Parappilly’s successful initiatives was to create a LEGO race cars exercise for an introductory physics course that previously seen high drop-out rates. Parappilly is also the founder of the STEM Women Branching Out at Flinders University, designed to make science and other technical subjects more inclusive.

Fish save energy by swimming in schools

Swimming in schools helps fish avoid predators, but it also allows them to conserve energy. This is the finding of researchers in Germany, China and Hungary who used fish-like robots to investigate how real fish might gain from the watery vortices that other fish generate as they swim. The phenomenon they observed is known as vortex phase matching, and the researchers say that understanding how it works could inspire the development of more efficient fish-like underwater vehicles.

While scientists have long suspected that swimming fish might exploit the swirling hydrodynamic flows created by their neighbours, it was not clear how individual fish coordinated their movements to benefit from these vortices. Researchers led by Iain Couzin at the Max Planck Institute of Animal Behavior and the University of Konstanz studied this question by constructing fish-like robots and measuring how much energy they consume when swimming in pairs (the most common swimming configuration in natural fish populations) compared to when they swim alone. Such measurements would, they say, be impossible to perform on real animals.

“Robofish”

The researchers’ robotic fish are 45 cm long with a mass of 0.8 kg. Each boasts three sequential servo-motors that are connected to joints covered in a soft, waterproof, rubber skin, and controlled using a “central pattern generator” that enables the robot to mimic the undulations of real fish.

Over the course of 10 080 trials (representing 120 hours of swimming time), the researchers monitored how their “robofish” behaved when placed at different distances from a “lead” robofish. They found that paired-up fish consume much less energy than loners because the followers adjust their tailbeats to match the induced flow of the vortices shed by the leader. The matching takes place with a time lag that varies with distance: when the follower fish is beside the leader, the tailbeats are synchronized, but when the follower falls behind, its tailbeats go out of sync, with a delay that increases the further away it gets.

This vortex phase matching allows the follower fish to exploit the “Kármán vortices” – chains of swirling vortices shed by a blunt object as it travels through a liquid – that the leader fish leaves in its wake. The result is a reduction in the follower’s energy consumption, but team member Máté Nagy of the Hungarian Academy of Sciences and Eötvös University notes that it’s not just about saving energy. “By changing the way they synchronize, followers can also use the vortices shed by other fish to generate thrust and help them accelerate,” he explains.

Comparison to real fish

To find out whether real fish use vortex phase matching too, Couzin and colleagues made observations on 32 freely-swimming pairs of goldfish. While the real fish constantly changed positions relative to each other, the researchers observed that they nonetheless adopted a type of hydrodynamic interaction that could be described by a simple mathematical model that incorporates swim speed and the amplitude and frequency of the leader’s tailbeats. This model, Couzin tells Physics World, was then used to predict how real fish would behave if they were using vortex phase matching, and tested using artificial-intelligence-assisted analysis of the body posture of goldfish as they swim together.

The results indicate that real fish do indeed exploit vortex phase matching. According to the researchers, this strategy “may be a result of preflex or proprioceptive responses to neighbour-generated hydrodynamic cues”, since neither the fishes’ visual nor their lateral-line systems are required for them to behave this way. This leaves fish free to process other important information from their environment, including flows created by creatures other than their school mates – such as predators.

The team, which reports its work in Nature Communications, concludes that real fish use vortex phase matching “at least in part” to save energy. This simple and robust natural strategy might now be applied to improve the collective swimming efficiency of fish-like underwater vehicles, the researchers say.

Members of the team, which also includes scientists from Peking University, now plan to develop more advanced robots and to study “hybrid swarms” composed of real and robotic fish. In doing so, they hope to uncover further collective benefits of schooling.

Stonehenge acoustics enhanced music and speech, driving quickly is best on bumpy roads

It may look like something from the mockumentary This is Spinal Tap, but this 1:12 scale model of Stonehenge was built by researchers in the UK to find out how speech and music would have been affected by the stone circle. The model represents how archaeologists believe the monument appeared about 4200 years ago on the Salisbury Plain of southern England – and was studied inside a semi-anechoic chamber.

Trevor Cox and Bruno Fazenda at University of Salford and Susan Greaney of English Heritage found that reflections from the stones made someone speaking inside the monument sound louder to others inside. This, they suggest, could have been engineered by the ancient builders to improve communications during rituals and perhaps boost the status of speakers.

Wooden pipes and animal horns

The trio also found that music played inside the monument would be enhanced by reverberations from the stones – however, the reverb is much less than that engineered into modern concert halls. Nonetheless, they say that it would affect the sound of bone flutes, wooden pipes, animal horns and drums that are likely to have been used in Neolithic Britain.

They also looked for focused echoes in the monument that could enhance sounds at certain locations – something suggested by other researchers. However, they found no evidence for this.

You can read more about the study in an open access paper published in the Journal of Archaeological Science.

“Driving at low speed on a bumpy road is dangerous” is something you might expect to hear from a petrolhead like Jeremy Clarkson – but it is the conclusion of Kazem Reza Kasyzadeh and colleagues at the Peoples’ Friendship University of Russia in Moscow.

The researchers developed a computer model that describes vehicle body damage that is caused by fatigue failure. They used the model to calculate the damage to welds in a car moving at 5, 10 and 15 km/h over a very bumpy road. At the slowest speed, the model predicted that 100 spot welds would be damaged, whereas 40 and 50 were predicted for 10 and 15 km/h respectively.

You can read more about this counterintuitive result in this press release.

Without sound and fury, signifying something: acoustics and batteries

Want to learn more on this subject?

Watch now

Acoustics and batteries. (Courtesy: Columbia Electrochemical Energy Centre)

Although classic battery engineering is firmly rooted in chemical engineering and chemistry, the last decade has seen a significant increase in research on mechanical and chemo-mechanical properties of batteries for both reversible and irreversible behaviours. Characterization of structural evolution and degradation on a cycle and calendar basis is necessary to increase the lifetime of high-energy density cells while decreasing cost, and a range a chemistry-specific methods have been applied to probe chemical mechanical couplings. In theory, and increasingly in practice, acoustic interrogation reveals mechanical correlations across any chemistry and cell form factor due to its exploitation of the required mass balances for cell operation. Beyond being generalizable to all battery chemistries, acoustic methods are scalable, high rate and readily operando: sub-ms acquisition times are possible on full cells using desktop equipment.

In this webinar, the physical principles of electrochemical acoustic interrogation will be introduced, and example experiments will be described for lithium ion, lithium metal, and zinc alkaline systems. Statistical correlations of acoustic behaviour and “battery state” will be discussed (state of charge, state of power, state of safety, state of health), as will newer studies on structure and property extraction, with emphasis on degradation due to thermal and fast-charge events. Current spatial and temporal limits will be discussed, as will asymptotic capabilities.

Want to learn more on this subject?

Watch now

Daniel Steingart is the Stanley Thompson Associate Professor of Chemical Metallurgy and Chemical Engineering, and the co-director of the Columbia Electrochemical Energy Center. His group studies the systematic behaviours of material deposition, conversion and dissolution in electrochemical reactors with a focus on energy-storage devices.

His current research looks to exploit traditional failure mechanisms and interactions in batteries, turning unwanted behaviours into beneficial mechanisms. His efforts in this area over the last decade have been adopted by various industries and have led directly or indirectly to five electrochemical energy-related start-up companies, the latest being Feasible, an effort dedicated to exploiting the inherent acoustic responses of closed electrochemical systems.

Steingart joined Columbia Engineering in 2019 from Princeton University where he was an associate professor in the department of mechanical and aerospace engineering and the Andlinger Center for Energy and the Environment. Earlier, he was an assistant professor in chemical engineering at the City College of the City University of New York. Even earlier, he was an engineer at two energy-related start-ups. He received his PhD from the University of California, Berkeley, in 2006.

‘Hell-like’ exoplanet experiences rocky rain and supersonic wind

A fiery exoplanet located barely a million kilometres from its parent star is covered by magma oceans and has an atmosphere of vapourized rock on its “day” side, whilst its “night” side remains cold enough for glaciers to form. This is the finding of scientists at McGill University and York University in Canada who, with colleagues at the Indian Institute of Science Education, performed computer simulations using data from NASA’s Kepler Space Telescope and Spitzer Space Telescope to investigate the atmospheric dynamics of the exoplanet K2-141b. The scientists say that the complex interactions they uncovered reveal new information about the early years of other rocky planets – including Earth, which likely passed through a similar molten phase as it formed.

At just 0.00716 AU (astronomical units, the distance from the Earth to the Sun), the orbit of K2-141b is so small that the exoplanet’s parent orange dwarf star blots out a 50-degree chunk of its sky, compared to 0.5 degrees for the Sun as viewed from Earth. This close proximity means that the exoplanet most likely has a tidally locked orbit such that its host star never moves in its sky. It therefore has a permanent day side where temperatures soar to 3000 °C, and a night side where they plunge below –200 °C.

A magma ocean and rocky rain

In a study published in the Monthly Notices of the Royal Astronomical Society, researchers led by Nicolas Cowan and Giang Nyugen show that K2-141b’s extreme temperature difference causes large pressure gradients. Consequently, supersonic winds sweep across its surface, transferring material from one side of the exoplanet to the other. This transfer process exists alongside evaporation and sublimation, which – much like the water cycle on Earth – help to circulate material through the lava planet’s atmosphere.

When heat on K2–141b’s day side evaporates the molten rock at its surface, the resulting cloud of mineral vapour is blown to the night side at a rate of 5000 km/hr. There, it falls onto the exoplanet’s magma ocean and beyond the shore as rain or snow. From there, the material will eventually arrive back at the day side via ocean circulation, but the researchers predict, based on their simulations, that the return flow will be very slow. They also predict that conditions on K2-141b, as on other terrestrial planets, will change over time. Further constraining the composition and dynamics of K2–141b’s atmosphere could therefore help determine the evolution of other exoplanets, and suggest ways of observing them, they say.

Candidate for further study

Because K2-141b is so close to its host star, the researchers conclude that it would be a perfect candidate for transit spectroscopy. This technique relies on the fact that when an exoplanet passes in front of its parent star, the starlight must travel through the exoplanet’s atmosphere to get to Earth. In the process, molecules in the atmosphere absorb certain light wavelengths whilst letting others pass through unhindered; hence, a careful analysis of the star’s spectrum reveals information about the composition of the exoplanet’s atmosphere. The fraction of stellar light that reaches Earth after such transits is, however, extremely small, which restricts the telescopes and instruments that can be used to observe it. Studies like this one therefore help constrain which systems are best-suited to transit spectroscopy.

Arecibo Observatory will be decommissioned, says US National Science Foundation

Officials at the US National Science Foundation have decided to decommission the iconic Arecibo Observatory in Puerto Rico after a second cable failure caused fresh damage to the telescope’s metal platform, which is suspended above the 305 m-wide reflecting radio dish. According to a statement from the University of Central Florida, two of the remaining main cables seem to have wire breaks, increasing the likelihood of the tower platform falling and destroying the telescope.

Opened in 1963, the observatory is currently the world’s second-largest single-dish telescope. On 10 August one of the six 8 cm-wide auxiliary steel cables added in a 1990s upgrade failed. This tore a 30 m gash through the main reflector dish. Then on 6 November one of the dish’s four main cables snapped, transferring the load onto the remaining cables and so making them more likely to fail. The observatory has also suffered hurricane and earthquake damage in recent years.

Yesterday, the National Science Foundation – one of the organizations that manages the observatory together with the University of Central Florida (UCF), Universidad Ana G Méndez and Yang Enterprises – announced that the damaged areas could not be stabilized safely.

Safety a priority

“NSF prioritizes the safety of workers, Arecibo Observatory’s staff and visitors, which makes this decision necessary, although unfortunate,” NSF director Sethuraman Panchanathan said in a statement.

Ralph Gaume, director of NSF’s Division of Astronomical Sciences, adds, “Leadership at Arecibo Observatory and UCF did a commendable job addressing this situation, acting quickly and pursuing every possible option to save this incredible instrument”. “Until these assessments came in, our question was not if the observatory should be repaired but how. But in the end, a preponderance of data showed that we simply could not do this safely. And that is a line we cannot cross.”

Astronomers and members of the public have been sharing their thoughts on the closure using the Twitter hashtag #WhatAreciboMeansToMe. The Planetary Society posted, “We are sad to say goodbye to Arecibo but we’re grateful for its phenomenal contributions to space science and planetary defence,” referring to the observatory’s role in tracking near-Earth asteroids.

Hollywood star

Many people have been highlighting Arecibo’s role in the 1997 film adaptation of Carl Sagan’s novel Contact, which is about humanity’s first contact with an extra-terrestrial civilization. Others have referred to the 1974 “Arecibo message” – an interstellar radio signal carrying basic information about humanity that was sent from a radar transmitter at the observatory to the globular star cluster M13.

Some have posted personal messages on Twitter, including the educator, physicist and former astronomer Emily Alicea-Muñoz who got married at the telescope and shared some of her wedding photographs (see below).

#Arecibo has always meant so much to me, growing up as an astronomy-obsessed little kid in PR. As an undergrad I got to attend an observing session which was beyond cool. 10yrs ago I married @AstroAhura at the observatory. The decommissioning is so sad 😭#WhatAreciboMeansToMe pic.twitter.com/g4JlYq3tk0

— Emily Alicea-Muñoz, PhD (@drealiceam) November 19, 2020

Gold nanotubes and infrared light could treat asbestos-related cancer

Gold nanotubes can destroy cancer cells, according to physicists and medical researchers at the University of Cambridge and the University of Leeds. They found that their nanotubes, which were tuned to have strong near-infrared absorption, can enter mesothelioma cells and destroy them when heated with laser light.

Every year, more than 2700 people in the UK are diagnosed with mesothelioma. This cancer usually grows in the pleural membrane, a thin lining that surrounds the lungs. The vast majority of cases are caused by exposure to asbestos dust. When damaged, asbestos releases microscopic fibres that can be inhaled. These fibres can then migrate through lung tissue into the pleural membrane and cause mesothelioma to develop. Asbestos has been banned in the UK since the late 1990s, but mesothelioma can take from 15 to 60 years to develop.

“Mesothelioma is one of the ‘hard-to-treat’ cancers, and the best we can offer people with existing treatments is a few months of extra survival,” says Arsalan Azad at the University of Cambridge. “There’s an important unmet need for new, effective treatments.”

To develop a potential treatment, the researchers turned to gold nanotubes. They hypothesized that if absorbed by mesothelioma cells and then heated using near-infrared light, these hollow tubes – one thousandth the width of a human hair – would destroy the cancer cells.

Tests in an aqueous solution showed that, when heated with a laser at a wavelength of 875 nm with a power density of 1.9 W/cm², the nanotubes increased in temperature by up to 9°C, high enough to cause localized killing of cancer cells. Next, the team added the nanotubes to mesothelioma cell cultures and tracked them using various microscopy techniques, observing that the nanotubes were absorbed by the cells.

The researchers then exposed mesothelioma cell cultures with and without the gold nanotubes to the near-infrared light for 10 min. Laser irradiation alone did not cause cell death, but laser exposure combined with the nanotubes killed roughly half of the cells. They report their results in Small.

Key to the nanotubes’ potential to destroy cancers within the human body is their tunability. Stephen Evans, a physicist at the University of Leeds, says that there are two near-infrared windows in which light has good optical penetration through tissue. He explains that you need to tune the nanoparticles so that they best absorb – and convert to heat – light at those wavelengths.

The optical properties of gold nanomaterials are controlled by the density of free electrons that the light couples to and causes to oscillate, Evans explains. “That is dependent on the size, in our case, the thickness of the wall of the gold,” he says. “If we made thicker walled gold, we would shift the absorbance wavelength, and if we made the walls thinner, we would shift it in the opposite direction.”

The nanotubes are created using a solution-based technique. First, a silver seed particle is grown into a silver nanowire, and then a gold salt is added to the water-based solution. The gold deposits onto the surface of the silver, which oxidizes and becomes water soluble. The silver then dissolves away leaving the gold nanotube.

Gold nanotubes from silver nanowires — (a) Preparation of gold nanotubes using silver nanowires as templates; SEM images of (b) silver nanowires and (c) gold nanotubes (inset scale bars: 200 nm); (d) TEM image of a gold nanotube, with the dark edge showing the wall thickness. (Courtesy: CC BY 4.0/*Small* 10.1002/smll.202003793)

Evans tells Physics World that the thickness of the gold is tuned by controlling the size of the silver nanowires. In the study, the silver nanowires were around 100 nm in diameter, which led to a gold thickness of 12 nm. The team also used nanowires with average diameters of 65 and 193 nm as templates to prepare gold nanotubes with thinner and thicker walls. “We showed that we could make silver nanowires of different diameters and thereby produce gold tubes with different wall thicknesses,” Evans says.

Part of the attraction of using these gold nanoparticles to treat mesothelioma is that they have similar dimensions to asbestos fibres. The hope is that that they will also get trapped in the same part of the body, aiding the delivery of treatment. “We are sort of mimicking what causes the disease so we can use that as a treatment,” Evans explains.

Optical clock sets new constraints on dark matter

An optical clock has been used to set new constraints on a proposed theory of dark matter. Researchers including Jun Ye at JILA at the University of Colorado, Boulder and Andrei Derevianko at the University of Nevada, Reno, explored how the coupling between regular matter and “ultralight” dark matter particles could be detected using the clock in conjunction with an ultra-stable optical cavity. With future upgrades to the performance of optical clocks, their approach could become an important tool in the search for dark matter.

Although it appears to account for about 85% of the matter in the universe, physicists know very little about dark matter. Most theoretical and experimental work so far has been focussed on hypothetical dark-matter particles, including WIMPS and axions, which have relatively large masses. Alternatively, some physicists have proposed the existence of “ultralight” dark matter particles with extremely small masses that span many orders of magnitude (10⁻¹⁶–10⁻²¹ eV/c²).

According to the laws of quantum mechanics, the very smallest of these particles would have huge wavelengths, comparable to the sizes of entire dwarf galaxies – meaning they would behave like classical fields on scales we can easily measure.

Atomic energy levels

If these fields coupled to normal matter, these ultralight particles would generate oscillations in the values of fundamental physical constants. This includes the mass of the electron and the fine structure constant, which defines the strength of the electromagnetic interaction between charged particles. Both constants affect the energy levels of atoms, which are used in the operation of atomic clocks. Therefore, subtle variations in these fundamental constants would affect the timekeeping of an atomic clock.

In their study, Ye and colleagues measured the ratio between time signals produced by a strontium optical lattice clock (a type of atomic clock) and a resonant optical cavity made of silicon. This ratio should be extremely sensitive to the effects of ultralight dark matter particles. They also compared the frequency of the silicon cavity to that of a hydrogen maser – a microwave device that is used as a very precise frequency standard and has been used in previous dark-matter searches.

As expected, the maser was the noisiest device, and the team has concluded that the optical clock and silicon cavity combination is best for searching for dark matter. Indeed, they have set new constraints on the masses of ultralight particles, which they hope will improve as optical clocks get better. With suitable modifications, the detectors could even be integrated into satellites, producing the first global-scale telescopes for dark matter.

The research is described in Physical Review Letters.

In the Physics World Weekly podcast, Andrei Derevianko explains how atomic clocks and other quantum sensors could be used to detect dark matter.