The black phosphorus composite material connected by carbon-phosphorus covalent bonds has a more stable structure and a higher lithium ion storage capacity. Credit: DONG Yihan, SHI Qianhui and LIANG Yan
A new electrode material could make it possible to construct lithium-ion batteries with a high charging rate and storage capacity. If scaled up, the anode material developed by researchers at the University of Science and Technology of China (USTC) and colleagues in the US might be used to manufacture batteries with an energy density of more than 350 watt-hours per kilogram – enough for a typical electric vehicle (EV) to travel 600 miles on a single charge.
Lithium ions are the workhorse in many common battery applications, including electric vehicles. During operation, these ions move back and forth between the anode and cathode through an electrolyte as part of the battery’s charge-discharge cycle. A battery’s performance thus depends largely on the materials used in the electrodes and electrolyte, which need to be able to store and transfer many lithium ions in a short period – all while remaining electrochemically stable – so they can be recharged hundreds of times. Maximizing the performance of all these materials at the same time is a longstanding goal of battery research, yet in practice, improvements in one usually comes at the expense of the others.
“A typical trade-off lies in the storage capacity and rate capability of the electrode material,” co-team leader Hengxing Ji tells Physics World. “For example, anode materials with high lithium storage capacity, such as silicon, are usually reported as having low lithium-ion conductivity, which hinders fast battery [charging]. As a result, the increase in battery capacity usually leads to a long charging time, which represents a critical roadblock for more widespread adoption of EVs.”
New black phosphorus anode material
The anode in most lithium-ion batteries is made of graphite. Researchers led by Ji at USTC and Xiangfeng Duan at the University of California, Los Angeles, made their new anode material by combining graphite with black phosphorus. This 2D layered material had been considered before as a candidate for anodes, but tests showed that its electrochemical performance was far below its theoretical potential.
One reason for the shortfall is that the material’s structure deforms during battery operation. This deformation, which begins at the edges of the black phosphorus layers, reduces the material’s quality to such an extent that lithium ions cannot easily transfer through it.
By combining black phosphorous with graphite, Ji, Duan and colleagues showed that the chemical bonds between the two materials stabilize the edge structure and prevent unwanted edge changes. To overcome the continued formation and build-up of an ionically less conductive solid-electrolyte interphase, the team applied a thin polyaniline gel coating to the electrode materials – a strategy that also reinforced the transport path for lithium ions.
Towards higher energy density and fast charging
The researchers tested the charging-cycle performance of their new electrode material by preparing sample electrodes using a method that is compatible with industrial fabrication processes. They found that their test devices had reversible capacities of 910 mA.hour/g, 790 mA.hour/g and 440 mA.hour/g after more than 2000 cycles at 2.6 A/g, 5.2 A/g and 13 A/g, respectively. For context, an anode material that can charge at 13 A/g with a reversible capacity of 440 mA.hour/g implies that an advanced lithium-ion battery made with this technology could be charged in less than 10 minutes.
“If scalable production can be achieved, this material may provide an alternative, updated graphite anode, and move us toward a lithium-ion battery with an energy density of higher than 350 watts-hour per kilogram,” says Sen Xin, a researcher at the Institute of Chemistry, Chinese Academy of Sciences, and one of the study’s co-first authors.This figure, he adds, means that an electric vehicle equipped with such a battery could travel 600 miles on a single charge – making it competitive with conventional combustion-engine vehicles. By way of comparison, the Tesla Model S can travel 400 miles on one charge.
Full details of the research are published in Science.
Honey and other highly viscous fluids can flow faster than water in specially coated capillary tubes. This surprising discovery was made by Maja Vuckovac and colleagues at Finland’s Aalto University who also show that the counterintuitive effect stems from suppressed internal flows within more viscous droplets. Their results directly contradict current theoretical models of how liquids flow in superhydrophobic capillaries.
The field of microfluidics is concerned with controlling the flow of liquids through tightly-confined capillary regions – often to create devices for medical applications. Low-viscosity fluids are best suited for microfluidics because they flow fast and effortlessly. More viscous fluids can be used by driving them at higher pressures, but this increases mechanical stresses in delicate capillary structures – which can lead to failure.
Alternatively, flows can be sped up using superhydrophobic coatings, which contain micro- and nanostructures that trap cushions of air. These cushions significantly reduce the contact area, and subsequently the friction, between liquids and surfaces – boosting flow by up to 65%. According to current theories, however, these flow rates are still consistently reduced as viscosity increases.
Superhydrophobic inner coatings
Vuckovac’s team put this theory to the test by observing microdroplets of different viscosities as gravity pulled them down through vertical capillary tubes with superhydrophobic inner coatings. As they travelled at constant velocities, the droplets compressed the air below them, creating pressure gradients comparable to those found in a piston.
Although the droplets displayed the expected inverse relationship between viscosity and flow velocity in open-ended tubes, the rule was entirely reversed when one or both of the ends were sealed. The effect was most pronounced for droplets of glycerol – which flowed over 10 times faster than water, despite being three orders of magnitude more viscous.
To uncover the physics behind this effect, Vuckovac’s team introduced tracer particles to the droplets. Over time, the motions of the particles revealed rapid internal flows within the less viscous droplets. These flows caused the fluid to penetrate into the microscale and nanoscale structures in the coating. This reduced the thickness of the air cushion, preventing pressurized air beneath the droplet from squeezing past to level out the pressure gradient. In contrast, internal flows were barely perceptible in glycerol, suppressing its penetration into the coating. This resulted in a thicker air cushion, allowing air below the droplet to move aside more readily.
Using their observations, the team developed an updated fluid dynamics model that can better predict how droplets travel through capillary tubes with different superhydrophobic coatings. With further work, their discovery could lead to new ways of creating microfluidic devices capable of handling complex chemicals and medicines.
“Ghosts don’t haunt us. That’s not how it works. They are present among us because we won’t let go of them.” M is for Malice by Sue Grafton
Vic Tandy – an engineer at Coventry University in the UK – once described how he worked for a medical-equipment manufacturer whose laboratory included a room that was believed to be haunted. Sure enough, when Tandy was holed up in that room late one night, he felt uneasy and uncomfortable, and kept seeing and hearing odd things. As it turns out, there was a faulty extraction fan in the room that made the air vibrate at 19 Hz. This sort of infrasound has been shown to produce a number of physiological effects, including breathlessness, shivering and feelings of fear. Scientists studying the effects of wind turbines and traffic noise near residences have found that low-frequency noise can cause disorientation, feelings of panic and other effects that could be associated with being “visited” by a ghost.
Despite many such examples of natural phenomena mistakenly being interpreted as ghosts, belief in them is widespread. At the last count, almost half the UK population believe a house can be haunted and 9% claim to have made contact with the dead, with believers often thinking that science is on their side. According to Albert Einstein, and the first law of thermodynamics, energy in the universe can neither be created nor destroyed – it merely changes from one form to another. When someone dies, so the argument goes, their energy must live on in some way. And this energy, according to believers of the supernatural, is converted into a ghost. As the website for US-based Tri County Paranormal investigations asks: “When we’re alive, we have electrical energy in our bodies. What happens, when we die, to the energy that was in our bodies, causing our heart to beat and making breathing possible?” Well the answer, of course, is that electrical energy stops flowing when you die, like switching off a light bulb; and the source of the electrical energy, our bodies, loses energy as heat and organic matter is transferred into the worms and bacteria that eat us, until there is nothing left.
Faraday was highly sceptical about ghosts, spirits and so-called psychic phenomena, and devised experiments to discredit these hypotheses
In the 19th century most people believed in the supernatural. Michael Faraday, however, was highly sceptical about ghosts, spirits and so-called psychic phenomena, and devised experiments to discredit these hypotheses. A popular “supernatural phenomenon” was table moving or table rotation, in which a group of people stand around a small circular table and place their hands flat on the table surface. After a while the table starts to move due, it is believed, to the life force in the table. The sceptical Faraday constructed a table with two surfaces separated with ball bearings and held together with rubber bands. If spirits were indeed responsible for the movement, and people’s hands were simply following, then the top surface should lag behind the lower one. In fact, the opposite was observed. The upper surface moved first followed by the lower one, indicating, pretty conclusively, that the participants were unconsciously pushing the table. When the sitters were shown the result and the experiment was repeated, no movement was observed.
A similarly popular device used for “contacting” the dead is the Ouija board, which features letters, numbers and usually the words “yes” and “no”. Participants put their hands on a pointer that moves and spells out the answers to questions. It’s easy to discredit this by repeating the séance when the participants are blindfolded, in which case the pointer often aims to where there are no letters or wanders off the board completely, demonstrating that spirits were not doing the driving. This shows that the participants were executing unconscious involuntary movements – a phenomenon called “ideomotor action” by physiologist William Carpenter in 1852. He wrote that “honest intelligent people can unconsciously engage in muscular activity that is consistent with their expectations” and believed this was the principle that explained the underlying mechanism of a variety of psychic phenomena such as Ouija boards, moving tables and the divining rod.
These days it is still possible to buy Ouija boards. Indeed, Amazon is currently advertising more than 100 models, some as cheap as £9.99, which seems like a bargain if you can use it to contact your dead granny and find out where she hid the cash. Being Amazon, of course, there are customer reviews, many of which are very favourable. One satisfied customer wrote “I have spoken to Elvis, Hitler and the demon that steals my socks. Apparently, my socks have been taken as an offering to Cthulhu.”
For the final word on ghosts, we go to physicist and TV presenter Brian Cox. “If we want some sort of pattern that carries information about our living cells to persist, then we must specify precisely what medium carries that pattern and how it interacts with the matter particles out of which our bodies are made,” he said on a recent edition of his podcast The Infinite Monkey Cage. “We must, in other words, invent an extension to the Standard Model of particle physics that has escaped detection at the Large Hadron Collider (LHC). That’s almost inconceivable at the energy scales typical of the particle interactions in our bodies.” Fellow presenter and celebrity scientist Neil deGrasse Tyson checked whether Cox was really claiming that the LHC disproved the existence of supernatural spirits. “If I understand what you just declared, you just asserted that CERN disproved the existence of ghosts,” he asked. “Yes,” replied Cox.
My phone’s alarm awakens me (JE) at the ass crack of dawn. Bleary-eyed and annoyed, I roll over to dismiss it for the fifth time and do my morning doom scroll of social media before convincing myself that I do, in fact, need to attend my morning Zoom call. I’ve been working from home for seven months, and the commute is great: my office is in my kitchen, 10 steps from my bed (yes, I counted). Still, amid the pandemic; the police killings of Black people followed by no justice; and the death of my dad in June, I find getting up to work difficult. As I’m brushing my teeth, I see dark circles under my eyes, and my springy curls – thinning from the stress, sadness and anxiety – jut out in all directions like a dented crown. I splash some water on my face and tell myself that this is going to have to do, but as I scroll to find the Zoom link, I see a pile of dishes in the sink behind me, papers strewn across the counter and groceries that we were just too tired to put away last night. I need a Zoom background, but the one I carefully picked from West Elm’s site to mimic my dream kitchen can’t figure out how to position itself around my curls. Even in the COVID-19 era of virtual meetings, I’m subliminally told that I don’t belong.
While a failed Zoom background is a relatively trivial problem, the lack of representation of Black individuals in artificial intelligence (AI) has major consequences. AI systems are designed to mimic human intelligence, and they have taken hold of our lives, from the targeted ads we see when we are online shopping to the “recommended” section on a Netflix home page. While these systems offer significant societal benefits, too many of them have detrimental effects on the Black community. We can joke about automated soap dispensers being “racist” – their infrared technology cannot recognize darker complexions – but the same technology is also used in pulse oximeters. Here, encoded racial bias could mean life or death for Black individuals, especially in the time of COVID-19 when monitoring oxygen levels is important.
Google searches are also powered by AI, and even apparently innocuous searches can have noxious side-effects. As the writer and communications expert Safiya Umoja Noble details in her essay “The enduring anti-Black racism of Google search”, over-sexualization of Black girls is widespread, as is the idea that naturally textured Black women’s hair is somehow “unprofessional”. While some may think these errors have been fixed, in June 2020 the Google Ad portal was still perpetuating the objectification of Black girls, and in July the Massachusetts Institute of Technology permanently pulled a widely used, 80-million-image AI dataset because so many of the images were labelled in racist and misogynistic ways. These datasets are used to train image-recognition software that feeds into technology such as self-driving cars, social media and facial-recognition software used by police.
The role of physicists in AI
My graduate work in particle physics focused on developing a form of AI called a convolutional neural network that can distinguish charged-current muon–neutrino interactions from neutral-current interactions at low energies. Before my work, these interactions were seen as essentially identical. My motivation for developing this algorithm was to suss out a very complicated signal in the mounds of data the MicroBooNE detector at Fermilab creates, but historically, basic research also motivates innovations and new technology. The fact that my research, my proximity to AI, could have detrimental effects on the Black community is discombobulating.
Physicists like me who study such abstract concepts don’t tend to dwell on how our research might work its way into society. However, I’ve come to realize that we can’t just bury our heads in the sand. We cannot develop AI because we want to find physics beyond the Standard Model without also recognizing that some aspects of our innovations could be used to harm the Black community. Black scientists need to be in the room when AI algorithms are developed, and as a physics community, we need to be discussing the ethics of our role in that work.
The coming quantum revolution
Like AI, quantum information science and engineering (QISE) is a rapidly evolving field with tremendous potential benefits. Quantum mechanics describes the laws of physics at microscopic length scales, and QISE researchers like me (CB) endeavour to control the systems that obey these fundamental laws. By doing so, we hope to engineer novel quantum devices such as quantum sensors, networks or computers, as well as novel quantum materials with properties that don’t exist at everyday length scales.
What makes a quantum sensor, computer or network unique is its reliance on quantum superposition. Superposition allows the bits in a quantum computer – qubits – to encode much more information than classical bits. This should help researchers solve some currently intractable problems, while quantum networks should enable fast and secure information transfer between quantum devices. Superpositions are also prone to collapse when measured, meaning that, in principle, if an eavesdropper tried to intercept information encoded in a superposition, their spying would be immediately detectable. It is precisely this sensitivity to measurement, or perturbation, that enables researchers to create quantum sensors that are far more sensitive than classical ones.
QISE could revolutionize our technology and understanding of nature, and it is important for Black physicists to have a stake in that revolution. Yet I have been part of the QISE world for nearly a decade, and throughout its rise I have rarely seen other Black physicists. Fellow QISE researchers (and all physicists): who do you see in your Gordon Research Conference photograph? Who is in the bustling hallways and jam-packed QISE sessions at the APS March Meeting, or in the massive banquet room at the APS DAMOP meeting? If you can only remember a few (or zero) Black physicists, that is not surprising: in 2017 Black students were awarded about 3% of physics undergraduate degrees and 2% of physics PhDs in the US, according to data from the National Center for Education Statistics.
Representation matters
The low number of Black physicists, particularly in QISE, presents several issues that must be addressed. First, representation matters. It’s important that future or younger Black physicists are empowered by established, visible Black physicists making an impact in QISE. Second, early-career doctorate holders (those who have held a PhD for 10 years or fewer) bring new skills and knowledge to the STEM workforce, and are considered important drivers of technological progress and knowledge generation – but few of us are Black (figure 1), and Black representation is low throughout science and engineering occupations (figure 2).
Figure 1 Race, ethnicity and sex of early career doctorate holders with a science and engineering degree, 2014. Hispanic may be any race. Other includes American Indian or Alaska Native, Native Hawaiian or Other Pacific Islander, and respondents who selected more than one race. (Courtesy: National Science Foundation, National Center for Science and Engineering Statistics. 2017. Women, Minorities, and Persons with Disabilities in Science and Engineering: 2017. Special Report NSF 17-310. Arlington, VA. Available at www.nsf.gov/statistics/wmpd)
The $100m IBM-HBCU Quantum Center is a good example of the ongoing effort required to change this unsatisfactory status quo. The centre was set up as a partnership between IBM and historically Black colleges and universities (HBCUs) in the US, and it aims to help HBCU students and faculty get involved in QISE in a way that promotes belonging. The centre will also provide research opportunities to (under)graduates and connect HBCU talent to the QISE community. In doing so, it will have a direct and positive impact on Black representation in QISE, both in academia and in industry.
The money pouring into QISE offers many other opportunities to increase Black representation. The US National Science Foundation (NSF) recently created three $25m Quantum Leap Challenge Institutes that aim to advance the state of the art in quantum sensing, networking and computation. Another NSF-funded centre, Quantum Foundry, is designed to do the same for quantum materials. The US Department of Energy recently created the $115m Quantum Systems Accelerator to advance technological solutions to problems in QISE. The quantum technological revolution these institutions help to bring about will be transformative for science and society, and there is an incredible opportunity here to ensure that Black students are included in the research and training they offer, and to give established Black physicists the financial and infrastructural resources they need to participate.
Figure 2 Scientists and engineers working in science and engineering occupations, 2015. Hispanic may be any race. Other includes American Indian or Alaska Native, Native Hawaiian or Other Pacific Islander, and respondents who selected more than one race. (Courtesy: National Science Foundation, National Center for Science and Engineering Statistics. 2017. Women, Minorities, and Persons with Disabilities in Science and Engineering: 2017. Special Report NSF 17-310. Arlington, VA. Available at www.nsf.gov/statistics/wmpd)
A call for action
We both dream of a “quantum leap” in Black representation in physics, and the bountiful funding for both AI and QISE research places existing and future research centres in a unique position to help bring that about. If these centres implement swift action to develop and include Black researchers – perhaps by adopting the goals of the IBM-HBCU Quantum Center – then the researchers they support will be positioned to make important contributions to these developing fields.
In AI and QISE, especially, the richness of experience that Black physicists bring to research teams improves problem solving and provides a much-needed perspective on how technologies should be implemented. Without a concerted effort to address the current under-representation, however, the scientific community risks missing out on this perspective, perpetuating harm to the Black community, and unjustly excluding Black people from technologies that will wholly transform society.
Halloween is tomorrow, so I could not resist reading an article about the physics of giant pumpkins. And these really are giant, the biggest ever tipped the scales at more than 1190 kg. What is more, a pumpkin of that size puts on about 15 kg a day when growing. That is the weight of a two-year-old child every day. Now here is the physics bit – how does the pumpkin plant transport enough food and water through a relatively narrow stem to sustain this growth?
Another question is why are the most humungous squashes squashed? While normal-sized pumpkins tent to be spherical in shape, giant pumpkins tend to flatten-out and have a small upward arch on the bottom (see figure).
Small craft warning: a tiny boat made at Leiden University. (Courtesy: Leiden University)
Shifting length scales by several orders of magnitude, the physicists Rachel Doherty and Daniela Kraft at Leiden University in the Netherlands and colleagues have created a tiny boat that is just 30 micron long. About one third the thickness of a human hair, the vessel was made using an electron microscope and a high resolution 3D printer.
Kraft and colleagues are primarily interested in studying how tiny objects travel through fluids, with the goal of creating devices that could deliver drugs within the human body. You can read more on CNN: “Scientists used a 3D printer to create the world’s smallest boat”.
Officials at the Culham Centre for Fusion Energy (CCFE) in Oxfordshire, UK, have announced that they have achieved “first plasma” on the upgraded Mega Amp Spherical Tokamak (MAST). Following seven year of upgrade work on MAST at a cost of £55m, the machine was fully powered up for the first time yesterday allowing the experimental programme to begin. Over the coming years, MAST-U will seek to demonstrate that high plasma performance could be scalable to an actual fusion reactor.
Operated by the United Kingdom Atomic Energy Authority (UKAEA), the CCFE is already home to the Joint European Torus (JET) tokamak. This is involved in testing the materials that are to be used in the ITER fusion experiment, which is currently being built in Cadarache, France. Whereas JET – like ITER – has a doughnut-shaped plasma, MAST on the other hand has a spherical plasma, shaped much like a cored-out apple. This design allows for a much more compact – and cheaper – device and it is hoped that this kind of tokamak could one day be used as a potential fusion reactor.
The MAST tokamak was operational between 1999 and 2013 before being shut down for the upgrade, which was completed last year. Known as MAST-U, it is expected to be able to create a plasma of deuterium with a timespan of around 2–4 s, compared with just 0.5 s before. A major feature of the upgrade is a new exhaust system – known as a divertor – that it is hoped will show that the tokamak is able to handle the intense exhaust heat emerging from the plasma more effectively than existing designs, including that used on ITER. The MAST-U divertor is intended to take a 50 MW/m2 heat load and reduce it to just 5 MW/m2.
The CCFE has also received £21m from the European Fusion Research Consortium and the UK’s Engineering and Physical Sciences Research Council to further enhance the upgrade. This will include doubling the neutral beam injection into the plasma from 5 MW with MAST-U to 10 MW. Work on that is expected to be complete by the end of next year.
A step up
Research on MAST is also expected to inform the conceptual design of the UK’s prototype fusion power plant – Spherical Tokamak for Energy Production (STEP). Work on the design is set to be complete by 2024 with the aim to build STEP by 2040. “MAST Upgrade ensures the UK is in the premier league of countries working on fusion – and will be vital in achieving UKAEA’s goal of building the STEP fusion power plant,” says Ian Chapman, UKAEA chief executive.
Black physicists are an incredibly diverse group of people. Although we are often discussed as a monolith, we hold a range of ascribed identities, a spectrum of genders, sexualities and disability statuses. We come from a variety of socioeconomic backgrounds and geographical contexts, some of us are multigenerational university graduates while others are first generation. Many of us are descendants of kidnapped and enslaved Africans who were brought to the Americas; others are not and can trace their roots directly to a specific place on the African continent. In other words, we represent a rich array of histories, all under the proud moniker “Black”. Even so, we have a few things in common: a passion for the challenging intellectual work of physics, the burden of representation, and the social responsibilities of a physicist.
The question of representation is often reduced to playing mentor and role model. When we discuss what our responsibilities are as Black people in physics, they are usually along the lines of what we can do to “uplift the race” and show off our capacity to make contributions to physics. Even when we acknowledge that we must transcend above convincing white people that we too are made up of the exact same baryons that they are – with the same varieties of capability – we think primarily in terms of how to promote the individual dreams of other Black and under-represented groups in physics. We work to steward people through the obstacle course of academic and industrial racism and misogynoir (gendered anti-Black racism) and we may have elders, cousins and other family members who are depending on us to be financially stable and successful. We do all of this while fearing for our lives in the face of anti-Black police and vigilante violence on top of government policies that indicate our lives simply don’t matter.
These are hefty responsibilities and yet they are not the totality of our obligations. We are not just Black people in physics: we are Black physicists. This means the problems and responsibilities that fall at the feet of any physicist are also our own. In addition to being a Black queer and Jewish femme, I am also a theoretical particle cosmologist, meaning that my academic lineage is tied to the nuclear weapons development programme known as the Manhattan Project. The reason my area of research could develop with significant support from the US government was its ability to contribute to the US military-industrial complex. In this context, it is important to think carefully and clearly about the ethical implications of the work that I do, who I do that work with and with what money.
Because we face an array of structural barriers that sometimes feel Biblical in scale, it can be easy to say, “I can’t worry about that.” But worry about it we must. The same police forces for whom a routine traffic stop means not just giving someone a ticket but also a bullet wound have, in my lifetime, become more heavily armed and militarized. US police have received weapons and gear from the same Department of Defense that many physicists work for after they finish their degrees in physics and that many academic physicists take funding from to support their research. Police then deploy their new equipment against protestors who pour into the streets calling for justice for our brutalized and murdered community members.
Black physicists have a unique perspective that is rooted both in our experiences with oppression at the hands of our own government and our histories of triumph
I often tell my mentees that their responsibility is to survive and try to thrive because it is so incredibly hard to be a Black person in physics. Getting through the day while properly taking care of ourselves already feels like a major victory in a white supremacist, patriarchal world. Because I understand these challenges, for over a decade and a half, I volunteered for the National Society of Black Physicists, chairing the cosmology and gravitation committee and one year co-chairing our 500-person joint annual conference with the National Society of Hispanic Physicists. During those years, the military sponsored our meetings, military representatives attending the conference recruited Black students for summer internships and I made friends with people who were or have since become military contractors. Black communities, including historically Black colleges and universities, are fiscally robbed and underserved, a reality that the defence industry is aware of in its recruitment and partnership efforts. I knew that the money had helped a lot of people, me included. So, I made the choice to say nothing, despite being the sort of person who spoke to thousands at the Boston march against the Iraq War the night the invasion began.
Nearly 20 years later, that war seems endless. Many people have died, and my love for my community and desire to support myself and fellow Black physicists means that I have silenced my own moral impulses. But there is a price to pay for my silence, which as the Black feminist writer Audre Lorde said, will not protect us. Black physicists are perhaps particularly well positioned to understand this: from the impact on Black lives, we can see very clearly how social forces and economic circumstances can shape generations, and we know the specific cost of militarization to our communities.
A unique perspective
I now think more critically about the advice to survive and to individually thrive. Increasingly, I understand that it is not particularly progressive to support the presence of Black people in physics if all we do is assimilate into power structures without challenging the arrangements that underpin them. I don’t mean to suggest that it’s ever been the goal to look and sound like white physicists – although we do have our own internal debates about that. I’m talking about integrating into a scientific culture that has accepted the production of death as a tangential, necessary evil in order to gain funding. One that will march for science without asking what science does for or to the most marginalized people. One that still doesn’t teach ethics or critical history to its practitioners.
Importantly, a Black physicist can simultaneously be a barrier breaker and be part of a scientific story that should give us pause. A powerful example is Carolyn Beatrice Parker, the first Black woman to earn a postgraduate degree in physics. In 1953 Parker earned a Master’s degree from the Massachusetts Institute of Technology (MIT) in nuclear physics and six decades later, I joined MIT as a postdoctoral fellow. Parker almost certainly is one reason why that door was open to me. But it’s also the case that during the 1940s, Parker worked on the Dayton Project, the part of the Manhattan Project that focused on plutonium research.
What does it mean for me to proclaim that Black Lives Matter, to believe colonialism in Africa (where materials for nuclear weapons have been mined) is wrong and to simultaneously claim a nuclear weapons researcher like Parker as an ancestor of great significance? What are the ethical implications of the new ideas we Black physicists are working on today – and the funding sources that support our work? We urgently need to have that discussion.
Black physicists have a unique perspective that is rooted both in our experiences with oppression at the hands of our own government and our histories of triumph. We can bring an important new ethical sensibility to the table. I don’t expect the conversation to be simple or for all of us to agree because we are a diverse community. But if we are going to talk about what it means to be Black in physics then what it means to wield the power of the physicist in a white supremacist, patriarchal, and colonialist world must be part of the conversation.
Researchers in Germany and Australia have grown two-dimensional materials directly on optical fibres for the first time, creating a new hybrid platform with possible applications in a host of optoelectronic devices, including photodetectors and non-linear light converters.
In their work, team members led by Falk Eilenberger, Andrey Turchanin and Antony George of the University of Jena and Markus A. Schmidt of the Leibniz Institute of Photonic Technology focused on a family of crystals known as transition metal dichalcogenides (TMDCs). These materials have the chemical formula MX2, where M is a transition metal such as molybdenum or tungsten and X is a chalcogen such as sulphur, selenium or tellurium. In their bulk form, TMDCs act as indirect band-gap semiconductors. When scaled down to monolayer thickness, however, they behave as direct band-gap semiconductors, capable of absorbing and emitting light at high efficiencies.
This property means that TMDCs in their 2D form are attractive building blocks for devices such as light-emitting diodes, lasers, photodetectors and solar cells. They could also be used to make circuits for low-power electronics, sensors or flexible electronic devices, and combining them with optical fibres could lead to further applications in non-linear optical devices and quantum technologies. But there is a catch: the task of transferring fragile, atomically-thin layers of material onto optical fibres is far from easy, and until now it had to be done manually, layer by painstaking layer.
The best of both worlds
The breakthrough came when the team, which also includes scientists from Sydney Nano, the Fraunhofer Institute for Applied Optics and Precision Engineering IOF and the University of Adelaide, developed a new growth process for 2D TMDCs. “By analysing and controlling the growth parameters, we identified the conditions at which the 2D material can directly grow in the fibres,” Turchanin explains. “The technique is based on chemical vapour deposition at a temperature of 700°C, which while high does not affect the properties of the optical fibres, which are heat-resistant up to 2000°C.”
The hybrid platform “combines the best of both worlds”, George says. The tiny thickness of TMDCs and planar substrates, he explains, means that the length over which light and matter can interact is usually restricted to less than a nanometre. This short interaction length reduces both the optical response of the TMDCs and the types of applications possible. Although coupling the TMDCs with optical resonators enhances the light-matter interactions, this strategy is limited to narrowband resonance, meaning that broadband, ultrafast operation remains challenging. In contrast, integrating TMDCs directly onto waveguides or optical fibres greatly increases the interaction length, even for broadband light.
While previous attempts at integration proved unsuitable for large-scale applications, the new technique overcomes this limitation. Growing the 2D materials directly on the optical fibres is a scalable process that, in effect, turns the fibres into 2D-functionalized waveguides, and produces interspersed monolayers of high-quality TMDC crystals that are around 20 microns long on fibres few centimetres in length.
Potential application areas
According the team, there are two main areas where the new hybrid system could find applications. The first is gas sensing. Here, the light-emitting properties of the TMDC would change as a gas is absorbed onto the functionalized fibre, leading to a change in the colour of light in the fibre. Since the fibres are so thin, a gas sensor based on this technology might be suitable for biotechnology or medical applications, the researchers say.
Another possible application would be to use the composite optical fibres as nonlinear optical converters, capable of transforming laser pulses into white light for spectroscopy. The Jena researchers also mention applications in quantum electronics and quantum communication. “These applications would exploit the tendency of TMDCs to form single photon emitters around point defects or their large nonlinearity for spontaneous parametric conversion,” Eilenberger tells Physics World.
The researchers, who report their work in Advanced Materials, attribute their success to the “diverse expertise” in their highly interdisciplinary team. They have filed a patent application for their invention, and now plan to optimize the light-matter interaction by tuning the fibre’s geometry before demonstrating a sensing application with their platform.
Drug-infused microneedles that dissolve upon insertion could make photodynamic therapy (PDT) more effective at treating skin cancers, according to a study in the Journal of Biophotonics.
Microneedle developers from Queen’s University Belfast and PDT researchers from the University of São Paulo fabricated arrays of 500-µm-long needles by mixing a water-soluble polymer and a precursor to a PDT photosensitizer. In experiments with mice, these dissolving microneedles proved better than topical cream-based administration – the conventional approach for PDT – at delivering the therapeutic agent to a tumour’s surface. The researchers say that the results are especially promising for the treatment of thicker skin lesions.
PDT works on the principle that certain chemicals that are innately non-toxic can be “activated” when irradiated with the right frequency of light. When this happens, these photosensitizing chemicals convert molecular oxygen in the surrounding tissue into reactive species, which damage nearby cells. By shining the light only on the tumour, clinicians can target cancerous cells while sparing adjacent healthy tissue.
The challenge in applying this technique lies in getting the photosensitizer to where it can have the greatest effect. Michelle Barreto Requena, at the University of São Paulo, and colleagues adopted a commonly used solution to this problem in that they delivered not the photosensitizer itself, but its precursor, aminolevulinic acid (ALA).
ALA is a naturally occurring metabolite involved in the biosynthesis of haem. When an external supply of ALA is introduced, the chemical is taken into cells, where it is processed to form the photosensitizer protoporphyrin IX (PPIX). Although this produces the required therapeutic levels of PPIX in the intracellular environment, applying ALA as a cream means that only the more superficial cells of the tumour are treated. This is the limitation that Requena and colleagues sought to overcome with their dissolving microneedles.
The team created their ALA microneedles by mixing the precursor with a combination of methyl vinyl ether and maleic anhydride. Shaping and drying the mixture in silicone moulds yielded arrays of microneedles hosted on square patches about 7 mm across. Each array’s 361 needles were 500 µm long, 300 µm wide at the base, and were spaced 50 µm apart.
To investigate the effectiveness of the microneedle arrays, the researchers tested them on live mice in which skin cancers had been induced. They secured the arrays against the animals’ lesions using tape, then waited 60 min for the ALA to penetrate the cancerous tissue and be converted to PPIX. They then measured the concentration of PPIX in the live animals using widefield fluorescence imaging and fluorescence spectroscopy. After the mice had been euthanized, the team measured the distribution of PPIX with depth using fluorescence confocal microscopy.
Compared with animals in which ALA had been delivered topically as a cream, the mice treated with microneedle arrays exhibited a 200% increase in mean PPIX fluorescence intensity according to spectroscopy analysis, indicating a much greater uptake of the precursor. Fluorescence microscopy of the tumour sections also showed that the microneedles produced diffuse distributions of PPIX down to depths of around 0.5 mm, whereas the cream concentrated the photosensitizer at the lesions’ surfaces.
As well as achieving greater penetration than conventional cream-based PDT, treatments based on dissolving microneedles could be more cost-effective, because they don’t need such high concentrations of ALA. They could also lead to shorter procedures, as ALA delivered at depth is absorbed more quickly. Given these benefits, the researchers are hopeful that their technique could extend access to skin-cancer treatment, and that it could be rolled out soon.
“Due to its low cost and simplicity, our approach may fit in many countries that need to increase the availability of treatment for the population,” says Requena. “There are always optimization studies that can be explored, such as decreasing the time for the dissolution of the microneedles. However, with these very promising results, we believe that this technology is potentially ready to be explored in clinical studies.”
The question of how much heat is given off when erasing a single bit of information has excited scientists for decades, given its fundamental implications for thermodynamics and computation. Physicists in Ireland and the UK have now asked this question of quantum-mechanical bits (qubits) and found that the qubits’ coherence can lead to surprisingly high heat dissipation. They say that the result has important implications for protecting thermally sensitive quantum hardware and also shines further light on the paradox of Maxwell’s demon.
James Clerk Maxwell proposed his famous demon in 1867 to show how it might be possible to violate the second law of thermodynamics, which states that the entropy of a closed system tends only to increase over time. Maxwell envisaged a tiny intelligent being controlling a door in a partition dividing up a box of gas, which is initially at a uniform temperature. He argued that by opening and closing the door at just the right times, the demon could sort the gas molecules according to their speeds so that one half of the box contains faster molecules and is therefore hotter than the other half. This would decrease entropy without directly transferring energy to the particles.
In 1961, Rolf Landauer at IBM put forward a principle stating that any logically irreversbile computation produces entropy. In particular, he found that erasing a single bit of information necessitates the release of a certain minimum amount of heat – kTln(2), where k is Boltzmann’s constant and T is the temperature. Drawing on this idea, Charles Bennett, also at IBM, then argued in 1982 that this principle explains Maxwell’s paradox. The demon relies on a memory to sort the molecules, with that memory needing to be continually updated and scrubbed – thereby boosting entropy.
Is qubit erasure more expensive?
In the latest work, John Goold, Giacomo Guarnieri and Mark Mitchison at Trinity College Dublin and Harry Miller at the University of Manchester, investigated whether qubit erasure dissipates more heat than does scrubbing a classical bit. Qubits can be in superposition states, allowing them to exist as a one and a zero at the same time and so forming the basis of quantum computers.
As Goold and colleagues point out, heat dissipation can be minimized by erasing information over an extended time in order to keep the system close to thermal equilibrium. Nevertheless, there will always be some thermal fluctuation, meaning that in practice there is a chance that any given bit erasure dissipates significantly more heat than the Landauer limit.
Physicists have done several experiments looking at how low this dissipation can be pushed – typically using particles undergoing Brownian motion and confined in double-well potentials. This research showed that when erasing information repeatedly, the amount of heat dissipated forms a Gaussian distribution with an average close to the Landauer limit.
To see whether the statistical distribution of dissipations looks any different in the quantum case, Goold and colleagues extended the theoretical analysis of double-well potentials. In these two-level systems, the stored bit value simply corresponds to which of the wells the particle occupies. A demon can raise the energy level of either well so that its potential is nearly flat and then allow thermal fluctuations to tip the particle into the other well – if it is not there in the first place. This re-sets the system to a pre-defined state, so erasing the bit value and releasing some heat to the environment.
Tunnelling option
In the case of a qubit that is a coherent superposition of the two well states, however, there are two ways to overcome the remaining energy barrier – one is to hop over it via thermal fluctuations and the other is to tunnel underneath it. This latter mechanism can generate large quantum fluctuations in the dissipated heat, meaning that on average some small fraction of erasure cycles will release a large amount of heat to the environment.
Doing their sums, Goold and colleagues found, as expected, that the average amount of heat dissipated when erasing a qubit is like that given off in the case of a classical bit. However, they discovered that the energies involved in the most wasteful erasures in the two cases are very different. In one simulation they found that the most heat given off in any single classical cycle was about four times the Landauer limit, whereas in erasures that involved tunneling – roughly one in every thousand for a qubit – heat dissipation could exceed the Landauer limit by more than a factor of 30.
They reckon that even this small fraction of erasure cycles could pose a serious problem of potential overheating in future quantum-scale devices, given that modern computers irreversibly process many billions of bits each second. They also say that it provides further perspective on the paradox of Maxwell’s demon, given that a demon with a quantum memory will end up dissipating even more heat than one with a classical memory.
The results for the moment remain purely theoretical, but the group says that they provide markers for experimentalists to distinguish between quantum and thermal fluctuations – the fact that only the former can generate two consecutive events involving emission of energy quanta. State-of-the-art quantum technologies such as superconducting circuits could be used to look for such dual events, they add.
