Images gathered by NASA’s OSIRIS-REx mission to the near-Earth asteroid 101955 Bennu show the asteroid ejecting small amounts of material in three distinct events. The observation was made by an international team of astronomers led by Dante Lauretta at the University of Arizona, who believe that it suggests that all similar asteroids may be at least partially active. If correct, this insight could improve our understanding of the evolution of asteroids, and the origins of interplanetary dust.
The OSIRIS-Rex team has also decided that the spacecraft will land at a site on Bennu called “Nightingale”, which is located in a crater high in the asteroid ’s northern hemisphere. There, the spacecraft will obtain a soil sample before taking-off in 2021 for a scheduled return to Earth in 2023. The site was chosen from four perspective locations on the asteroid because it appears to be devoid of large rocks and boulders. It is, however, only about 140 m wide, so NASA scientists must be careful not to set the spacecraft down onto one of the large boulders just outside the perimeter of Nightingale.
Made of boulders
Bennu is a roughly spherical asteroid around 500 m in diameter that circles the Sun in an orbit that allows it to get relatively close to Earth. It is believed to be composed of boulders held together by gravity. Earth-based observations have indicated that Bennu’s composition resembles some active asteroids, which undergo continual mass loss.
NASA’s OSIRIS-REx mission was launched in 2016, with the goal of returning a sample from Bennu’s surface back to Earth in 2023. In preparation, the spacecraft is currently conducting a thorough scan of the asteroid.
When studying the images sent back by OSIRIS-REx, Lauretta’s team spotted evidence for three distinct ejection events between January and February 2019. Each event occurred in the late afternoon in local solar time, and produced around 100 particles with a range of energies and trajectories. By tracking individual particles across multiple images of the largest event, they determined that the ejecta typically were centimetre-sized particles travelling at speeds up to 3 m/s.
Crashing back down
The particles originated from widely varied sites across the asteroid. While some particles escaped into interplanetary space, others orbited Bennu for several days, before crashing back to its surface.
By studying the events, Lauretta and colleagues were able to rule out two ejection mechanisms typically seen in active asteroids. These are ice sublimation, which creates the tails of comets, and rotational disintegration.
That left a variety of possible mechanisms in contention, including impacts from micrometeorites and re-impacting particles, and the loss of water trapped in silicate materials within the asteroid. Furthermore, thermal fracturing could result as Bennu’s boulders are subjected to temperature changes of up to 100 K, over rotational periods of just 4.3 h.
The team’s surprising findings suggest that all asteroids could be active to at least some extent. The theory will be further scrutinised in 2020, when JAXA’s Hayabusa2 mission is expected to return a sample from Ryugu – an asteroid with similar characteristics to Bennu, but with a different composition. Through such future missions, in combination with further Earth-based observations, astronomers could gather important insights into how asteroids evolve, and the ways in which they supply dust to interplanetary space.
In addition to all the talks here at the UN climate summit in Madrid (COP 25), there is plenty of climate-inspired artwork. The most eye-catching is this 3-m version of Support by the internationally-renowned Italian artist Lorenzo Quinn. It’s a reproduction of the sculpture he originally made for the 2017 Venice Biennale, where two giant hands emerged from the city’s Grand Canal to appearing to prop up the historic Ca’ Sagredo Hotel.
Working with the UN, Quinn will be producing 25 replicas of the work for auction, with proceeds going towards climate programmes. I caught up with Quinn at COP 25 to find out what inspired the work and whether he believes artists have a role to play in raising climate change awareness. “I visualised the problem, I rethought the way I have to live my life, at least in part… in the end we’re humans and humans need to collaborate and come together,” said the artist and former actor, who played the role of surrealist artist Salvador Dalí in the 1991 film Dalí.
Quinn signs a replica artwork in support of climate action (Courtesy: James Dacey)
Biopsy: an examination of tissue removed from a living body to discover the presence, cause or extent of a disease.
This clinical definition obscures much of a patient’s experience during and after a biopsy. Local tissue injury, bleeding, inflammation, stress and elevated risk of metastasis are not uncommon.
What if there were a less invasive method for performing a biopsy, especially for potential cancer diagnoses? And what if this method could provide information about tumour heterogeneity that conventional biopsies cannot?
The researchers’ system uses electroporation – the application of pulsed electric fields to tissues – as a type of molecular biopsy that could be performed even when the precise locations of tumours in a tissue or organ are unknown.
The technique works in two stages. First, researchers apply short electric field pulses at high voltages to make the membranes of cells in tissues vulnerable to external influences. Next, they use longer-duration pulses at low voltages to extract molecular components of interest from these cells. Electroporated molecules are then removed from the tissue using a solvent or other fluid, and pre-existing methods used to analyse molecules of interest.
While the technique sounds simple in principle, a lot of work goes on behind the scenes to optimize and validate the method. Extracting delicate proteins and RNA requires researchers to manage dynamic variables, including electric field strength, pulse duration, pulse number and pulse frequency, to name but a few. In the current work, the researchers performed numerical modelling and related studies to optimize parameters and choose which parameters they would use. Then, they applied their technique to study the molecular signatures of excised mouse livers, kidneys and liver tumours.
The researchers were able to distinguish between the genes and proteins of different tissues and conclude that their technique maintains gene expression and protein functionality, propelling the technology to the next stage.
Work on the horizon
Now, the researchers must learn how their electroporation technology impacts human tissues and the structural integrity of RNA. They must also decide which clinical cases and tumour types would benefit most and apply the technology to whole organs ex vivo and, eventually, in vivo.
Still, electroporation could be a major step toward tumour profiling and precision medicine as it might provide critical information about tumour heterogeneity that conventional biopsies cannot.
“Identifying tissues not only by their structure and morphology, as is done today, but also by their unique molecular signatures can assist in diagnosis and decision-making,” says lead author Alexander Golberg, professor of environment and earth sciences at Tel Aviv University. “We hope to rapidly find commercial partners to bring this method to the clinic.”
Extracurricular: (From left) Leïla Haegel, Jana Lasser and Salma Sohrabi-Jahromi at the 2019 Lindau meeting. (Courtesy: Jude Dineley)
If you’re aspiring to a life-long career in science, it can often feel like there aren’t enough hours in the week. Finding time to look beyond your own research, however, can be rewarding. At the 69th Lindau Nobel Laureate meeting in Germany this July, I spoke to three early-career scientists active in projects to make academia healthier, more productive and equitable, while picking up valuable experience along the way. Here are their experiences.
Tackling toxic work environments
Originally from Austria, Jana Lasser was a postdoc at the Max Planck Institute for Dynamics and Self-Organisation in Göttingen, Germany, until the end of November this year. In the spring, she finished her PhD investigating the formation of curious geometric patterns on the surface of salt flats such as the Badwater Basin in California. Beyond the lab, last year she served as the elected spokesperson of the Max Planck PhDnet, a network of about 5000 PhD candidates, making her acutely aware of the challenges that early-career researchers face.
With hours approaching that of a full-time job, Lasser’s spokesperson role was tough to balance with her PhD, but well worth it. “It was the best thing I did in my life so far,” she says, adding that it was “like a crash course in everything – leadership, communication, negotiation, conflict management, time management and project management”.
There is mounting evidence that large numbers of young scientists suffer from stress and mental illness
Mental health was a hot topic during Lasser’s term. There is mounting evidence that large numbers of young scientists suffer from stress and mental illness and, typically, problems at work play a significant role. Consequently, when the PhDnet became involved in a Max Planck task force on employee health, Lasser and her colleagues pushed to increase the focus on mental wellbeing. A direct outcome was a 24-hour mental-health crisis hotline for all staff, which launched in April. It has sparked plenty of interest. “Many people asked me, ‘Is it already there? We want it. We need it’,” she says.
Though valuable, Lasser describes the hotline as, ultimately, a “really good band-aid”, naming the publish-or-perish mentality and job insecurity as critical underlying factors. “If you know that you’re probably going to move every second year, then it’s hard to build a network, it’s hard to have a life outside of work…relationships are destroyed,” she says. “I’ve seen that in my circle quite a number of times.”
Supervisor behaviour, too, can have a decisive impact on young researchers, with bullying at one extreme. In 2018 allegations of bullying involving senior researchers at two Max Planck institutes hit the mainstream media. After assisting in the immediate aftermath, Lasser became part of a second task force addressing the issue at a systemic level. She spoke to hundreds of doctoral researchers, many of them victims, about their experiences, informing a white paper she wrote with her colleagues. Among a long list of recommendations were robust institutional frameworks for handling disputes promptly and mandatory management training for PIs. “When you become a group leader or professor, you are not necessarily educated to be a good manager” says Lasser. “We have to recognize that and train these people.”
Breaking down barriers
Also based in Göttingen, Salma Sohrabi-Jahromi is a PhD candidate at the Max Planck Institute for Biophysical Chemistry. Barriers – and breaking them down – is a theme that has popped up repeatedly in her career. She is a computational biologist, who switched from biotechnology. A true interdisciplinarian, her research sits at the boundary between biology and physics, investigating the behaviour of biomolecular condensates and their physical properties in cells.
Political barriers – albeit indirectly – inspired Sohrabi-Jahromi to become active in the academic community as an undergraduate. Studying in Tehran in her home country Iran, she experienced the isolation of the local science community caused by political sanctions. Then, the city was chosen to host a rare international meeting in her field. She jumped at the chance to help organize it and network with visiting scientists. “It was really rewarding,” she recalls. Guests included a researcher from her current institute and, through them, she learned about the Max Planck international graduate programme, which she eventually ended up joining.
In Germany, outside of her research, Sohrabi-Jahromi has helped tailor computer-science and statistics teaching in her institute, to better engage graduate biologists. Spurred on by her earlier experience in Tehran, she has also helped organize events for early-career researchers, from seminars on careers and mental health for new PhDs, to interdisciplinary gatherings. “I’m particularly interested in bridging between different fields in science and somehow bringing people from different backgrounds together,” she says. One such event Sohrabi-Jahromi helps organize is Horizons, a local conference that brings scientists together from across the life sciences. There, she’s found it particularly gratifying to see collaborations sprout at the event, some of them lasting several years.
When two fields collide, the rewards can be large. “In biology, a lot of amazing discoveries that have happened are by physicists,” she says, citing research by biophysicist and Nobel laureate Joachim Frank as a favourite example. He continues to advance cryogenic electron microscopy – an imaging technique he pioneered last century – to observe cell dynamics, facilitating research in molecular medicine.
Like Lasser, Sohrabi-Jahromi has no regrets about getting active in the scientific community. “It has helped me, really a lot, not only in meeting really amazing people and networking with them, but also to develop myself personally,” she says. “[I’ve learned] how to form groups and communicate with people and how to lead when things are not moving on.”
Empowering young scientists in Africa
Based at the University of the Balearic Islands in Palma, theoretical physicist Leïla Haegel analyses gravitational-wave data from the prestigious LIGO experiment. Outside of her research, it’s been a busy year for the French postdoc. She welcomed the arrival of her first child – a little boy – and founded ADARA, the Association for the Development of Arab Research.
Haegel’s goal is to help empower young people in Arab countries who are interested in scientific research. She established ADARA to formalize her efforts after organizing a four-week internship in her group for a high-school student from Morocco – Haegel has strong family ties in the country. The student was interested in both science and engineering, but was unsure which path to follow.
Earlier in her career, Haegel did internships at CERN and her current university. “There should be more internships, just to see how things are [in research]” she says. “It was a good experience for me.” The student’s internship was a success. She wrote her first Python code and got a taste of how astrophysicists work day-to-day – often without telescopes, as she was to discover. She is now keen to pursue a career in physics.
For Haegel, the experience highlighted how, without support, students often don’t make fully informed decisions when opting for a career in research – even more so in countries such as Morocco. “[These] problems that we experience everywhere in science are stronger in countries with less investment in research.”
Through ADARA, Haegel plans to expand her activities beyond internships, starting in Morocco. “The idea is to reach more Moroccan students and researchers by providing [scientific] seminars there, as this is a country where little money is available to invite researchers.” ADARA’s first events begin this month in universities in Casablanca and Rabat. She is also planning round table events on careers and specific issues, such as grant applications, featuring local and international researchers.
While paying forward her privilege motivates Haegel, she also sees ADARA as one small step towards a more inclusive – and ultimately more productive – community that she would like to work in. “I am not interested in science being an elitist club but in spreading it as much as possible.”
More broadly, there are other aspects of academic culture that need be addressed, she says – including the pressure to publish, systemic biases against researchers who are carers, and financial barriers and visa difficulties that prevent non-Western researchers from attending conferences. Early-career scientists should play a part in driving change, she believes. “We don’t have to accept those drawbacks, and as young scientists, I feel that it is also our responsibility to solve these issues, so we can do better science as a non-elitist community.”
As the UN climate climate summit (COP 25) enters its final scheduled day on 13 December, global energy markets have taken centre stage in Madrid. One of the key goals is to reach international agreement around article six of the 2015 Paris accord – putting a price on carbon and restructuring the way carbon is traded. But away from the negotiating tables, there has been no shortage of prominent voices sharing their vision for the future of global energy.
On 10 December, former US vice-president Al Gore delivered an 80-minute speech, lamenting the failure of the international community to curb greenhouse-gas emissions despite scientific consensus on the impacts of climate change. Gore, who won the Nobel Peace Prize in 2007, was positive, however, about the speed at which nations are investing in renewable energy at the expense of fossil fuels.
“If you are tempted to believe that we simply don’t have the will to survive that we simply don’t care what happens to our children and future generations, that we simply don’t have the will to act – remember that the will to survive and the will to act are themselves renewable resources,” Gore told delegates in Madrid.
A subprime carbon bubble?
Speaking about investment markets, Gore spoke of a “subprime carbon bubble”, comparing the current fossil-fuel industry to the subprime mortgage market in the US – the collapse of which contributed to the global financial crisis of 2007–08. Gore says that fossil-fuel companies have seen a decline in financial returns greater than 10% over the past three years, a trend he believes will continue as nations divest in coal at increasing rates.
“Reality is beginning to set in”, he said. “In fact BP just acknowledged that some of its reserves will never see the light of day. That’s the beginning of the reckoning.”
Al Gore says the will to act is a renewable resource
Al Gore says the will to act is a renewable resource
Al Gore says the will to act is a renewable resource
On the flip side, Gore pointed to the faster-than-expected growth of renewables within the global energy mix, especially wind energy and solar. “Yesterday [9 December] the continent of Europe got 30% of all of its electricity from wind…because the cost has been coming down so rapidly and this cost decline is continuing”.
Seeking a just transition
While most at COP 25 agree that renewables will continue to grow its market share, there is an active debate about how to ensure that the energy transition does not leave some communities behind. In Europe, the new Green Deal set out by the European Commission in Brussels on 11 December includes a €100bn “just-transition” fund to support communities currently reliant on carbon sectors, including coal, oil and gas. Poland in particular stands to benefit from the fund as it accounts for more than 50% of EU coalminers.
At the Madrid summit, the UN also chaired a session yesterday to consider how the global energy transition can be accelerated, in line with its sustainable development goals and the Paris agreement. Francesco La Camera, the general-secretary of the International Renewable Energy Agency (IRENA), said renewable energy had created 700 000 jobs in 2018 – a figure that exceeded the number of jobs lost in the fossil fuel industry.
La Camera said that renewables combined with energy efficiency measures can achieve 90% of the required emissions reductions to meet the Paris target of carbon neutrality by 2050. In September, IRENA launched the Climate Investment Platform to direct the flow of capital towards renewables projects, especially in the developing world. “Enhanced ambition needs to meet with increasing investment. We are now at the level of $35bn per year in renewable investments. We need to double this amount of investment,” he said.
The nuclear question
In the same session, Rafael Mariano Grossi, the new director-general of the International Atomic Energy Agency (IAEA), made the case for nuclear energy. “We are part of the solution, we are not the solution. We are working with others and we are note a replacement for other sources of energy,” he said.
Argentina’s ambassador to Austria, Grossi became the first Latin American to head the IAEA when he assumed office on 3 December. He pointed out that despite some perceptions, nuclear power has in fact increased in the world energy mix during the past five years, with 37 new nuclear power plants coming online and nearly 50 in construction.
“We deploy a solid steady baseline energy with no greenhouse gas emissions that is there all the time, come rain come shine,” said the career diplomat. “It is actually very difficult to imagine how countries, especially countries using nuclear energy at the moment, could ever meet their climate commitments in terms of the agreement reached in Paris without nuclear energy.”
Grossi also emphasised how the IAEA is contributing to climate change mitigation through its scientific research programme. That included developing heat-resistant crops, and using nuclear and isotopic techniques to better understand the impacts of ocean acidification on marine ecosystems.
Calling graphene a “wonder material” may sound trite, but it is an eminently suitable moniker for a material that continues to amaze condensed-matter physicists like MIT’s Pablo Jarillo-Herrero. I chatted with him on the phone from Cambridge, Massachusetts last week to find out what has happened since his team revealed last year that “magic angle graphene” is both a high-temperature superconductor and a Mott insulator.
Before I update you on the state-of-the-art, a little background on the breakthrough. Graphene is a free-standing sheet of carbon just one atom thin that was first isolated just 15 years ago. Layers of graphene stack upon each other to make the familiar material graphite. Twisted graphene can be made from two sheets of graphene by rotating one sheet away from the usual stacking angle.
Our discovery is the tip of the iceberg, there is so much more underneath
Pablo Jarillo-Herrero
Jarillo-Herrero has been working on twisted graphene since 2009. Last year his team reported how they had stacked two sheets of graphene on top of each other and then twisted the sheets so that the angle between them was 1.1°. At this theoretically-predicted “magic angle”, the team had expected to observe a range of interesting physics. This is because carbon atoms in the two overlapping graphene crystals create a moiré superlattice (see figure).
Two very important discoveries
They were not disappointed and made two very important discoveries. First, they found that magic-angle graphene is a Mott insulator. This is a material that should be a metal but is instead an insulator because of strong interactions (correlations) between electrons.
Then, they added a few extra charge carriers to this insulator state by applying a small electric field, which turned the material into a superconductor at temperatures below 1.7 K. Despite the low temperature, the proximity to the Mott insulator state and low electron density of the material mean that the material resembles a high-temperature superconductor.
So Jarillo-Herrero had created a system where small adjustments in terms of angle and electric field creates two iconic states of condensed matter physics – a Mott insulator and a high-temperature superconductor.
What happened next?
Despite it being less than two years since Jarillo-Herrero’s team described its results in two papers in March 2018, the work has been cited over 1000 times. He says that most of the citations are from theorists and that there are now hundreds of theory groups working on the system. It takes longer for experimentalists to learn how to make and characterize magic-angle graphene, but Jarillo-Herrero reckons the are about 20–25 experimental groups that already have results on the material.
Despite this huge effort, the physics of magic angle graphene still in its infancy and Jarillo-Herrero says there are many things to learn and study. Beyond superconductivity and the strongly correlated states – both of which occur at low temperatures – he says that there is much to learn about the material at higher temperatures.
He is also keen at studying other 2D materials that can be twisted. One possibility is bilayer graphene on top of bilayer graphene with a twist between the two bilayers. Twisted bilayers are expected to have interesting correlated-electron physics with magnetic properties that are different to those of the original twisted graphene.
Moiré magnetism
His team is also looking at twisting materials unrelated to graphene such as 2D superconductors and 2D magnets. There are theoretical predictions, for example, that a special kind of magnetism called moiré magnetism occurs when 2D magnets are twisted on top of each other.
“Our discovery is the tip of the iceberg, there is so much more underneath,” he says, “There is a lot of work for many years to come”.
I also asked Jarillo-Herrero about the technological applications of magic-angle materials and his response was frank and honest. “Don’t expect any applications in for 30-40 years – which is the normal time scale for a new material.”
Superconducting transistor
He believes that one technology that could emerge is a superconducting transistor that can be switched between superconducting and normal states. Such devices could be used in “cryogenic classical computers”, which would run at very low temperatures to avoid the power dissipation problems associated with high-speed silicon processors. Other options include using twisted materials to make superconducting singe-photon detectors or superconducting quantum bits for quantum computing.
However, he points out that the technology is still in its infancy in terms of making large and consistent samples of twisted materials.
In the nearer future, Jarillo-Herrero says that magic-angle materials have a wide range of fascinating physics that will be explored. One avenue is “quantum simulation”, whereby twisted graphene is used as a proxy for a more complicated material such as a high-temperature superconductor.
And, of course, he points out that there should be lots of interesting physics – including topological properties -- lurking in twisted graphene itself – which will keep researchers busy for years to come.
Scientific meetings evolve over time, just like plants and animals.
Take the annual Fall Meeting of the American Geophysical Union, the largest organisation of Earth and space scientists, with around 60,000 members worldwide, now wrapping up its centennial year.
I've attended almost every one of the AGU's Fall Meetings since 1998, when I was appointed its first full-time public information manager, a post I held until retirement in 2007. Although changes have been gradual, this year’s meeting, which I atteneded as a freelance science writer, bears only the most basic resemblance to that in 1998.
Take the facility: the Moscone Convention Center in San Francisco, California. Back in 1998, it consisted of two buildings, North and South, connected underground, but the meeting was entirely in Moscone North. At some point, Moscone West was added to the mix, and over the years AGU has expanded into all three.
Stairway to success: the 2019 Fall Meeting of the American Geophysical Union in San Francisco (Courtesy: Harvey Leifert)
Then there's the press room, which I ran for nine years. In my day, anywhere from 75 to 150 reporters and institutional public-information officers signed up. The reporters represented a range of daily newspapers from several countries, as well as magazines and radio networks. We provided them with 10 landline phones to send in their stories. In the press-conference room, the only equipment we needed was a 35 mm slide projector, an overhead transparancy projector, and microphones for the scientists presenting their latest discoveries.
This year’s press room offers no landline phones, as everyone has a mobile device, and stories are transmitted electronically. There are now 330 press registrants; thankfully, not all are present at any one time. But few traditional newspapers are represented. Even the San Francisco Chronicle, located a block away, which used to cover AGU with two or even three science reporters, sent no one this year. Online media, like Physics World, abound.
For press conferences, PowerPoint presentations are now preloaded, and reporters who cannot make it to the meeting can listen and watch online and even submit questions, which are put to the presenters in real time.
Do I feel nostalgic for the old days? Not really. The meeting itself is more exciting than ever. The 26,000 scientists who are attending hail from 100 countries. After the US, the greatest representation is from China. Japan, Canada, the UK, and South Korea round out the top six. They find a wide array of activities beyond the traditional oral and poster sessions of yore. There are lectures on geophysical topics in informal claimed spaces, high-tech presentations in the exhibition hall by institutions ranging from NASA to Google, workshops, town halls, tutorials and five keynote addresses.
Personally, I prefer covering poster sessions, as authors are happy to discuss their research in detail, which is not possible in oral sessions. At AGU, unlike some organisations, posters are not second-class citizens.
With more than 27,000 abstracts submitted, there is simply not enough time or rooms to present all important findings orally. In fact, around two-thirds are posters, lined up in rows in a hall larger than a football pitch.
This now-iconic image shows the doughnut-shaped ring of radio emissions surrounding a supermassive black hole that lies at the centre of a galaxy 55 million light-years from Earth. Event Horizon Telescope astronomers are the first to obtained images of the region near the event horizon of a black hole – the point beyond which matter and energy cannot escape the object's intense gravity. This was done by combining the outputs of eight radio dishes in six different locations across the globe, which itself is an engineering triumph.
The black hole has a mass 6.5 billion times that of the Sun. The illuminated ring in the image is gas and dust surrounding the black hole in an accretion disc. It is heated to billions of degrees and therefore glows brightly with radio waves. Einstein's general theory relativity predicts that a black hole will have a “shadow” around it that is about three times the radius of the event horizon -- and this is clearly evident in the image. The shadow is of great interest as its size and shape depend mainly on the mass of the black hole – and to a lesser extent, the rate at which the black hole is rotating.
“We are giving humanity its first view of a black hole – a one-way door out of our universe,” said Sheperd Doeleman of the Haystack Observatory at the Massachusetts Institute of Technology (MIT) who was EHT’s lead astronomer when the observation was announced on 10 April 2019.
This year’s Top 10 Breakthroughs were selected by a crack team of five Physics World editors, who have sifted through hundreds of research updates published on the website this year. In addition to having been reported in Physics World in 2019, our selections must meet the following criteria:
Significant advance in knowledge or understanding
Importance of work for scientific progress and/or development of real-world applications
Of general interest to Physics World readers
Here are the nine runners-up that make up the rest of the Physics World Top 10 Breakthroughs for 2019, in the order in which we covered them this year.
Neuroprosthetic devices translate brain activity into speech
Shared equally by Hassan Akbari, Nima Mesgarani at Columbia University’s Zuckerman Institute and colleagues and Edward Chang, Gopala Anumanchipalli and Josh Chartier of the University of California San Francisco for independently developing neuroprosthetic devices that can reconstruct speech from neural activity. The new devices could help people who cannot speak regain their ability to communicate with the outside world. Beneficiaries could include paralysed patients or those recovering from stroke. Beyond medical applications, the ability to translate a person’s thoughts directly into speech could enable new ways for computers to communicate directly with the brain.
First detection of a “Marsquake”
To scientists working on NASA’s InSight mission for detecting a seismic signal on Mars. The first “Marsquake” was detected on 6 April 2019 and the researchers believe that the tiny tremor originated from within the planet rather than being the result of wind or other surface phenomena. The Red Planet now joins the Moon as a place where extraterrestrial seismic activity has been detected – and like the Moon, Mars does not have tectonic plates and therefore is expected to be much quieter than Earth when it comes to seismic activity. Studying the seismology of Mars should provide important information about the interior of the planet and how it was formed.
CERN physicists spot symmetry violation in charm mesons
To physicists working on the LHCb experiment on the Large Hadron Collider at CERN for being the first to measure charge–parity (CP) violation in a charm meson. The team spotted CP violation by measuring the difference in the rates at which the D0 meson (which contains a charm quark) and the anti-D0 meson decays to either a kaon/anti-kaon pair or a pion/anti-pion pair. Since the D0 and anti-D0 decays produce the same products, the big challenge for the LHCb team was working out whether an event was associated with a D0 or an anti-D0. While this latest measurement is consistent with our current understanding of CP violation, it opens up the possibility of looking for physics beyond the Standard Model.
“Little Big Coil” creates record-breaking continuous magnetic field
To Seungyong Hahn and colleagues at the National High Magnetic Field Laboratory (MagLab) in Tallahassee, Florida for creating the highest continuous magnetic field ever in the lab. The 45.5 T record was set using a compact, high-temperature superconductor magnet dubbed “Little Big Coil”. Whereas the previous record of 45 T was set by a magnet that weighs 35 tonnes, the MagLab device is a mere 390 g. The magnet was designed to achieve even higher fields but was damaged during its record-breaking run. The breakthrough could lead to improvements in high-field magnets used in a range of applications including magnetic resonance imaging for medicine, particle accelerators and fusion devices.
Casimir effect creates “quantum trap” for tiny objects
To Xiang Zhang of the University of California, Berkeley and colleagues for being the first to trap tiny objects using the Casimir effect – a bizarre phenomenon in which quantum fluctuations can create both attractive and repulsive forces between objects. Zhang and colleagues used tuneable combinations of attractive and repulsive Casimir forces to hold a tiny gold flake between gold and Teflon surfaces with no energy input. Measuring the tiny forces involved in the trapping process was a triumph of optical metrology and provides a better understanding of how Casimir forces affect the operation of micromechanical devices. If the forces can be further controlled, there could even be practical applications involving trapped particles.
Antimatter quantum interferometry makes its debut
To the Quantum Interferometry and Gravitation with Positrons and Lasers (QUPLAS) collaboration for doing the first double-slit-like experiment using antimatter. Their experiment involved sending a beam of positrons (antielectrons) through a period-magnifying two-grating Talbot–Lau interferometer and showing that the antiparticles behave like waves and undergo quantum interference. They observed a diffraction pattern that changed as they changed the energy of the positron beam – something that is predicted by quantum theory and cannot be explained by classical physics. The breakthrough could lead to other experiments that look for differences between the quantum natures of matter and antimatter.
To Hartmut Neven, John Martinis and colleagues at Google AI Quantum and several other US research institutes and universities for being the first to do a calculation on a quantum computer in a much shorter time than if done on a conventional supercomputer. This “quantum supremacy” over conventional computers was achieved by a quantum computer comprising 53 programmable superconducting quantum bits. It performed a benchmark calculation in about 200 s, whereas the team estimates that a supercomputer would take about 10,000 years to do the same calculation. While critics have since claimed the actual supercomputer execution time is more like 2.5 days, the team has still shown a clear advantage for quantum computing.
Trapped interferometer makes a compact gravity probe
To Victoria Xu and colleagues at the University of California, Berkeley for creating a new and more compact means of using trapped atoms to measure the local acceleration due to gravity. Their “quantum gravimeter” relies on the interference pattern generated when clouds of atoms are first vertically separated in space, and then allowed to recombine. Whereas most gravimeters measure the effects of gravity on atoms as they fall through space, the Berkeley device suspends the atoms in an optical trap where they interact with the gravitational field for up to 20 s. This improves the sensitivity of the measurement, paving the way for applications ranging from geophysical exploration to sensitive tests of fundamental forces.
Wearable MEG scanner used with children for the first time
To Ryan Hill, Matthew Brookes and colleagues at the University of Nottingham, the University of Oxford and University College London for developing a lightweight “bike helmet” style magnetoencephalography (MEG) scanner that measures brain activity in children performing everyday activities. Traditional MEG systems measure the tiny magnetic fields generated by the brain using cryogenically cooled sensors in a one-size-fits-all helmet that is bulky and highly sensitive to any head movement. Instead, the team used lightweight optically pumped magnetometers on a 500 g helmet that can adapt to any head shape or size. The scanner was used on a two year old (the hardest age to scan without sedation), a five year old watching TV, a teenager playing computer games and an adult playing a ukulele.
Upon stimulation, two photons emerge from the quantum dot (QD) giving detailed information about the dynamics of the excited charges within the QD. Courtesy: ICFO
A new technique for imaging single molecules that does not rely on fluorescent emitters could find a host of applications in nanotechnology, photonics and photovoltaics. The technique, which was developed by researchers in Barcelona, works by detecting stimulated emission from single quantum dots at room temperature. Its speed makes it possible to trace charge-carrier populations through the entire absorption and emission cycle.
Single-molecule imaging techniques are widely employed in biology. To date, they have been entirely based on detecting spontaneous fluorescence from the sample being imaged. In these fluorescence-based techniques, researchers typically excite the sample at wavelengths at which it absorbs light and then detect redshifted (lower energy) fluorescence signals. This makes it relatively simple to block the background light from the excitation beam and detect only the fluorescence.
However, fluorescence imaging is far from perfect, since it is limited to molecules that fluoresce efficiently. Fluorescent light is also both incoherent and prone to "bleaching", where the signal fades after the molecule can no longer fluoresce. A third drawback is that spontaneous emission is a relatively slow process, occurring on a timescale of nanoseconds. This means that fluorescence-based imaging can only provide information on the lowest excited state of the target molecule, since more highly excited states have shorter lifetimes on the order of femtoseconds or picoseconds.
Detecting stimulated emission
Techniques based on detecting stimulated emission (SE) offer several advantages. All molecules exhibit SE, even those that do not fluoresce. SE also avoids bleaching, since the molecule spends little time in the excited state, and is much faster, as the light is emitted on femtosecond timescales – meaning that SE can provide information on the dynamics of excited states. The downside is that the laser beam that drives the stimulated emission also produces a significant amount of background light.
Researchers at the ICFO in Barcelona, along with Lukasz Piatkowski and colleagues in Niek van Hulst’s group at ICREA, have now overcome this issue by using ultrashort laser pulses to image individual colloidal nanocrystals, or quantum dots (QDs). By applying the laser pulses, the team showed they could force individual QDs (single colloidal CdSe/CdS rod-in-rods made in Iwan Moreels’ group in Ghent) to emit through a SE process, rather than waiting for them to spontaneously emit light.
The researchers begin by using a laser to “pump” the QD into a highly excited state in the conduction band. Afterwards, the excited charge carriers (electrons and holes) decayed through the excited state manifold, eventually ending up in the lowest exited state (the band edge) in the core of the nanostructure. The scientists then apply a second (probe) laser pulse at the resonant frequency of the core band-edge transition. This makes the charge carriers recombine, causing the QD to relax back to the ground state and producing a photon via stimulated emission. Because this emitted photon is in phase with the one that stimulated it, all the light produced is coherent.
Technique beats the limit of the background signals
Piatkowski explains that the team’s technique works because the probe pulse train used to modulate the excited state population operates at high, MHz, frequencies, allowing for phase sensitive detection (using a lock-in amplifier) and beating the limit of the background signals. The pump and probe pulses are also synchronized because they derive from the same broadband laser, which means that the researchers detect stimulated photons on top of a very intense stimulation beam. The stimulation beam is therefore a background signal to the stimulated emission signal. The technique is ultrafast, too and works on the femto- to picosecond scales, which allows the researchers to image their QDs at any moment during the photocycle.
Thanks to this work, which is detailed in Science, the researchers have started to investigate electron-hole core-shell dynamics in their QDs. Such studies will make it easier to understand and engineer trap-free, non-blinking, photorobust QDs for future applications in optoelectronics.
Extensions to biology
Studies of biological systems could also benefit, van Hulst says, since the ultrafast response will allow researchers to study the dynamics of important energy transfer systems in biological cells. “At ICFO, we are investigating coherent energy transfer in photosynthetic light-harvesting complexes (which are used by plants to enhance collection of incoming light),” Piatkowski says. “The new technique could allow us to do this on the single light-harvesting complex level.”
The ICFO team says that it would now like to extend its technique to molecules and biomolecular complexes. “We are also working on three- and four-pulse schemes to combine 2D-spectroscopy with SE and luminescence detection,” van Hulst tells Physics World. “Each technique will complement the other and provide different information: luminescence will report on the ground state of a molecule while SE its ultrafast excited state dynamics.”
Accurate estimates of CT dose, tailored for individual patients, can now be made within clinically acceptable computation times, report researchers at Duke University. Shobhit Sharma and colleagues used an automatic image-segmentation method to create custom anatomical models from patients' CT data. Combined with a Monte Carlo simulation of photon transport, and implemented on multiple parallel processors, the team's technique calculates the radiation burden imposed by the scan in less than half a minute – hundreds of times faster than non-parallelized Monte Carlo methods. Having access to patients' personalized dose histories will help clinicians make cost–benefit decisions about any subsequent procedures that would deliver an additional dose (Phys. Med. Biol. 10.1088/1361-6560/ab467f).
When estimating the radiation dose that a CT scan delivers to a patient’s organs, clinicians have to balance accuracy against practicality. The most reliable estimates are derived from Monte Carlo simulations that model the individual patient's anatomy and the geometry and other properties of the scanner – but such simulations can take hours to compute, putting them beyond the reach of time-constrained clinics.
Quicker results can be achieved by using dedicated graphics processing units (GPUs) to perform the simulation, but, says Sharma, “existing tools that use GPU computation to speed up the process lack either patient or scanner specificity, which limits their relevance towards providing dose estimates for a particular patient”.
Off-the-shelf computer models – which come in a range of sizes and body types – can be used instead, but no generic model quite matches a patient’s unique anatomy, so these can only ever yield an approximation of the delivered dose.
The solution that Sharma and his colleagues explore in their proof-of-principle experiment – based on actual CT datasets from 50 adult subjects – is to follow up the scan with an automatic procedure that maps out the patient’s internal organs. This segmentation process is performed in two steps: first, an artificial neural network produces an initial model from the patient’s CT dataset; then, gaps in the model are filled in by comparing it to a library of pre-existing computational phantoms.
Equipped with a detailed 3D map of their subject, the researchers then run a Monte Carlo radiation-transport simulation to work out where in the patient the X-ray photons’ energy was deposited during the CT scan. As well as accounting for the anatomy of the patient, this simulation can also handle any peculiarities of the CT setup. “Our active collaborations with major CT manufacturers enable us to accurately model proprietary components, which makes the dose estimates from our tool highly scanner-specific,” says Sharma.
Using a code that is optimized for parallel computation by multiple GPUs, the team found that the simulation took an average of 24 s to run for each patient, meaning the procedure can be implemented without disrupting existing clinical workflows.
Having demonstrated the speed of their method, Sharma and colleagues tested its accuracy by comparing it to dose measurements in physical phantoms – the gold standard for validating simulations. The team placed thermoluminescent dosimeters (TLDs) at various positions within two anthropomorphic phantoms, one approximating a five-year-old child, the other an adult male. As they would with a human patient, the researchers acquired CT images of the phantoms, then segmented them (manually this time) to build bespoke computational models. Overall, the calculated doses agreed with those measured by the TLDs to within 10%, which the researchers consider a good match.
One limitation of the technique in its current form is in dealing with secondary electrons produced when X-rays are absorbed in the body. The Monte Carlo code that the researchers use assumes that photons deliver their energy at the point of initial interaction. In reality, secondary electrons carry the energy a short distance before depositing it by exciting and ionizing atoms in the medium. While not a problem at the scale demonstrated so far, a full treatment of secondary electrons is crucial for calculating dose on microscopic scales.
“Since our code doesn’t include any models for sampling electrons generated from photon interactions, our method in its current form is not the best candidate for calculating dose where the range of electrons is greater than the voxel size used for the anatomical model,” explains Sharma. “Having said that, incorporating additional models for electron generation and transport in MC-GPU is feasible and can be accomplished if the need arises.”
