This week the LIGO-Virgo collaboration announced the detection of gravitational waves from the most massive black-hole merger ever seen. This podcast features an interview with LIGO–Virgo member and University of Portsmouth astrophysicist Laura Nuttall, who explains why scientists are puzzling over the origin of one of the black holes involved in the merger. She also looks forward to the next observing run of the LIGO–Virgo detectors and what astrophysical events they might capture.
The podcast also includes a round-up of what is new in physics this week, including two very different ways of dealing with decoherence in quantum computers, a new way of using X-rays to personalize the treatment of a serious eye disease and a breakthrough in our understanding of how bubbles pop.
We’re just 100 seconds from midnight on the Doomsday Clock – a metaphor created by the Bulletin of the Atomic Scientists in 1947.
The clock indicates how near we are to a humanity-ending catastrophe. We’ve never been this close to midnight before.
And what’s more, the last time the clock was set on 23rd January 2020, before the COVID-19 outbreak became a global pandemic. With the year we’ve had so far, who would bet against the clock ticking even closer to midnight in 2021?
But what is the Doomsday Clock and how is its time determined?
The clock emerged from the concerns of the physics community immediately after the Second World War.
Many scientists and engineers had taken part in the Manhattan Project, which developed the atomic bombs that the United States dropped on Hiroshima and Nagasaki in August 1945.
Just a few months after the war finished, two University of Chicago physicists – Eugene Rabinowitch and Hyman Goldsmith – launched the Bulletin of the Atomic Scientists.
This journal aimed to encourage scientists to engage in political issues. The war had made it painfully clear that even theoretical physics is no longer an abstract intellectual exercise, somehow divorced from the real-world.
Part of the journal’s remit was to consider future dangers. Or as Rabinowitch poetically put it: “to manage the dangerous presents of Pandora’s box of modern science”.
It was a desire to communicate these risks to the public that led to the Doomsday Clock being set up in 1947.
The idea emerged from the cover of the June edition that year, an artwork created by Martyl Langsdorf. Langsdorf placed the first clock at seven minutes to midnight for purely aesthetic reasons, but its subsequent positions were set by Rabinowitch.
When he died in 1973, a science and security board took over that responsibility, in consultation with the journal’s board of sponsors.
Over the years, the clock hands ticked further or closer to midnight depending on prevailing nuclear concerns.
Prior to this year, the closest the Doomsday Clock had been to midnight was in 1953. Then, it was set to 11:58 after both the US and the Soviet Union carried out hydrogen-bomb tests the previous year.
Its furthest distance from midnight came in 1991 when the clock was moved back to 11:43. That optimism followed the end of the Cold War and the signing of the Strategic Arms Reduction Treaty, which led to deep cuts in US and Soviet nuclear-weapon arsenals.
Another key change occurred in 2007 when the Doomsday Clock started factoring in the risk of climate change. Since then it has also started considering new disruptive technologies, including artificial intelligence and gene editing.
Each year, the Bulletin’s panel of experts meet in Chicago in November to vigorously debate whether the time should be reset – with the decision confirmed and announced in January.
The decision to set the 2020 clock so close to midnight was based on a combination of factors.
They included the continuing existence of nuclear arsenals coupled with the lapse of several major arms-control treaties, and America’s decision to quit the Iran nuclear deal.
Given the clock’s gloomy connotations, it’s not surprising that it has attracted some criticism over the years.
Commentators have accused the Bulletin of everything from inconsistency, historical pessimism, lack of clarity around its methodology, and political bias. Though it should be noted that the clock has moved forward and back during both Democratic and Republican administrations in the US.
Whatever people think of the clock, it has consistently met its goal of triggering public debate about the role of science in society.
There’s no doubt the COVID-19 pandemic will play into 2021 decision. Progress towards vaccines will surely be crucial, as might the tensions surrounding the US presidential elections in November.
Find out more about the Doomsday clock in the September 2020 issue of Physics World.
Tinted solar panels could allow land to be used to grow crops and generate electricity simultaneously, with financial gains, according to researchers in the UK and Italy. The orange solar panels absorb some wavelengths of light, while allowing those that are best for plant growth to pass through. The team even claim that their setup can produce crops offering superior nutrition.
Agrivoltaics uses land to simultaneously grow crops and produce electricity from solar panels. Usually opaque or neutral semi-transparent solar panels are used. Now, Paolo Bombelli, a biochemist at the University of Cambridge, and his colleagues used orange-tinted, semi-transparent solar panels to see if selective use of different wavelengths of light for plant growth and electricity production could offer additional benefits. The solar panels allow orange and red light to pass through, as these wavelengths are the most suitable for plant growth, while absorbing blue and green light to generate electricity.
The researchers grew basil and spinach in greenhouses in northern Italy with the glass roofs replaced with semi-transparent, orange-tinted solar panels. Even though the yield of both crops was reduced compared with plants grown in standard greenhouses, the agrivoltaic system offered a financial advantage over standard growing conditions, they report in the journal Advanced Energy Materials.
Financial gains
Overall, the spinach and the electricity produced in the agrivoltaic greenhouses were worth about 35% more than a spinach crop grown in a standard greenhouse. While basil and the electricity generated offered financial gains of around 2.5%. This was based on the wholesale global market price of the crops and the local feed-in-tariff for selling electricity to the Italian national grid. According to the researchers, the substantial difference in financial gains occurs because basil sells for about five times the price of spinach. In other words, such an agrivoltaic systems offers more financial reward when used with lower value crops.
The yields of basil grown under the orange-tinted solar panels dropped by 15% compared to those in the standard greenhouses, while spinach yields fell by 26%. However, the researchers noticed some interesting differences between the agrivoltaic and traditionally grown plants. The plants grown beneath the solar panels demonstrated a more efficient photosynthetic use of light, and they produced more tissue above ground and less below ground. This resulted in differences in plant morphology, with the basil producing larger leaves and the spinach longer stems.
Additionally, laboratory tests found that both basil and spinach plants grown under the solar panels contained more protein than those grown in standard greenhouses. The researchers suggest that the changes in morphology, redirection of above and below ground metabolic energy, and the increases in protein could be adaptations to improve photosynthesis under reduced light conditions. They add that the accumulation of more protein is interesting “in view of the need for alternative sustainable protein sources to substitute animal proteins, for example, in plant-based artificial meats”.
More experiments needed
Bombelli told Physics World that the technique might be applicable in locations besides Italy’s Mediterranean climate, depending on the chosen conditions. He says, “It depends on the percentage of the land cover by solar panel and the type of crop chosen”, adding that the only way to know for sure is to conduct additional experimental work. Indeed, the team is now hoping to run a trial in the UK.
Earlier this year Brendan O’Connor and colleagues at North Carolina State University published a modelling study in the journal Joule looking at how much energy could be produced with the addition of the solar cells on greenhouses. Like Bombelli’s work, the study analysed solar panels that harvest energy from the wavelengths of light that plants do not use for photosynthesis.
O’Connor explains: “We found that there are greater opportunities in hot and moderate climates. Yet, the heating energy requirements in colder climates result in a significant cost to greenhouse growers, and offsetting those energy costs is critical. If the solar cells can be designed to minimize losses in plant yield, there should be benefits across different climate zones.”
O’Connor says that the latest study is impressive. “While there were some losses observed in crop biomass, they demonstrate a net economic benefit of the system, which is a very exciting result for the concept,” he explains. O’Connor adds that research on integrating solar cells with greenhouse structures is growing rapidly as “there is a need to reimagine producing food to meet human needs in the most environmentally friendly and sustainable manner possible”.
As new and improved radiotherapy technologies emerge, treatment conformality – the level of dose delivered to the tumour target and not to the rest of the body – increases alongside. For photon-based radiotherapy, for example, the development of intensity-modulated radiotherapy (IMRT) and the recent introduction of MR-guided systems has ramped this conformality. In proton therapy, meanwhile, the shift to pencil-beam scanning had a similar beneficial effect.
But since the early 2000s, progress in proton therapy has stalled somewhat. In particular, there’s still a general lack of the high-quality image guidance that’s available for photons. “Protons are up to three times more expensive in terms of cost, time and manpower, but reimbursement is only 1.3 times; and that is the reason that the proton space has flatlined,” says Niek Schreuder, president of proton therapy at Leo Cancer Care. “There’s just no R&D money in existing proton therapy facilities.”
Schreuder points out that various technologies with potential to enhance proton and other particle therapies are under development and shown to be feasible. These include, for example, prompt gamma measurements for range verification, proton radiography, dual-energy CT and optical guidance. But the large size of particle therapy gantries and the lack of space around the isocentre makes installing image-guidance technologies extremely difficult.
Upright approach
The solution, says Schreuder, is upright radiotherapy, where the patient is treated in an upright position and rotated in front of a static treatment beam. Upright treatment systems have lower installation costs and space requirements, freeing up resources for technology developments. “It also provides more comfortable treatment positions for patients with pulmonary problems, pain or claustrophobia issues,” he adds.
As such, Leo Cancer Care is developing a range of products to enable this novel treatment approach, including an automated patient positioning system. “All radiation treatments include two parallel processes – patient positioning and beam delivery,” explains Schreuder. “95% of the treatment time involves patient positioning, but up to now, in proton therapy, less than 10% of the system cost goes into patient positioning.”
To address this shortfall, the company is developing software that incorporates all of the imaging technologies used to position the patient into one user interface, with a treatment room control system focused on the systems needed to position the patient. “Of course, beam control is super important,” Schreuder adds, “but we argue that beam control has been developed to such a level of maturity that, as a company, we don’t have to worry much about this part.”
The positioning system can place the patient in appropriate postures according to the cancer type being treated. This includes, for example, seated vertical for head-and-neck treatments, with gravity pulling the shoulders naturally downward to better expose the head nodes; leaning slightly backward for lung and liver radiotherapy and slightly forward for breast cases; or perched, supported by the back of the thigh and a knee rest, for prostate and pelvic treatments.
Previously, all of these cases were treated the same, with the patient lying down, explains CEO Stephen Towe. “What we’re doing, for the first time, is using patient posture as a degree-of-freedom to truly optimize treatment delivery,” he says. “This is the first time that the radiation therapy problem is being addressed by starting at what the patient needs to be treated optimally and more comfortably.”
The company has also developed a vertical dual-energy CT scanner coordinated with the upright positioning system that scans the patient in any of these orientations and can perform a dual-energy scan in less than one minute. The idea is that the two devices will be placed together in a fixed-beam treatment room, within an existing proton or carbon-ion therapy facility, for example. Treatment is delivered by rotating the patient in the beam, rather than rotating a large gantry.
A major advantage of this approach is the reduced shielding requirements. If radiation is being delivered through 360°, this requires 360° of shielding. If the radiation is only sent in one direction, it’s possible to significantly reduce the room size and costs. It also reduces design complexity, which is particularly vital for developing markets, where expertise to design radiation shielding rooms may not be available.
Schreuder points out that, compared with a classical rotating proton gantry, an upright fixed-beam setup requires around 19 times less shielded volume. Even with a superconducting gantry, the shielding requirements are about 10 times less for the fixed proton beam. Ultimately, this could allow installation of a proton therapy system in an existing linac vault.
“The weight of a radiotherapy system ranges from six up to 600 tonnes for a carbon ion facility,” adds Towe. “Trying to rotate that mass around the patient with an accuracy of less than 1 mm makes no sense. It’s like changing a lightbulb by rotating your house.”
Clinical benefits
Rock Mackie, board chairman and co-founder of Leo Cancer Care, says that it’s not just a matter of cost, but that upright radiotherapy also confers some important clinical benefits. A study looking at thoracic cancer patients, for instance, showed that lung volume was on average 25% greater with upright rather than horizontal positioning, which reduces breathing motion. This is because the lung is more inflated as gravity pulls the diaphragm down. This could prove particularly useful for scanned proton treatments where motion can cause interplay effects, explains Mackie.
Reducing patient motion could also remove the need for motion management, making the treatment much faster. And where breath holds are required, these are easier for patients to perform when upright. A more inflated lung also reduces the dose to normal lung for proton therapy.
The team is also studying the impact of positioning on prostate motion. They saw that when a patient is upright, the level of bladder filling does not impact prostate position as it does when they are lying down, which is a huge benefit. “We started by focusing on making patients more comfortable, and this resulted in something that’s really clinically significant, the reduction in motion that comes from being positioned upright,” says Towe.
Leo Cancer Care is now working with proton therapy vendors to integrate the Leo technologies into existing fixed beam rooms and with one vendor to install its first system in a new proton therapy facility that will start construction at the end of this year. The company is also building a compact self-shielded linac for upright photon-based radiotherapy, which incorporates the positioning device and CT scanner. All systems should achieve FDA approval and CE marking by the end of 2021.
“There’s a problem that needs to be solved right now in the proton therapy space by reducing cost,” adds Schreuder. “Our approach could lead to more proton therapy systems across the world. And as a result of the huge reduction in the total facility costs, we believe that these facilities will have the financial means to support R&D projects to further develop many other aspects of proton beam delivery with endless benefits to patients.”
Customer choice is right up there as a competitive differentiator for the vacuum specialist Edwards – and beyond that, how best to match its diverse user base with the right product and the right functionality for the job in question. A case in point is the manufacturer’s growing family of dry vacuum pumps, a core enabling technology for analytical instrumentation OEMs (in their electron microscopy and mass spectrometry systems), high-energy physics laboratories (across their accelerators, beamlines and laser systems) and a range of R&D and industrial applications (including coating systems, gel drying, gas recovery and plenty more besides).
To service that broad spectrum of requirements, Edwards’ portfolio of dry vacuum pumps must be similarly broad in scope – more choice, more options ultimately translating into more scientific, OEM and industrial end-users. That’s the rationale underpinning the manufacturer’s latest family of dry pumps – the nXRi series – with initial variants providing pumping speeds of either 60 or 90 m3/h. Equally significant are the nXRi’s compact footprint (494x301x217 mm) and weight (29 kg) which, taken together, enable the pumps to fit easily under a laboratory benchtop while ensuring a mobile vacuum solution for changing R&D workflows and environments.
“The nXRi has the highest pumping density on the market, delivering up to four times more pumping speed versus similarly sized dry pumps,” claims Dave Goodwin, senior product manager for scientific vacuum at Edwards. “Our aim is to offer customers greater choice, with many of them already bought into the advantages of dry pumps – not least the fact that they’re quiet, oil-free and easy to maintain.” The nXRi pump, for example, is maintenance-free for up to five years, with no tip-seal or oil change translating into extended uptime and lower operating costs for research labs and industry users.
Bridging the gap
The nXRi product line is, of course, the latest chapter in a running back story. Up until 2017, the Edwards scientific dry-pump portfolio (small, single-phased, air-cooled units) was dominated by scroll pumps (the XDS and nXDS product lines), which provide pumping speeds in the range from 6 to 40 m3/h. At this point, however, the vendor introduced the nXLi family of multistage roots pumps, offering pumping speeds from 110 to 200 m3/h, with all the associated benefits of dry-pump technology. That diversification was driven, in large part, by instrumentation OEMs seeking higher-performance pumping products for integration into their commercial mass spectrometers.
Alexander Kaiser: “Plug, play and forget – that’s the essence of nXRi.” (Courtesy: Edwards)
As such, the nXRi product line fills an obvious gap in the Edwards dry-pump portfolio – between the largest scroll pump (at 40 m3/h) and the smallest multistage roots pump (at 110 m3/h) – with a compact-footprint platform that’s easy to move around the lab. “We’ve now reached a point where the nXRi pumps have an even smaller footprint than our compact nXDS scroll pumps, providing up to a factor of four improvement in terms of pumping density,” explains Alexander Kaiser, Edwards’ global product manager for primary pumps (including responsibility for nXRi and nXLi).
There are also the running costs and environmental impacts to consider. The nXRi pumps are efficient, low-power units that generate a lot less heat and noise compared with traditional oil-sealed pumps – a win all round and in line with Edwards’ commitment that new pumping technologies should be more environmentally friendly versus the previous-generation products that they replace.
“For large labs, running banks of mass spectrometers 24/7, the nXRi pumps will mean a significant reduction to their operational costs for electrical power and air-conditioning units,” notes Kaiser. A laboratory operating a mass spectrometer using two big 40 m³/h rotary vane pumps, for example, can now think about streamlining that pumping set-up with a single nXRi unit (operating at 90 m³/h). “The nXRi is more compact, creates less noise and can save the customer upwards of £1000 per year in energy costs,” Kaiser adds.
Synchronized roadmaps
Operational benefits notwithstanding, it’s evident that the Edwards approach to new product innovation – for the nXRi pumps as well as the rest of the dry-pump portfolio – is rooted firmly in a continuous-improvement mindset and ongoing dialogue with the customer base. Think collaborative product development. “We’re in regular contact with customers to ensure our near- and long-term technology roadmaps align with their requirements,” Goodwin explains. “The goal is to synchronize so that we’re bringing through the right products, features and functionality when they need it.”
At the heart of that collective conversation is Edwards’ Global Technology Centre (GTC) in Burgess Hill, UK. Within a diversified and global R&D effort, the GTC employs a team of scientists and engineers dedicated to core technology development and validation across all of Edwards’ product lines, including the dry-pump portfolio. While part of the GTC’s remit is to address technical requirements coming in from the end-users, the centre also spends at least half of its time working on long-range blue-sky R&D – with a chunk of the latter driven by the GTC team eyeing more speculative commercial opportunities.
“Often the customer will come to us with a technical challenge,” says Goodwin. “We’ll take those inputs on board and endeavour to solve their problems by adapting existing products or developing new ones.” For the dry-pump portfolio, the response is shared across the wider innovation ecosystem at Edwards, pulling in product managers like Goodwin and Kaiser, GTC engineers, as well as Edwards’ experts located around the globe. “The task is to get a technology demonstrator product into the customer for feedback at an early stage, so that we can be sure we are heading in the right direction,” adds Goodwin.
Kaiser, for his part, is encouraged by the commercial reception to date for the nXRi family of pumps – and that’s despite the product launching in February as the coronavirus pandemic accelerated across Europe. Given the ongoing Covid restrictions, Kaiser and colleagues are focusing much of their promotional efforts on customer webinars as well as online and social videos. “When users see the nXRi pump running and realize it’s operating with speeds of up to 90 m³/h – this is what really impresses them,” he concludes. “It’s a completely different proposition to what they’re expecting in terms of the compact size, noise levels, maintenance, running costs and hence the overall environmental footprint.”
Gravitational waves from the most massive merger of two black holes ever seen have been detected by the LIGO–Virgo observatories. Dubbed GW190521, the event was spotted in May 2019 and involves the creation of a black hole with a mass of about 142 Suns. This is the first intermediate-mass black hole to be observed using gravitational waves, with its mass falling between that of stellar-mass black holes and the supermassive black holes that dominate the centres of most large galaxies.
The initial pair of black holes are thought to have weighed in at 85 solar-masses and 66 solar-masses – making the heavier object the first black hole observed in the pair instability mass gap where black holes are not thought to form from collapsing stars.
The two black holes orbited each other, getting closer and closer together before they merged. In the last moments of this inspiral, the pair broadcast gravitational waves that were observed by the LIGO–Virgo detectors. These are three huge interferometers – two in the US and one in Italy.
A long time ago
Physicists working on LIGO–Virgo have calculated that the merger took place 17 billion light-years from Earth when the universe was half the age it is today – making this one of the most distant mergers seen by LIGO–Virgo to date. Detecting the gravitational waves at such a large distance is possible because of the huge amount of energy they carried away from the merger – the mass–energy equivalent of eight Suns.
GW190521 is of particular interest to black hole experts because of the large masses of the three objects. Stellar-mass black holes are created when a large star collapses under its own gravity, creating a huge supernova explosion that leaves behind a black hole.
Astrophysicists believe that stars of up to 130 solar masses will collapse in this way to create black holes with a maximum mass of about 65 solar masses. The same applies to stars heavier than about 200 solar masses, which will collapse to create black holes of greater than 120 solar masses.
Electron–positron pairs
However, stars in the 130-200 solar mass range experience an effect called pair instability as they collapse. Highly energetic photons within the star are converted into electron-positron pairs, which generate less outward pressure than photons. This causes a rapid gravitational collapse of the star and an extremely violent explosion that leaves no black hole behind.
As a result, astrophysicists believe that there should be a gap in the mass spectrum of black holes ranging from about 65 to 120 solar masses. Therefore, it looks like one – or possibly both – of the GW190521 progenitor black holes lies in this gap, leading astrophysicists to speculate on their possible origins. One possibility is that the progenitor black holes may themselves have been formed in black hole mergers.
Although a black-hole merger is the most likely source of GW190521, the way in which the event was detected opens the door to other intriguing possibilities. LIGO–Virgo looks for signs of gravitational waves in two different ways. One method involves looking for signals that resemble the gravitational waves that astrophysicists expect to be given off by merging black holes or neutron stars. The other involves looking for anything out of the ordinary – and is called a “burst” search.
In the case of GW190521, a burst search was slightly better at identifying the signal, which opens the enticing possibility that the source of the signal is not a distant black-hole merger but rather the collapse of a star in the Milky Way. Another possibility, says the LIGO–Virgo team, is that the signal was created by a cosmic string that was produced just after the inflationary epoch of the early universe.
Earlier this year an international team of astronomers suggested that GW190521 may have been accompanied by a flare of light from a distant quasar, making it the first gravitational-wave signal from merging black holes to have an electromagnetic counterpart. However, more recent work suggests that the quasar is not in the same part of the sky as GW190521.
SARS-CoV-2 has infected more than 22 million people globally, leading to ~800 thousand deaths in just 10 months. However, significant uncertainty still remains regarding the prevalence of asymptomatic and mild cases of COVID-19, the disease caused by SARS-CoV-2, as well as the magnitude, effectiveness, and duration of antibody responses. Gaining a better understanding of population immunity is critical to improving predictive models of infection spread and safely reopening economies worldwide. However, to fill knowledge gaps in these areas requires population-scale testing using low-cost, non-invasive, and highly specific and sensitive assays that can be deployed broadly and serially to characterize antibody responses to SARS-CoV-2.
Benchmark detection approaches are based on sandwich immunoassays relying on optical readouts of fluorescence emission or colour change to report antibody levels. These technologies can be costly, often require centralized facilities with trained personnel and are, therefore, not amenable to at-home testing.
More affordable technologies, such as lateral flow assays have been found to be inaccurate or prone to user misinterpretation. Motivated to circumvent such barriers, the Arroyo lab has undertaken a journey to develop an at-home electrochemical assay.
This presentation will report the results of our initial, three-month effort to produce a portable immunoassay:
Learn about SARS-CoV-2 immunoassays.
Discover how electrochemists help fight COVID-19.
Learn the challenges behind developing antibody-based sensors.
Netzahualcóyotl Arroyo Currás (Netz Arroyo) is an assistant professor in the Department of Pharmacology and Molecular Sciences at Johns Hopkins University School of Medicine. He obtained his PhD degree in analytical chemistry from the University of Texas at Austin, where he worked with Allen J Bard on electrochemical energy storage and studies of electrocatalysis employing scanning electrochemical microscopy. He graduated in 2015 and moved to California to complete his postdoctoral training with Kevin W Plaxco at the University of California Santa Barbara, where he developed electrochemical biosensing platforms supporting real-time measurements of specific molecular targets in the body. His current research focuses on the development of electrochemical biosensors for pharmacological applications.
What first sparked your interest in quantum physics?
I had an interest in physics at high school and just before the Chinese New Year in 1998, our school invited Jian-Wei Pan to give a public science lecture that was held in the largest cinema in Dongyang in Zhejiang province. At the time Pan was in Anton Zeilinger’s group at Innsbruck University in Austria and they had just reported their first quantum-teleportation experiments. The lecture was intriguing but to some extent it all sounded a little crazy.
But it was fascinating enough for you to study physics?
Yes. I then studied physics at the University of Science and Technology of China (USTC) in Hefei and joined Pan’s group where I worked on a number of interesting problems such as six-photon entanglement, quantum simulation of anyons and teleportation of quantum-logic gates. In 2008 I moved to the University of Cambridge in the UK to do a PhD before moving back to USTC in 2011.
What are you currently working on?
In the past few years, I have focused on the development of scalable quantum-light sources for “boson sampling” – an intermediate quantum-computing model. My current research covers a variety of topics from blue-sky research to emerging quantum technologies such as large-scale quantum entanglement, quantum teleportation and quantum computing. I am still young enough to start exploring new fields such as atom arrays in optical tweezers and superconducting circuits.
In 2015, you and colleagues were awarded the Physics World Breakthrough of the Year for your work teleporting two quantum properties of a photon. How has research moved on since then?
We have made steady progress, for example, making quantum teleportation more “complete” by carrying out the first experimental demonstration of “high-dimensional” teleportation of a quantum spin-1 system (Phys. Rev. Lett.123 070505). My colleagues have also achieved teleportation over longer distances by exploiting the Micius satellite. This has allowed us to go from about 100 km to 1400 km via a ground station to space (Nature549 70). We have also proposed and demonstrated a teleportation-inspired method to efficiently simulate random quantum circuits that redefine the limits of “quantum advantage” (Phys. Rev. Lett.124 080502).
How has COVID-19 affected your work and are you now starting to reopen the labs?
Students had already returned home for the Chinese New Year when the COVID-19 outbreak emerged. During the height of the pandemic, people showed a remarkable ability to come together. Thanks to the selfless dedication of medical workers and the strong measures taken by the Chinese government, COVID-19 was effectively controlled within a short time. Graduate students have returned to campus cautiously in a step-by-step and organized way. As far as I know, not a single student or faculty member of USTC was affected by the COVID-19 virus.
As chair of the Quantum 2020 conference in October, what do you hope it will achieve?
Quantum 2020 aims to bring together the international quantum-science community to learn, share and collaborate on the latest research and emerging areas of interest. Taking place over four days from 19 to 22 October, it is co-organized by the USTC, the Chinese Physical Society (CPS), the UK’s Institute of Physics and IOP Publishing, which publishes Physics World.
The virtual event gathers early-career and established researchers from universities, industry and governments worldwide. Beyond a high-profile programme of more than 30 invited talks from world-leading figures in the field, Quantum 2020 will also feature two special interactive panel sessions covering industry and worldwide initiatives in quantum technology.
The meeting is now virtual given the COVID-19 pandemic. How has the organization gone?
We began to organize the Quantum 2020 event at the end of last year. The original plan was to hold the conference at USTC’s Shanghai campus. But because of the COVID-19 situation, in March I suggested changing it to a virtual conference. Organizing an online meeting has been a new experience, but it has gone well thanks to the help of the advisory committee and the quantum-science community. The virtual format delivers several benefits in terms of time and efficiency and we have seen a real appetite from invited speakers and panellists to get involved. We are very excited by the calibre and diversity of the scientific programme.
Have there been any benefits of it going online?
I believe that virtual conferences will remain an important part of the scientific community beyond the current COVID-19 situation. We anticipate that the online format will have several benefits for participants including improved accessibility, reduced cost and personal disruption, as well as an overall reduction in carbon footprints from travel.
Are there any benefits of this conference to wider society?
In partnership with the CPS, we plan to translate some of the plenary and invited talks to raise the awareness of quantum physics and quantum technology in China. In addition, Quantum 2020 plans to establish new international awards to promote and encourage early-career researchers in quantum technologies. These will run alongside other high-profile awards in the field such as the Micius Quantum Prize awarded by the Micius Foundation, the biennial International Quantum Communication Award given out at the QCMC conference and the Rolf Landauer and Charles H Bennett award in quantum computing by the American Physical Society.
Quantum computing has accelerated recently. How do you see its development?
Scientists are gaining exquisite control at a fundamental level and pushing the physical boundaries. Such efforts will not only deepen our understanding of physics but also unlock unbelievable potential for new technology. In the not-too-distant future, they will become a reality to control and entangle thousands or even millions of quantum bits at will. Quantum simulators and computers will become first useful for physicists, chemists and engineers for applications of material and drug design. Many more surprises will emerge but, of course, they are hard to predict.
What excites you about the future of quantum technologies?
We are now beginning the second quantum revolution. Exploiting quantum superposition and entanglement offers a radical new way to transform our technologies from communication and computation to metrology. Comparing quantum computers with their classical counterparts is like comparing lasers to light bulbs. I believe that the development of quantum computers will follow a similar trajectory as lasers – first as a useful tool inside the laboratories then finding applications in many different areas. Inspired by what we know about the history of the laser, I believe what we have already discovered – for example, quantum key distribution, and quantum algorithms – is just the tip of the iceberg of what is to come.
With many countries pushing the implementation of quantum technologies, is is it driven more by co-operation or competition?
Quantum technology has a long way to go before it can be widely used. So, international co-operation and open exchanges are imperative. Just like how we all need to work closely together to confront the COVID-19 pandemic. Likewise, quantum technology is for all and not just a single country. The potential outcome from quantum research, such as solving energy problems, new materials, and new medicine, will too benefit all people. Diseases have no borders and neither should research.
That’s according to the Doomsday Clock – a device created by the Bulletin of the Atomic Scientists in 1947 as a metaphor to indicate how near we are to a humanity-ending catastrophe. The clock started out at 11:53 p.m. and over the years has shifted backwards and forwards as the global situation has worsened or improved. But on 23 January 2020 the clock was moved closer to midnight than at any other time in its near 75-year lifetime.
This year’s historic decision was announced to “leaders and citizens of the world” at the National Press Club in Washington, DC by members of the Bulletin of the Atomic Scientists. In setting the clock to 100 seconds to midnight, they cited risks such as worsening nuclear threats, a lack of climate action, and the rise of “cyber-enabled disinformation campaigns that undermine society’s ability to act”.
The annual resetting of the Doomsday Clock is these days a major media event, providing grist for politicians, policy-makers and commentators around the world. But the clock actually emerged from the concerns of the physics community immediately after the Second World War, when two University of Chicago physicists – Eugene Rabinowitch and Hyman Goldsmith – started to think about the consequences of their work. They were among the many scientists and engineers who had taken part in the Manhattan Project, which developed the atomic bombs that the US dropped on Hiroshima and Nagasaki in August 1945.
Chicago was where the Italian physicist Enrico Fermi had in 1942 designed and built the first reactor that could achieve a self-sustaining nuclear reaction and where much of the science of the Manhattan Project was incubated. “Within three or four months of the bombs being dropped, Rabinowitch and Goldsmith created a publication called the Bulletin of the Atomic Scientists,” says current Bulletin president and chief executive Rachel Bronson.
According to Bronson – who is not a physicist, but an expert in international relations – many of the Manhattan Project physicists were already politically conscious, but their main concern had been to acquire a nuclear bomb before Germany. In the event, the Germans never built such a weapon and it was only after the war that the Manhattan researchers began to debate nuclear risk and proliferation with the wider physics community.
“The Bulletin was established to set a flag and say here is where scientists should engage in political issues,” says Bronson. But the journal was also set up to consider future dangers or – as Rabinowitch poetically put it – “to manage the dangerous presents of Pandora’s box of modern science”. And it was a desire to communicate these risks to the public that led to the Doomsday Clock being set up in 1947, two years after the first edition of the Bulletin.
The idea of the clock emerged from the cover of the June 1947 edition of the Bulletin, created by the artist Martyl Langsdorf whose husband was a nuclear physicist. Langsdorf placed the first clock at seven minutes to midnight for purely aesthetic reasons but its subsequent position was decided by Rabinowitch, the Bulletin’s founding editor. When he died in 1973, a science and security board took over that responsibility in consultation with the journal’s board of sponsors.
Originally set up by Albert Einstein with Robert Oppenheimer as its first chair, the board of sponsors currently includes 13 Nobel laureates – including the particle theorists Sheldon Glashow, Steven Weinberg and Frank Wilczek – as well as astronomer Martin Rees and theoretical physicist Lisa Randall. As for the science and security board, its composition has evolved over the years. It currently has 19 members and is chaired by Robert Rosner – a physicist and former head of the Argonne National Laboratory near Chicago.
“On the nuclear side we have physicists who know about all things nuclear, weapons and reactors, but also people who have been involved in negotiations with the government,” says Rosner, who has served on the board for a decade and currently leads the process to set the Doomsday Clock each year. “There’s a strong political aspect to it,” underlines Bronson, who points to the need for “people who understand the political process of different countries and how the science wraps up in what we’re doing”.
Closer and closer
Before 2020 the closest the Doomsday Clock had been to midnight was in 1953, when it was set to 11:58 p.m. after both the US and the Soviet Union carried out hydrogen-bomb tests the previous year (figure 1). Its furthest distance from midnight came in 1991 when the clock was moved back to 11:43 p.m. That heady moment followed the end of the Cold War and the signing of the Strategic Arms Reduction Treaty, which led to deep cuts in US and Soviet nuclear-weapon arsenals.
The nuclear threat is still with us, however. In 2019 there were almost 13,900 nuclear warheads in the world (albeit down from a high of more than 70,000 in the mid-1980s). And it was the continued existence of these arsenals – coupled with the lapse of several major arms-control treaties and America’s decision to quit the Iran nuclear deal – that shaped the Bulletin’s current risk assessment.
1 The time according to the Doomsday Clock Starting out in 1947 at 11:43 p.m., the Doomsday Clock has changed position depending on the threats to the world from nuclear proliferation and, more recently, from other risks too.
However, since 2007 the journal’s Doomsday Clock deliberations have also factored in the risks facing humanity from climate change. Bronson admits that adding climate change to its remit was “probably one of the most controversial decisions” her organization has made given its reputation as primarily an authority on the threat from nuclear weapons. “[But] if you believe that the Bulletin was founded to manage ‘the dangerous presents of Pandora’s box of modern science’, then the answer is, of course, you have to include climate change,” she says.
In recent years the Bulletin has also begun to focus on new disruptive technologies. “Take synthetic biology, for example, or gene editing,” says Steve Fetter, a physicist-turned-policy-expert from the University of Maryland, who serves on the Bulletin’s science and security board. “While this technology has tremendous promise for curing currently incurable diseases, you don’t have to have too much imagination to see how someone could misuse it.” The risks, Fetter adds, are compounded by how easy these technologies are to develop. “You needed a huge factory to make plutonium or highly enriched uranium, but gene editing, or artificial intelligence; these are things that can be done by an individual.”
Hands-on process
Setting the Doomsday Clock is a process the Bulletin says it takes very seriously. “We pride ourselves in it being consultative,” says Bronson. “Our leaders and experts can come together with different perspectives and put them out on the table and have them examined.”
The formal process begins in November each year, when members of the science and security board meet in Chicago for a day and a half, with its deliberations centred on two fundamental questions. First, is humanity safer or at greater risk this year compared to the year before? And second, is humanity safer or at greater risk this year than in all the years since the Bulletin began its deliberations in 1947? Rosner says discussions are not for the sensitive, with board members being “pretty hard-headed people who are vigorous in expressing their opinions”. With the calibre of minds involved, he says, “you can’t be a lazy thinker”.
To set the clock the closest it’s ever been to midnight, the Bulletin of the Atomic Scientists had to be convinced we are living in the most dangerous period since 1947
After the November meeting, board members start drafting a public statement that explains their decision, refining it with staff from the Bulletin in a process that lasts until the announcement of the clock’s new position in early January. To set the clock the closest it’s ever been to midnight, Bronson insists that the board this year had to be convinced we are living in the most dangerous period since 1947. She cautions, though, that setting the clock is not an exact science. “You’re not shovelling data into a big algorithm that spits out a time; it’s really a judgement,” she says.
The annual clock-setting is intended to serve as a challenge to politicians to do better in the year ahead. However, members of the board are currently concerned that the world lacks the strong leadership and co-operation to deal with the risks they have flagged. “You can see it now playing out in the coronavirus pandemic,” says Fetter, who points out how many governments have ignored the results of their regular security and pandemic-readiness exercises.
Bronson denies accusations – mostly from the right wing – that its risk assessment is arbitrary and politically motivated, pointing out that the clock has been moved forward and back during both Democratic and Republican administrations in the US. “We take criticism on both sides, quite frankly, and we take that as a sign that we’re doing something correct.” If anything, Bronson feels that the clock provides many with a sense of sanity. “It’s empowering, if it helps people feel that they’re not crazy by noting that something is not right [and that] we’re not where we should be in 2020.” She also laughs off criticism that the clock is just a scare tactic. “That’s fair, it’s called the Doomsday Clock!”
Closer than ever The press launch of the Doomsday Clock in January 2020, when members of the Bulletin of the Atomic Scientists announced in Washington, DC, that it had been set to 100 minutes to midnight. From left to right: the journal’s executive chair Jerry Brown, former Irish president Mary Robinson, and former UN secretary-general Ban Ki-moon. (Courtesy: Lexey Swall Photography/Bulletin of the Atomic Scientists)
Members of the Bulletin are also increasingly concerned by the rise of fake news and conspiracy theories, which have led to the anti-vaccination movement and other scare stories. Social media has allowed such disinformation to spread, making it much harder to address real risks. But could this move away from a pure focus on nuclear weapons and the arms race dilute the message behind the Doomsday Clock?
Stuart Parkinson, a physicist and executive director of the UK-based Scientists for Global Responsibility (SGR), thinks the Doomsday Clock may have become too simplistic given the variety of risks it now tries to encompass. “I’d like to see a more multi-dimensional risk-communication device, so maybe for each issue you have a traffic-light system or a five-point rating.” Bronson knows the clock is a blunt instrument but says that’s also its beauty, generating conversations at the highest levels of power and in the most local classrooms.
Engaging physicists
It’s true, though, that the issue of nuclear risk has suffered an overall decline in interest, including among physicists. “There was a very special period during the war and at least 20 years after, when physicists played a very important role in public policy,” says Rosner. “That’s changed dramatically.” The policy agenda has become more crowded, with the climate crisis forcing the Bulletin to consult not just physicists, but chemists, biologists and risk-assessment experts too. Indeed, he feels many of today’s physicists shy away from public or political debate and are uncomfortable dealing with the uncertainties around climate risk.
One physicist who is unafraid to contemplate the risks posed by scientific advances is Anthony Aguirre, a cosmologist from the University of California, Santa Cruz. Together with Max Tegmark from the Massachusetts Institute of Technology, in 2014 he founded the Future of Life Institute – a non-profit centre that investigates how to safely develop new technologies. “[The institute addresses] questions that aren’t part of the regular academic day-to-day research discourse, and what can we tangibly do to increase the probability of things going well,” Aguirre says.
The COVID-19 pandemic has convinced Aguirre that scientists need to think even more seriously about risk. “It’s woken up a lot of people to the idea that risks are not purely theoretical. I’m even more concerned about the lack of effective national and international institutions to both prevent and deal with risks as they arise.” And although the Future of Life Institute does look at nuclear risk, its current focus is on transformative artificial intelligence (AI), which he feels could compound existing risks.
If used in weapons, for example, AI could rapidly and inadvertently escalate minor incidents into nuclear wars without humans being able to stop them. And that, he feels, is exactly the kind of problem physicists are well placed to examine. “Within the physics and cosmology community there is a tendency to think on bigger scales and longer timescales,” Aguirre points out, which he believes gives them an ability to understand how small risks can still become significant over time.
Aguirre cites the physicists on the Manhattan Project, who – even before they had built a bomb – worried that a nuclear explosion could create such extreme temperatures that hydrogen atoms in the air and water would fuse to form helium. Literally igniting the atmosphere and oceans, this process would – they feared – generate a runaway reaction that could engulf the globe. Fortunately, when this possibility was studied, it proved unfounded.
For its part, the Bulletin of Atomic Scientists is now looking to re-engage the physics community, given the nuclear threat posed by the current unravelling of Cold War agreements. The last remaining bilateral nuclear arms control treaty, New START, is scheduled to expire in February 2021, leaving nuclear weapons proliferation unconstrained for the first time in 50 years. Those events prompted physicists at seven US universities to launch the Physicists Coalition for Nuclear Threat Reduction earlier this year, with support from the American Physical Society. Although the Bulletin is not directly involved with this initiative, it is supportive of these efforts according to Bronson.
Dangerous times The Doomsday Clock began in 1947 to assess the risks from nuclear weapons but these days also considers the threat of climate change, cyber-security and biosecurity risks – including pandemics. (Clockwise from top left: iStock/Gerasimov174; iStock/piyaset; iStock/martin-dm; iStock/matejmo)
As part of the initiative, Fetter and others are giving talks at university departments in the US and hoping to attract physicists to advocate for nuclear-threat reduction, through public engagement and lobbying their local Congressional representatives. He points to the effective role that they played in disarmament policy well into the 1980s, including arguing against America’s proposed ballistic-missile defence systems, which had been touted as a solution to the nuclear threat. Physicists pointed out that in space all objects regardless of mass travel along the same trajectory of launch, meaning it would be very hard to distinguish warheads from decoys.
Over at SGR, Parkinson agrees that physicists have become complacent about nuclear risks and how their own work might endanger the world. That’s why his organization thinks ethics and responsibility must be embedded within scientific education. Physics, he feels, is not a pure, abstract exercise but has consequences that cannot just be ignored. Indeed, he calls for more protection for scientists who speak out, and thinks professional institutions should debate the risks of new and existing technologies more openly with their members and with the public. Parkinson also thinks such organizations should end their links with fossil-fuel and defence firms.
Aguirre even feels better incentives and rewards are needed for individuals who help the world to avoid catastrophes, citing the case of Soviet air-defence officer Stanislav Petrov, who in 1983 was on duty in a command centre near Moscow when a radar screen on a satellite early-warning system seemed to suggest the US had launched five nuclear missiles. Petrov refused to alert the authorities, suspecting – correctly as it turned out – a malfunction. Despite having potentially saved the world from nuclear war, he was reprimanded by the Soviet authorities.
The current COVID-19 pandemic, while not a threat created by humans, is clearly a warning of what can happen when risks are ignored
The current COVID-19 pandemic, while not a threat created by humans, is clearly a warning of what can happen when risks are ignored, which Aguirre says is something we continue to do with existing and new technologies. Indeed, he feels the effort we put into planning for all risks is just not enough. “We’ve all seen the catastrophic things that can in fact happen to us [with COVID-19],” he says, “[and] this is pretty minor in the spectrum of catastrophes.” For Aguirre, we need to put far more intellectual thought into other significant and potentially even more catastrophic risks.
Whether 2021 will bring us closer to midnight, we’ll find out soon.
The list of surprising behaviours in “twisted bilayer” graphene (TBG) just keeps getting longer. The material – which is made by stacking two sheets of graphene on top of one another, and then rotating one of them so that the sheets are slightly misaligned – was already known to support a wide array of insulating and superconducting states, depending on the strength of an applied electric field. Now researchers in the US have uncovered yet another oddity: when TBG is exposed to infrared light, its ability to conduct electricity changes. According to Fengnian Xia of Yale University, Fan Zhang of the University of Texas at Dallas, and colleagues, this finding could make it possible to develop a new class of infrared detectors using these stacked carbon sheets.
A single layer of graphene consists of a simple repetition of carbon atoms arranged in a two-dimensional hexagonal lattice. In this pristine state, the material does not have an electronic bandgap – that is, it is a gapless semiconductor. However, when two graphene sheets are placed on top of each other and slightly misaligned, they form a moiré pattern, or superlattice. In this new arrangement, the unit cell of the 2D crystal expands to a huge extent, as if it were artificially “stretched” in the two in-plane directions. This stretching dramatically changes the material’s electronic interactions.
From magic angle to twistronics
The misalignment angle in TBG is critically important. For example, at a so-called “magic” misalignment angle of 1.1°, the material switches from an insulator to a superconductor (that is, able to carry electrical current with no resistance below 1.7 K), as a team at the Massachusetts Institute of Technology (MIT) discovered in 2018.
The existence of such strongly correlated effects – which were first theoretically predicted in 2011 by Allan MacDonald and Rafi Bistritzer of the University of Texas at Austin – kick-started the field of “twistronics”. In this fundamentally new approach to device engineering, the weak coupling between different layers of 2D materials, like graphene, can be used to manipulate the electronic properties of these materials in ways that are not possible with more conventional structures, simply by varying the angle between the two layers.
Infrared light affects TBG’s conductance
Xia and Zhang’s teams have now studied how TBG interacts with infrared light – something that has never been investigated before. In their experiments, they shone light in the mid-infrared region of the spectrum, with a wavelength of between 5 and 12 microns, onto samples of TBG and measured how the electrical conductance varied at different twist angles. They found that the conductance reached a peak at 1.81° and that the photoresponse of the material was much stronger compared to untwisted bilayer graphene. This is because the twist significantly enhances the interactions between light and the material and induces a narrow bandgap (as well as superlattice-enhanced density of states). They also found that this strong photoresponse fades at a twist angle of less than 0.5°, as the bandgap closes.
Further investigations by the team revealed that the TBG absorbs the incident energy of the photons from the infrared light. This increases its temperature, which, in turn, produces an enhanced photocurrent.
The results suggest that the conducting mechanism in TBG is fundamentally connected to the period of the moiré pattern, and the superlattice produced, which is itself connected to the twist angle between the two graphene layers, Zhang explains. The twist angle is thus clearly very important in determining the material’s electronic properties, Xia adds, with smaller twist angles producing a larger moiré periodicity.
Towards a new class of infrared detectors
The researchers, who report their work in Nature Photonics, now hope to find out whether they can combine photoresponsivity and superconductivity in TBG. “Can shining a light induce or somehow modulate superconductivity? That will be very interesting to study,” Zhang says.
SARS-CoV-2 has infected more than 22 million people globally, leading to ~800 thousand deaths in just 10 months. However, significant uncertainty still remains regarding the prevalence of asymptomatic and mild cases of COVID-19, the disease caused by SARS-CoV-2, as well as the magnitude, effectiveness, and duration of antibody responses. Gaining a better understanding of population immunity is critical to improving predictive models of infection spread and safely reopening economies worldwide. However, to fill knowledge gaps in these areas requires population-scale testing using low-cost, non-invasive, and highly specific and sensitive assays that can be deployed broadly and serially to characterize antibody responses to SARS-CoV-2.
