Inside the NRU. (Courtesy: Atomic Energy of Canada)
By Hamish Johnston at the CAP Congress in Edmonton
In 1957 Atomic Energy of Canada built “a reactor that can do everything” at Chalk River, Ontario. Dubbed the National Reactor Universal – or NRU – that facility will shut down for good in 2018 and Canada’s neutron-science community is now pondering its future.
In the short term, physicists will have to travel abroad to use neutron sources, such as those at Oak Ridge in the US and Grenoble in France. The challenge during this 10–15 year period will be to keep the research community together and make sure that vital skills and expertise built up over decades at the NRU will be retained. In the longer term, there are calls for Canada to build a new neutron facility, but it is by no means clear whether that will happen.
Greetings from Edmonton on the western edge of the Canadian prairies, where I am starting my “Physics across Canada” tour. The nation’s physicists are gathering here for the annual Canadian Association of Physicists Congress at the University of Alberta.
The congress opens today with a session that promises to be out of this world. Exoplanet expert Sara Seager of the Massachusetts Institute of Technology is talking about the search for habitable worlds beyond our blue planet. I am really keen to learn more about the latest techniques for studying the atmospheres of exoplanets and I plan to record an interview about that very subject later this week.
The European Space Agency’s Philae lander has woken up following seven months in hibernation mode. Controllers at the agency received a signal from the lander at 22.28 CET on 13 June, and are now hopeful that the mission will soon be able to restart science operations.
Philae was part of the Rosetta mission that was launched in 2004 to study Comet 67P/Churyumov–Gerasimenko. While Philae successfully landed on the comet last year, it touched down in an awkward position, meaning that its solar panels could not charge. The craft still managed to carry out experiments using its onboard instruments, but after around 60 hours of observations – with the battery fully depleted – it entered hibernation mode on 15 November.
As the comet moved closer to the Sun this year, researchers were hopeful that Philae would slowly emerge from the shadows, allowing its solar panels to charge its battery. It is only when Philae receives around 19 W of power that it can start to reboot and then make contact. Since 12 March, the communication unit on orbiter Rosetta has been turned on to listen for the lander.
Late on Saturday, the Lander Control Center at the German Aerospace Center announced it had received a signal from Philae, when around 300 “data packets” were sent from the probe during an 85 s period. “Philae is doing very well: it has an operating temperature of –35 °C and has 24 W available,” Philae project manager Stephan Ulamec said in a statement. “The lander is ready for operations.”
Starting science
The Rosetta researchers will now try to piece together what happened to the lander in the past few days, as Philae would have woken a couple of days before it sent the signal. Indeed, there are still more than 8000 data packets in Philae’s memory that will give the team crucial information about the status of the lander. “We have only had brief interaction with the lander, but it seems in very good shape,” Matt Taylor, Rosetta project scientist, told physicsworld.com.
Taylor adds that the first priority is to get Rosetta in the best location for it to be able to communicate with Philae. “This is a major challenge, given that the comet is active and the dusty environment is challenging to navigate safely,” says Taylor. “Once we optimize things, then we can start lander science.”
With Philae receiving around three hours of sunlight each day, it is hoped that the craft will be able to spend this time doing experiments. In particular, a high priority will be drilling into the surface of the comet to obtain a sample that can then be analysed.
Taylor adds that he is hopeful that Philae can then spend a couple of months doing science before the comet starts heading back towards the outer solar system. “We will have to see how things evolve in the next days from the analysis of housekeeping data from the lander,” he says. “But I hope for a few months [of measurements].”
The Rosetta mission was awarded the Physics World 2014 Breakthrough of the Year, for being the first to land a spacecraft on a comet. Watch the Google Hangout video below, where physicsworld.com editor Hamish Johnston talks to Rosetta Mission manager Fred Jansen
According to the World Health Organization, cardiovascular disease (CVD) is the number-one killer in the world today. In 2012 around 17.5 million people died from CVDs, such as heart disease and stroke, accounting for more than 30% of all global deaths. CVDs can be diagnosed using various tools, including chest X-ray, electrocardiogram, Holter monitoring and cardiac magnetic resonance imaging. However, for early predictions of CVD risk in large populations, these tools are either too complicated or too expensive. As a result, individuals with a risk considered to be low or moderate are often undiagnosed.
A consortium of companies and research institutes, led by IMEC in Belgium, has recently been awarded €3.6m from the European Commission’s Horizon 2020 programme to develop a mobile, low-cost, non-invasive, point-of-care screening device. Called CARDIS, the project kicked off in February and will run for 42 months with the aim of making CVD mass screening possible. It is based on technology called laser Doppler vibrometry (LDV) – a non-contact technique that directs a laser at a moving surface and then uses the Doppler shift of the reflected light to infer its vibration amplitude and frequency. This technique has already been used in applications such as aircraft inspection and the monitoring of hard-disk drives, but only recently has it been used for CVD studies.
As part of the CARDIS project, LDV is used to detect vibrations of certain locations on the skin close to arteries or on the chest. The idea is to retrieve a number of useful parameters used for CVD screening, one of the most important being arterial stiffness. A stiffer artery leads to a higher pulse wave velocity (PWV) – the velocity of the pressure pulse in the arteries caused by the beating of the heart – so by measuring the PWV we can assess arterial stiffness. This measurement is typically made by recording how long it takes a pulse wave to travel between two different locations along the same artery. Although various vibration sensors can be used for this measurement, LDV can operate without physical contact and with higher accuracy.
Laboratory success
Studies have shown that LDV is sensitive enough to pick up additional information linked to CVD risk, such as stenosis-induced vibrations and cardiac-contraction abnormalities. Since commercial LDV devices contain discrete optical components or fibre systems that make them bulky and expensive, such LDV-based assessment has so far been limited to laboratory conditions. But this type of assessment has already been included in the 2013 guidelines for CVD risk prediction drawn up by the European Society of Hypertension and the European Society of Cardiology.
The feasibility of the LDV technique was demonstrated in a small-scale study several years ago by the present authors and colleagues (American Journal of Hypertension21 1280). Following this, Yanlu Li – a graduate student from Ghent University in Belgium – worked for four years to develop an on-chip version of LDV (Optics Express21 13342). CARDIS is the natural successor to this work and will bring the technology to a European level. The key innovation behind it is the use of silicon-based photonic integrated circuits, which can be manufactured in high volume and at low cost in the same fabrication plants that make computer chips. Indeed, the small (<1 mm2) footprint of a single on-chip device means it is just as easy to make a chip holding, say, 10 LDV circuits as it is to make a single circuit.
In 2013 we demonstrated in vivo PWV measurement with an on-chip double-LDV system (Biomedical Optics Express4 1229), where the “double” refers to its need to measure skin movement at two locations. With CARDIS, in contrast, the proposed PWV measurement device will be a photonic chip with two groups of six LDV systems. A custom-designed micro-optic system will focus the six beams on the skin to ensure that at least one spot is located on top of the artery, offering a user-friendly device that allows a doctor or a nurse to assess CVD risk in less than a minute.
Silicon photonic integrated circuits have a lot of potential for other demanding sensor applications for clinical diagnosis, such as sensing glucose levels in the interstitial fluid of diabetes patients. In this application, which our group has shown could work, the chip would be the heart of a compact subcutaneous implant.
The CARDIS project is well placed to succeed, boasting institutes with a track record in cardiovascular research and clinical validation, including Ghent University, Maastricht University in the Netherlands, INSERM in France and Queen Mary University of London in the UK. On the technology side, IMEC, SIOS Messtechnik in Germany and the Tyndall National Institute at University College Cork in Ireland will work to design the system and the photonic chip, and to integrate all the components in a compact device. Meanwhile, Netherlands-based Medtronic brings expertise in developing medical devices and bringing them to market.
The outcome of CARDIS will be a complete, functional prototype. Turning this into a commercial device that can be manufactured in high volume is beyond the remit of the project, but our results should encourage industry to invest on its own.
Imagine you were to measure the distance between the Earth and the Sun so precisely that you could observe changes in their separation equivalent to just a single atomic radius. As challenging as it sounds, this degree of precision is currently being realized at several sites across the globe in the search for gravitational waves. Predicted by Einstein’s general theory of relativity a century ago and generated by the acceleration of mass, these fluctuations in the curvature of space–time are so weak that only the most extreme astronomical systems – pulsars, supernovae, neutron stars and black holes – can produce a detectable signal.
Waiting to catch them is a series of “long-baseline” gravitational-wave detectors: LIGO in Lousiana and Washington State, US; Virgo in Italy; GEO600 in Germany; and the KAGRA detector under construction in Japan. The first-generation LIGO detectors began operation in 2001 and several runs have been carried out since then, most in conjunction with GEO600 and many with Virgo and TAMA – the predecessor of KAGRA in Japan.
Indirect evidence
Although no direct detection has been made, we have excellent indirect evidence for the existence of gravitational waves. The orbital period of the binary pulsar PSR 1913+16, for instance, is decaying at precisely the rate predicted by models of gravitational-wave emission. A direct discovery of these space–time ripples would not only remove any remaining doubt, but also usher a new era of gravitational-wave astronomy. This is the source of great excitement for a second generation of detectors – in particular an upgraded version of LIGO seven years in the making – that are due to come online starting this year.
As a gravitational wave propagates, it alternatively compresses and stretches space–time and exerts tidal forces on objects in its path. Long-baseline detectors search for these effects using laser interferometry to measure changes in the separation of test masses located at the ends of two long, perpendicular arms. (In LIGO, which is the largest of the long-baseline detectors, the arms are 4 km long and the test masses weigh 40 kg apiece.) A beamsplitter divides the laser lightinto two components and directs them along the arms to be reflected from the mirrored surfaces of the test masses, after which the reflected light is recombined to produce an interference pattern. Since a passing gravitational wave would increase the length of one arm relative to the other, it would cause a detectable change in the interference pattern.
To achieve the fantastically high sensitivity required to detect such minute displacements, a range of noise sources must be combatted. Seismic noise arising from the motion of the ground is reduced by suspending the mirrors by glass fibres, for example, while thermally induced vibrations in the mirrors and suspension fibres is another key noise source in low to mid-range frequencies. Seismic waves also cause “gravity gradient noise” – a direct gravitational force on the mirrors that effectively excludes searching for gravitational waves at frequencies below about 1 Hz.
In addition, quantum noise arising from radiation pressure at lower frequencies and photoelectron “shot noise” at higher frequencies combine to form what is known as the standard quantum limit. Advanced techniques for surpassing the standard quantum limit, such as “squeezed light” or speed-meter interferometry, may be implemented in future LIGO upgrades.
Following their initial operational period, a programme of upgrades to the LIGO and Virgo detectors was initiated in 2010. The Advanced LIGO (aLIGO) interferometers are now being commissioned, while the Advanced Virgo detector is in the final stages of construction. GEO600, meanwhile, has been used to test cutting-edge techniques and to provide cover in case a large gravitational-wave event occurs while the other detectors are offline. KAGRA is being built in a mine under Mount Kamioka in Japan to reduce seismic and gravity-gradient noise. Taken together, all of these widely separated detectors can operate as a network in order to pinpoint the source of any detected gravitational waves.
Thanks to upgrades to all the interferometer subsystems in LIGO – including a 200 W laser for improved shot noise, larger optics, improved optical coatings and silica suspension fibres, and more aggressive vibration isolation – the aLIGO detectors are designed to be 10 times more sensitive to gravitational waves than in the past. This translates to a rise inthe expected event rate by a factor of 1000, leading to optimism that aLIGO will make a first detection of gravitational waves in an early observation run some time between now and 2020.
While this will be a momentous event, the ultimate goal is to make “gravitational-wave astronomy” a mainstream tool. The weak nature of gravity means that gravitational waves, unlike electromagnetic waves, are not readily scattered or absorbed. In addition to offering new insights into the physics of supernovae, neutron stars and black holes, gravitational-wave astronomy could therefore reveal information about the early universe and let us explore unknown astronomical objects that are massive but do not emit light, such as dark matter.
Optics challenge
Meeting aLIGO’s strict specifications has required state-of-the-art optics. At its heart are the interferometer mirrors, which must exhibit low thermal noise, high reflectivity and ultra-low optical loss. Thermal noise can be cut by using materials with a low mechanical loss: just as a good wine glass will ring with a pure note when tapped, so a gravitational-wave detector must be stable against thermal noise that causes the optics to vibrate. Careful design of aLIGO and the use of low-loss materials such as fused silica allow thermal noise to be low enough to detect gravitational waves.
Surfing gravity The Hanford/Livingstone LIGO detector site showing the 4 km-long perpendicular interferometer arms and the central station housing the beamsplitter, input and output optics. (Courtesy: LIGO)
To allow sufficient light power to build up in the arm cavities, which magnifies the gravitational-wave signal and reduces shot noise at high frequencies, highly reflective optical coatings made from multiple layers of dielectric materials were applied to the front of the mirrors. Unfortunately, these coatings typically have a mechanical loss several orders of magnitude higher than the fused-silica-mirror substrate, resulting in thermal noise that limits aLIGO’s performance at its most sensitive frequencies. Understanding these issues has been a crucial part of the R&D for aLIGO and potential further upgrades.
In addition, the mirror coatings must also have exceptional optical performance so that high levels of power can build up in the interferometer arm-cavities and minimize thermal distortions in the mirrors. Extraordinary levels of optical absorption lower than 0.5 parts per million (ppm) and less than 2 ppm of the incident light scattered have been achieved for aLIGO.
Construction and installation of aLIGO began in 2008 and finished in March this year. Most of the $205m price tag has been paid for by the US National Science Foundation, with Australia, Germany and the UK contributing $30m. Some 10% of the overall budget has been devoted to aLIGO’s test masses, beamsplitters and other core optical components. Heraeus in Germany provided the silica glass; Tinsley did the shaping and polishing; and Laboratoire des Matériaux Avancés and the Commonwealth Scientific and Industrial Research Organization did the coating.
The first optical components were delivered to the LIGO Louisiana site in early 2012 and it took a few months to instal them. The first step was to weld the test mass to the silica suspension fibres. Each one is supported by four fibres, which at their narrowest point are only 0.4 mm in diameter. The entire cartridge of suspension, optic and seismic isolation was then lowered into a vacuum chamber by a crane, where technicians aligned the optics in situ.
There are still challenges with charge build-up on the mirrors, for instance, because their positions are controlled via electrostatic actuators, and also with mechanical excitation of the installed optics. However, none of these issues has changed the expectation that aLIGO will have the sensitivity to detect a gravitational wave.
In 2016 aLIGO will be joined by Advanced Virgo, followed by KAGRA in 2018 and a possible third aLIGO detector in India in 2022. Thanks to this truly international effort, there is a growing sense that a direct detection of a gravitational wave is on the horizon – one that may just chime with the centenary of Einstein’s prediction.
The development of light microscopy in the 17th century led directly to the field of cell biology and transformed our view of life. It was quickly realized, however, that biological samples are optically transparent and therefore difficult to image using visible light. Indeed, in 1873 Ernst Abbe showed that there was a fundamental diffraction limit to optical resolution – a result that would later be more deeply understood in terms of Heisenberg’s uncertainty principle. For most of the 20th century the resolution dogma remained unchallenged and cell microscopy witnessed a constant struggle to improve image resolution.
Contrast, on the other hand, can be improved by using “exogenous” agents – those introduced from outside the body – to label structures of interest. Stains are essential for histopathology, for example, while fluorescent dyes lie behind many recent advances in cell biology. Although these techniques provide high-contrast images, samples tend to suffer from phototoxicity or photobleaching, which can damage the cells under investigation and limit continuous measurement times, respectively. An alternative way to improve contrast is to engineer optical systems that exploit the scattering interaction between light and the sample to reveal structure.
Imaging breakthroughs
The first breakthrough for such label-free, or “endogenous”, contrast imaging came in the 1930s with the invention by Dutch physicist Frits Zernike of phase contrast microscopy (PCM). This allowed fine biological structures to be visualized with sub-cellular resolution. Although PCM enabled biologists to observe living cells for long periods, and earned Zernike the 1953 Nobel Prize for Physics, the phase information is not retrieved quantitatively and therefore provides little more than a high-contrast image. If we are to truly characterize biological systems, we need high-throughput, quantitative methods that perturb samples as little as possible and which can quantify dynamic behaviour at temporal scales ranging from milliseconds to days, and spatial scales ranging from nanometres to centimetres.
A new technology called spatial light interference microscopy (SLIM) incorporates all of these key capabilities. SLIM is a type of quantitative phase imaging (QPI) – a field born out of Dennis Gabor’s pioneering work in holography in the 1940s. It measures phase shifts through the specimen at each point in the field of view to provide unique, quantitative insight into biological systems in a completely non-invasive manner (Optics Express19 1016). In order to retrieve phase information, two waves are required: one that propagates through the sample and one that does not, after which the two wave fronts can be compared. While such interferometry is already used in cell microscopy, for example via existing PCM technology, these are not quantitative.
In traditional interferometry, the sample and reference fields travel through different optical paths. That makes it hard to extract phase information because noise experienced by the reference field is uncorrelated with that experienced by the sample field. Furthermore, interferometers traditionally use lasers – a highly coherent source of light that creates random interference patterns, known as “speckle”, which makes the technique less spatially sensitive.
SLIM avoids uncorrelated noise by making the reference and sample fields travel through the same path, such that the noise cancels out when they interfere. By using a broadband source with a shorter coherence length, it also addresses the problem of speckle. Taking the form of an add-on module for existing commercial PCM devices, the SLIM system builds on these basic principles of the phase-contrast microscope and transforms it from a qualitative to a quantitative instrument.
Back in the 1930s, Zernike had appreciated that light emerging from the sample is spatially decomposed into two parts. There is a low-frequency portion that passes through the sample undeviated, which falls in the centre of the back focal plane of the objective, and a high-frequency component surrounding the non-deviated light. In the case of thin biological specimens, the phase shift induced by the sample relative to the background barely modifies the image intensity. But Zernike realized that by introducing a thin phase plate to create a 90° phase delay to the background light, minute changes in phase translate to a detectable change in intensity.
SLIM in action
SLIM turns Zernike’s phase plate into a tunable element. It uses a lens to project the “pupil plane” of the microscope onto a spatial light modulator that imparts a phase delay to the incident field. A second lens is then used to project the modulated image onto a CCD camera. Four intensity images are acquired at 90˚ phase-delay increments between the scattered and unscattered fields, allowing the quantitative phase image to be calculated. The apparatus therefore offers all the bells and whistles of the modern microscope – such as automated positioning, stage-top incubation and fluorescence capabilities – while providing unprecedented quantitative insight into cell and tissue physiology.
Since the measured phase shift is proportional to the product of the relative refractive index and the thickness of the sample, SLIM has two natural modes: to measure topography in situations where the refractive index is known, and vice versa. One of the first samples we measured, in 2008, was a carbon film grating with a known refractive index, which proved that the topography measured by SLIM agrees with measurements using atomic force microscopy to within a fraction of a nanometre. The broadband illumination led to a spatial sensitivity of 0.3 nm, while SLIM’s common path geometry enabled a temporal sensitivity of 0.03 nm.
Remarkably, when it comes to living cells, the phase information can also be used to calculate the non-aqueous content, or “dry mass”, at the single-cell and even sub-cellular level – a fact first realized by biologist Robert Barer in the 1950s. Our experiments show that SLIM is sensitive to changes in dry mass at the femtogram level and so can be used to study the growth characteristics of single living cells. Since cells weigh on the order of picograms and only double their mass over the cell cycle before dividing, femtogram accuracy is vital to know if a single cell is growing exponentially or linearly, for instance. Recently, we used SLIM to measure the growth of a single E. coli bacterium and verified that our results agreed well with previous growth measurements from flow cytometers and microresonators (Lab on a Chip14 646).
Mapping mammalian cell growth
We then turned our attention to the growth of single mammalian cells as a function of cell cycle – a problem that at the time could not be addressed with any other system. Such cells undergo a cell cycle that typically starts off with a growth phase, followed by a DNA replication phase, then a second growth phase, and finally mitosis or cell division. Researchers typically use samples with artificially synchronized population cycles, the effects of which are transient and perturbative. We used SLIM to acquire fluorescent images using a reporter that indicates when cells are in the DNA replication phase, and found that mammalian cell growth is much more variable than the traditional textbook picture (Proc. Natl Acad. Sci.108 13124).
Cells in action A pair of human osteosarcoma cells growing within a 3.2 × 2.4 mm region of a cell culture dish (lower image) that was imaged every 10 minutes for 48 hours. (Courtesy: Mustafa Mir and Gabriel Popescu)
More recently, we have used SLIM to characterize the mass growth during neural‑network formation in mature human neurons over a period of 24 hours. The results revealed a period of growth lasting 10 hours, during which the SLIM images showed neurons extending and exploring their environment. This was followed by a plateau characterized by an aggregation of clusters and finally larger-scale reorganization. We then analysed the data using a new technique called dispersion-relation phase spectroscopy (DPS). This technique analyses fluctuations of mass as a function of time and space, providing a quantitative picture of mass transport in cells (Optics Express 19 20571).
Applying DPS at 0 and 24 hours we found that the mass transport characteristics of the neuron culture shift from being dominantly diffusive to dominantly directed as the network matures and forms more connections. This indicates that the network at some point “decides” to divert energy from growth and exploration to transport and communication (Scientific Reports4 4434).
The broadband nature of the illumination in SLIM, coupled with a high numerical aperture, also enables high-resolution single-cell tomography by mapping the refractive index in 3D using a method called white-light diffraction tomography. As recently demonstrated by our group, this allows the study of 3D biological structures in living cells – a feat that was previously only achievable using fluorescence-based methods such as confocal or light-sheet microscopy (Nature Photonics8 256).
Although many biological applications of QPI have already been explored, we believe that the field is just getting started. Recent results on SLIM imaging of biopsies, for instance, indicate that clinical information can be extracted from the nanoscale morphology of tissues (Scientific Reports at press). Until now, SLIM has been used mainly by optical engineers and physicists, so we need to further distill the data into parameters that directly provide insight to biologists and to identify relevant research questions. The recent commercialization of SLIM technology by Phi Optics – a spin-out from the University of Illinois – is a major step forward in this regard, since it adds QPI capabilities to any existing microscope.
As well as getting the instruments into the hands of more biologists, we also need to develop mathematical tools to analyse the wealth of information in QPI data. One such approach aims to connect the large, multi-parameter space measured by QPI to cellular phenotype, which is currently measured using antibody-based labels. If successful, this could ultimately remove the need for fixing and staining, in the process saving researchers thousands of dollars. Given the pace of growth and rapid advancements in the field, we believe that QPI techniques will play a dominant role in the future for many biologists and biophysicists working to elucidate the mysteries of cellular life.
The formation of “atmospheric rivers” – thin corridors that transport moisture away from the tropics – may, in some cases, be linked to coherent structures that occur in Earth’s large-scale wind patterns. That is the conclusion of a team of scientists in Spain and Switzerland. The researchers believe that their work could help to improve the identification and classification of atmospheric rivers, and better predict which ones will lead to extreme weather conditions.
The movement of water vapour from the Earth’s tropics to higher latitudes is an intermittent process, with 90% of moisture occurring in the form of atmospheric rivers. These are narrow corridors of moisture – each typically a few thousand kilometres long but only 400–600 km wide – that often carry more water than the Amazon river. There are usually four to five atmospheric rivers present in each hemisphere at any given time. A famous example is the “Pineapple Express”, which carries moisture from Hawaii to the west coast of North America.
When such a river makes landfall, the resulting precipitation can be extreme, with the largest causing sizeable floods. Despite both their impact on human activity and importance to Earth’s water and heat cycles, however, the formation of these structures is poorly understood.
Dynamic skeletons
Recently, the concept of Lagrangian coherent structures has emerged as a way of looking at transport within large-scale fluid flows. These structures are distinct surfaces of trajectories in a flow, and form the basic skeleton of the larger dynamic system. The structures last long enough to form separate areas of the fluid, each with distinct transport properties. Studying these structures has furthered our understanding of a variety of fluid flows, including clouds of volcanic ash, blooms of plankton and offshore oil spills.
The Lagrangian coherent structures serve as a kind of temporary scaffolding around which an atmospheric river can grow and lengthen
Vicente Pérez-Muñuzuri,University of Santiago de Compostela
“Given that atmospheric rivers over the Atlantic and Pacific oceans appear as coherent filaments of water vapour lasting for up to a week, and that Lagrangian coherent structures have turned out to explain the formation of other geophysical flows, we wondered whether Lagrangian coherent structures might somehow play a role in the formation of atmospheric rivers,” says team member Vicente Pérez-Muñuzuri, a physicist at the University of Santiago de Compostela.
To test this idea, Pérez-Muñuzuri and colleagues studied data on the wind-speed and water-vapour flux in atmospheric rivers running over the Atlantic from the Caribbean to the Iberian Peninsula. These data were compared with the predictions of a computer model that simulated the movement of thousands of air particles.
The results revealed a close similarity between the Lagrangian coherent structures formed in the simulations and the patterns of the atmospheric rivers that form over the Atlantic in the winter months. “The Lagrangian coherent structures serve as a kind of temporary scaffolding around which an atmospheric river can grow and lengthen,” says Pérez-Muñuzuri, with the wind field forming shear, filamentous jets that act as a transport barrier, separating regions of strong and weak horizontal moisture flow.
In contrast, strong Lagrangian coherent structures were not found to be associated with the shorter, less-defined and short-lived atmospheric rivers that typically form in the summer. The researchers suggest that in these rivers the water-vapour balance could be dominated by local sources. Team member Daniel Garaboa explains that the stronger and more persistent winds that occur in the winter lead to more homogeneous moisture-transport patterns.
“This new work has important implications for the definition and detection of atmospheric rivers,” comments Bin Guan, a climate physicist at NASA’s Jet Propulsion Laboratory in California who was not involved in the study. Guan explains that, as atmospheric rivers can have different shapes and moisture loads, it has previously proved challenging to develop universal criteria for defining these features. A better understanding of how the rivers form – and a classification of their types – may help to predict which types lead to extreme meteorological conditions.
The capacity of computer memory could be boosted by exploiting tiny magnetic vortices known as skyrmions. That is the vision of physicists in the US, who have made individual skyrmion “bubbles” at room temperature by pushing magnetic domains through a narrow gap in a thin ferromagnetic-metal film – just as bubbles can be made by blowing on a soap-covered gap.
Skyrmions are particle-like regions within a field where all of the field vectors point either towards or away from a single point – a bit like the way in which spines are arranged on a hedgehog. Skyrmions were originally proposed in the 1950s by British physicist Tony Skyrme, who found they could explain the emergence of protons and neutrons from the field that mediates the strong nuclear force. But while skyrmions never really took off in particle physics, their underlying mathematics has been applied to other areas of physics.
The crucial point about a skyrmion is that it is topologically stable. This means that, while its shape can be altered, the “twistedness” of a skyrmion cannot be broken without first removing the singularity that holds it together. This resembles the behaviour of a Möbius strip, which must be cut if it is to be transformed into a normal loop – no amount of bending will do the job.
Tiny yet stable memory
This is a quality that could make skyrmions ideal for use in computer memory. Hard disks work by encoding 0s and 1s using the direction of atomic magnetic moments within small regions, or domains, on the surface of a ferromagnetic material. But there is a limit to how small these domains can be made before the magnetization becomes unstable. The great stability of skyrmions means they could potentially occupy far smaller areas – having dimensions of just a few nanometres, rather than the roughly 50 nm of today’s best drives – and the very small electric current needed for their operation means they would consume less energy.
In the latest research, Axel Hoffmann of the Argonne National Laboratory in Illinois and colleagues have made a device consisting of a very thin layer of a cobalt-iron-boron alloy – a ferromagnet – sandwiched between layers of tantalum metal and tantalum oxide. They pattern this multilayered film so as to create a wire 60 μm wide with a narrow (3 μm) constriction about halfway along.
Normally, the magnetic moments of electrons within a thin film of ferromagnetic material point along the plane of the film, but the layered structure of this device orients those moments perpendicular to the plane. The researchers apply a magnetic field to create a long “stripe” of domains with upward-pointing magnetization, surrounded by areas of opposing magnetization, and then switch on an electric current flowing from left to right along the wire.
That current exerts a force on the magnetic domains, moving them to the right. But since the current cannot flow outside of the wire, it is constrained to funnel through the constriction, thus introducing a vertical component to its left–right motion. That in turn pushes the sides of the stripes outwards as the latter emerge from the narrow gap. This expansion continues until the head of each stripe forms a bubble shape that then breaks off and continues to move rightwards (see figure).
“This bubble disconnects, just like an expanding soap bubble or a drop from a dripping tap,” explains Hoffmann. “It has the exact topological structure of a magnetic skyrmion.”
Everyday materials
A significant advantage of this scheme over alternative ways of generating skyrmions, says Hoffmann, is its use of transition metal alloys and other metallic films that are already found in commercial memory devices. The device also operates at room temperature, so doing away with the need for expensive refrigeration equipment. In addition, he says, whereas skyrmions are usually manipulated within lattices, here they have been isolated.
According to Hoffmann, these skyrmions could have a number of uses. One would be to make logic gates. An AND gate, for example, could potentially consist of two joined wires that would generate an output only if skyrmions were present in both wires. However, the most straightforward application would be encoding data using the presence or absence of skyrmions. In particular, “racetrack memory”, which involves reading and writing to mobile magnetic domains on a fixed wire, suffers from the fact that stripe-shaped domains can easily be corrupted by defects in the wire. Point-like skyrmions, in contrast, could move around such defects, he says.
Kirsten von Bergmann of the University of Hamburg in Germany, whose group in 2013 reported having created and destroyed skyrmions on a thin film of palladium and iron, praises the latest work, which she says is “a significant step towards implementation” of real devices. But she cautions that significant hurdles remain, including the need for better control over the positioning and movement of individual skyrmions. Also crucial, she says, is size. The American group has produced skyrmions with a diameter of about 1 μm, which is one to two orders of magnitude too large to be useful, she estimates.
Hoffmann acknowledges this shortcoming but believes it can be overcome. He says that other research groups have been able to make room-temperature skyrmions as small as 100 nm across (the greater thermal energy at higher temperatures perhaps making it more difficult to prevent small skyrmions from unwinding). Reducing the scale by another factor of 10, he says, “is not so unrealistic”.
Bruno Pontecorvo was the only nuclear physicist who defected from the West after contributing to Allied nuclear research during the Second World War. His flight to the Soviet Union in 1950 rang alarm bells throughout the physics community, but especially in the UK, where he had been employed in the Atomic Energy Research Establishment at Harwell. Since then, the questions of whether Pontecorvo passed restricted information before his defection, and what contribution he made to Soviet atomic energy research after it, have been the focus of several enquiries. The recent declassification of documentation produced by the British security services has revamped the search for answers, and Frank Close’s Half-Life: the Divided Life of Bruno Pontecorvo, Physicist or Spy is the latest book to take advantage of this newly available information.
At this point, I must offer a caveat. Mine is one of two previous works that Close describes as “excellent books” about Pontecorvo (thank you, Frank), and his biography of this Italian-born scientist stems from his reading (and dissecting, as I now realize) of both my published work and several conversations we have had on the subject of “Bruno”. Reading Close’s book was thus an unusual experience for me (at one point, I declared “This is not true!” only to turn to the book’s endnotes and gasp; the source cited was my own book), and so writing this review is a little unusual as well. So, reader, I beg your pardon: I will describe Close’s book, but I cannot comment on it.
Close has carefully examined the declassified papers and gained additional information from a number of other sources. The result is a rich account, containing details on Pontecorvo’s life and career that previous works on atomic espionage overlook. What struck me especially was the description of Pontecorvo’s research on the ethereal “quadium”: a hydrogen isotope wrongly assumed to have potential as an ingredient for a new type of atomic bomb.
It made me wonder whether Ponte-corvo’s one-way trip to Moscow was solicited precisely because of this useless -component. If this is the case, then the Pontecorvo episode would probably fit better in a Monty Python sketch than in the latest James Bond film: in this version of the story, the physicist was headhunted to complete the Soviets’ ultimate nuclear weapon, only to produce a device that fizzled out even before being weaponized. One may also wonder if the Soviets were then, in effect, stuck with “Dr Quadium” (a.k.a. Pontecorvo) and his family, but could not say so.
Close’s book has other surprising twists that would, however, hardly feature in a thriller by Ian Fleming. He describes, for example, how Pontecorvo’s mamma sent him a letter just before his flight to Russia in which she urged him to tell her the truth – as if he were a naughty child caught with his hands in the cookie jar. There is also a shy and mysterious Swedish woman, Marianne Nordblum, among the supporting characters – but she is hardly a deceitful temptress intending to steal Pontecorvo’s secrets. Instead, she is his wife, a woman prone to depression who Bruno loved “to bits”.
By now, you will have realized that I lied when I wrote that I would not comment on Close’s biography. The truth is that his book has the invaluable merit of taking the reader away from the expected cast of covert spies intending to steal secrets (and intelligence agents eager to catch them at it) and placing real human affairs at the centre of the narrative instead. Bruno Pontecorvo thus appears as the ingenuous genius that he was; the naive and lumbering main character of his life’s drama.
On the other hand, when Close does focus on the “spy trail”, I wonder whether his book perhaps gives more credit to espionage literature than the genre deserves. In particular, Close alleges that, before his defection, Pontecorvo might have been the one who smuggled uranium powder to the “Mata Hari” of Massachusetts, Leontine Theresa Cohen. Close also claims that the notorious double agent Kim Philby warned Moscow about Pontecorvo, thus instigating the latter’s defection. He thus renews the doubt that has been haunting many before: was the physicist a spy?
Even if we consider Pontecorvo’s naivety to be apocryphal, and regard as misleading the lack of non-anecdotal evidence on his Communist background, an examination of what Robert De Niro (as ex-CIA operative Jack Byrnes in the film Meet the Parents) calls “circles of trust” gives -reasons to doubt that he was. Spy rings existed in both the US and Canada, but their operatives, with the notable exception of Klaus Fuchs (an exception nonetheless), were recruited among English-speaking Communist Party members. Could Pontecorvo trust them? Could they trust him?
Yes, atomic spies did exist, and, yes, there are still more unidentified codenames in the US counterintelligence “Venona Project” than there are known agents with names and surnames. But the allegory of modern crusaders fighting against scheming Communist infidels to protect a “holy grail” of atomic secrets has rapidly decayed. This narrative was constructed after the first Soviet atomic test in order to cast a negative light on the Communist atomic -programme, but it actually hid the fact that Allied governments encouraged the exchange of scientific information with Soviet Russia right up until 1944.
After the war, other nations joined the race for atomic energy, and they too exchanged “secrets”, above and beyond ideologies and legal protocols. In 1970, for example, a French colleague of Pontecorvo, Lev Kowarski, revealed that many physicists had learned through the grapevine about a Canadian reactor called ZEEP, which became a prototype for the Norwegian JEEP and the Swedish SLEEP. Pressed by his interviewer, though, Kowarski denied committing security violations. “A certain amount of leakage is unavoidable,” he said. One wonders whether he was thinking about his Norwegian colleague Gunnar Randers, who first learned about atomic energy in wartime Britain, was recruited by US officials to acquire secrets of the Nazi nuclear project and, finally, unbeknownst to the Americans, consulted with the French in order to complete a Norwegian reactor. Was Randers ever accused of espionage? Of course not: he was appointed NATO’s science adviser.
Thanks to his contagious enthusiasm, Close has gone a long way towards reconstructing Pontecorvo’s life, and thereby uncovering the real man behind the fictional spy. The acknowledgments at the end of the volume show how many people he has contacted to find the fresh evidence that makes this important appraisal possible. Pontecorvo’s flight to the Soviet Union is still a mystery, but it is thanks to Close that the veil that shrouds a crucial episode in the history of the Cold War may soon fall.
