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Physics World 2015 Focus on Optics & Photonics is out now

By Matin Durrani

With the 2015 International Year of Light now in full swing, it’s time to tuck into the latest focus issue of Physics World, which explores some of the latest research into optics and photonics.

The focus issue, which can be read here free of charge, kicks off by looking at the giant laser interferometers underpinning the latest searches for gravitational waves. We also report on recent efforts to use optical instead of radio waves for satellite communication and have an interview with Ian Walmsley from the University of Oxford about the vital role that optics and photonics play in the UK’s new £270m Quantum Technologies Programme.

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US cosmic-ray observatory set for expansion

The Telescope Array observatory in Utah, US, is set for a $6.4m upgrade that will see some 400 detectors added to the facility. This will quadruple its collecting area from 730 km² to 2500 km². Increasing the size of the northern hemisphere’s largest ultra-high-energy cosmic-ray detector will allow astronomers to learn more about the origins of the most energetic particles in the universe.

When a cosmic ray hits the Earth’s atmosphere, it produces a cascade of secondary particles. The Telescope Array currently has 507 scintillation detectors, which generate light in response to incident radiation. By detecting the cascade of particles, astronomers can then obtain information on the direction and energy of the original ray.

Scintillating science

Many astrophysicists believe that ultra-high-energy cosmic rays are just protons, although some argue that they may include helium and nitrogen nuclei. Possible sources for the highest energy rays include active galactic nuclei, supernova remnants and colliding galaxies. The Telescope Array, which started collecting data in 2008, is able to observe cosmic rays with energies greater than 1 × 10¹⁸ eV.

The observatory involves institutions from Belgium, Japan, Russia, South Korea and the US. Japan has announced it will provide ¥450m ($4.6m) to fund the majority of the expansion, with researchers seeking to find the remaining $1.8m to complete the upgrade. One possible source could be the National Science Foundation in the US. The new detectors will be built over the next three years.

“These experiments are very large because the flux of cosmic rays at the highest energies is very low, about two per square kilometre per century,” says Gordon Thomson, co-principal investigator for the Telescope Array and an astrophysicist at the University of Utah.

Cosmic hotspot

The Telescope Array has previously identified a possible “hotspot” of ultra-high-energy cosmic rays centred on the constellation Ursa Major. The observatory detects around 15 events per year, with a quarter in the hotspot – although this could be a statistical fluctuation. “If cosmic rays were [uniform], we would expect 0.9 events per year in the hotspot area of the sky, whereas we see about 3.5 events per year on average,” says Thomson. To confirm and study the hotspot, researchers need to collect more data. “With the fourfold increase in data rate from a four-times-larger detector, we expect to answer interesting questions about the origin of cosmic rays,” he adds.

The expansion, when complete, will make the Telescope Array similar in size to the Pierre Auger Observatory in Argentina, which is currently the world’s largest cosmic-ray detector with a 3000 km² collecting area. “Having observatories of a similar size in both the northern and southern hemispheres will enable full-sky surveys of ultra-high-energy cosmic rays,” says Karl-Heinz Kampert, a particle physicist at the University of Wuppertal in Germany who is co-spokesperson for the Pierre Auger Observatory. “The Telescope Array suffers a lot from the relatively low rate of events set by the present area, so expanding the detector array is a natural step – this should strengthen indications of a hotspot seen in the northern sky.”

Debating warp bubbles and quark novae over beer and samosas

Photograph of Miguel Alcubierre lecturing in Edmonton — Time traveller: Miguel Alcubierre works the crowd in Edmonton.

By Hamish Johnston at the CAP Congress in Edmonton, Canada

The first day of the Canadian Association of Physicists (CAP) Congress at the University of Edmonton closed yesterday on the theme of time travel. Surely that is science fiction, you are thinking? But Miguel Alcubierre of the National Autonomous University of Mexico (UNAM) wasn’t joking when he delivered the Herzberg Memorial Lecture yesterday evening (although he did giggle a lot during his talk, which was very endearing). The session was called “Faster than the speed of light” and it was a fascinating romp through some of the more bizarre implications of Einstein’s general theory of relativity (GR) – which is 100 years old this year.

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Lament for ‘the reactor that can do everything’

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.

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'Physics across Canada' tour kicks off in Edmonton

By Hamish Johnston

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.

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Philae comet lander wakes up from hibernation mode

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

Space Station vacancy, lead balloons on the fly and more

By Michael Banks and Tushna Commissariat

You may remember that the “classical crossover” soprano star Sarah Brightman had been undergoing strenuous training at Moscow’s Star City complex before hitching a ride on a Soyuz rocket to the International Space Station (ISS) in September. The singer was set to pay a whopping £30m for a ticket that would have seen her embark on a 10-day journey into space. Brightman even recorded a special song in March that she was planned to perform on the ISS itself – you can watch the 5 News report above. But Brightman has now postponed the trip, putting out a brief statement on her website citing “personal family reasons” for the decision. One beneficiary of Brightman’s no-show will be the Japanese entrepreneur Satoshi Takamatsu, who had been training as Brightman’s back-up. Whether he’ll do his own version of her planned performance isn’t clear.

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New study could help predict floods caused by atmospheric rivers

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 study is described in the journal Chaos.

Physicists blow magnetic bubbles

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”.

The research is reported in Science.

Tinker, tailor, physicist, spy?

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.

2015 Basic Books £20.00/$29.99hb 400pp

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Physics World 2015 Focus on Optics & Photonics is out now