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Home » Page 1340

Germany sends optical-clock signal over nearly 1000 km

Physicists in Germany have sent a burst of light over a distance of 920 km down an optical fibre – with its frequency remaining stable to the 19th decimal place. As well as supporting the development of highly accurate “optical clocks”, the breakthrough could also be used in a range of commercial and scientific applications including precision spectroscopy, geodesy and very-long-baseline astronomy.

Optical clocks are like conventional atomic clocks but operate at much higher frequencies – about 1015 Hz rather than 1010 Hz. They are therefore much more accurate than atomic clocks, which use a specific electronic transition frequency of an atom as a frequency standard, with the “ticks” being the oscillations between two energy states in an atom. Indeed, the best optical clocks have fractional uncertainties in their frequencies of around 10–18.

But for any clock it is important to be able to compare the frequencies of two or more instruments. In particular, nations set their time standard according to their own atomic clocks, which are compared to others around the world via satellite links to ensure that they all produce the same result. Comparisons are also important for basic research, particularly for testing the fundamental physical laws and constants that are involved in the operation of atomic clocks.

Flying clocks impractical

Unfortunately, satellite links are only able to compare clocks with fractional uncertainties of about 10–15 and therefore cannot be used to compare optical clocks. One alternative being investigated is the use of “flying clocks” that could be shuttled between facilities to compare time. However, unlike atomic clocks, which are amenable to miniaturization, optical clocks tend to be room-sized systems involving lasers and vacuum chambers.

Other groups are looking at how to use commercially available optical fibres to compare clocks. In 2009 Gesine Grosche and colleagues at the PTB standards lab in Braunschweig, Germany, transmitted an optical signal 146 km over a fibre with a fractional uncertainty of 10–19. Then in 2011 Atsushi Yamaguchi of Japan’s National Institute of Information and Communications Technology and colleagues compared two atomic clocks separated by 120 km of optical fibre at a fractional uncertainty of 10–19.

In the new work, a team including Grosche and Katharina Predehl at the Max Planck Institute for Quantum Optics (MPQ) near Munich has sent an extremely stable optical signal 920 km along fibres linking PTB with MPQ. The two fibres run in a buried conduit next to an underground gas pipeline and one is used to send signals from PTB to MPQ and the other to send signals in the opposite direction.

Amplitude and frequency

Predehl told physicsworld.com that transmitting the optical signal over 920 km posed two significant technological challenges. The first was to amplify the relatively weak signal at several points along the route to ensure that it could be measured upon arrival. The second was to ensure that physical disturbances to the optical properties of the fibre – caused by local changes in temperature or vibrations – do not shift the frequency of the signal.

Solving the first problem was relatively straightforward, according to Predehl. This was done by installing nine pairs of erbium-doped fibre amplifiers at nine locations between PTB and MPQ. Although each amplifier introduces some noise into the optical signal, this is distributed symmetrically across the frequency spectrum and therefore does not affect the frequency of the signal, Predehl explains.

Local changes to the optical properties of the fibre can shift the frequency of the transmitted signal and, if left unchecked, would limit the link to a fractional uncertainty of 10–15. The team corrected for these local changes using a feedback loop. Some of the light that arrives at MPQ, for example, is reflected back to PTB – a round trip that takes about 10 ms. This light is then compared with the light that is being sent and the frequency of the latter adjusted so that the two match – the result being that the same frequency is sent and received. By doing this, the team managed to limit the fractional uncertainty of the one-way trip to 4 × 10–19.

Precision spectroscopy

While the link is good enough to compare two optical clocks, that probably will not be its first application – for the simple reason that there is no optical clock at MPQ. Instead, its first practical use is likely to be precision laser spectroscopy, which requires access to an extremely stable frequency standard. While a signal from an atomic clock is currently suitable for MPQ’s spectroscopy research, Predehl says that they are approaching the point where they will need a signal from an optical clock. At that point the lab could use a signal from an atomic clock at PTB for its spectroscopy experiments.

Looking further into the future, Predehl believes that the network could be extended to other labs in Europe. This could allow researchers in standards labs in London, Paris and other locations to compare their optical clocks. Indeed, Giuseppe Marra, who works at the UK’s National Physical Laboratory, calls the PTB–MPQ link an “important step towards a pan-European fibre network for connecting optical clocks”. Indeed, in some European countries, scientists have already taken steps to ensure that they have access to the appropriate fibre networks.

While fibre networks should be able to connect optical clocks within Europe and within Japan, connecting these facilities with those in the US – and even connecting widely spaced labs within the US – could be a challenge. According to Predehl, the technique should work in principle over transatlantic distances – but not on existing links, which don’t have the appropriate amplifiers.

In addition to spectroscopy and metrology, such optical networks could be used in a range of fundamental science. The rate at which an optical clock beats is determined by the value of the fine structure, which should not vary by location or time. Comparing optical clocks in different locations could reveal variations in this constant, which could point to new physics. Optical clocks are also governed by Einstein’s theory of relativity and, for example, tick at slightly different rates when at different altitudes. Comparing clocks could reveal deviations from what is predicted by theory, and also lead to new physics.

The link is described in Science 336 441.

Quirky solar cell sets new efficiency record

Researchers in the US have built a new type of solar cell that emits light as well as absorbs it, making it the most efficient single-junction device ever developed. The efficiency of their prototype cell allows it to convert 28.6% of the Sun’s energy into electricity. This is a considerable increase from the previously recorded highest efficiency of 26.4%, which was achieved in 2010.

Scientists have known since 1961 that the absolute limit for the amount of energy that can be harvested from sunlight hitting a typical solar cell is about 33.5%. However, for almost five decades researchers have been unable to come close to achieving this theoretical efficiency. But now, Eli Yablonovitch and his graduate student Owen Miller from the University of California, Berkeley have designed and built a new type of solar cell that gets closer to that limit by mimicking the behaviour of a light-emitting diode. That is to the say the solar cell is highly capable of absorbing light as well as emitting it. In fact, it is the controlled emission of light that has boosted the efficiency.

The researchers have shown that the better a solar cell is at emitting photons, the higher its voltage is and the greater its efficiency. “[The result] is almost paradoxical and counterintuitive. It can be quite confusing to grasp at first,” says Yablonovitch, as he tells physicsworld.com that he and his colleagues discovered the connection while trying to resolve the large gap between the theoretical and achieved limits for solar-cell efficiency.

Managing photons

The solution lay in a mathematical connection between absorption and emission of light – a phenomenon better understood as “photon management”. Conventionally, photon management involves controlling the photons incident on a solar cell so that a photon ejects as many electrons as possible, thereby generating the maximum amount of electric current. “But there is another aspect to photon management, in that we manage not only the incident light, but also the emitted light. Emitted photons sometimes get ‘lost’ within the cell, so what we do is make sure those photons are emitted,” explains Yablonovitch. In a conventional solar cell, photons from the Sun hit a semiconductor material, knocking electrons loose and allowing them to flow freely. But this process can also generate new photons, in a process known as “luminescent emission”. As there is a fundamental thermodynamic link between absorption and emission, designing solar cells to emit light causes an increase in the voltage produced by the device.

The researchers’ novel concept has been put into practice by a company called Alta Devices, which was co-founded by Yablonovitch and California Institute of Technology physicist Harry Atwater in 2007. The firm was set up specifically to produce economic and high-energy solar cells. The new prototype solar cell is made of gallium arsenide, a material often used to make solar cells for satellites. The result is a device that operates at 28.6% efficiency.

First to put into practice

An image of the solar cell
The prototype solar cell

While the theory of luminescent emission causing an increase in voltage has been known for a while, it has never been put into practice. “It is somewhat puzzling why it has never been used in the field of solar-cell development until now. But a lack of certain requirements might explain that,” says Yablonovitch. He goes on to say that solar cells are “grown” on substrates that are generally of poor quality and act as “sinks” for the emitted luminescent photons, which are then lost. The new cell made by Alta Devices is separated from the substrate, which delivers a much better performance. “In fact, we separate the substrates on which the cells are grown and then re-use them. This not only helps with efficiency, but it also brings the cost of producing our cells down, and so it is a key factor,” says Yablonovitch. He explains that the cells are still as thin (1 µm) as traditional cells and so people are genuinely shocked to know the devices have been developed cheaply using gallium arsenide. Alta Devices is already producing the cells on an industrial scale, with samples being shipped to customers.

Yablonovitch says he hopes researchers will be able to use this technique to achieve efficiencies close to 30% in the coming years. And given that the work applies to all types of solar cells, the findings have implications throughout the field.

The team will present its findings at the Conference on Lasers and Electro Optics to be held in early May in California in the US.

The research is to be published in Journal of Photovoltaics.

Who is most likely to reach the next significant milestone in manned space exploration?

By James Dacey

50yearlogo.jpg
Today is being heralded as the 50th anniversary of the UK in space. That is because on 26 April 1962 the Ariel 1 satellite was launched, marking the nation’s first step into the final frontier and making it the world’s third space-faring nation. The Ariel 1 mission also signified the world’s first international space mission – while the mission carried six scientific experiments designed and built by UK space scientists, the satellite itself was built and launched on US soil, by NASA.

This bilateral mission may come as a surprise, because the space race of the 1960s is usually seen as a two-horse affair between the US and the USSR. The collaboration between the UK and the US resulted from the Eisenhower administration’s 1959 offer to launch allies’ space-science instruments, free of charge, on US rockets.

Of course, it was indeed the two superpowers, acting unilaterally, that reached the two most significant milestones of the space race. The Soviets had stunned the Americans when Yuri Gagarin became the first person in space in 1961. But a generation of US national investment paid off when Neil Armstrong took that first small step on the Moon in 1969.

Today, space exploration is no longer the domain of just two superpowers. Many other nations have space programmes, and satellites have been launched by more than 50 nations – everyone from Italy to Israel, via Morocco and Mauritius. The space industry has also matured, with the majority of space missions now take place for strategic or commercial purposes, such as launching global positioning satellites and instruments to collect environmental data. Space missions are also no longer carried out exclusively by national agencies; an increasing number of private enterprises are starting to invest in the space industry.

Looking to the long-term future of space exploration, there have been murmurings of a return to the Moon or even a manned mission to Mars, linked with both NASA and the China National Space Administration (CNSA). But many commentators have noted that without the “simplicity” of the political situation in the 1960s, it will be hard to generate the incentive to pour vast amounts of national money into expensive manned missions. And given the ongoing global financial crisis, the prospects for state-sponsored manned missions to space is unlikely to improve any time soon.

But those with a Promethean view of the space industry will be confident that humans will one day resume manned exploration of space. And when we do, who will be in the driving seat? In this week’s Facebook poll, we want you to let us know how you think this quest will continue.

Which of these is most likely to reach the next significant milestone in manned space exploration?

The US
Russia
An emerging space nation such as China or India
An international collaboration
A private company

Have your say by casting your vote on our Facebook page. And feel free to post a comment to explain your choice or to make an alternative suggestion. You can also read how US astrophysicist and science popularizer Neil deGrasse Tyson makes the case for space exploration in this recent interview with physicsworld.com.

In last week’s poll, we looked at a topic close to the hearts of many condensed-matter physicists. We asked you to select your favourite quasiparticle from a list of five. The most popular, picking up 48% of the vote, was the phonon. Others opted for the hole (19%), the spinon (14%), the exciton (13%) and the wrinklon (7%).

Given its amusing name, several people actually questioned whether the wrinklon is a real particle, including Facebook user Heather Williams, who legitimately asked “Is there really such a thing as a wrinklon? It sounds like some nonsene they’d put on anti-aging cream.” But despite its name, I can confirm that the wrinklon does exist, at least according to these physicists, who described the new quasiparticle in a paper published last June in Physical Review Letters.

Thank you for all your participation and we look forward to hearing from you in this week’s poll.

Einstein, the travelling physicist

Albert Einstein did not normally keep a diary, but he often wrote in travel notebooks. From 1921 to 1933 his itinerary included trips to New York, Hong Kong, Singapore, Malacca, Penong, Palestine, Rio de Janeiro, Buenos Aires, Havana, Palm Springs, Oxford, Panama, Honduras, Salvador and beyond. Josef Eisinger’s Einstein on the Road tells the story of these journeys, drawing mostly from Einstein’s unpublished notebooks.

After newspapers trumpeted the eclipse observations that supported Einstein’s theory of gravity, turning him into an international celebrity in 1919, people around the world clamoured to see him. The worsening political situation back home in Germany also led him to travel. At first, Einstein had appreciated the Weimar Republic as his political dreams come true, but growing hostilities there – including the 1922 assassination of his Jewish friend Walther Rathenau, the republic’s foreign minister – meant that he soon found reasons to travel outside “woeful Europe”.

The title Einstein on the Road is something of a misnomer, since most of Einstein’s journeys took place aboard cruise ships. Einstein did not want to give guest lectures anywhere. He despised photograph sessions, tiresome receptions and the barrage of journalists’ inane questions: “Define the fourth dimension in one word”, “Define relativity in one sentence”. But travel on board ship was different. Away from reporters and fans, he had some leisure time to think about physics, and to pay attention to little things. He noticed, for example, that younger people are more prone to being seasick than old people, and women more susceptible than men. Once, when his ship was in a storm, he stood on a bathroom scale and noted with interest that his weight oscillated between heaviest and lightest in the ratio of 3:2. From this, he computed the ship’s acceleration as it dropped into the trough between waves.

Einstein’s notes contain some clever moments, but also biases. I was particularly amused by the way he seemed to subscribe to the old theory that regional climates determine native behaviours. While travelling through the Strait of Messina, towards the Mediterranean, Einstein reacted to the heat and “severity” of the landscapes by speculating that the climate must have been different in antiquity, such that the Greeks and Jews inhabited a temperate zone more suitable for intellectual work. He also thought that the people of the island of Ceylon (Sri Lanka) and China were primitive and miserable because of their tropical climates. Tepid water in the equator, he argued, spread serenity and drowsiness.

On land, Einstein’s experiences and impressions were varied. In person, he was gracious, patient and clever. But in his travel books he recorded snippy thoughts too. He was delighted by the enthusiasm and friendliness of the people of Japan, but he disliked their music and inferred that the Japanese were more artistic than intellectual by nature. He was deeply affected by visiting Palestine, yet when he saw many Jews praying at the Wailing Wall he thought “the dull-witted fellow-members of the tribe” made a deplorable scene. Einstein thought that the Chinese, though modest and gentle, were the most unfortunate people on Earth: listless, cruelly abused and treated worse than cattle. Meanwhile, in Pasadena, California, people seemed to him like scentless flowers.

Thousands of admirers swarmed around Einstein, on docks, in the streets, in lecture halls. They bought expensive tickets to see him. Most did not understand what he said, in German or in French, yet they were fascinated. Einstein complained that he did not know why people were so interested in his theories. He made few public pronouncements at events and receptions, and told his wife, Elsa, that he felt like a con artist who did not give people what they expected. Unlike the work of Copernicus, he said, his theories of relativity did not effect any radical change of perspective on humanity’s place in the universe. Although he accepted honorary degrees, he did not wear the medals. He tolerated countless handshakes and journalists as a slow form of torture, recalling the German proverb “Anyone can get used to being hanged”.

Eisinger’s book records such amusing complaints, but it also gradually demolishes the old impression that Einstein wanted to be a recluse. He said that he did, but his actions give a different impression. He met scores of individuals and carried out a voluminous correspondence with them. He socialized until it made him physically ill. Though he claimed to be indifferent to social standing, he took especial care to befriend people who were wealthy and successful. He hobnobbed with presidents and royalty. He was accessible to famous musicians. With them, many times, Einstein played Mozart on his violin, to the extent that Mozart could have been a secondary character in the book.

Einstein on the Road is easy to read, and Eisinger, an emeritus professor who has worked on nuclear physics and molecular biology, makes a pleasant narrator. At its best, his book is an interesting travelogue. But at its worst, it illustrates a little too well the tedium Einstein suffered by constantly meeting boring strangers. The book is thin on scientific content, with just faint glimpses of Einstein’s work on physics, though perhaps more relating to astronomy and cosmology, such as his support of Richard Tolman’s model of a pulsating universe. Similarly, most of Einstein’s more intriguing encounters are mentioned much too briefly. Passing descriptions of meetings with Clarence Darrow, Winston Churchill and many others are gone in a blink. Eisinger also mentions books that Einstein read at sea – on Chinese wisdom, Jewish history and so on – but lacks discussion of their substance. It would have been better to select fewer anecdotes and develop them more.

Einstein on the Road includes 42 photographs but unfortunately most are already well known: Einstein as a child; at the patent office; with his first wife Mileva Mari? with Charlie Chaplin. Photographs of Einstein in the many countries he visited would have been much better. The book also suffers from minor mistakes, mostly in the background material. For example, Eisinger states that Einstein was recognized as a child prodigy entering the Zürich Polytechnic (he was not), that his daughter was “quietly given up for adoption” (we do not know what happened to her) and that his first paper of 1905 showed the equivalence of mass and energy (it was on the photoelectric effect). Nonetheless, Einstein on the Road is a welcome contribution to the literature as it illuminates how Einstein gradually departed from the isolated individual he once was.

Neil deGrasse Tyson makes the case for space exploration


One of the most common arguments against space funding is “We shouldn’t be spending money on space exploration until we fix things down here.” How do you respond that?

Often the person uttering that statement is missing important information. Let’s look at how much money we are spending “down here”. Typically, the person involved is concerned about the plight of the human condition, so maybe you would look at the US federal budget for monies that are allocated to social services or education. When you combine just those two, for example, you find that the US government allocates about 50 times as much money to those programmes than it does to NASA. So it is not an either/or. We are spending vastly more money on these things than we are on NASA. NASA is getting, as a fraction of the tax dollar, one-half of 1%. That paid for the space station, the space shuttles, the NASA centres, the astronauts, the Hubble Space Telescope and the Mars rovers. Now, you may not want to spend that money on space, but the very premise of the question is false.

What are the benefits of space exploration?

I know of no force of nature or culture as great as the urge to explore the cosmic unknown. If a nation says “We’re going to do this in a big way”, and in so doing advances the space frontier, that is a call to all the innovators who are out there who previously had no place to put their innovative energies. The NASA portfolio involves biologists, chemists, planetary geologists, astrophysicists, physicists, plus mechanical, electrical and aerospace engineers. All these frontiers are represented. If you stand in front of a classroom and say “We are going to Mars and I need all of these frontiers, who’s coming with us?”, you are going to get the best.

It is my opinion that if you go to space in a big way – and people know that requires innovation, discovery and achievements that are writ large in the daily newspapers – it will influence the culture in such a way that even if you are not personally engaged in space exploration, you will still want to innovate. Big, grand visions have the power to trigger a wave of innovation. If you go into space in a big way, it creates a seductive dream for the educational pipeline. You won’t need programmes to convince people that science is an interesting thing to do, they will be compelled to want to do it simply by reading the day’s headlines.

Does that also hold for other countries? There are a lot of other countries that have the economic base to reach for space and some are actually trying…

[Interrupting] No, no. They have the vision to reach for space, and their economic base grew. Yes, it helps if you have money in advance, but China has held designs on space for a long time. Their first astronaut was in 2003. They were thinking about space in the 1990s. In the late 1980s and early 1990s, were any of us thinking about China as an economic powerhouse? Well, they were! They also knew that investments in science and technology – and big, grand visions of your nation – will pump up that innovation culture and economy. It is no accident that they are making big inroads into space, and that they have the leading growth rate in the aerospace industry.

India also has plans for space, although not a manned programme that I know of. But you see India’s economy coming out of the doldrums. India was one of the big laggards among the democracies of the world. You look at graphs of their space and technology growth, and you see it is slower than that of China, but it is on its way up. They have a billion people to make it happen. Western Europe is also very active in space. Their astronauts are local heroes in every one of their countries, which is something we took for granted coming out of the Apollo programme here in the US.

The book cites the disturbing fact that one in five Americans think that the Sun revolves around the Earth. Your response is to say that “There is no excuse for thinking that the Sun, which is a million times the size of the Earth, orbits the Earth.” How do attitudes such as this change, and what do you see as your role in that process?

Front cover of Space Chronicles: Facing the Ultimate Frontier
Tyson’s latest book

People say we need better education. Yes, that’s an eternal truth. I claim that if you put big visible goals up, people will want to get educated. They will not be content to be steeped in ignorance. By the way, you can’t deduce that the Sun is a million times bigger than Earth without the tools of science. That’s why it took so long for anyone to understand that. In the Bible, the Earth is created before the Sun, so this bias that the Earth is some significant object goes deep within our culture. It is a bias that is understandable because we live on the Earth, we don’t live on the Sun. We had no clue how big the Sun is or how far away it is – that only came after millennia of studies and research. So I can blame an incomplete education for the fact that someone doesn’t know the answer to that, but it’s not necessarily because they aren’t observant. You can make better teachers, but that is not going to solve the problem. What you need is to create the grand vision and then everyone will say “The universe is awesome, tell me more about it!” They will want to come to you to get the answers. Even the poet will want to understand it. They will be steeped in a culture that values that adventure.

You have this passage from a 2006 essay where you mention “Earth’s inexhaustible supply of things to notice”. What drives your curiosity?

I am certain it is because I’ve never grown up. Kids notice everything. You bring a kid into a new home – someone else’s home that doesn’t have kids – where the breakables are not protected. The kid will come in and everything is an exploration. What is this? What is that? Can I pick this up? Will this break? How much does this weigh? Can I get this dirty? Can I get this clean? Can I pull on the curtain? Kids are born curious. We beat it out of them by telling them “sit down, you might break it”. I think a scientist – speaking to other scientists – is a kid who has never grown up. It is not a question of what you have to do to keep a kid interested, it is what you have to do to the adults to get them out of the way so that the kid never stops being interested. That’s the challenge.

Buy Space Chronicles now with 25% discount for physicsworld.com readers (enter promo code WN207 at checkout)

Graphene emits infrared light

Physicists in the US have discovered yet another useful property of the wonder material graphene – it can function much like a laser when excited with very short femtosecond light pulses. The team has shown that the material has two technologically important properties – population inversion of electrons and optical gain. The findings suggest that graphene could be used to make a variety of optoelectronics devices, including broadband optical amplifiers, high-speed modulators, and absorbers for telecommunications and ultrafast lasers.

Graphene is a sheet of carbon atoms arranged in a honeycomb-like lattice just one atom thick. Since its discovery in 2004, the material has continued to amaze scientists with its growing list of unique electronic and mechanical properties. Graphene could find use in a number of technological applications – even replacing silicon as the electronics industry’s material of choice in the future thanks to the fact that electrons whizz through graphene at extremely high speeds, behaving like “Dirac” particles with no rest mass.

Ideal for photonics?

The material could also be an ideal candidate for photonics applications – especially optical communications, where speed is all-important. For example, it has an ideal “internal quantum efficiency” because almost every photon absorbed by graphene generates an electron–hole pair that could, in principle, be converted into electric current. Thanks to its Dirac electrons, it can also absorb light of any colour and responds extremely fast to light, which suggests that it could be used to create devices much faster than any employed in optical telecommunications today.

Researchers have already shown that they can make basic devices, such as solar cells, light emitters, touch screens and photodetectors from graphene. However, few studies have looked at what happens when the material is excited with femtosecond (fs) light pulses that create so-called non-equilibrium charge states – particularly the state consisting of extremely dense Dirac electrons. Materials that harbour such states have nonlinear optical properties that are important for making real-world optical devices, such as ultrafast modulators, amplifiers and wavelength converters.

Inversion and gain

In their experiments, Jigang Wang and colleagues at Ames Laboratory and Iowa State University excited high-quality, epitaxially grown graphene monolayers with pump laser pulses just 35 fs long and photon energy of around 1.55 eV. They then measured how much light was reflected by the samples. Because graphene is just one atom thick and has a zero-energy electronic bandgap, this measurement provides information on the amount of light absorbed by the material. This in turn depends on the optical conductivity of graphene, explains Wang.

The researchers found that the optical conductivity changes from being positive to negative as the intensity of the pump pulses increases. “This means that more light is coming out of the material than going in, something that indicates optical gain,” says Wang.

The team demonstrated that the intense external pump laser pulses excite electrons in graphene so that more of these charge carriers exist in the upper “Dirac cone” – the conduction band of the material – than in the lower cone. Once such a population inversion has occurred, a probe photon then stimulates these excited states to emit infrared light in a coherent cascade. “The coherent light emitted shows gain on the order of about 1%, a value that is much greater than those seen in conventional semiconductor optical amplifiers – a surprising result since graphene is merely one-atom thick,” says Wang.

A wide energy range

The team found that this optical gain could be observed over a wide range of energies – up to hundreds of millielectronvolts below the pump photon energy. Such a broad optical gain might be unique to graphene and related to the fact that photoexcited electrons in the material scatter extremely fast among themselves. What is more, an ultrashort pulse just 35 fs long is sufficient to produce this broadband gain – something that has never been seen before in any material.

The population inversion and resulting optical gain in the infrared part of the electromagnetic spectrum confirms graphene’s potential for applications such as broadband optical amplifiers, lasers and in telecommunications. However, there is still much to do before this happens, says Wang, who is now looking at further characterizing the photoexcited graphene states in the near-infrared to the mid- and far-infrared spectral regions. “We are also studying the effects of different sample configurations and growth methods,” he reveals.

The current work is reported in Phys. Rev. Lett. 108 167401.

Gamma rays hint at dark matter

Using a new statistical technique to analyse publicly available data from NASA’s Fermi Space Telescope, an astrophysicist in Germany says he may have spotted a tell-tale sign of exotic particles annihilating within the Milky Way. If proved to be real, this “gamma-ray line” would, he claims, be a “smoking-gun signature” of dark matter.

There is a wide body of indirect observational evidence that an invisible substance accounts for some 80% of the matter in the universe. Although physicists can measure the effects that this dark matter has on the visible universe, they have very little understanding of what this mysterious stuff actually is. As well as looking for direct evidence of dark matter by detecting it – or even producing it – here on Earth, researchers are also scouring the skies for signs of the particles that dark matter might produce when self-annihilating. An excess of high-energy positrons (anti-electrons) observed by the Italian-led PAMELA spacecraft in 2008, and confirmed by Fermi last year, might be such a signature. However, it is possible that these positrons are produced by processes unrelated to dark matter.

In contrast, say astrophysicists, a gamma-ray line would leave little room for alternative explanations. The dark-matter particles believed to exist in a halo surrounding our galaxy are slow moving because they have been slowed down as the universe has expanded. As a result, the total energy of the photons produced by the collision and annihilation of two such particles is, essentially, twice the rest mass of a dark-matter particle. Conservation of momentum requires that the energy of each photon equals the mass of one dark-matter particle – and would appear as a very narrow peak, or line, in gamma-ray spectra. This is unlike the radiation emitted by all standard astrophysical phenomena, which have much broader energy distributions.

Something at 130 GeV?

In the latest work, Christoph Weniger of the Max Planck Institute for Physics in Munich looked for such lines in about 3.5 years’ worth of gamma-ray observations carried out by the Fermi satellite’s Large Area Telescope (LAT). To increase his chances of success he only considered data from those regions of the Milky Way that should generate the highest ratios of dark-matter photons to photons from background sources – according to five different models for the distribution of dark matter within the halo. He also restricted the data to within the 20–300 GeV energy band.

In regions close to the centre of the galaxy, Weniger found that the gamma rays collected by Fermi showed evidence for a line, at about 130 GeV, with a statistical significance of 4.6σ. This dropped to 3.3σ after allowing for the fact that he searched for such a line across finite ranges of space and energy. Put another way, there should be only about a 1 in 1000 chance that the line is due to a statistical fluctuation.

Most dark-matter models predict that this line should be very faint because dark matter does not couple directly with electromagnetic radiation. Photons are instead produced by the annihilation of intermediate particle pairs such as electrons and positrons, but such secondary annihilation is generally considered improbable because the extremely high energies of the particles involved means that they would almost certainly fly apart before they have the chance to combine. There are a few models, however, in which such annihilation is enhanced – one such model, for example, allows for the creation of virtual pairs of particles and antiparticles that are unable to fly apart. “If this is a dark-matter signal it would imply a model where the line is surprisingly strong,” says Weniger. “This would allow us to reduce the number of possible models considerably.”

Weniger acknowledges that his gamma-ray line is provisional, pointing out that it consists of data points from only about 50 photons, and that reaching the roughly 5σ level needed to claim a discovery is likely to need several more years’ worth of data. He also points out that because his analysis is based only on publically available data he does not know all there is to know about possible instrumental errors.

Fake gamma-ray lines

In fact, according to Elliott Bloom of Stanford University in the US and Jan Conrad of Stockholm University in Sweden, both members of the Fermi LAT collaboration, instrumental biases associated with identifying photons against a background of charged particles and sifting those photons according to their energy has previously created fake gamma-ray lines, and overcoming these problems, they say, “still requires considerable additional work”.

If Weniger’s gamma-ray line is real but turns out to be significantly broadened, he believes it could be caused by a more conventional astrophysical process. One potential candidate, he says, which was only identified in 2010 using Fermi data, is a pair of enormous gamma-ray-emitting “bubbles” extending outwards from the plane of the Milky Way. Because the energy of the photons within the bubbles may have a sharp cut-off at around 130 GeV, he suggests that the mechanism responsible for the bubbles might also generate the line that he has identified. Conversely, he says, dark matter might cause the cut-off seen in these bubbles.

John Wefel, an astrophysicist at Louisiana State University in the US, points out another tantalizing possibility: that the peak at about 125 GeV seen in data from the Large Hadron Collider (LHC) at CERN last year, and which is reckoned to be a signature of the Higgs boson, may in fact be related to the line identified by Weniger. “Do we have a halo of Higgs bosons around the galactic centre, or is the LHC observing some new particle likely to be the dark matter particle, and not the Higgs at all?” he asks, playfully.

Weniger’s analysis is described in arXiv:1204.2797.

Asymmetry born out of symmetry

Chiral lattices


Two snub hexagonal tilings that were generated in a 2D simulation of point particles interacting via an isotropic potential. The patterns are chiral and are mirror images (plus a rotation) of each other. (Courtesy: APS)


By Hamish Johnston

The chirality – or handedness – of many biological molecules plays an important role in their function. The amino acids that make up proteins only exist in the left-handed form, for example, while the sugars found in DNA are exclusively right-handed.

Why nature seems to favour one handedness over another has long puzzled physicists – particularly because the relevant physical laws that govern the synthesis of such molecules are symmetric and should not be biased towards right- or left-handedness.

The emergence of molecules with a specific chirality in a chemical process is usually understood in terms of chiral-specific catalysis, which accelerates the production of molecules of one handedness over the other. However, it’s also possible that chirality can emerge in much simpler systems that don’t involve complicated chemical reactions.

In order to understand how chirality emerges from symmetrical interactions, Martin Nilsson Jacobi and colleagues at Chalmers University in Sweden have done computer simulations that reveal how point particles acting under a spherically symmetric force can form chiral patterns in 2D. According to the team, the system begins with “maximal a priori symmetry” and therefore the emergence of asymmetric chiral patterns is surprising.

The team began with what it describes as the simplest form of chiral lattice in 2D. This is made from identical scalene triangles – a triangle with no sides of equal length. Such a lattice can be made in two ways, each being a mirror image of the other. However, one lattice cannot be transformed into the other by rotation or translation.

Nilsson Jacobi and colleagues first calculated the Fourier transform of the lattice, which gives its reciprocal lattice. Then, using a technique introduced by the team last year, they were able calculate a potential energy between pairs of lattice points that would result in the creation of the desired chiral lattice. The amazing thing about this potential is that it is spherically symmetric – looking a bit like a 1/r potential with a number of wiggles in it.

To confirm that the potential would indeed result in a chiral structure, the team then used a Monte Carlo simulation to determine what lattice would form if point particles were subject to such a potential. The resulting lattice was indeed a chiral pattern of scalene triangles.

The team then set its sights on a more complicated – and visually appealing – 2D chiral lattice called “snub hexagonal tiling” (see images above). Again, the chiral pattern emerged from the simulation.

While the team has shown that in principle chiral patterns can emerge from simple symmetric systems, this could prove to be very difficult to achieve in a real system. The problem is that the required potentials would be very difficult to recreate in a real-life system and “are not likely to appear in the near future,” according to the physicists.

The simulations are described in this paper in Physical Review Letters.

Cosmic-ray theory gets the cold shoulder

One of the leading theories describing how the most energetic cosmic rays are produced may need a rethink in light of a new study by physicists at the IceCube Neutrino Observatory in Antarctica. The team had set out to detect the extremely energetic neutrinos that are expected to be produced alongside high-energy cosmic rays in the violent explosions that mark the deaths of massive stars – but after looking at hundreds of these explosions, no such neutrinos have been found.

High-energy cosmic rays are charged particles such as protons with kinetic energies in excess of 1018 eV – a million times more energetic than the particles collided by the Large Hadron Collider at the CERN particle-physics lab.

The mystery of where these cosmic rays originate has baffled astrophysicists for decades. Likely candidates include gamma-ray bursts (GRBs), which occur when a massive star explodes. In the tens of seconds that they usually last, a GRB can outshine everything else in the universe by releasing as much energy as the Sun will produce in its entire lifetime. Other possible sources of high-energy cosmic rays are the active galactic nuclei at the centre of galaxies.

However, it has proved very difficult to test these theories because tracing cosmic rays back to their source is not easy. This is because, as charged particles, they are deflected during their long journeys to Earth by the strong magnetic fields present in space.

Fireball theory

Out of a number of possible GRB scenarios, the fireball model is “the one most widely believed”, according to Nathan Whitehorn, a physicist at the University of Wisconsin in the US who was involved in the IceCube study.

This theory describes a series of internal shocks that accelerate protons through the burst as it expands. Protons collide with gamma rays and form pions, which spontaneously fall apart to give neutrinos with energies of about 1014 eV. Unlike cosmic rays, neutrinos are not affected by magnetic fields and therefore their trajectories should point straight back to the GRB.

IceCube comprises an array of thousands of photomultiplier-tube detectors buried under 1.5–2.5 km of ice. The detectors look for the tiny flashes of light that are produced when a neutrino interacts with the ice. The researchers scoured two years’ worth of IceCube data for evidence that a pulse of high-energy neutrinos arrives on Earth whenever a GRB was spotted by a network of about 100 satellites, each containing gamma-ray detectors. Instead of seeing high-energy neutrinos arriving with all the gamma rays, not even one neutrino was seen in coincidence with the 300 GRBs that were detected during the two years.

Surprising result

The null result is “really surprising”, according to Darren Grant, a physicist with the University of Alberta in Canada who works on the IceCube project. “[It] changes what was thought to be the best modelled understanding for these incredible astrophysical objects,” he says.

The lack of neutrinos suggests that either GRBs do not produce cosmic rays, or that current ideas about how the cosmic-ray and neutrino production mechanisms are related is flawed.

The researchers calculated that production of energetic neutrinos was a factor of at least 3.7 lower than any variation of the fireball models they tested. According to Whitehorn, this latest research “challenges, though does not rule out, the idea that GRBs may be the sole sources of the highest energy cosmic rays”.

GRBs not completely ruled out

Theoretical astrophysicist Kohta Murase of Ohio State University in the US agrees that it “demonstrates that neutrino observation has become an important tool to address the long-standing [cosmic ray] mystery”. However, Murase – who is not a member of the IceCube collaboration – cautions that GRB sources should not be ruled out just yet, pointing out that there are “theoretical issues” with some models.

“In some scenarios, it is also expected that high-energy cosmic rays are produced without many neutrinos,” he says. “Or they may be produced during the afterglow phase, where the neutrino flux is expected to be lower.”

The study is described in Nature.

Supercomputers provide new insight into charge–parity violation

An international team of scientists has, for the first time, simulated the decay process of a kaon into two pions with extreme precision, using some of the fastest supercomputers available today. This calculation could provide further insight into charge–parity (CP) violation and help to explain why there is more matter than antimatter in the universe.

The new work, published this month in Physical Review Letters, involved researchers from the Brookhaven National Laboratory, Columbia University, Washington University and the University of Connecticut in the US; the University of Edinburgh and the University of Southampton in the UK; and the Max-Planck Institute in Germany. The calculation took 54 million processor hours on the IBM BlueGene/P supercomputer at the Argonne National Laboratory in the US.

Violated theories

CP symmetry dictates that a process involving a particle and a process involving the mirror image of its antiparticle should be identical – particles and their antiparticles should decay at exactly the same rate. This is tested either by creating the particles and watching them decay or seeing the process naturally occurring as certain particles decay into their antiparticles.

The kaon decay process has been explored since the 1960s – in fact, it won physicists James Cronin and Val Fitch the 1980 Nobel Prize for Physics for a 1964 experiment where they had set out to prove CP symmetry but instead found the first “indirect” experimental evidence for CP violation as a kaon decayed into two mesons.

Today, the Standard Model of particle physics is the most successful theory that describing how three of the four fundamental forces – the strong, weak and electromagnetic forces, but not gravity – affect subatomic particles. But certain underlying fundamental questions remain unanswered. One way is to test the model at large-scale particle accelerators such as the Large Hadron Collider (LHC) at CERN, Geneva. The other is to look at what one of the team members, physicist Chris Sachrajda from the University of Southampton, calls “rare processes”, which very precisely test predictions of the Standard Model.

Super simulations

When kaons decay into pions, the constituent subparticles – quarks – are affected by the weak force, and as the quarks move apart, they exchange gluons. But one problem is the huge computing power that is required to simulate quark–gluon interactions. “It has taken several decades of theoretical developments and the arrival of very powerful supercomputers to enable physicists to control the interactions of quarks and gluons, the constituents of the elementary particles, with sufficient precision to explore the limits of the Standard Model and to test new theories,” explains Sachrajda. “But we now have the computing power and advanced algorithms that are required to simulate these rare processes.”

In the simulation, a technique known as lattice quantum chromodyamics (QCD) is used to carry out the computation. The parameters of the decay are input into a computer as a finite grid or lattice of space–time points. “Using lattice QCD was tricky, as the lattice box has a finite size and this means that the quarks cannot separate infinitely,” says Sachrajda. He goes on to explain that the process the researchers considered involved the kaon decaying into two mesons with isospin 2 (a quantum number related to the strong interaction). “This isospin has a real and imaginary part – the real part has been predicted and experimentally verified, and our value was in good agreement with that. The imaginary part, on the other hand, is not known from experiment. This is the first time it has been experimentally determined,” explains Sachrajda. He also explains that it was important to repeat this calculation using a second lattice spacing to eliminate any possible errors. The quantum fluctuations of the decay are calculated by a statistical approach called the “Monte Carlo” method, which outputs the most likely fluctuation.

Extreme scales

An illustration of the varying distance scales for kaon decay
Varying scales

A novel feature of the computation is that the decay process was simulated over a vast scale. The researchers looked at the process from distances as minute as a 1000th of a femtometre, which allowed them to follow the decay of individual quarks and some other subatomic particles. They also looked at it from a few tenths of a femtometre, where the picture would be obscured by a sea of quark–antiquark pairs and a cloud of the gluons that holds them together. It is at this distance that the gluons begin to bind the quarks into the observed particles. So the actual kaon decay described by the calculation spans distance scales of nearly 18 orders of magnitude – this range is similar to a comparison of the size of a single bacterium and the size of our entire solar system. While the simulation reported here has determined the fundamental kaon decay process, it also marks the beginning of the next phase of the collaboration’s work, which involves improving the precision of the computations and extending the range of physical quantities for which the effects of the strong nuclear force can be quantified. The researchers believe that lattice QCD will continue to be an important technique in these studies and that even more computing power will be required.

“What we are trying to do now is ‘break’ the Standard Model,” explains Sachrajda “as that will be the only way to really understand the underlying physics.”

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