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Home » Page 1467

Now you see it…now you see it again

cloak.jpg


By Hamish Johnston

If you type the word “invisible” into the search engine on arXiv.org you get a very curious result. Two papers with nearly identical titles, uploaded three days apart.

A quick scan of both papers, which are by separate groups, reveals that they are both about roughly the same thing – the first invisibility cloak that works on large objects illuminated with visible light.

This promises to be a major breakthrough in the world of cloaking and I understand that one paper is destined for a prestigious journal; I’m not sure about the other.

We have a crack reporter looking into it. More later…

In the meantime you can read the papers here and here.

NASA spies storm stretching across the Sun

New findings from NASA’s Solar Dynamics Observatory (SDO) reveal that the Sun’s surface is an even more complicated web of physical and magnetic processes than previously thought. The finding was unveiled this week in San Francisco at the American Geophysical Union (AGU) Fall Meeting and could lead to better forecasts of radiation levels experienced by satellites.

The surface of the Sun is an incredibly volatile environment, which frequently ejects intense radiation and clouds of energetic radiation into space. These emissions pose a serious threat to astronauts and if they reach Earth they can wreak havoc with telecommunications satellites.

The research focuses on the analysis of an event that occurred on 1 August 2010 when almost the entire Earth-facing side of the Sun erupted in a tumult of activity, including solar flares and coronal mass ejections (CMEs). The event was captured by equipment on board the SDO, launched in February to investigate the causes of solar variability and how this creates a weather system in space.

Connected phenomena

While earlier missions have returned data from isolated active regions of the Sun, the SDO and its twin STEREO spacecraft were specifically designed to study magnetic activity over almost the whole star. This enabled Karel Schrijver and Alan Title of Lockheed Martin’s Solar and Astrophysics Laboratory to deconstruct the activities of 1 August to look for connections between the different phenomena.

We can see that solar storms can be global events, playing out on scales we scarcely imagined before Karel Schrijver, Lockhead Martin

The breakthrough came when the researchers discovered that bursts of solar activity appear to be connected via a system of magnetic fault zones known as “separatrices”. In a paper due to be published in the Journal of Geophysical Research, Schriver and Title break down the activity into 12 significant events over a 28-hour period spanning 180 degrees of solar longitude. “The 1 August event really opened our eyes,” says Schrijver. “We can see that solar storms can be global events, playing out on scales we scarcely imagined before.”

The researchers admit, however, that much work remains in order to unravel the causes and effects in these dynamic processes, and for this they will need to study more events. “Nor all eruptions are going to be global,” notes Title. “But the global character of solar activity can no longer be ignored.”

A more comprehensive understanding of solar processes could also lead to more accurate forecasts of space weather conditions, a development welcomed by Rodney Viereck of the US’s National Oceanic and Atmospheric Administration. “Solar flares can be particularly hazardous as they disturb high-frequency radio communications and GPS and the disruptions occur very quickly as the dangerous X-rays travel at the speed of light”.

Satellites reveal strain on Earth’s biological resources

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By James Dacey at the AGU in San Francisco

NASA satellite images have revealed that the biosphere is being placed under increasing strain as rising population on a global scale is accompanied by increased consumption of crops and animals per capita. If population and consumption continue to grow at present rates then by 2050 more than half of the new plant material generated on Earth each year will be required for humans. These findings were presented on Tuesday by NASA scientists at the American Geophysical Union (AGU) Fall Meeting here in San Francisco.

Marc Imhoff of the NASA Goddard Space Flight Center presented the results of a global survey for 1995–2005. Using data from NASA’s AVHRR and MODIS satellites, Imhoff and his colleagues tracked the amount of plant material produced on Earth. These satellites scan the Earth at 600 km per second, monitoring the colour of light emitted from the surface. Light near the green part of the spectrum is taken to indicate the presence of vegetation. A MODIS image of some of North America is shown above (image courtesy of NASA).

To create a “currency” for natural consumables, the researchers considered plants and animals in terms of the amount of carbon that they draw from the atmosphere – referred to as “net primary-production (NPP) carbon”. They discovered that between 1995 and 2005 the amount of NPP carbon used for human consumption rose from 20% to 25% of the total generated on land.

“These images tell us very dramatically that we do need to look at what kind of impact human consumption rates have on the ability of the biosphere to generate the supply,” said Imhoff.

He believes that the need for more plant products will have big implications for land management. As more land is required for agriculture, planning authorities will be faced with difficult decisions as they try to protect important ecosystems, such as boreal forest.

Rama Nemani, another member of the NASA team, is keen to stress that it’s not the role of Earth-monitoring programmes to suggest what should be done with global land use. He believes, however, that the next generation of Earth-monitoring satellites will play a key role in informing these discussions. These will include NASA’s National Polar-Orbiting Operational Environmental Satellite System (NPOESS) Preparatory Project and ESA’s Sentinel satellites.

Nemani told me that he would also like to see the creation of an international body to monitor global biodiversity, in the same way that the climate is assessed by the Intergovernmental Panel on Climate Change (IPCC).

Quantum theory survives latest challenge

Since quantum mechanics was first formulated, a string of physicists including Albert Einstein have been uncomfortable with the idea of entanglement – whereby a group of quantum particles have a closer relationship than allowed by classical physics. As a result, some physicists have proposed alternative theories that allow such close relationships without the need for quantum mechanics. While it has been difficult to test these theories, researchers in the UK have used “twisted light” to make an important measurement that backs up quantum theory.

Quantum theory seems foreign to our everyday experience because it defies our idea of “realism” – the expectation that objects have definite properties whether we’re looking at them or not. Quantum theory also seems to call for entities that can instantly react to an event occurring elsewhere – apparently defying the principle of locality, which forbids communication faster than the speed of light.

These oddities were expressed mathematically by the physicist John Bell in his famous inequality. Bell showed that a particular combination of measurements performed on identically prepared pairs of particles would produce a numerical bound (or inequality) that is satisfied by all physical theories that obey realism and locality. He also showed, however, that this bound is violated by the predictions of quantum physics for entangled particle pairs.

In Bell experiments two distant observers measure, for example, the polarization of entangled particles along different directions and calculate the correlations between them. This was done in the 1970s by Stuart Freedman and John Clauser and in the 1980s by Alain Aspect, who used entangled photons to confirm quantum theory.

Sacrificing locality for realism

Physics has generally accepted that the quantum world flouts “local realism”, but in 2003, Anthony Leggett of the University of Illinois at Urbana-Champaign tried to restore realism by sacrificing locality. If two entities can arrange their correlations through instantaneous communication, then perhaps it is still possible that they each have definite properties. Leggett’s real but non-local scenario passes the Bell test, but could it really describe the quantum world?

Four years later, physicists in Austria, Switzerland and Singapore answered with data. Instead of measuring the linear polarization states used to violate Bell’s inequality they looked for correlations between elliptical polarizations – combinations of linear and circular states. Even assuming that entangled photons could respond to one another instantly, the correlations between polarization states still violated Leggett’s inequality. The conclusion being that instantaneous communication is not enough to explain entanglement and realism must also be abandoned.

This conclusion is now backed up by Sonja Franke-Arnold and collegues at the University of Glasgow and University of Strathclyde who have performed another experiment showing that entangled photons exhibit entangled photons show stronger correlations than allowed for particles with individually defined properties – even if they would be allowed to communicate constantly. But rather than polarization, they studied the properties of each photon’s orbital angular momentum.

Twisting light

In photons, orbital angular momentum can be understood by imagining that the wave twists around the beam axis. It can draw a simple corkscrew pattern, a double helix or more complex helices with increasing angular momentum. Franke-Arnold and her team focused on the double-helix pattern.

Glasgow student Jacquie Romero did the experiment by firing an ultraviolet laser into an optical crystal designed to split the high-energy photons into pairs of entangled infrared photons. These went on to computer-controlled holograms, which were set to filter out roughly complementary orbital angular momentum states. Photons that passed the holograms were then counted by a single-photon detector.

The correlation between two entangled photons, one with a clockwise orbital-angular momentum while the other twists anticlockwise, is predicted by Bell’s and Leggett’s proposals as well as quantum theory. “We deliberately misalign our holograms from the complementary states and measure the resulting correlations,” explained Franke-Arnold. The coincidence counts in the detector occured too often to agree with Leggett’s theory. They did, however, match quantum predictions.

‘A philosophical result’

“The main outcome is really a philosophical result,” says Franke-Arnold. Entangled particles can’t be described as individual entities, not even with a telepathic connection to their partners.

Simon Gröblacher of the University of Vienna points out that these experiments rule out realism only for a large class of nonlocal theories – still others aren’t described by Leggett’s inequality. His team first showed the violation of Leggett’s inequality through photon polarisation, and he says that it’s nice to see the violation verified with another property of photons. “The experiments seem to be simpler,” he adds, noting that orbital-angular momentum offers options to test superpositions of more than two states.

The work is described in New Journal of Physics 12 123007.

How to walk through walls

Imagine being able to walk through a solid wall. That sort of trick might sound far-fetched, but it’s a little closer to reality now that researchers in China have created what they call an “invisible gateway”.

Huanyang Chen at Soochow University, Jiangsu, says that the effect is a bit like “platform nine and three-quarters” – that is, the fictional area of King’s Cross railway station in the Harry Potter books that is only accessible through a secret, illusionary wall. Although the researchers’ current demonstration is based on an electrical circuit for radio waves, Chen claims that it could also work for visible light.

The idea for the invisible gateway stems from so-called transformation optics, which gave us the first invisibility cloak back in 2006. Yet the invisible gateway is almost the opposite of a cloak: rather than bend light round an object to make the object invisible, the device makes an object – a wall – appear that isn’t really there. It is, according to Chen’s group, the first demonstration of illusion optics.

Network of capacitors and inductors

Chen, whose colleagues are based at the Chinese Academy of Sciences, Beijing, and the Hong Kong University of Science, created the invisible gateway using a network of capacitors and inductors. The network forms a channel that separates two electric conductors – the walls – one of which contains a slab of material with a negative index of permittivity and refraction. The combination of these two materials allows collective waves of electron, called plasmons, to form on the surface. The plasmons prevent electromagnetic waves from passing through the channel. To an observer, the channel looks like a continuation of the walls – so long as they are looking at electromagnetic radiation between 45 and 60 MHz.

This does demonstrate that the principle works Tom Driscoll University of California, San Diego

Tom Driscoll, a researcher who studies novel electromagnetic devices at the University of California, San Diego, calls the demonstration a “good step”, although he notes that the progression to devices that work with visible light and at human scales are “decades or more away”. “The total sample size is quite small compared with the wavelength used, so I would like to have seen a bigger example,” he says. “However, this does demonstrate that the principle works.”

Martin McCall, a theoretical physicist at Imperial College, London, also thinks that the invisible gateway is an interesting development. “It’s a viable addition to the pile of interesting electromagnetic structures being produced,” he says.

Chen and colleagues’ invisibility gateway is one of many ideas to have been realised used transformation optics in recent years. Last year, groups at Cornell University and the University of California at Berkeley independently created 2D cloaks that operated at optical wavelengths. Earlier this year, a team at the Karlsruhe Institute of Technology in Germany went one step further to produce a 3D optical cloak.

In 2008, Chen’s group proposed what might be the next step on these lines – a device that can cloak objects at a distance.

The research is available in Phys. Rev. Lett. 105, 233906

Nuclear reaction defies expectations

A novel kind of fission reaction observed at the CERN particle physics laboratory in Geneva has exposed serious weaknesses in our current understanding of the nucleus. The fission of mercury-180 was expected to be a “symmetric” reaction that would result in two equal fragments but instead produced two nuclei with quite different masses, an “asymmetric” reaction that poses a significant challenge to theorists.

Nuclear fission involves the splitting of a heavy nucleus into two lighter nuclei. According to the liquid-drop model, which describes the nucleus in terms of its macroscopic quantities of surface tension and electrostatic repulsion, fission should be symmetric. Some fission reactions are, however, asymmetric, including many of those of uranium and its neighbouring actinide elements. These instead can be understood by also using the shell model, in which unequal fragments can be preferentially created if one or both of these fragments contains a “magic” number of protons and/or neutrons. For example, one of the fragments produced in many of the fission reactions involving actinides is tin-132, which is a “doubly-magic” nucleus, containing 50 protons and 82 neutrons.

The latest work, carried out by a collaboration of physicists using CERN’s ISOLDE radioactive beam facility, investigated the interplay between the macroscopic and microscopic components of nuclear fission. It used what is known as beta-delayed fission, a two-step process in which a parent nucleus beta decays and then the daughter nucleus undergoes fission if it is created in a highly excited state. This kind of reaction allows scientists to study fission reactions in relatively exotic nuclei and was first studied at the Flerov Laboratory in Dubna, Russia, about 20 years ago, although the Dubna measurements did not reveal the masses of the fragments produced.

Firing protons at uranium

The experiment at ISOLDE involved firing a proton beam at a uranium target and then using laser beams and a magnetic field to filter out ions of thallium-180 from among the wide variety of nuclei produced in the proton collisions. These ions then became implanted in a carbon foil, where they underwent beta decay and some of the resulting atoms of mercury-180 then fissioned. Silicon detectors placed in front of and behind the foil allowed the energies of the fission products to be measured.

The researchers were expecting the fission reaction to be symmetric, with the mercury-180 splitting into two nuclei of zirconium-90, a result thought to be particularly favoured because these nuclei would contain a magic number of neutrons (50) and a “semi-magic” number of protons (40). What they found, however, was quite different. The energy of the fission products recorded in the silicon detectors did not peak at one particular value, which would be the case if only one kind of nuclei was being produced in the reactions, but instead showed two distinct peaks centred around the nuclei ruthenium-100 and krypton-80.

Collaboration spokesperson Andrei Andreyev of the University of Leuven, Belgium, (and currently at the University of West of Scotland) says that this asymmetric fission was unexpected because the observed fragments do not contain any magic or semi-magic shells. His colleague, theorist Peter Möller of the Los Alamos National Laboratory in the US had in fact devised a model of the nucleus that predicted that mercury-180 would undergo asymmetric fission. But he wasn’t able to explain why that is, having plotted a three-dimensional potential energy surface for the fission of mercury-180 and then identified a minimum in that surface, but he couldn’t identify which of the three variables were responsible for that minimum.

‘Beautiful experimental achievement’

Phil Walker of the University of Surrey in the UK, who is not a member of the collaboration, describes the research as a “beautiful experimental achievement” that has “an impressive theoretical outcome”. He says that the result will be mainly of interest to academics but believes that it might just have practical implications. “Much of our energy generation depends on nuclear fission,” he points out, “and if we want to make reactors safer and cheaper we need to be able to trust the basic theory of the fission process. I would say that the theory has been found to be sadly lacking, and it needs to be fixed.”

Andreyev agrees. “I hope that as a result of our paper theorists will start to think about this problem and tell us what is happening,” he says. “For the moment we don’t know.”

The research appears in Physical Review Letters.

Eric Cornell: an experimental maestro

Cornell won the 2001 Nobel Prize for Physics “for the achievement of Bose–Einstein condensation in dilute gases of alkali atoms, and for early fundamental studies of the properties of the condensates”. He shared the prize with his University of Colorado colleague Carl Wieman and Wolfgang Ketterle of the Massachusetts Institute of Technology. BECs are formed when identical bosons – particles with integer spin – are cooled until all particles fall into the same quantum state and behave as a single quantum particle. In June 1995 Cornell and Wieman succeeded in creating this extreme state of matter for the first time by using the newly developed techniques of laser cooling to take a cloud of rubidium atoms to near absolute zero. Ketterle repeated this feat a few months later with sodium atoms.

Was there a ‘eureka moment’ when you saw a BEC for the first time?

By the standards of physics experiments, it was not at all long and drawn out. The day we saw it, we also believed in it. It really seemed that it was a very clear signature. It was a more dramatic moment than these things usually are – over the course of a morning we came to believe it was there.

But did it take a long time to convince the rest of the community?

The day we saw it, we also believed in it.

Interestingly, when we first saw it, it was just a couple of weeks before a meeting in atomic physics that was happening on the island of Capri near Naples, so we had just a couple of weeks to convince ourselves that this was real enough to go public.

A week after that, there was a specialized talk on the topic of BECs near Strasbourg, and basically anyone who was interested in the BECs was there. It was a really rigorous meeting and there were a lot of probing and sceptical questions, but by the end I think we had pretty much everyone convinced.

Did you know this would be a Nobel-prize winning discovery?

I’ll say that the thought had crossed my mind. But in those days, as now, typically a discovery is made and the Swedes wait maybe 20 years before they decide they are convinced enough…so I certainly wasn’t expecting to get a telephone call early in the morning so soon.

How have BECs furthered our understanding of physics?

Maybe the most active area of research is to use the condensate to explore model systems in quantum mechanics. Basically, they put the atoms in a lattice of interfering beams of laser light and you get this optical lattice. And you get a small number of atoms, maybe one of two atoms in each optical lattice site, and then using clever analogies you can say this system has the same underlying physics as the source of magnetism in an exotic material, for example.

How did winning the prize change you – both personally and as a scientist?

What I’ve found I have to be careful about is that before the Nobel prize I was a young, slightly brash, not particularly cautious physicist. You know, someone would be describing an experiment and I’d say to my friends: “That’s stupid! It’s probably wrong.” And now, if I say that, it’s like: “Oh! Cornell says it’s wrong. Scandal!” So I have to be a little bit more cautious in that respect because people take me more seriously and therefore I have to be more serious, which is a little too bad.

Do you find that you spend a lot of more time doing things outside of science now?

Yes, but I don’t especially enjoy it. I mean, I like travelling, I like meeting people. But I don’t like getting more involved in administration. I’m to become chair of my institute in a couple of months and I can’t say that I’m looking forward to that. It seems to be something it’s hard to get out of.

What has your research focused on since winning the prize?

Apart from the BECs. I have another project going on, that I’ve been working on for six or seven years, which is getting at some of the same physics that they get at, at CERN, but using very different technology – looking for the asymmetry of the electron. As near as anyone can tell, the electron is a pretty symmetric particle, but we can’t tell that for sure. So we’re trying to do a much more careful study of whether the electron’s north pole and the south pole are the same or whether they could be slightly different.

Why does this fascinate you?

In nature, and in fundamental particles, there are some huge asymmetries and the biggest one is that if you just look around you the world is made up of electrons and protons and neutrons. It’s not made up of antiprotons and antineutrons and positrons – it’s a very imbalanced thing. If you look around the universe, there’s a very, very tiny amount of antimatter.

What can we learn from this?

We know that these basic violations of things like parity and charge conservation happen, and you can look around and mostly what you see are little tiny effects where you have to stare very hard at particles. Then you see immense, broad, crude effects, like we’re all made out of matter and not antimatter. And connecting those two is not easy to do. The people who try to do that in a theoretical way are convinced that there are more small asymmetries at the microscopic scale than we have found so far. One of their favourite predictions is that the electrons should have this asymmetry…it’s called the “electric dipole moment”.

How would you describe your approach to physics – are you motivated by the theory, or do you take a more practical approach?

I think if I’d ended up being an accountant, then I’d be a hobbyist – the sort of person who builds remote-controlled airplanes.

I’m not a chalk guy; I’m an oscilloscope and laser guy. But you can have experimentalists who are working on very practical or impractical things. And likewise, you can have theorists who are working on very practical things. For instance, my electron experiment has involved developing technology to measure things very precisely. So even if the research itself isn’t practical, maybe the equipment you develop might be.

If you had not become a Nobel-prize-winning physicist, what might you have done instead?

Well, I’ve always been a bit of a tinkerer. I think if I’d ended up being an accountant for a living, then I’d be a hobbyist – the sort of person who builds remote-controlled airplanes or something like that. But I was always interested in languages. I was never very good at it… I had a picture that I’d do something more involved in literature or politics. I’m still very interested in politics, but really just as a spectator sport these days.

So you’re not tempted to follow the same path as Carl Wieman who is now working as Obama’s associate director for science?

Carl Wieman and Steve Chu got out of the realm of the spectator sport and they put on their kit and their boots and they’re actually playing. I’ve never done that and I don’t really have an ambition to. But I do like to follow the game. My wife is much more involved in things like politics than I am, I wouldn’t be surprised if she went for office one day, and I could be sort of like Denis Thatcher. I could do that job!

‘Medusa front’ spotted in nanobatteries

 

Researchers at Sandia National Laboratories in New Mexico and the US Department of Energy’s Pacific Northwestern Laboratory are the first to observe how a nanobattery operates in real time using high-resolution transmission electron microscopy. The work could help make improved devices that will be used to power nanomachines and nanorobots of the future with applications in medicine and other fields.

Six years after Richard Feynman gave his famous lecture on nanotechnology, “There’s plenty of room at the bottom”, Twentieth Century Fox released a science fiction film called Fantastic Voyage in which a miniature spaceship just 1 µm in size travels inside a human body and removes a blood clot, all while evading the body’s immune system. Given that a typical human cell is about 10 µm across and the latest computer chips have feature sizes of just 45 nm, making such nanorobots is no longer out of the question. However, the biggest challenge will be how to power these autonomous nanomachines.

Lithium-ion batteries consist of two electrodes, the anode (negative) and the cathode (positive), separated by an electrolyte – a conducting material through which charged ions can move easily. During cell discharge, the positively charged lithium ions travel across the electrolyte to the cathode and so produce an electric current. When the batteries are recharged, an external current forces the ions to move in the opposite direction so that they can be stored at the anode.

Expanding anodes

Tin oxide is ideal for making the anode in lithium-ion batteries because it has a high energy density. However, upon charging, the material expands, which leads to cracking and reduced electrical conductivity, and eventually battery failure – a problem that is particularly serious in practical nanobatteries.

To investigate this expansion in detail, Jianyu Huang and colleagues made a working prototype of a nanobattery that comprises a single nanowire anode made of tin dioxide that can be charged and discharged. The battery also contains a specially designed ionic liquid electrolyte that can withstand the high vacuum of a transmission electron microscope (TEM) and a bulk lithium cobalt oxide cathode. The researchers loaded the device into the TEM to see exactly what happened when they applied –4 V against the lithium anode.

The team observed that when the tin dioxide nanowire was charged up, it swelled, twisted and then elongated (see video). These changes come about because of a process called lithiation, common to all lithium-ion batteries. This is when ions, extracted from the cathode during charging squeeze into the anode. By observing lithiation as it happens, Huang and colleagues say that their work might help design more advanced nanobatteries by understanding how the electrode accommodates volume changes associated with this process. “This will help us understand why a battery fails following cyclic charging and discharging,” says team member Chongmin Wang.

Surprising behaviour

The fact that the anode nanowire elongates to nearly twice its size came as a surprise, says Huang. Normally, the wire should expand in the radial direction rather than along its length. The nanowire twisting during charging and discharging is also unexpected and astonishing, he adds. “Such behaviour must be taken into account if we want to design and build standalone nanowire batteries, because it is electrical shorting as a result of these transformations that leads to battery failure.”

The researchers didn’t stop there; they also recorded how the microstructure of the battery evolved during charging. They observed that it changed from being an ordered, crystalline solid to being disordered and amorphous. “We saw that a high density of mobile dislocations nucleate and are absorbed at the chemical reaction (or ‘Medusa’) front with the dislocation cloud serving as a precursor to solid-state amorphization,” explained Huang. Electrochemical solid-state amorphization is a poorly understood process by which a crystalline material changes to an amorphous material. Amorphization will degrade a device, and controlling it is crucial for how well batteries perform and how long they last.

“While we ran short of demonstrating a fully packaged nanobattery, we believe we have made an important step towards an important goal in nanotechnology – building a single-nanowire battery consisting of a nanowire anode and cathode, and nanoscale electrolyte and packaging,” Huang told physicsworld.com.

The research, which was reported in Science (330 1515), will ultimately help create batteries with high energy densities, high power density and long cycle lifetimes.

CERN moves closer to antihydrogen spectroscopy

Physicists at CERN have taken a big step towards making the first spectroscopic measurements on a beam of antihydrogen atoms. The antihydrogen atoms, which consist of an antielectron orbiting an antiproton, were made by members of the lab’s ASACUSA group. The beams could be used to carry out the first detailed studies of the energy levels in antihydrogen.

Measuring in detail the energy levels in antihydrogen is important because the Standard Model of particle physics says they should be identical to those of hydrogen. Any slight differences in the “fine structure” of the levels compared to ordinary hydrogen could shed light on why there is so much more matter than antimatter in the universe.

The breakthrough comes just weeks after researchers in the ALPHA collaboration at CERN succeeded in trapping 38 antihydrogen atoms for about 170 ms. This was the first time antimatter atoms had been stored for long enough to measure their properties in detail and, taken together, the two results represent major advances in studies of antimatter.

Trapped in a cusp

The ASACUSA researchers, however, used an alternative technique for creating antihydrogen. Led by Yasunori Yamazaki of the RIKEN laboratory in Japan, they created their antiatom beams by combining antiprotons with positrons in a “cusp trap”.

The trap comprises 17 successive ring-shaped electrodes and two magnetic coils, which are wired to create magnetic fields in opposite directions (see figure). A cloud of antielectrons (also called positrons) from a radioactive source is first sent into the trap, where it is held as a plasma. A cloud of antiprotons – created in a nearby accelerator – is then fired into the plasma to create the antihydrogen atoms.

Charged particles remain stuck in the trap, while neutral antihydrogen atoms are able to move further along the apparatus to a “field ionization trap”. At this point, antihydrogen atoms in highly excited Rydberg states, in which the positron lies very far from the antiproton, are ionized and their antiprotons are trapped.

Detecting pions

The trapped antiprotons are then released and quickly annihilate upon contact with the walls of the trap. Each annihilation event creates pions, which are easily spotted by a bank of detectors surrounding the trap. By comparing the number of antiprotons injected into the trap with the number of annihilations detected, the team estimated that about 7% of antiprotons combine to form antihydrogen.

The team is now trying to improve the way in which antihydrogen is extracted from the trap before passing it through a microwave cavity in which hyperfine transitions between atomic energy states should occur. Making precise measurements of these transitions, which have not yet been carried out, could be used to study a fundamental quantum transformation known as the charge-parity-time (CPT) operation.

When applied to a physical system, a CPT transformation converts every particle to its antiparticle, reflects each spatial co-ordinate, and reverses time. Although is currently no experimental evidence that the CPT symmetry is violated, it could show up as a slight difference in the frequency of hyperfine transitions in hydrogen and antihydrogen atoms. The discovery of such a violation could also help physicists understand why there is much more matter than antimatter in the universe.

The work is reported in Phys. Rev. Lett. 105 243401.

Cosmic dark ages brought to light

It may look like a picnic table, but this humble piece of kit located in the Australian outback has revealed how the universe emerged from a period called the cosmic dark ages nearly 13 billion years ago. This was the time when the first stars and galaxies began to assert their influence on cosmic evolution, by bombarding the intergalactic medium with ultraviolet light until it had become a warm, ionized plasma.

The EDGES apparatus, a $30,000 radio antenna designed to detect the prominent 21 cm spectral signature of hydrogen, has allowed Judd Bowman of Arizona State University and Alan Rogers of the MIT Haystack Observatory in Massachusetts to place the first direct limit on how fast this cosmological phase transition took place.

“This result marks an observational milestone,” says Rennan Barkana of Tel Aviv University, who was not involved with the work. “Knowing when and how re-ionization happened teaches us a great deal about whatever population of objects existed 300–800 million years after the Big Bang, which could still turn out to be something more exotic than stars such as massive black holes or even decaying dark-matter particles.”

Plunged into darkness

The universe became neutral and transparent about 380,000 years after the Big Bang, when it had cooled sufficiently for protons and electrons to combine into hydrogen atoms. Photons, which before recombination were unable to travel far in the plasma, flew out in all directions and have since been stretched – or redshifted – by the expansion of space to constitute the cosmic microwave background (CMB). There were no luminous objects at that time because matter had yet to clump together under gravity, but 100 million years later the first stars and galaxies began to punch holes in the darkness and their light set about reionizing the intergalactic medium – mostly hydrogen.

Since at these early times (or red shifts) hydrogen’s 21 cm spectral line is stretched to roughly metre wavelengths, EDGES was designed to pick up signals with a frequency between 100-200MHz – in the VHF band. During three months of continuous observation Bowman and Rogers looked for the highly red-shifted 21 cm signal, which is expected to be extremely weak because it comes from transitions between the lowest energy states of hydrogen in which the spin of the electron and proton are either aligned or anti-aligned.

Having painstakingly subtracted the much stronger low-frequency signals coming from the magnetized plasma in the Milky Way and nearby galaxies, not to mention interference from terrestrial TV and radio transmissions, the team concluded that reionization did not end abruptly but took place over a red shift larger than 0.06. That translates to a period of at least 5 million years.

“This was expected in all theoretical models that forecast reionization by the first stars, but is now an observational statement,” says theorist Avi Loeb of Harvard University. “Most of the 21 cm community is involved in the construction of interferometers (whose cost is larger by two orders of magnitude) that image the sky as well, so it is gratifying to see that an innovative experiment succeeded in getting the first constraints.”

21 cm cosmology

Until now there have been two types of observational data on reionization: anisotropies in the cosmic microwave background as measured by WMAP and observations of the light from distant quasars. These data indicate that the universe was already fully ionized by redshift 6.5 (corresponding to about 1billion years after the big bang), but Barkana says that studying hydrogen’s 21 cm emission is very promising for going back to the period 200 million years after the Big Bang. “21 cm cosmology is opening up a new observational cosmic window,” said Barkana. “At least one, perhaps two more cosmic transitions should have left spectral fossils.”

“So far EDGES has only set limits and not made a detection,” says Rogers. “We hope that we might detect the early hydrogen and put some real numbers on the red-shift, but it remains to be seen how far EDGES can go.”

The research is published in Nature 468 796.

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