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COROT sees first light

Astronomers have already discovered more than 200 exoplanets using ground-based telescopes or general-purpose space telescopes like Hubble. Most of these are large, gaseous planets, sometimes called “failed” stars, about the size of Jupiter and Saturn. Exoplanets are found by looking for minute dips in the brightness of a star when the exoplanet passes in front of it. Turbulence in Earth’s atmosphere makes it extremely difficult for smaller, rocky planets, like Earth, to be detected in this way from the ground. As a result, only a handful of rocky planets have been discovered so far.

COROT, however, is 900 km above Earth’s atmosphere, where its CCDs can detect changes in star brightness as small as 0.01%. This is about 100-times better than the best ground-based telescopes. Still, planets the size of Earth will probably escape detection, but the craft should be able to spot planets twice Earth’s size circling stars like the Sun. Unlike large gaseous planets, these rocky planets are more likely to harbour life.

Of the four CCD arrays, two are dedicated to measuring seismic oscillations in stars. Just like the Sun, many stars vibrate, and these vibrations can be observed as small periodic fluctuations in their brightness. These fluctuations occur on timescales ranging from seconds to hours and provide information about the star’s mass and internal structure.

CNES has already performed a successful in-orbit testing and calibration of COROT’s CCD arrays. This was done in the dark with the help of flashes from light-emitting diodes. Now, astronomers can do more tests using starlight. Annie Baglin of the Observatory of Paris at Meudon and COROT’s principal investigator told Physics Web, “Everything worked out fine. We are starting to see stars… We will need a few months for further calibrations before we can present data to the public.”

Neutrino detector is compact yet sensitive

The soft recoiling of a nucleus after it has been struck by a low-energy neutrino is a mechanism that physicists have long been keen to get a grip on. Low energy neutrinos probe entire nuclei at once, and as a result there is a strong likelihood of these interactions occurring compared to other neutrino scattering processes, potentially enabling detectors to become very compact. But the catch, which has prevented the construction of such detectors until now, is that a successful detector would have to be acutely sensitive at low energies.

Juan Collar and colleagues from the University of Chicago and Canberra Industries in the US, however, have now built a prototype detector that has just that. Like existing germanium detectors, the new detector spots softly-recoiling nuclei as they ionize surrounding atoms in the semiconductor. But unlike existing germanium detectors, impurities distributed within the p-type semiconductor help the ionized charges to move, effectively “pulling” them towards the detector’s electrodes. The physicists also reduced electronic noise by using much smaller electrodes that minimize unwanted capacitance.

The most significant application presents itself in nuclear reactors, where there is increasing concern that fuel rods could be removed illicitly and used to make nuclear weapons. Although in principle a reactor’s neutrino signature would change following illicit behaviour, detectors proposed hitherto have been so large (over 30 cubic metres) and sensitive to noise that they cannot get within the 20 metre radius required to monitor with any reliability. Collar’s device, at over 1,000 times smaller (and roughly half a kilogram), is unobtrusive enough for it to monitor a reactor at close range.

But the applications don’t stop there. Because the detector responds in the same way to all known types of neutrino (electron, muon and tau), it could possibly provide the first concrete evidence for the much-debated fourth type: the so-called “sterile” neutrino.

Collar says that his team are now looking to lower the energy threshold of their prototype further still, so that they detect even more low-energy neutrinos using the same mass of germanium.

Pendulum swings away from dark energy

An important mystery facing cosmologists is that the rate of expansion of the universe appears to be increasing with time – physicists had expected this rate to decrease as the finite energy of expansion is depleted by the gravitational attraction that holds the universe together. Cosmologists have tried to explain this in terms of “dark energy”, which boosts the expansion of the universe by counteracting the effects of gravity. To be effective, dark energy must account for about 70% of all energy in the universe — but it has yet to be observed directly.

Cosmologists have calculated that a new force associated with dark energy should become apparent at relatively short length scales – about 85 micrometres. It turns out that one of the best places to look for evidence of dark energy at this length is not in the deepest reaches of outer space, but rather in a simple laboratory experiment that measures the gravitational attraction between two plates and looks for any deviation from the classical inverse-square law (see figure “Attractive plates”).

Dan Kapner and colleagues at the University of Washington have used a torsion balance to measure the force of gravity down to 55 micrometres and found that it still obeyed the inverse-square law well beyond 85 micrometres with 95% confidence. While this doesn’t rule out dark energy, it has allowed Kapner and colleagues to conclude that a new gravitational-strength force does not appear at this length scale.

Although other groups have measured the force of gravity on shorter length scales, Kapner told Physics Web that the Washington experiment offers the highest sensitivity at the length-scale associated with dark energy. This is because it employs more interacting mass at the required separations than other setups. The researchers are now building an improved balance that could boost the sensitivity of the measurements by a factor of 100.

Photonic Schroedinger cat breaks record

Mixing particles so that the measurement of one affects the state of another, otherwise known as “entanglement”, plays a crucial role in quantum information processing. Recently, physicists realized that entanglement could be conveniently related to an underlying graph, by creating a new type of state called a “graph state”. But while these special states have already provided fresh insights into phenomena such as non-locality and decoherence, further progress in the world of quantum computing relies on the generation of more numerous entangled states.

Now Chao-Yang Lu and colleagues from the University of Science and Technology of China and institutions in Austria and Germany have managed to create two types of six-photon graph state: a “Schroedinger cat” state, and a “cluster” state. Both of these break records for the number of photons entangled in such a manner: the cat state was previously five photons, and the cluster state was previously four.

The physicists began by using an intense ultraviolet laser to pump photons into a crystal. Every so often, one of the photons decayed spontaneously into two entangled photons. They then combined photons from three of these entangled pairs using beam splitters, resulting in six photons that could all be either vertically or horizontally polarized. These two states, according to Lu and colleagues, were analogous to the “dead” and “alive” states of Schroedinger’s cat – a legendary paradox thought up by Erwin Schroedinger to highlight the absurdity of quantum mechanics.

However, Lu says that the six-photon cluster state, which his apparatus can produce when a plate is inserted to transform the polarization of one of the photons, is likely to be of more importance.

“Standard quantum computation is based on sequences of unitary quantum logic gates, where a major difficulty lies in the implementation of dynamical operation,” he said. “Cluster-state quantum computation does not need any dynamical operation.”

GEANT4 helps hadrons destroy tumours

Beams of protons and other heavy particles (hadrons) are proving to be very effective at killing tumours that are close to sensitive areas, such as the brainstem, optic nerve and spinal cord. The key to success lies in delivering the required dose to a tumour while sparing surrounding healthy tissue and organs.

This involves performing dose calculations, which usually rely on deterministic methods to model particle transport in the human body. However, the availability of increased computing power means that Monte Carlo-based simulations are a viable option too. Now, Igor Pshenichnov, Igor Mishustin and Walter Greiner who are based at the Frankfurt Institute for Advanced Studies at Johann Wolfgang Goethe University, have shown how such an application derived from high-energy physics software can map the dose distribution from proton and carbon-ion therapy .

The trio built their MCHIT model using the GEANT4 toolkit (version 8.0), an open-source library of computational tools created by the high-energy physics community for basic research. “This again confirms that applied studies in medical physics can benefit from using theoretical methods and software designed in the field of basic nuclear and high-energy physics,” said Pshenichov.

He believes that the GEANT4-based application could be used for benchmarking in the growing field of hadron therapy. Most dedicated therapy facilities use either proton or heavy-ion beams to irradiate difficult-to-treat masses. However, a few sites now operate with both protons and carbon ions, and more facilities offering these two beam types are under construction throughout Europe. The model could also be used as an educational tool, he suggests.

The results presented to date are “quite satisfactory as a first attempt” and could be improved by fine-tuning the model, Pshenichnov says. However, because MCHIT is a general-purpose code, he is also keen to test the GEANT4-based model on other possible candidates for hadron therapy, including helium and oxygen nuclei.

Giant-magnetoresistance pioneers win Wolf prize

GMR occurs in materials consisting of two successive magnetic layers, separated by a very thin non-magnetic layer. In 1988 research groups led by Fert and Gruenberg noticed that these materials display very large changes in their electrical resistance when exposed to a magnetic field – much larger than the magnetoresistance seen in other materials.

An important feature of GMR devices is that they can be used to create a current of spin-polarized electrons in which the spin of most of the electrons points in the same direction. This has been harnessed by physicists around the world to create electronic circuits that exploit the spin of the electron — and could someday lead to the creation of quantum computers that could outperform conventional computers by using the spin of electrons to store, transmit and process information. On a much more practical level, GMR has also led to an enormous increase in the data-storage capacity of devices such as hard-disk drives, which are all now equipped with read heads that exploit GMR.

Peter Gruenberg

Albert Fert spent most of his career at the Université Paris-Sud in Orsay, where he did a PhD in physics in 1970 before becoming an assistant professor in 1970 and then professor of physics in 1976. Peter Gruenberg completed a PhD at the Technical University at Darmstadt, Germany in 1969 before joining the Institute for Solid State Physics at the Juelich Research Center 1972.

This is the fourth major award given to the pair for their GMR work. They have also been honored by the American Physical Society, the International Union for Pure and Applied Physics and the European Physical Society.

Fert and Gruenberg will share the $100 000 prize, which will be awarded later this year by the President of the State of Israel at the Knesset Building in Jerusalem.

Sound breaks the light barrier

Sound often comprises numerous superimposed waves of various wavelengths. At certain points, these constituent waves can all combine constructively to produce a pulse, which moves through the medium at a velocity known as the “group velocity”.

In a normal dispersive medium, the velocity of a wave is proportional to its wavelength, resulting in a group velocity that is slower than the average velocity of its constituent waves. But in an “anomalously” dispersive medium — one that becomes highly absorbing or attenuating at certain frequencies — velocity is inversely proportional to wavelength, meaning that the group velocity can become much faster.

Indeed, the group velocity of light has already been shown to travel faster than the speed of light in a vacuum. But until now, superluminal acoustic waves have existed only in theory, and would require the group velocity to increase almost a million times over.

William Robertson and colleagues from Middle Tennessee State University in the US have managed to produce “faster than light” sound, however, by putting a sound pulse through a surprisingly simple waveguide. Inside, a loop filter splits the signal along two unequal length paths, and then recombines it to produce large amounts of anomalous dispersion. As they interfere with each other, they replicate the shape of the original pulse, only farther ahead. This gives the impression that the sound has travelled farther, and thus faster, in the same space of time.

Robertson says that such split-path interference can also occur naturally when a sound source is located near a hard wall: some of the sound reaches the listener directly, and some reaches the listener from a slightly longer path as it bounces off the wall. Therefore, he says, superluminal sound is an “everyday” occurrence, although it is mostly too subtle to notice.

Proponents of Einstein’s special relativity need not worry, though. The underlying waves that make up the pulse remain at subluminal velocities, so no information, matter or energy actually travels faster than light. (See related link: “Subluminal”.)

“The effect is the same as that observed in previous electrical or optical experiments,” Robertson told Physics Web. “The only somewhat startling difference is that the acoustic waves making up the pulse move so much more slowly than light.”

Climate-change breaks records in Prague, not Philadelphia

If Earth’s atmosphere was not warming, statistics predicts that the incidence of record-breaking hot and cold days would decrease as time progressed, being inversely proportional to the time that has elapsed since records began. While common sense dictates that global warming should lead to more record-breaking hot days, little is known about how warming affects record-breaking events.

Sid Redner of Boston University and Mark Petersen of Los Alamos National Laboratory have addressed this gap in the knowledge. They calculated that if the average daily temperature rises at a constant rate, inverse proportionality will dominate at first. However, the effects of global warming will begin to win out with time and when this occurs the probability of record-breaking will stop falling and approach a constant value (Phys Rev E 74 061114).

Redner and Petersen then set about looking for this effect in climate data. This crossover from statistical variation to a global-warming constant was not observed in the first set of data analysed by the physicists — 126 years worth of temperature data for Philadelphia, US. Undeterred, the researchers then focussed on a much longer set of data from the Czech capital Prague, which covers 231 years. Redner told Physics Web that a preliminary analysis reveals that the probability of having a record-breaking day in Prague appears to approach a constant value after about 130 years. This suggests that global warming is affecting the number of record-breaking days in Prague — and that this effect may soon become apparent in Philadelphia and other locales with records covering about 130 years or more.

NMR finds holes in nuclear waste storage

Integrating radioactive material into mineral-based ceramics is a leading contender for the disposal of nuclear waste. Some of these ceramics, such as “zircon” (ZrSiO₄), already occur naturally with slowly-decaying radioactive isotopes incorporated into their crystalline structure. Nevertheless, they have remained intact over billions of years despite the damage caused by the onslaught of high-energy alpha particles produced in the decay process.

Some scientists had hoped that zircon could withstand much higher doses of the radioactive plutonium isotope ²³⁹Pu, which is found in spent nuclear fuel. The risk is that increased exposure to alpha particles would displace too many atoms and damage the crystalline structure irrevocably. But this damage had been difficult to measure and in the past scientists relied on vague empirical calculations based on the assessment of large defects to predict how long the ceramics would last.

Mineral physicists Ian Farnan and colleagues at the University of Cambridge may now have the answer, however. They used a technique called “magic-angle spinning” NMR on zircon, showing that each alpha-particle displaces up to 5000 atoms in the crystal lattice, rather than the 1000 to 2000 estimated before. The technique enhances the resolution of the NMR spectrum by spinning a sample at high speeds and at a certain angle to the applied magnetic field. This is the first time individual damage events have been witnessed, and could put an end to the “back of the envelope” calculations that had prevented scientists from accurately determining a material’s lifespan.

Unfortunately this means that zircon containing 10% of ²³⁹Pu (roughly the dose required for radioactive waste storage) would break down after just 1400 years – nowhere near the 250 000 years that regulation dictates. Although the technique has ruled-out zircon, it could pave the way for characterizing other materials over long timescales.

“The main issue with siting a nuclear waste repository is that there are many uncertain factors,” said Farnan. “When you extrapolate these into the future you get a very large uncertainty, which can make the idea of a repository intractable. But we feel that by working on the material itself, that’s where you are going to get the biggest effect.”

Light squeezes through nano coax

Coaxial cables comprise an inner and outer conductor separated by an insulating dielectric layer and are used to transmit all manner of electromagnetic waves from radio to microwave. They are extremely useful because they can transmit waves with wavelengths much greater than their diameter, making cable television and other technologies possible.

Light is an electromagnetic wave so there is no reason why it cannot be transmitted in a similar manner via a coaxial cable — but conventional wisdom had held that light could not travel through a cable of diameter less than its wavelength. Now, Boston College’s Jakub Rybczynski, Mike Naughton and colleagues realized that a coaxial could carry sub-wavelength light waves if it were miniaturized.

Their coaxial cable is based around a carbon nanotube, which forms the central conductor (see “Building a nano coax”). The nanotube is surrounded by a concentric ring of transparent aluminium oxide — which acts as the dielectric layer — and finally a concentric metal ring that acts as the outer conductor. The separation between the inner and outer conductors is about 100 nm.

Some of the central conductor protrudes from the cable and acts like an “antenna”, gathering light and sending it down the cable. The cable works exactly like a conventional coax, constraining the transverse electric and magnetic fields of the light wave between the two conductors, thereby guiding the light along the cable for distances of up to 50 µm. While this is not very far, it could allow the structures to be exploited in a number of ways.

Naughton and Rybczynski told Physics Web that the cable’s ability to control light over sub-wavelength distances could be exploited to solve a wide range of technological problems. For example, the dielectric material could be replaced by a photovoltaic material like silicon, which would convert the light to electricity. This could be used to create better solar cells that exploit the cable’s ability to constrain the light wave into an area smaller than its wavelength, thereby boosting the efficiency of the conversion process.

This ability to constrain light could also be exploited in new optical microscopes and optical techniques for processing computer chips that can resolve features smaller than about half the wavelength of light – something that conventional optics cannot do. The researchers also believe that the technology could someday be used to fabricate components for optical communications including switches that stop the flow of light by applying an electrical signal across the inner and outer electrodes.

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