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IBEX spies a ribbon at the edge of the solar system

A “bright narrow ribbon snaking its way through the sky” is just one of the puzzling features of the first all-sky maps of the edge of the solar system made by NASA’s Interstellar Boundary Explorer (IBEX) – according to one of the scientists working on the project.

Launched one year ago into very high altitude Earth orbit, IBEX has been busy measuring how ions in the solar wind interact with the plasma from interstellar space. IBEX will tell us more about the shape of the solar system’s protective “bubble” – called the heliosphere – which is created by the solar wind and shields us from harmful galactic cosmic rays.

‘Truly remarkable’

“The IBEX results are truly remarkable, with emissions not resembling any of the current theories or models of this never-before-seen region,” said David McComas of the Southwest Research Institute and IBEX principal investigator. “We expected to see small, gradual spatial variations at the interstellar boundary,” he explained.

The ribbon is the source of intense emissions of energetic neutral atoms (ENAs), which IBEX specializes in detecting. Its presence appears to be at odds with the current model of the heliosphere, which scientists believed is shaped like a comet by the collision of the outgoing solar wind and the galactic wind, which blows outside the heliosphere.

Origins a mystery

According to McComas, the ribbon seems to be full of charged particles, which seem to have been concentrated along its length – but how they got there is a mystery.

IBEX data suggest the alignment of the ribbon is related to the local interstellar magnetic field, which could mean that its origins lie outside of the solar system. The ribbon also appears to have a fine structure, suggesting that the ion concentrations vary along its length.

The results are published in a series of five papers in Science

Charges band together in graphene

Electrons in graphene show collective behaviour similar to that observed in superconductors, magnets and superfluids – according to experimental results published by physicists in the US. Collective behaviour had been predicted by theory and its confirmation could lead to a better understanding of the complex physical properties of graphene, which is often touted as the material of choice for replacing silicon in future electronics devices.

Graphene is a one-atom thick layer of crystalline carbon that was first isolated in 2004 by University of Manchester scientists, Andre Geim and Kostya Novoselov. Graphene is different from other materials in that its charge-carrying electrons move at extremely high speeds, behaving like relativistic particles with no rest mass. As a result graphene has all sorts of unusual properties, including the highest room temperature conductivity of any known material, extremely high sensitivity to chemicals (it can sniff out a single molecule) and optical properties that allow it to go from being transparent to opaque when a voltage is applied.

Physicists have predicted that the relativistic charge carries in graphene are correlated – that is, they interact strongly with each other. “Such correlations often lead to unexpected collective phenomena, where the whole is more than the sum of the parts, and fundamentally new properties,” explains team leader Eva Andrei at Rutgers University. “Examples of such collective effects include superconductivity, magnetism and superfluidity.”

Can relativistic particles be correlated?

Before graphene was discovered, it was difficult to observe correlations in relativistic particles. For example, collective phenomena cannot be observed in neutrinos, which come from the Sun or are produced in high-energy colliders, because they are too sparse – at least here on Earth, says Andrei. Some researchers even believe that correlations might not occur at all in these particles.

Now, Andrei’s team has confirmed that charge carriers in graphene do interact strongly with each other and show collective behaviour that can be detected as the fractional quantum Hall effect (FQHE). This effect occurs when charge carriers like electrons are confined to moving in a 2D plane, as in graphene, and subjected to a perpendicular magnetic field. The charge carriers then form new quasiparticles with a fraction of the charge on an elementary electron.

The FQHE is fundamentally different from the integer quantum Hall effect and forms thanks to strong interactions between electrons. In this state, the electrons and flux lines from the magnetic field form a coherent “liquid” of composite particles each consisting of an electron and an even number of captured flux lines.

Fractional statistics

“The FQHE represents an entire family of quantum phases, the most robust of which is the 1/3 FQHE described as two flux lines captured by each electron,” Andrei told physicsworld.com. “These fractionally charged quasiparticles obey so-called fractional statistics, a feature that may be important for developing future quantum computers.”

The researchers obtained their results by first depositing a graphene flake on a standard semiconductor wafer consisting of a silicon crystal capped with a thin silica layer. The flake was produced by rubbing graphite on the wafer – the same process that occurs when you write with a pencil. Andrei’s team then used scanning electron lithography and thin film deposition to define electrical contacts made of gold and titanium on the flake.

Fractional quantum Hall effect

The next step was to expose the sample to a strong acid that etched away the silica but which did not affect the graphene, electrodes or silicon, to produce a graphene strip suspended in air. Finally, the sample was cleaned by heating it to very high temperatures to “boil off” any impurities that may have landed on it during fabrication.

Cleanliness the key to success

The FQHE is detected by measuring the resistance of graphene (cooled to about 2K and in an applied magnetic field) as a function of applied voltage. The appearance of “plateaus” in the resistance – which become more pronounced as the field increases – revealed the FQHE state.

“Suspending and cleaning graphene were crucial steps that isolated it from its environment and removed impurities,” said Andrei. “They allowed the electrons to interact with each other rather than with other charges and impurities in their surroundings, as in previous work.”

The researchers also had the bright idea of using an unconventional non-invasive two-terminal lead geometry to probe the FQHE. This configuration minimizes interference between the electrodes and the measurement process while allowing the graphene to remain mechanically stable. “We will describe our two-terminal measurement technique and how it allows us to access the Hall effect in mesoscopic-sized samples in a forthcoming publication,” revealed Andrei.

By demonstrating that relativistic particles do interact strongly, our work implies that new phases of matter will emerge that have unexpected collective properties very different to conventional materials with non-relativistic charge carriers, she added.

The work is reported in Nature.

‘Magnetic charge’ measured in spin ice

Researchers in the UK and France have measured the charge and current of “magnetic monopoles”, which were recently shown to exist in materials called spin ices. While the team didn’t actually create the magnetic analogue of an electrical circuit, they showed that magnetic monopoles respond to magnetic fields in much the same way as charged particles respond to electric fields.

Earlier this year two independent groups of physicists provided the best evidence yet that magnetic monopoles – free “north” and “south” magnetic poles – can exist in magnetic materials called spin ices.

The magnetic moments in a spin ice do not line up like those in a ferromagnet. Instead physicists believe that they join up to create magnetic flux lines within the material that resemble a knotted mess of strings. However, if a moment is flipped – a string is broken and the magnetic flux spills out in a manner resembling a monopole.

In September, teams led by Jonathan Morris at the Helmholtz Centre in Berlin and Tom Fennell at the Institute Laue-Langevin (ILL) in Grenoble measured the thermodynamic and other properties of two different spin ices – and their results suggested the existence of monopoles.

Charge and current

Now Fennell has joined forces with Steve Bramwell of University College London and Sean Giblin of Rutherford Appleton Laboratory (RAL) to make the first measurements of the magnetic charge and current associated with magnetic monopoles in the spin ice Dy₂Ti₂O₇.

They did this by “mapping” the problem onto Lars Onsager’s 1934 theory of electrolytes – which describes how an applied electric field causes molecules in solution to ionize, thus boosting the conductivity of the solution. Working at RAL’s ISIS muon facility, the team applied a magnetic field to the spin ice, which creates north and south monopoles that drift apart to create magnetic currents in the material.

The presence of these currents was detected using muon spin resonance (µSR) – whereby a beam of muons is fired at the material and some of the particles take up residence in the crystal lattice. The muons decay to produce positrons, which exit the material and are detected. The direction of the emitted positron is related to the magnetic polarization of the muon.

Wobbling muons

In the absence of monopoles, a muon’s magnetic moment will wobble about an applied magnetic field, and the µSR signal resembles a decaying sine wave. The creation of monopoles leads to random, local magnetic fields, which disrupt the rotation and hasten the decay of the µSR signal.

The team related the µSR decay rate to the magnetic conductivity of the spin ice – which increased with stronger applied magnetic fields, just as predicted by Onsager’s theory. This allowed the team to use the theory to determine the elementary magnetic charge of the monopoles – which was in good agreement with the theoretical value.

Claudio Castelnovo of Oxford University, who was not involved in the research, described the work as “an important paradigm shift in spin ice research” and “the first step in generating a current of monopoles”.

Explaining the inexplicable

Fennell told physicsworld.com that the study could also explain the “inexplicable behaviour” that has been seen in previous µSR studies of spin ices. “The experiments could have been seeing monopoles all along,” he said.

The spin-ice monopoles have very different origins from the monopoles famously predicted by Paul Dirac’s work on quantum electrodynamics. But, because the monopoles occur in magnetic materials, understanding their properties could help with the development of magnetic memories and other spintronic devices.

The results are published in Nature.

Entangled electrons do the splits

Physicists in Switzerland and Denmark have created a device that can separate pairs of entangled electrons. The device, which is based on a superconducting “Y” junction, should pave the way for tests of the so-called non-locality of quantum mechanics in the solid state.

In the theory of quantum mechanics, when two particles are entangled the measurement of one can affect the state of the other, no matter how far they are separated. Such non-locality would seem to go against Einstein’s theory of relativity, which implies that no information can travel faster than light. Even so, tests of non-locality using entangled pairs of photons have so far shown quantum mechanics to be correct.

But tests of non-locality using electrons – that is, matter in the solid state – has proved trickier. Unlike photons, which are relatively easy to create and manipulate in isolation, electrons in materials reside en masse in a “Fermi sea”, making it difficult to isolate a well defined pair.

On solid ground

“It is important [to check non-locality] for electrons in a solid because these are so-called quasi-particles that live in an environment of many electrons,” explained Christian Schönenberger at the University of Basel. “Quantum phenomena in a background of strongly interacting matter are very different from the existing studies with photons in a vacuum.”

Schönenberger’s group, which includes others at Basel and at the University of Copenhagen, has found a way to extract entangled pairs of electrons, and separate them, using a superconducting Y-junction. An important property of the superconductor is that electrons can exist in entangled “Cooper pairs”. Such pairs cannot enter the Y-junction without passing through a barrier. Because of the low probability of passing this barrier, Cooper pairs tend to enter the junction one at a time.

The next step is to ensure that the pairs split, rather than having both electrons travelling down just one arm. They do this by placing a tiny piece of semiconductor – a quantum dot – at the end of each arm. A lone electron can pass through a quantum dot but it is unlikely that two electrons (which repel each other electrically) will squeeze through at once.

Non-local correlation

The team confirmed that entangled Cooper pairs were indeed being separated by adjusting the resistance of one of the quantum dots while monitoring the conductance of each arm. When the electron source was in a superconducting state, a “non-local correlation” between these parameters was seen suggesting that entangled pairs were being separated. However, when a magnetic field was applied to the electron source – destroying its superconductivity and Cooper pairs – the non-local correlation vanished.

Takis Kontos, whose group at the École Normale Supérieure in Paris has submitted a similar study to Physical Review Letters (preprint at arXiv:0909.3243) that uses carbon nanotubes in place of superconducting wires, thinks Cooper-pair splitting is “an important step forward”.

“It opens the avenue for the implementation of much more advanced quantum-optics-like experiments in electronic systems,” he said. “One could, for example, envision correlation experiments together with the use of spin filters in order to probe quantum entanglement in a very elegant way…The findings presented in this paper bring on very exciting perspectives and are very likely to generate a renewed and intense experimental and theoretical activity.”

Schönenberger told physicsworld.com that his and other researcher groups are now pursuing tests of non-locality, in particular using statistical studies of so-called Bell’s inequalities, which reveal whether the behaviour of two entangled particles is correlated.

The research is published in Nature.

Into the darkness

ZEPLIN-III lies deep in Boulby mine, Northeast England

By James Dacey

Lurking quietly in the bowels of Europe’s second deepest mine, ZEPLIN-III will come alive this month as it resumes its search for that mysterious substance called dark matter.

I recently caught up with Alex Murphy, leader of the University of Edinburgh’s contribution to the project.

Sitting in a cafe that resembled a fishtank, Murphy explained to me at length why he has placed his faith in dark matter. He also described the form he imagines this substance should take, and why he believes that ZEPLIN is now in prime position to make the first internationally-recognized detection of dark matter… possibly within months!

We wrapped things up by chatting about the aspects of his job that he loves and the aspects that he hates, as well as how he deals with the rivalry in the international search for dark matter.

You can read the full interview here.

Electrons flow forever in metal rings

If you want an electrical current to flow around a normal metal ring you have to supply enough energy to overcome the metal’s resistance – right? Not always, according to physicists in the US and Germany who have the best experimental evidence yet that currents can flow forever in micrometre-sized metal rings. The research involves measuring the tiny magnetic fields associated with the currents and validates a theory of persistent currents that was proposed in 1969.

Physicists are familiar with persistent currents in superconductors – in which electrons can flow forever, unhindered by resistance. But even the best normal conductors such as copper or gold have electrical resistance due to electrons scattering from defects, which should make persistent currents impossible.

However, if a metal ring is very small – about 1 μm diameter or less – quantum mechanics says that its electrons should behave in much the same way as electrons orbiting an atomic nucleus. And in the same way that electrons in the lowest energy configuration of an atom maintain their orbits without the constant input of energy, electrons in such “mesoscopic” rings should flow forever – even if the ring has resistance due to defect scattering.

Indeed, a 1 μm diameter ring cooled to 1 K should support a current of about 1 nA.

Breaking the ring

Nanoampere currents can be measured with an ammeter, but this would involve breaking the ring to include the ammeter in the circuit. Even if the ammeter could be shrunk down to micrometre size, its presence would destroy the quantum coherence that allows the current to flow.

Instead physicists have used superconducting quantum interference devices (SQUIDs) to try to measure the tiny magnetic fields created by persistent currents. This is very difficult because SQUIDs are sensitive to magnetic impurities in the rings. In addition, a magnetic field must be applied along the ring’s axis to cause the persistent current to flow in one direction. Such a field makes it difficult to operate a SQUID – but without an applied field some electrons would flow clockwise and others anti-clockwise, resulting in zero net current.

Because of these problems, experimental results have been inconsistent and at odds with theoretical predictions. Now, Jack Harris and team at Yale University along with a colleague at the Free University of Berlin have invented a completely new way of measuring persistent currents that is about 100 times more sensitive than SQUID-based experiments.

Diving boards

Tiny diving boards

The team grew aluminium rings on a silicon chip and then used lithographic processes to create 300 nm-thick diving-board-like cantilevers with one or more rings at the tips.

To measure the current in the rings a cantilever is aligned at an angle of about 45° to a strong magnetic field of several Tesla. The component of the applied magnetic field perpendicular to the cantilever causes the persistent current to flow in one direction – resulting in an additional magnetic field perpendicular to the cantilever. The parallel component of the applied field is at right angles to the field created by the persistent current, resulting in a torque on the cantilever.

The cantilever has a natural frequency of oscillation, which changes as a result of this torque. By comparing the frequencies with and without the applied field, Harris and colleagues can work out the size of the persistent currents in the rings.

The team studied several different cantilevers decorated with a single ring or arrays of hundreds or thousands of identical rings. The rings on different cantilevers had diameters varying from 616 nm to 1.59 μm.

Arrays of rings

Closing a chapter

By measuring the size of the persistent current while changing the magnetic field, the team confirmed that the persistent current is a periodic function of the magnetic flux quantum h/e – as predicted by Yoseph Imry of Israel’s Weizmann Institute and colleagues in 1969 and 1983. Imry described the study as a “very thorough study” that has “closed a chapter on persistent currents”.

He pointed out, however, that Harris and colleagues did not study persistent currents at low magnetic fields, where the currents are expected to be a periodic function of h/2e. “The explanation is akin to the simpler one involving ‘coherent backscattering’, the addition of paths encircling the ring in opposite, time reversed, orbits”, explained Imry. Imry told physicsworld.com that Harris should be able to do experiments at lower field strengths in order to see the h/2e regime.

Harris intends to do further experiments at different magnetic field strengths and angles, as well as different temperatures and ring size. He also said that he is keen to put tiny structures into the rings such as quantum dots or Josephson junctions – and study the resulting mesoscopic circuits remotely using the cantilever technique.

The work is described in Science.

Prince of darkness

Alex Murphy

What is dark matter and why does physics need it?

Start with an easy one eh? OK, so first of all, if we look up at the stars and galaxies, and try to apply the laws of gravity that we know work well at the small scales of planet Earth, and even the solar system, they fail. For the laws of gravity to work when looking at things of the size of galaxies or larger, there needs to be much more matter “out there” than we can see. Because it must be quite heavy, and we can’t see it, we call it dark matter.

There’s a pretty good chance that we could discover dark matter in the next few months!

But is there any actual evidence that it exists?

Well there are many different types of measurement that all suggest exactly the same thing. For example, the speeds with which stars rotate about galaxies, or the way that the light that we see from very distant stars is distorted by this “something” in between.

So what do you think this dark matter is — what form would it take?

The answer looks like it comes from particle physics, and a general consensus is building that the answer lies in a framework called “supersymmetry”. This theory suggests that for every particle type we know about, there should be a heavier version – hence the name, supersymmetry. These particles would have been created in much the same way as normal matter at the time of the big bang, and the lightest ones should still be floating around today – this is what we call dark matter.

Why can’t we see it?

It’s because it doesn’t interact at all with light, and only very rarely interacts with anything else. This is the reason why the favoured candidate for dark matter is called “weakly interacting massive particles”, or WIMPs.

Photomultiplier array covered by electrode grid

So ZEPLIN is looking for WIMPs?

Yes. The astronomy suggests that our galaxy is sitting in the middle of a region of dark matter. This isn’t unusual – since there’s roughly ten times as much mass in dark matter as there is in normal matter, the dark matter distribution is in fact what has determined where the galaxies have formed. The dark matter particles interact so weakly with anything that they can pass right through the Earth, but because there’s so many of them, a very sensitive detector in a place where nothing else can reach it, should be able to see a few of them.

And what does your machine do exactly – how does it work?

It’s really just a bucket of liquid with some very sensitive light detectors in it. But the liquid is xenon, which is one of the noble gas elements, like helium or argon. This has the nice property that if a particle passing through it scatters, as we expect WIMPs to do occasionally, it creates a small flash of light.

We’d also expect some of the xenon atoms to be broken up by such events, separating the xenon atom from some of its electrons. We put almost 20,000 volts across the liquid, which allows us to sweep up these released electrons, which we then use to generate a second flash of light in a layer of gaseous xenon. The combination of the first flash of light and the signal from the electrons tells us that there’s been an event in the detector.

We have focussed on keeping everything as clean and pure and getting the clarity of signal as high as possible

Why is ZEPLIN located deep underground in a salt mine?

Ahh, that’s because of cosmic rays. Inevitably, a detector that’s very sensitive to WIMPs is also very sensitive to many other things. Sitting here in a café we’re being bombarded all the time by cosmic rays coming from space. These are coming from the sun, from supernovae, and even a few from the cores of very distant galaxies. They cause things like the aurora, but for us they’re a pain. Cosmic rays are fast moving particles like electrons and muons, and these can go through quite a bit of material so it’s hard to shield against them. But going deep underground, in a mine, we’re able to get rid of about 99.9999% of them. That’s few enough so as not to be a problem.

But can you really be sure that you are seeing dark matter, not some other source of background noise?

Even down in the mine, there’s still natural radiation coming from the rock itself, from the detector components, from people, in fact everything has some level of radioactivity. We’ve placed ZEPLIN within a cocoon of very low radioactivity plastic and lead, and the device itself is made of very clean materials, but still there’s a few things flying around. Mostly these are gamma rays.

How does the upgrade make ZEPLIN better than other detectors in the world?

We have focussed on keeping everything as clean and pure and getting the clarity of signal as high as possible. I think we’re doing pretty well at that. We are now using newer photomultiplier tubes, those are the light collection devices, and these have much lower levels of intrinsic radiation than the ones we used before.

Recommissioning the xenon gas handling system

Also, we’ve built a completely new additional detector around the outside of ZEPLIN. The idea is that if we see an event that interacts in both this new detector, and in ZELPIN, then it can’t be a WIMP. WIMPs interact so rarely that there’s no chance whatsoever of them scattering in both detectors. Background neutrons could, so it’s an extra measure to be sure of what we are seeing.

So you’re looking forward to this latest run — what’s the timetable?

There’s a pretty good chance that we could discover dark matter in the next few months! There is nothing more exciting and more timely in UK physics at the moment…

We finished calibrating the detector in September and we hope to start running for dark matter data starting in December or even as early as November. The plan is to run it for a year without looking at data until end of the experiment. We do this “blind” so as to minimize the extent to which we could influence the experiment. History tells us that it’s very easy to unwittingly alter things so as to get the result you want and this is a standard technique to avoid that.

How do you handle the competitive element of the dark matter search – is there much international collaboration or is it all very secretive?

A lot of them are good friends, it’s just a rivalry…I mean…it’s a big prize…right?

Well, I guess it works as well as one could expect. I mean there are currently three groups in the world that are leading in terms of limits – there’s the Cryogenic Dark Matter Search in the US, there’s the Xenon100 team based in Italy, and then there’s us, and we all have similar limits at the moment. We don’t talk that much to each other because it’s all quite secret. All the time we’re thinking of clever ideas to tune the instruments and usually these are closely guarded secrets.

So there is a strong sense of rivalry?

Well, we’re not going to go around putting plutonium jabs into people to bump off the enemy players! A lot of them are good friends, it’s just a rivalry…I mean…it’s a big prize…right? But I think in general the rivalry and the difficulty of getting funding means that the science advances quicker than it would do otherwise.

Do you feel there are any areas of the science that could benefit from a more open approach?

We could definitely improve the way in which data is shared. Every group tends to present their results via a different style and this makes it more difficult than one would hope to compare results.

Can you tell me a little bit about your particular role?

I’m head of the Edinburgh contribution to the ZEPLIN III project. This means that probably 95 percent of the time behind a computer – updating spreadsheets, applying for money etc – that’s the bit I like the least… it’s always a last minute rush.

The ZEPLIN-III team

And which bits do you enjoy the most?

Well, it’s got to be the diversity – the way I do something different every day. The best bits of time are with PhD students guiding them with their projects. And, I do a fair bit of public understanding of science events. I’ve done a café scientifique in Moscow [informal science talks in café and bars], which was pretty scary, and run a few schools events, hopefully enthusing younger kids into science, showing them its not some kind of incredible magic, but that they can understand it and make a contribution. I also run a lot of talks back in Edinburgh.

Do you think the public can understand topics as difficult as dark energy?

Given the opportunity I think they can. The biggest problem is not in the conveying of ideas, it’s constraining the imagination of people listening not to go further. You can show them one little bit and they’re bright enough to start thinking about how the whole thing may fit together – but trying to answer some of those questions – that’s not an hour’s talk – that’s a lifetime’s talk!

ZEPLIN-III is a joint project of the University of Edinburgh, Imperial College London, the Rutherford Appleton Laboratory, and international partners in Portugal and Russia. To learn more about the project please visit the ZEPLIN III homepage.

Nanopores sequence DNA

Researchers at the Delft University of Technology have developed a new technique that can measure both the charge and diameter of a single molecule for the first time. The method, which employs solid-state nanopores, can clearly distinguish between molecules of DNA that have a protein coating and those that do not – something that could be useful for DNA sequencing and detecting markers for genetically inherited diseases.

“Our technique will eventually allow us to rapidly differentiate spots along an individual molecule, for example, on multiple DNA-bound proteins,” team member Adam Hall said.

The scientists, led by Cees Dekker, began by attaching DNA molecules coated with RecA proteins on a microbead. Next, they placed the molecules near the opening of a solid-state nanopore in ionic solution. By then applying an appropriate voltage across the pore, a single molecule was pulled into the pore where it was statically held by the bead tether.

Changes in current

The presence of the molecule changes the current measured through the pore, which allows its size to be determined, explains Hall. And, varying the applied force across the pore produces a 1D force curve on the molecule that then provides information about the overall charge on it.

The net force measured is mainly the electrostatic force acting on the charged molecule due to the applied voltage. Since DNA filaments coated with protein have a greater net charge, they feel a larger force than bare DNA molecules for the same applied voltage.

By using the trapped bead as a “handle”, the researchers can also control the position of the molecule inside the nanopore. They can thus choose the position at which to perform force measurements and locate a region of interest.

Faster protein mapping

“Our study provides a possible route towards fast, direct mapping of the entire library of proteins that bind to genomic DNA,” said Hall. “This could have important implications for sequencing, as well as detecting markers for genetically inheritable diseases.”

The team will now look at sequence-dependent proteins rather than co-operatively binding proteins that coat the DNA entirely. “Such an approach will allow us to push the limits of our resolution and rapidly detect structures that are important in a wide range of disease, such as cancer.”

The work was published in Nano Letters.

Microwaves live on the edge

In the week that three pioneers of optical communications have been awarded with the ultimate prize in physics, new research outlines an experimental breakthrough that could lead to a new generation of highly efficient optical fibres.

Researchers in the US have unveiled a new version of a device known as a photonic crystal, which they claim can transport microwaves with significantly less loss due to scattering. If the device could be scaled down to operate at optical wavelengths it could improve the quality of optical communications.

In addition to its practical potential, the function of the device also represents an analogy of a well known quantum phenomenon – the quantum Hall effect.

Guiding light

“This is an exciting development! It potentially allows construction of channels, which allow photon modes to travel along winding paths with no back-reflection at bends,” said Duncan Haldane at Princeton University who predicted the effect last year but was not involved in this latest research.

Photonic crystals are structures that are designed to trap and guide light in a similar manner to the way electrons are manipulated inside a semiconductor. Where semiconductors contain electronic bands in which electrons of a specified energy range cannot reside, photonic crystals are also layered to prevent transmission of light at certain wavelengths – creating a “photonic band gap”.

These materials have found widespread application in the optics industry, with optical fibres being one particularly promising application. Because light can be trapped and guided along channels roughly the same width as the wavelength of light, it means that less energy is lost through scattering by the intrinsic roughness of the fibre.

Asymmetry is the key

Now, Zheng Wang and his colleagues at the Massachusetts Institute of Technology (MIT) may have discovered a way of reducing the scattering further still. Their new type of photonic crystal could make it possible to confine light to the outer edges of the material where it loses its “time reversal-symmetry”. In simple terms this means that photons only move in one direction and therefore cannot be back-scattered. This would mean that the only way light could be attenuated is from non-linear effects or absorption.

The key to achieving this effect was to make a photonic crystal containing iron-based rods. Wang and his team realized that applying a magnetic field perpendicular to the direction of electromagnetic radiation forces photons into so-called chiral edge states (CESs). This optical effect is analogous to the phenomenon experienced by a 2D electron gas when it is exposed to a strong magnetic field, known as the quantum Hall effect.

To demonstrate the effect, the physicists created a periodic structure that works at microwave frequencies. In principle, the same structure could now be scaled down to trigger the same effect at optical frequencies.

Referring to the realization of Haldane’s prediction, Wang told physicsworld.com, “We were inspired by his theory and found that CESs exist in a more general class of photonic crystals and performed numerical calculation to support that.” Adding, “Because of the generality of our theory, we were able to construct a design that is practically feasible and use off-the-shelf materials”.

This research appears in the latest edition of Nature .

Water-seeking rocket smashes into the Moon

NASA’s Lunar Crater Observation and Sensing Satellite (LCROSS) smashed into the Moon today as planned.

At 07:31 EDT, its 2200 kg Centaur rocket was first to collide, kicking debris high above the lunar surface. A few minutes later, a second “shepherding” spacecraft – that will be used to collect scientific data – also collided with the surface.

NASA’s mission control has confirmed the thermal signature of impact and predict that imaging and spectroscopic data will be returned to Earth within the next few hours.

The $80m unmanned mission is searching for water, salts, hydrocarbons and other signatures of habitable conditions in the lunar surface.

The impact site was a 98 km wide crater called Cabeus near the Moon’s south pole, chosen because scientists have predicted that large quantities of water-ice could exist in these lunar “shadow-lands”.

LCROSS is part of a dual mission launched on 19 June this year that aims to shed new light on the Moon. Its sister rocket, NASA’s Lunar Reconnaissance Orbiter (LRO), has been orbiting the Moon to produce maps of its surface with the highest resolution yet.

The missions are important precursors to NASA’s Constellation programme, which aims to send astronauts to the Moon and to create a lunar outpost as a stepping stone for a trip to Mars. As well as determining if water or other useful substances can be found on the Moon, the missions could help identify possible sites where a future manned mission could land.