Page 1247 – Physics World

Graphene comes of age

Since its discovery in 2004, graphene has rapidly attracted the attention of academic and commercial institutions, which are looking to develop new technologies from the material. These include speedy electronics and communications devices, and a new generation of highly efficient solar cells.

The fundamental science of graphene and its potential applications are subjects of great interest to Patrick Soukiassian, a scientist who has spent several decades researching nanotechnologies and semiconductor materials. In this video interview, Soukiassian, of the University of Paris-Sud/Orsay and CEA-Saclay, explains that the vast potential of graphene has now been recognized by the European Union, which is starting to invest in several initiatives to support the development of these technologies.

In a separate video recorded last year, I visited the University of Manchester in the UK to learn how graphene can be made using nothing more than a sheet of graphite and some sticky tape.

But in order to develop applications, scientists are now seeking ways to produce high-quality graphene in large quantities. Soukiassian identifies one promising approach that involves growing sheets of graphene crystals on an underlying substrate of silicon carbide. The fact that this method is silicon-based means that it can borrow techniques that have already been refined over many years in the computing and electronics industries.

This field of research, known as epitaxial graphene, is the subject of a recent special issue of Journal of Physics D: Applied Physics, of which Soukiassian is an associate editor. The collection of papers, guest edited by Walt A de Heer of Georgia Institute of Technology, US, and Claire Berger of Georgia Institute of Technology and CNRS-Institut Néel, France, addresses the science and technology behind epitaxial graphene, both its production and its physical properties. The issue is free to read until the end of June 2012.

Graphene compilation Discover the must-read research on this expanding topic across eight leading physics journals from IOP Publishing

Photon-polarization qubits stored in atomic combs

Three independent teams of physicists have created the first solid-state quantum memories that store the polarization states of photons. While the three systems each use different materials, they are all based on the concept of an “atomic comb” – whereby a photon is stored in a collective excitation of the atoms within the solid.

Photons have proved to be a good choice for transmitting quantum bits (qubits) of information because they can travel for long distances without interacting with their surroundings. Their other advantage is that quantum information can be stored in a photon’s polarization state, with “1” represented by horizontal polarization and “0” by vertical, for example.

Unfortunately, it is very hard to make a quantum memory that can store photon qubits, which has been bad news for anyone wishing to make a quantum computer or build “quantum repeaters” that allow quantum information to be transmitted over long distances. Physicists have been able to create solid-state memories that can store and re-emit qubits based on the temporal properties of light – “1” for the presence of a photon and “0” for its absence, for example – but not the photon’s polarization itself.

One problem is that many solids tend to cause bifringence – a refractive effect that sends light of different polarizations along different paths. The other snag is that the degree to which many solid materials absorb light depends on the polarization of the light. The bottom line is that it is very difficult to make a device that stores a qubit with an arbitrary polarization.

Triple pronged

To get round this problem, the three groups took surprisingly simple approaches. Two teams – one led by Chuan-Feng Li at the University of Science and Technology of China in Hefei and the other by Mikael Afzelius at the University of Geneva – made their memories from two crystals of the same material that are rotated by 90 degrees to each other.

Meanwhile at the Institute of Photonic Sciences in Barcelona, Spain, Hugues de Riedmatten and co-workers used one crystal but split the incoming light into two separate beams, one of which has its polarization rotated by 90 degrees. The two beams are then sent through the crystal, and the emerging rotated beam is then rotated back to its original polarization before the two beams are recombined.

In all three cases, the basic idea is the same – by rotating one crystal or beam polarization by 90 degrees, the problems of bifringence and directional absorption are effectively eliminated. Quantum information is stored in each case in an “atomic frequency comb” (AFC) – a phenomenon first identified in 2009 at the University of Geneva by a team that included Afzelius and De Riedmatten. An AFC is a collective excitation of atoms in a crystal that includes a large number of frequency modes that are equally spaced in energy, like the teeth on a comb.

Drifting out of phase

The storage process begins when the photon is absorbed and excites the AFC modes. The modes then evolve in time such that they drift out of phase with each other. While this occurs, the AFC is unable to re-emit the photon. However, after a fixed amount of time – typically tens or hundreds of nanoseconds depending on the material – the comb modes are back in phase with each other and the photon is re-emitted with the same polarization as the absorbed photon.

The Geneva and Barcelona groups used yttrium-orthosilicate crystals that were doped with neodymium and praseodymium, respectively, while the Hefei team used yttrium-orthovanadate crystals doped with neodymium.

All three teams are currently trying to improve their devices. Li told physicsworld.com that the Hefei team is working on how to store multi-photon polarization entanglement in several different quantum memories. Afzelius’ team is investigating how the storage time can be extended to millisecond or even second timescales by using different materials, which is important given that storage times of seconds are needed for practical quantum repeaters. Meanwhile in Barcelona, De Riedmatten and colleagues are looking at how the AFC excitation can be transferred to a spin excitation in the solid – a process that they believe could be used to boost the storage time to a few seconds.

The research is described in three papers in Physical Review Letters.

Physicist convicted of terrorism leaves prison

Adlène Hicheur, the particle physicist convicted of terrorist charges, has been released less than two weeks into his four-year prison sentence, according to his former supervisor. Jean-Pierre Lees, a researcher at the Particle Physics Laboratory in France, told physicsworld.com that Hicheur was released from a French jail on the night of Tuesday 15 May, just one day after the deadline for him to appeal his guilty verdict had elapsed. According to Lees, Hicheur had considered appealing the verdict but in the end had decided against it because it would have required him to remain in prison for an additional eight months.

Hicheur, a 36-year-old French-Algerian, was arrested by French police on 8 October 2009 on suspicion of having links with the organization Al-Qaida au Maghreb islamique (AQMI). Prior to his arrest, Hicheur was working as a postdoc at the Swiss Federal Institute of Technology in Lausanne and spent time at the Large Hadron Collider at CERN. Hicheur remained in custody without charge for almost two and a half years – the maximum time permissible under French anti-terrorism laws

In November 2010 Lees sent a letter to the French Physical Society expressing concerns about the continued imprisonment of Hicheur without charge that was signed by 19 physicists including the Nobel laureate Jack Steinberger. Hicheur also received support from an “international defence committee”, consisting of about 100 scientists, which wrote to the French authorities, including the then French president Nicolas Sarkozy.

Focus on e-mail exchange

Hicheur’s first hearing took place on 16 February this year and the case came to a close on 4 May as Hicheur was found guilty of “participation in a conspiracy to prepare an act of terrorism”. The trial’s official summary document is centred on an exchange of e-mails between Hicheur and a contact with an alleged affiliation to AQMI, which took place between 2008 and 2009 while Hicheur was based in French territory. The trial document states that Hicheur appears to have given “financial support” to his contact in AQMI by offering to transfer between €5000–8000 to them. The summary also states that a folder was discovered on Hicheur’s personal computer with the name AQMI.COMS.

But Hicheur’s supporters, including Lees, feel that the sentence was a miscarriage of justice. “No proof that he sent money to this people [sic] has been shown, while all his bank accounts have been carefully scrutinized,” says Lees. He also questions the decision of the court not to summon the suspected AQMI terrorist to ascertain the true identity of this person and to establish whether Hicheur was fully aware with whom he was liaising.

“The instruction judge had no choice but to demonstrate by any means the involvement in a terrorist project, and so had to condemn Adlène to justify the two years and eight months he had already spent in jail,” says Lees. “The justice, like the medicine, rarely recognize its errors.”

‘A Kafkian situation’

Francesco Spano, a particle physicist at Royal Holloway, University of London, is also highly critical of the French justice system in its handling of the Hicheur case. “I am outraged by the fact that a person can be kept captive for about two and a half years without being judged, in a Kafkian situation,” he said. “During the trail it was hard for me to find the evidence for the specific material facts that would support the accusation”. Spano, who is one of 342 members of the Adlène Hicheur International Support Committee (CISAH), says that he would like this group to now focus on two goals: to help Hicheur to rebuild his life, and to try to understand the implications of this trial to society.

Another group that helped to raise the profile of the Hicheur case is ConCERNed for Humanity, which was seeking the support of the CERN management and the lab’s international community of users. One of the group’s members Michael Dittmar, a researcher based at ETH Zurich and CERN said that he will continue to support Hicheur and echoed the criticism of the French anti-terror laws. “Perhaps one might have some very remote hopes that the new French government can also lead to a more open position of the management,” he said.

A cracking approach to nanotechnology

For most manufacturers, cracks are usually something to be avoided – and the semiconductor industry is no exception. But now engineers in South Korea have shown how initiating and then controlling the spread of nanometre-sized cracks can be used to create pre-designed patterns in a silicon wafer. They say that their approach offers a potentially faster and cheaper alternative to conventional lithography for the fabrication of integrated circuits.

Cracks can form when two materials with mismatching crystalline structures are placed on top of one another. Stress builds up at the interface between the materials, deforming the crystal structures and creating a crack that spreads throughout both materials if the deformity builds up enough potential energy to break atomic or molecular bonds. This can happen when a thin layer of silicon nitride is deposited on a silicon substrate, with cracks spreading uncontrollably through one or both of the layers.

Koo Hyun Nam of the Ewha Womans University in Seoul and colleagues have controlled the formation of such cracks to create elaborate patterns within a silicon substrate. To do this they etched tiny structures at particular positions and with specific orientations within 0.5 mm-thick silicon wafers. The idea was that these “micro-notches” would concentrate the stress resulting from the deposition of a thin film of silicon nitride on the substrate. They also carved out step-like structures within the substrate to halt the spread of cracks or to isolate certain regions of the wafer from cracks.

Wavy cracks

Using chemical vapour deposition to lay down the silicon nitride, Nam and co-workers found that the cracks formed and propagated spontaneously. They were able to make the cracks either straight or wavy by changing the orientation of the crystal planes in the wafers as well as adjusting other parameters such as the temperature and pressure of the vapour. By laying down a film of silicon dioxide between the substrate and the silicon nitride they were able to generate a third shape – “stitch-like” cracks, which are straight cracks with short, parallel, angled branches.

The width of the cracks varied between about 10–120 nm, with the wavelike variety generally wider than the straight cracks. In addition, the researchers found that they could change the direction of a crack, causing it to “refract” much like a light wave passing into and then out of a block of glass, by separating only a part of the wafer and the silicon nitride with the silicon-dioxide film. Where there was no silicon dioxide, the crack penetrated more deeply into the silicon substrate and aligned itself more closely with the substrate’s atomic planes, whereas this alignment was weaker where there was silicon dioxide, causing the crack to change direction in this region.

Writing in Nature, Nam’s team says that this method of controlled cracking could offer a faster and cheaper alternative to conventional lithography for microchip fabrication. In an accompanying article, Antonio Pons of the Polytechnic University of Catalonia in Barcelona, Spain, agrees. He says that lithography, which allows patterns to be etched in silicon using a mask created via beams of light, electrons or ions, is often complex, expensive and time-consuming.

Hours, not weeks

Pons told physicsworld.com that the advantage of the new approach is that the time needed to form the pattern “is simply the time taken for the crack to propagate”, estimating that it should take only a few hours altogether to prepare the substrate, deposit the film and create the pattern, compared with the “days or weeks” needed using standard lithography. He admits, however, that he does not know how long it would take to make the micro-notches and other features. He also says it remains unclear how closely the cracks can be positioned to one another, something, he points out, “that is crucial when making small structures”.

But Pons believes that the new technique should also find applications beyond the semiconductor industry. One possibility, he says, is making microfluidic devices. These are networks of tiny channels within which fluids, containing molecules such as DNA, can be manipulated for study. He also wonders whether it might prove useful at larger scales, perhaps allowing buildings in earthquake zones to fracture more safely. “The answer to that is not necessarily yes,” he says. “Scale is very important, and we would be going from atomic-level interactions to the size of a house. But maybe this work will inspire people in other fields.”

Zhenan Bao, a chemist at Stanford University in the US, says that the strength of the latest work is in showing the formation of controlled cracking, pointing out that other groups have previously used cracks to create nanoscale patterns, but that they were not able to carefully control where the cracks formed. Bao cautions, however, that such controlled cracking would only be possible with certain combinations of materials, which may mean the technique has more limited appeal than standard lithography. “It would be nice to see a demonstration of this method for device application,” she adds.

Quantum dots give graphene photodetector a boost

Researchers in Spain have fabricated a new, highly sensitive photodetector from graphene and semiconducting quantum dots. The device is a billion times more sensitive to light than previous graphene-based photodetectors and might be ideal for a variety of applications, including light sensors and solar cells, infrared cameras for night vision and in biomedical imaging.

“In our work, we managed to successfully combine graphene with semiconducting nanocrystals to create complete new functionalities in terms of light sensing and light conversion to electricity,” says Gerasimos Konstantatos, who was co-leader of the team at the Institute of Photonic Sciences (ICFO) in Barcelona. “In particular, we are looking at placing our photodetectors on ultrathin and flexible substrates or integrating the devices into existing computer chips and cameras,” adds co-leader Frank Koppens.

Graphene is a sheet of carbon atoms arranged in a honeycomb-like lattice just one atom thick. The material could find use in a number of technological applications and in the future could even replace silicon as the electronic industry’s material of choice, thanks to its unique properties. These include extremely high electrical conductivity, which comes about because electrons whizz through graphene at extremely high speeds, behaving like “Dirac” particles with no rest mass.

Ideal internal quantum efficiency

Graphene also shows great promise for photonics applications because it has an ideal “internal quantum efficiency” – almost every photon absorbed by the material generates an electron–hole pair that could, in principle, be converted into electric current. Thanks to its “Dirac” electrons, it can also absorb light of any colour.

However, all is not perfect because graphene’s “external quantum efficiency” is low – it absorbs less than 3% of the light falling on it. Another problem is that useful electrical current can only be extracted from graphene-based devices that have electrical contacts with an optimized “asymmetry” – something that has proven difficult to achieve.

Koppens, Konstantatos and colleagues began with a piece of high-quality graphite from which they obtained graphene flakes using the now-famous “Scotch tape” technique. This involves placing the graphite sample on a piece of sticky tape then folding and unfolding the tape a few times until some grey transparent pieces of material can be seen among the original black and shiny fragments of graphite. Next, the researchers press the tape with the transparent graphite onto a substrate and then remove the tape.

“We can see many pieces of graphite with varying thicknesses on the substrate, and look for the ones with the lowest contrast – these are graphene,” explains Koppens. “It is remarkable that we can see a one-atom-thick layer of material with the naked eye, but that is thanks to graphene’s unique interaction with light.”

Hybridizing with quantum dots

The teams then used nanoscale lithography to contact a sample of graphene to two gold electrodes for subsequent electrical measurements. The electrodes are positioned with sub-micron precision in a process whereby gold is evaporated onto a resist mask especially drawn for the flakes with an electron-microscope gun. The next step is to hybridize the graphene with colloidal semiconducting quantum dots that photosensitize the carbon material.

“We chose quantum dots because of their unique optoelectronic properties,” says Konstantatos. The materials can be tuned to absorb a wide range of light wavelengths simply by changing the size of the nanocrystals. They also absorb light very strongly. Moreover, the dots can be processed in solution and can thus be sprayed, spin cast or ink-jet-printed onto any substrate, including graphene, at low temperatures and in air – non-negligible advantages that could bring down costs and simplify fabrication.

The researchers used lead-sulphide quantum dots because the band gap in these semiconductors can be tuned in the technologically important short-wavelength infrared (SWIR) and near-infrared (NIR) ranges.

Tailored connections

“A critical part of our experiment involves ligand exchange to cross-link the quantum dots with short (around 0.2 nm) molecules and attach these to the graphene,” explains Konstantatos. “This step was needed to passivate the surface states in the quantum dots and so allow for efficient charge-carrier transfer to graphene and suppress unwanted recombination of electrons and holes – which would have lowered the amount of current eventually produced by the finished photodetector.” The challenge was to tailor the electrical connections between the semiconducting nanocrystals and graphene while maintaining the high quality and exceptional electrical conductivity of this material, he adds.

After depositing a thin film of the quantum dots on the graphene, the ICFO researchers characterized their device by exposing it to light while probing its resistance at the same time. The photodetector responded to tiny amounts of light (almost complete darkness), as seen by the large change in its resistance.

Since the device was designed in a transistor arrangement, it was possible to change the carrier density in the graphene by varying the voltage on the gate electrode. “From these measurements, we could very precisely quantify the internal quantum efficiency of the device as being greater than 25% because the quantum dots absorb light so well and because charge transfer between the two materials is so effective,” says Koppens. “We found that we could also optimize this performance with the voltage on the gate or even switch off the response. This ‘switchability’ might come in useful for pixelated imaging techniques.”

Bendy solar cells

Many of today’s photonic devices rely on ultra-efficient conversion of light to electricity, Konstantatos adds. “Our detector could be used in digital cameras, for night vision and in biomedical imaging, as well as in sensing applications. Its flexibility also means that it might be ideal for bendy solar cells that could be placed on objects of any shape – an avenue that we are currently exploring.”

The team now plans to scale up the detector to large-scale imaging arrays. “We expect that most cars will be equipped with night-vision systems in the near future and our arrays could form the basis of these,” says Koppens. The researchers also aim to improve their device even further so that it can detect single photons.

The current work is detailed in Nature Nanotechnology.

How to cook up a new topological insulator

By Hamish Johnston
First predicted in 2005 and confirmed in the lab in 2007, topological insulators (TI) are perhaps the hottest material in condensed-matter physics these days. As well as constituting a new phase of quantum matter that should keep physicists busy for some time, the material has recently been shown to harbour quasiparticles resembling Majorana fermions. First predicted by the Italian physicist Ettore Majorana in 1937, such particles could be used to store and transmit quantum information without being perturbed by the outside world. As such, they could find use in the quantum computers of the future.

It’s not surprising that scientists worldwide are working hard to discover and study new variants of TIs. However, researchers at Duke University in the US believe that, until now, discoveries have been based on trial and error.

To encourage a more systematic approach, Stefano Curtarolo (right) and colleagues have created a “master ingredient list” that describes the properties of more than 2000 compounds that could be combined to make TIs. The clever bit of the work is a mathematical formulation that helps database users search for potential TIs that are predicted to have certain desirable properties.

The system is based on Duke’s Materials Genome Repository, which has already been used to develop both scintillating and thermoelectric materials.

According to Curtarolo, the system gives practical advice about the expected properties of a candidate material – for example, whether it will be extremely fragile or robust.

Commenting on the fragile materials, Curtarolo says “We can rule those combinations out because what good is a new type of crystal if it would be too difficult to grow, or if grown, would not likely survive?”

The research is also described in a paper published in Nature Nanotechnology.

Bow-shock no-show shocks astronomers

The Sun moves much more slowly relative to nearby interstellar space than was previously thought, according to scientists working on NASA’s Interstellar Boundary Explorer (IBEX) mission. Their study casts doubt on the existence of an abrupt “bow shock” where the edge of the solar system meets the interstellar medium – instead suggesting that the boundary between the two regions is much gentler than previously thought. The discovery could lead to a better understanding of how some cosmic rays can enter the solar system, where they pose a threat to space travellers.

The bow shock refers to the region where the heliosphere – the huge bubble of charged particles that surrounds the Sun and planets – is believed to plunge into the interstellar medium. The commonly accepted idea is that the solar system moves faster (relative to the speed of the interstellar medium) than sound itself, rather like the shock wave that forms ahead of a supersonic aircraft. Charged particles moving supersonically in the heliosphere therefore pile up at the front of the shock, with the density of charged particles dropping off rapidly where the heliosphere meets the interstellar medium.

Astronomers have always had good reason to believe the bow shock exists because similar structures can be seen surrounding nearby stars. But the new analysis of IBEX data – which has been carried out by David McComas of the Southwest Research Institute in Austin, Texas, and an international team – suggests that the bow shock does not exist after all. In other words, the solar system is not moving as fast as we though relative to the interstellar medium.

Fast-moving atoms

Launched in 2008, IBEX orbits the Earth and is designed to study fast-moving neutral atoms. What McComas and colleagues did was to use IBEX to characterize neutral atoms from the interstellar medium that cross into the heliosphere. Because these atoms are not electrically charged, they are not affected by magnetic fields – and so their speed should correspond to the relative velocity of the interstellar medium.

The study suggests that the relative speed is about 84,000 km/h, which is about 11,000 km/h less than previously thought. In addition, data from IBEX and earlier Voyager missions suggest that the magnetic pressure found in the interstellar medium is higher than expected. When these parameters were fed into two independent computer models of the heliosphere, both suggested that a bow shock does not exist, but rather a gentler “bow wave” occurs at the interface.

Gliding through space

“A wave is a more accurate depiction of what is happening ahead of our heliosphere – much like the wave made by the bow of a boat as it glides through the water,” explains McComas.

According to McComas, decades of research based on the presumption of a bow shock must now be redone using this latest information. “It’s too early to say exactly what these new data mean for our heliosphere,” he says. Given that the heliosphere shields the solar system from some cosmic rays, McComas says “there are likely [to be] implications for how galactic cosmic rays propagate around and enter the solar system, which is relevant for human space travel”.

The work is described in Science.

Physics in 100 seconds

Ready, steady, GO!

James Dacey

“What is dark matter?…you’ve got up to 100 seconds to answer…your time starts…NOW!”

This was the challenge facing Luke Davies (above) during a day of filming at the University of Bristol, where academics were asked to give super-condensed lectures on some of the biggest questions in physics. Participants at this UK university were armed with nothing more than a whiteboard and a couple of marker pens. And just to make the experience that bit more thrilling/nerve-racking, speakers were faced with a countdown alarm that sounded once their time was up.

The idea is to compile a series of short films for physicsworld.com that will provide introductions to topics across the whole spectrum of physics and its related disciplines. Films are presented by various physicists and cover everything from antimatter to fracking to black holes. Oh, and I almost forgot to mention the presentation about recognizing penguins in a crowd. From behind the camera, I certainly learned an awful lot about an awful lot!

The scientists appeared to get a lot from the day too. Several of them commented about what a vast departure it was from their usual experiences of presenting: standing in front of students and lecturing for an hour or so. Clearly 100 seconds is not very much time to explain topics as complex and detailed as dark energy or the Higgs boson, but everybody rose to the challenge and it was fascinating to observe the different styles that people adopted.

These films will be appearing on physicsworld.com over the coming weeks.

Astronomers lift the veil on hidden exoplanet

An international team of researchers has detected and characterized an “unseen” exoplanet, simply by looking at its gravitational effects on the orbit of another exoplanet within the same system. While the technique has been used before to confirm the existence of certain exoplanets found using other methods, this is the first time it has been used to discover an exoplanet.

NASA’s Kepler mission is currently looking at more than 15,000 stars, any of which could harbour their own planetary systems. Kepler looks for small dips in a star’s brightness as a function of time – its light curve – that would occur when a planet crosses the face of the star in a “transit”. It also looks for changes in the radial velocity of the star itself – the tiny shifts that occur because of the gravitational pull of a planet. In 2005 two different groups of researchers outlined a method that could help find exoplanets – especially very small ones – for which radial-velocity data are inconclusive and the transits of which are not visible from Kepler’s vantage point. The method focuses on irregularities in the patterns and transit timings of known exoplanets – dubbed “transit-time variations”.

Predicted twists

A lone exoplanet following a strict Keplerian orbit around its parent star would show an equally spaced and timed transit light curve that is periodic. Any variation in this could mean that the exoplanet is not alone – it may have a moon or there may be other unseen exoplanets in the system. Since the method was proposed, the Kepler team has used it to confirm its data in a number of systems where exoplanets did show their transits, rather than as a method to discover hidden exoplanets.

Graph showing the transit-time variations

Kepler data showing the variations in transit times

It is only now that an analysis of publicly available Kepler data has allowed David Nesvorny of Southwest Research Institute in Boulder, Colorado, and colleagues to successfully identify and characterize an exoplanet in a system previously thought to contain only one exoplanet. “We had been looking at hundreds of systems that could host possible exoplanets as identified by Kepler – each is known as a ‘Kepler Object of Interest’ [KOI] – and this one in particular seemed the best candidate as it showed a huge transit-time variation of almost two hours and was not periodic at all,” explains Nesvorny. The team looked at the data from 15 transits of the known exoplanet-b that has an orbit of 33.6 days around its parent star – known as KOI-872 – and then ran complex computer simulations to allow for all possible solutions that could cause the variations.

Hidden effects

The researchers found that the best solution for the gravitational perturbations seen for exoplanet-b is the existence of another near-by exoplanet, exoplanet-c, that orbits the parent star every 57 days. According to Nesvorny, the data supporting the existence of exoplanet-c are “very convincing and have a statistical significance of better than 3σ”. To put their findings to the test, the researchers have predicted the timing variations for numerous upcoming transits of exoplanet-b. Some of the data for this are already available to the Kepler team, while other data that be collected over the next three years. “The Kepler team can check for the predicted trend and they can even check for whatever faint radial-velocity data it can get for the effect of the other exoplanet in the system to test our predictions,” says Nesvorny.

Unexpected addition

As it turns out, soon after the researchers predicted exoplanet-c, a third exoplanet – a super-Earth approximately 1.7 times the mass of the Earth and with a very short orbit of 6.8 days – was found in transit data from Kepler. So could it be that this third exoplanet has an equal or greater effect on exoplanet-b? “We did immediately test for that too, but our simulations showed that for a smallish exoplanet like the super-Earth, its gravitational effects would be in the order of minutes and not hours. Of course, there is a chance that the super-Earth is extremely dense or that exoplanet-c is extremely light, but the chances of that are slim. For the observed two-hour transit-time variation, the super-Earth would have to almost be as dense as a neutron star, which is not going to be the case,” says Nesvorny. So the team is sticking to its original prediction for exoplanet-c.

In the months to come, Nesvorny and the team will be looking at more data from the Kepler mission to prove their predictions. Nesvorny, who is also part of the Hunt for the Exomoons with Kepler (HEK) project, will be looking at data on transit-tiem variations to search for exomoons. “Exomoons are the next big thing, I feel,” says Nesvorny. “They help determine the mass of an exoplanet and could be situated within the habitable zone of a star and be as interesting as their parent planets,” he says.

The research will be published in Science.

Frequency comb takes a measure of distance

A new method for measuring distance based on an optical frequency comb has been unveiled by physicists in the Netherlands. The main benefit of the technique, which involves passing the light from an optical comb through a Michelson interferometer and analysing the resulting interference patterns, is that it allows distances to be measured accurately without already knowing the value to within half a wavelength of the light used. The technique could be used to measure the distance between satellites or to make very precise measurements of the dispersion of light in optical materials.

A Michelson interferometer consists of two optical paths – or arms – that are at right angles to each other. Incoming light strikes a beamsplitter where the two arms meet and light is sent simultaneously along each arm until it hits a mirror at the end of each path and is reflected back. The two beams are then recombined and sent to a screen, where an interference pattern appears if the two arms are different lengths. By analysing the pattern, the difference in length between the arms can be determined in terms of a fraction of the wavelength of the light.

Half-wavelength limit

In its simplest form, a Michelson interferometer is used with monochromatic light. However, this limits its effectiveness because, before a measurement is made, the length of the distance to be measured must be known to within one half of the wavelength (λ/2) of the light used – typically less than 500 nm. The problem is that the distance being measured can be expressed as an integer multiple of λ/2 plus a fraction of λ/2 – but this integer multiple cannot be determined from the interference data.

Physicists have found two ways round this problem. One is to use several lasers at different colours to gain more information about the system. The other is to use a light source with a range of wavelengths and then look for phase differences in the interfering light, which can be related to distance. These techniques have their own problems, not least that thousands of different lasers would ease a measurement but installing them all in a lab would be impractical.

9000 different lasers

What Steven van den Berg and colleagues at the National Metrology Institute in Delft and the Technical University of Delft have done is to combine the two techniques to allow even better distance measurements. The team makes use of a frequency comb – a device that outputs laser light at regularly spaced wavelengths. The result is that the interferometer is effectively being operated using about 9000 different lasers at the near infrared in the 808–828 nm range.

The researchers demonstrated their new technique by firing pulses from their frequency comb into an interferometer with a measurement arm about 15 cm long. After being recombined, the light from the two arms was passed through an optical device called an etalon and then reflected from an optical grating. Both of these devices change the trajectory of the light according to its wavelength, creating an interference pattern that was captured using a CCD camera. This image was then analysed to produce a plot of the intensity of light as a function of wavelength – with each data point corresponding to a different wavelength of the comb. The result is a sine wave from which the measured distance can be calculated.

The team found that the distance could be found to within λ/30 – or about 27 nm. While this is a typical result for a Michelson interferometer, the new set-up allows any distance that is already known to within 15 cm to be measured – compared with the 400 nm prior knowledge required by a monochromatic device.

Measuring optical properties

Van den Berg believes that the system could find uses in space, where light can travel long distances without interference. As such, he says it could be used to make extremely accurate measurements of the distances between satellites. The fact that the system makes very sensitive measurements across a range of wavelengths could also see it being used to measure the properties of optical materials – particularly how the index of refraction changes with wavelength.

The team is currently working on improving the stability of the system and also investigating how it performs over longer distances.

The work is described in Physical Review Letters.

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