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Home » Page 1454

‘Quantum simulators’ revealed in fresh detail

 

Physicists in Germany have used fluorescence imaging to identify individual particles in an optical lattice for the first time. The breakthrough could allow researchers to create more advanced simulations of quantum phenomena and it might help in the quest for practical quantum computing.

Optical lattices are regular arrays of identical energy wells created by criss-crossing laser beams. By injecting ultracold atoms into the energy wells, the lattices can be used to create and study a range of materials and they are used to create large-scale replicas of quantum systems.

In this new research, Stefan Kuhr and Immanuel Bloch of the Max Planck Institute of Quantum Optics in Garching, together with colleagues at Ludwig-Maximilians University in Munich, created an optical lattice to contain a type of ultracold gas known as a Bose-Einstein condensate (BEC). These systems are formed when identical atoms with integer spin are cooled until all the atoms are in the same quantum state, meaning they behave as if they were a single quantum particle.

‘Dark and bright’ regions

Kuhr’s team created its BEC by cooling several thousand rubidium-87 atoms to close to absolute zero, and then configured the optical lattice to form a series of “dark and “bright” areas. Due to the energy levels, the rubidium atoms are much more likely to settle in the dark regions, and to hop into neighbouring sites each atom would have to overcome a significant energy barrier. This configuration is known as a Mott insulator, named after British physicist and Nobel prize winner Sir Neville Mott, because it resembles a solid in which conduction electrons are localized due to the strong interactions between the atoms.

According to theory, the number of atoms varies between lattice sites in a BEC. In the case of a Mott insulator though, this number is predicted to approach a fixed value at very low temperatures close to zero Kelvin – with the atoms arranged regularly in each lattice site. Kuhr’s team has now been able to directly observe this behaviour in an experiment for the first time.

Scientists regularly cool atoms using laser beams in quantum optics systems. But the key was to fire laser pulses at the system and then directly detect the atoms using a specially designed high-resolution microscope that collects the fluorescence photons from the atoms as they cool. It was using this technique that the researchers were able to count single atoms on each individual lattice site – a “sensational result”, says Kuhr, “and a prerequisite for using these systems as quantum registers with individually addressable quantum bits in future quantum computers”.

A quantum register

“A Mott insulator with exactly one atom per lattice site represents a very promising candidate for a quantum register of up to a few hundred atomic quantum bits,” adds Kuhr. “However, we needed to show that we really are able to manipulate each individual atom in the structure. This is crucial for encoding and reading out qubits and we are now at the beginning of setting up the first experiments of this kind.”

These studies might not only lead to quantum computers; they could also help to develop fundamental models of condensed matter physics. This is because the atoms in an optical lattice are analogous to the electrons in a solid state crystal. “Such investigations could aid in our understanding of unusual magnetic and electric phenomena, such as high-temperature superconductivity, and may even pave the way towards ‘tailor-made’ materials,” said Kuhr.

The work is described in Nature.

Controlling heart beats with lasers

A technique for regulating the tempo of heartbeats with short flashes of light could lead to a less intrusive type of pacemaker, say researchers in the US. A team led by Andrew Rollins at Case Western Reserve University in Cleveland, US, has shown for the first time how the beating heart within the body of an embryonic quail can be synchronized with pulses of infrared laser light.

The door to such devices was opened in 2008 when extremely short femtosecond pulses from a Ti:sapphire laser were able to regulate the activity of small groups of cardiomyocytes, the specialized cells in cardiac muscle that contract in unison to create a heartbeat. Unfortunately, the price was possible damage to the cells in the process.

In the new research, Rollins’ group took its cue from a study showing how pulsed infrared light could influence a cell’s “action potential”, the name given to rapid changes in potential difference within a cell, which is thought to be the first step towards muscle contraction. These effects were seen at exposures well below the damage threshold.

Beating bird hearts

Stepping up the light source to a millisecond-pulsed infrared diode laser operating at a wavelength of 1.88 μm, the team carefully targeted a 0.3 mm2 area of the heart tube in a quail embryo. The heart in avian embryos begins beating after approximately 40 hours of incubation, so the Cleveland team used embryos at 53 and 59 hours of incubation whose hearts normally beat approximately once every 2 seconds. Light was delivered via a 400 μm diameter fibre, positioned 500 μm away from the embryo, and the heart tube was illuminated twice every second.

The result was synchronization between the laser pulse and the embryo’s heart rate, with each pulse inducing a heartbeat. Increasing the illumination frequency to 3 pulses a second caused the heart rate to follow suit, and when the laser was turned off the heartbeat dropped back to almost its original level. As long as the exposure intensity was kept below an upper threshold, which the team determined to be about 0.81 J cm–2, pacing was consistently successful with no subsequent signs of harm seen by electron microscopy.

Avian embryos are important models for the general study of cardiac growth and congenital defects, a topic where much remains unknown. That is because birds tend to have a relatively simple anatomy that develops rapidly, and there are still tight regulations controlling work on “live vertebrate animals”. A non-invasive way to control the heart rate would give researchers a means of manipulating the forces at work in the “looping” of a single heart tube into a four-chambered organ, and provide new methods of exploring what happens during the process.

Releasing action potential

However, the prospect of replacing electrical pacemakers in humans with laser-equipped equivalents is still a long way off, not least because the reasons why the laser produces the effect are unclear. The US team believes that it causes a thermal gradient, opening up ion channels and inducing action potentials under the right conditions. Whether that would still apply in the more opaque tissues of a human heart is not certain.

They believe that optical pacemakers could have advantages over the electrical variety. They would not need to be in physical contact with the heart, and so avoid the damage that the electrodes themselves can sometimes do to the organ they are assisting. Plus, the laser can be tightly focused on a particular area of interest.

The research is described in Nature Photonics.

Shining a light on diabetes

 

A quick and painless way to measure blood sugar is highly sought-after by diabetes sufferers, who currently have to prick their fingers to draw blood several times a day. Now, researchers in the US may have found a solution – a device that works by simply shining a light on skin.

The vision is to create a laptop-sized device that could be kept at home or carried around. Rather than having to pierce the skin to obtain blood samples, the device measures sugar levels by simply placing a scanner against the skin. Because measurement is fast and easy, it is hoped that the device may encourage people with diabetes to check their blood sugar more often, giving them better control over their condition.

At the heart of the device is a Raman spectrometer, which can identify chemical compounds by measuring how near-infrared laser light scatters on contact with molecules. The idea of using Raman spectroscopy to measure sugar levels in blood was first suggested 15 years ago by Michael Feld at the Massachusetts Institute of Technology (MIT). Although Feld sadly passed away in April this year, his team is now starting to realize his vision.

Keeping up with the sugar rush

The problem until now has been that near-infrared light can only penetrate a short distance into the skin. The technique therefore detects glucose in the fluid surrounding skin cells (the interstitial fluid), rather than in the bloodstream. This is a problem because blood glucose levels can change rapidly, such as after eating, while there is a time lag of 5–10 minutes before the sugar changes can be seen in the interstitial fluid.

The MIT team has now resolved the problem by developing an algorithm to relate blood glucose to interstitial glucose levels. “We’ve incorporated a mass-transfer model into the overall Raman spectroscopic algorithm, which allows us to seamlessly transform between blood and interstitial fluid glucose,” explains Ishan Barman, lead author of the research.

Using an early version of the device, the team tested the blood-sugar levels of some human volunteers and found that the accuracy and precision of the test was just as good as conventional finger-prick tests. In addition, the new algorithm allows the test to predict impending episodes of high or low blood sugar (hyperglycemia and hypoglycemia) by extrapolating the rate of change of sugar concentration.

Downsizing

The next challenge is to strip down the Raman system and build a miniaturized device that would be suitable for home use. A prototype has been made and is already scheduled for clinical testing, but reducing the complexity of the system and shrinking down bulky components could take a while.

“We are in a proof-of-concept stage in terms of device development – and we envision a laptop-sized or hand-held unit that could cost as little as $200,” he told physicsworld.com. “It is difficult to predict due to market variations and FDA regulations, but one could anticipate an optical device for glucose monitoring in the next 5–7 years.”

Randall Jean, an expert in remote sensing at Baylor University in Texas, US, is impressed by the work. “This research addresses a real problem and appears to provide an important means for improving the calibration of non-invasive sensors,” he says. “It may also be helpful in the development of a so-called ‘artificial pancreas’ – where insulin can be dispensed automatically in response to sugar levels.”

This research is described in Analytical Chemistry.

Nicola Cabibbo: 1935–2010

The Italian physicist Nicola Cabibbo, who many said should have shared the Nobel Prize for Physics in 2008 for his contribution to understanding the mechanism of quark mixing, died yesterday at the age of 75.

Cabibbo held many high-profile positions throughout his career including president of the Italian National Institute of Nuclear Physics (INFN). At the time of his death he was working at the University of Rome “La Sapienza”, and was president of the Pontifical Academy of Sciences and chair of the scientific council at the Abdus Salam International Centre for Theoretical Physics (ICTP).

Only last week Cabibbo, together with Ennackal Chandy George Sudarshan of the University of Texas at Austin, were awarded the ICTP’s 2010 Dirac medal for their “fundamental contributions to the understanding of weak interactions and other aspects of theoretical physics”. A friend or colleague of Cabibbo will now be invited to accept the award on his behalf when it is presented in November by Irina Bokova, the director general of the United Nations Educational, Scientific and Cultural Organization.

Weak force pioneer

Cabibbo was best known for his work on the weak interaction in quarks – a fundamental particle that makes up hadrons such as protons and neutrons – and had been recognized for his contribution to “quark-mixing” between different favours of quarks. In 1963 he introduced the “Cabibbo angle” that is related to the relative probability that down and strange quarks decay into up quarks.

Cabibbo’s 2×2 quark-mixing matrix was later extended to include a third generation of quarks by the Japanese physicists Makoto Kobayashi and Toshihide Maskawa, who then shared the 2008 Nobel Prize for Physics together with theorist Yoichiro Nambu. The resulting Cabibbo–Kobayashi–Maskawa (CKM) matrix describes how the strange quark and the down quark inside a kaon can switch to and fro into their antiparticles and, in doing so, occasionally violate charge–parity (CP) symmetry. This matrix also predicted the existence of new quarks – the charm, bottom and top – that were later discovered in experiments.

“Cabibbo was a giant of contemporary particle physics,” says Tim Gershon, a particle physicist at the University of Warwick in the UK. “The impact of his contribution to the development of the Standard Model is almost impossible to overstate.”

Some physicists, however, still feel that Cabibbo should have been awarded a share of the 2008 prize together with Kobayashi and Maskawa for laying the groundwork of the CKM matrix. “There has been an almost universal feeling in the community that Cabibbo was desperately unfortunate not to share in the prize,” says Gershon.

A life in physics

Born in Rome in 1935, Cabibbo graduated in physics from the University of Rome “La Sapienza” in 1958 and then worked as a researcher at the INFN until 1962. After spending some time at the Lawrence Berkeley National Laboratory, California and at Harvard University, he returned to Italy to work at the University of Aquila in 1965. In 1966 he moved back to “La Sapienza” where he remained affiliated for rest of his career. He became president of the INFN in 1983 and had been president of the Pontifical Academy of Sciences since 1993.

“Nicola Cabibbo had a very strong influence on his colleagues. He was highly respected and considered to be a true gentleman,” says theoretical physicist Giorgio Parisi from “La Sapienza” who was supervised by Cabibbo during his graduate studies. “I remember he always made Saturday morning a time to do physics. It was like a game for him to put together the pieces of a puzzle.”

Drab butterfly reveals its hidden colours

 

With its leafy-brown body, Pierella luna is a butterfly from the forests of Central America that appears to have nothing special about it. But when illuminated by a beam of light, the males reveal a rainbow marking on both of their forewings when viewed from certain angles.

First observed over 30 years ago, the mechanism behind this intriguing optical effect has now been described for the first time by a group of researchers in Belgium and Panama. If it could be mimicked in the laboratory, the effect might allow engineers to develop novel colour-changing surfaces.

Rainbow colouration has been noted previously in the wings of several other butterflies, and has been attributed to grooves in wing scales. These channels diffract light during reflection – much like the pits in the surface of a CD. But Pierella luna is different because its wings produce a reverse conventional colour spectrum with red light emerging almost perpendicular to a wing and blue light being almost parallel to the surface.

A flash of green

Annette Aiello, of the Smithsonian Tropical Research Institute in Panama, first spotted the rainbow effect in 1981 in the wings of a male Pierella luna as it was pursuing a female. “I could have sworn I saw a flash of green,” she recalls. Aiello went on to become the first scientist to describe the phenomenon in a publication.

Curled scales
Curled scales

Aiello has now teamed up with physicists at the University of Namur in Belgium, led by Jean Pol Vigneron, to investigate the mechanism behind the iridescence effect. The researchers collected a number of butterflies from two sites in Panama and another location in Mexico, before conducting a series of experiments.

Using scanning electron microscopy, Vigneron and his colleagues discovered an unusually stark deformation in the wings of Pierella luna, finding that the wing scales curl to form a vertical diffraction grating perpendicular to the wing surface. So rather than being reflected, light is diffracted as it passes through the butterfly’s wings with the perceived colour changing with viewing angle.

By illuminating the wings of Pierella luna from a fixed angle of 45° from the normal, Vigneron’s team observed the wing colours change from brown through to green through to blue, as their viewing angle varied from 45° through to 75° relative to the vertical.

Marking the males

It is not clear why male Pierella luna have evolved to possess this ability, but Aiello believes it is related to courting, perhaps to help males distinguish between the sexes to ensure they are “chasing” the right butterflies. “It is difficult to study because we so rarely see the males open their wings in the sunlight – they spend most of their time near the forest floor, camouflaged against the leaves,” says Aiello.

Producing an artificial version of this rainbow effect is described by the researchers as a “formidable” task, given the micro-engineering challenges. In theory, the effect could be exploited to create, say, clothes that can change colour or novelty fabrics for house interiors.

Mathias Kolle, a biological structures researcher at the University of Cambridge in the UK, believes that the effect could also be exploited in security printing to brand goods and to mark authentic bank notes from counterfeits. “This unique effect would extend the library of optical phenomena that are already exploited to create labels with unique optical signatures,” he says.

This research is described in Physical Review E.

US astronomers unveil 10-year plan

Astronomers in the US have identified the highest priority research activities in astronomy and astrophysics for the coming decade. The decadal survey, released today by the National Research Council, says understanding the nature of dark energy, studying the formation of galaxies and black holes, and seeking nearby habitable planets are the most important science objectives for the next 10 years and beyond. Their decisions are influenced by the opportunity of international collaboration and for the first time the decadal survey also takes into account the project’s technical feasibility as well as its cost and current schedule

The survey, which included the input of over 200 scientists, prioritizes projects in four categories – large and mid-size space-based missions, as well as large and mid-size ground-based telescopes.

Two projects that will study dark energy – a mysterious substance that accounts for 74% of the total mass-energy of the universe and is causing its rate of expansion to increase – are given the highest priority in the large space-based and the large ground-based categories.

The search for alien worlds

The Wide-Field Infrared Survey Telescope (WFIRST), which has an estimated cost of $1.6bn and is scheduled for launch in 2020, is given top priority in the large space-based class. WFIRST will take measurements of supernova distances, gravitational lensing and measure baryon acoustic oscillations – clustering of matter due to acoustic waves that propagated in the early universe – to determine the effect of dark energy on the evolution of the universe.

The committee chose WFIRST, which is a collaboration between NASA and the US Department of Energy, as top priory as it offers a chance for international participation. It also presents a relatively low technical and cost risk making its completion “feasible within the decade, even in a constrained budgetary environment”. The telescope is based on a design for the planned $650m Joint Dark Energy mission, which will now be amalgamated with a mission to search for exoplanets.

“WFIRST not only gets at all the dark energy [priorities], but it also has significant capability in exoplanet science and will do outstanding work in infrared survey science,” Michael Turner, a cosmologist at the University of Chicago and the Kavli Institute for Cosmological Physics told physicsworld.com. Turner, who served on the 23-member committee for the decadal survey, also notes that the survey did not reject the idea of a possible collaboration with the European Space Agency (ESA) to combine its planned Euclid dark-energy mission with WFIRST.

Top priority for large ground-based missions is given to another telescope that will attempt to study the nature of dark energy and dark matter. The $465m Large Synoptic Survey Telescope (LSST), which when complete in 2015 will survey the entire sky every three nights with an 8.4 m optical telescope in Chile. The telescope came top in the ground-based category due to its “capacity to address so many of the science goals” of the decadal survey.

“This is extremely good news for the project, and a great relief,” says astronomer Andrew Lawrence from the University of Edinburgh who was attending a meeting about the LSST in Arizona this week. “It does not guarantee that it will now get the rest of its funding, but it is a very good bet.” Construction of LSST is due to begin in 2013, which will take around two years. After a year of testing, science operations are then set to begin in 2015.

Setting priorities

Studying gravitational waves and the nature of black holes make up the other priority missions in the large space-based category. After WFIRST, second priority is given to the “explorer programme”, which supports small- and medium-scale missions such as the Wide-Field Infrared Survey Explorer that launched in December to probe the coolest stars in the universe and the structure of galaxies. These missions are launched within five-year time scales and the survey recommends that the budget for this programme is increased from $40m to $100m per year by 2015.

The Laser Interferometer Survey Telescope (LISA), a joint mission between NASA and the European Space Agency (ESA), is third priority for large space-based missions. To be launched in 2025 and costing roughly €2.4bn, LISA will consist of three identical spacecraft flying five million kilometres apart from each other in a triangular formation. Laser beams directed between each of the three pairs of spacecraft will measure minute changes in the distances between the craft caused by the passage of gravitational waves.

The fourth priority for large space-based missions is the $5bn International X-ray Observatory (IXO), which is planned for launch in 2021 and will study black holes and hot gas associated with galaxies and stars. The report says that there are still some cost and technical uncertainties concerning the IXO, which is reflected in the mission’s lower priority.

Astronomy from the ground

In the large ground-based category, the “mid-scale innovations programme” that will study new telescope and instrumentation designs and fund telescopes in the $4m to $135m range gets second priority after LSST with a recommended funding of $40m per year. While third priority is given to a large optical and near-infrared telescope – a Giant Segmented Mirror Telescope (GSMT) – to be built in the coming decade at a cost of around $1bn. Two telescopes – the Giant Magellan Telescope and the Thirty Metre Telescope – are currently being developed as a GSMT and the committee says that a choice between the two projects should be made “as soon as possible for a federal partnership at a level of about a 25% investment in one of them”.

Fourth priority in the large ground-based projects is the $400m Advanced Gamma-ray Imaging System (AGIS), which will study high-energy gamma rays from black holes and seek indirect evidence for dark matter annihilation.

At a glance: Decadal survey priorities

Space based

Large-scale: (in priority order): WFIRST; Explorer programme; LISA; and IXO.

Medium-scale: The “new worlds technology development programme” to design a future mission to study nearby Earth-like planets; “inflation probe technology development programme” to prepare for a potential cosmic microwave-background mission to study the epoch of inflation; and the Space Infrared telescope for Cosmology and Astrophysics (SPICA).

Ground based

Large-scale (in priority order): LSST; Mid-scale Innovations Programme; GSMT; and AGIS.

Medium-scale: The Cerro Chajnantor Atacama Telescope – a millimetre/submillimetre telescope that will study galaxies, stars, and interstellar gas.

Fractals boost superconductivity

Fractal patterns are ubiquitous in nature from the shape of a galaxy to the structure of a snowflake. They may also lie behind the mysterious phenomenon of high-temperature superconductivity, according to a group of physicists in Europe who have observed the characteristic scale-invariant patterns in the structure of a superconducting copper oxide, using a new kind of X-ray microscopy.

High-temperature superconductivity was discovered in a class of ceramic compounds known as cuprates, which consist of layers of copper oxide sandwiched between other elements, by IBM researchers Georg Bednorz and Alex Müller in 1986. Since the discovery, scientists have identified cuprates that remain superconducting at temperatures as high as 135 K, but the mechanism behind the phenomenon remains a mystery. Unlike conventional superconductors, such as elemental mercury or lead, high-temperature superconductors do not appear to create the pairs of electrons needed for zero-resistance conductivity via vibrations of the crystal lattice.

Could be in the oxygen stripes

Some theoretical physicists have suggested that high-temperature superconductivity may be linked to the distribution of oxygen ions in the layers between the copper oxide. They say it might be the formation of some of these ions into rows, or “stripes”, that is responsible for cuprates’ remarkable conduction properties.

To investigate these claims, a team including Antonio Bianconi at the University of Rome “La Sapienza” exposed a sample of the superconductor lanthanum copper oxide to X-rays generated at the European Synchrotron Radiation Facility in France. By using advanced X-ray optics, the researchers were able to focus the powerful beam down to a spot one-millionth of a metre across and perform X-ray diffraction analysis over this tiny area. Scanning the beam across the sample and obtaining a scattering pattern for each square micron of the sample, Bianconi and colleagues obtained an extremely detailed picture of the superconductor’s structure, with high scattering intensity corresponding to greater structural order.

What they found was that this intensity followed a power-law distribution, in other words that the superconductor was made up of a small number of very high-ordered regions and larger numbers of disordered regions. This, they say, is the hallmark of a scale-free distribution, which is typical of a fractal pattern – with the oxygen stripes forming a similar structure on all scales up to 400 µm.

Fractal scales with temperature

In addition, the researchers found that this fractal distribution increases the temperature up to which the lanthanum copper oxide remains superconducting. They altered the transition temperature of the sample by heat treatment and then recorded its X-ray diffraction image. Carrying out this process at five different transition temperatures, they found that the higher this temperature the more closely the intensity pattern resembled a power law.

To try and explain this correlation, Bianconi suggests that the fractal distribution of oxygen ions makes the ordered and disordered regions of the superconductor very highly interconnected, which maximizes the interference between the wavefunctions of the superconducting condensates of these two regions. This, he says, increases the stability of the quantum coherence that is responsible for superconductivity, rendering the superconducting state robust at higher temperatures.

Nigel Hussey of the University of Bristol describes the research as a “beautiful example of self-organization and complexity in a transition metal oxide” but is not convinced that the current result sheds further light on the cause of high-temperature superconductivity. “Crystalline order does not cause superconductivity, but it can improve it,” he says. “Likewise, disorder can weaken it.”

This research is described in Nature.

Electrons caught moving on the edge

Researchers in Germany and the US have used flashes of laser light to follow the motion of valence electrons as they are excited and ejected from atoms of krypton. They say that their technique could lead to the control of chemical reactions by steering the motion of individual electrons, allowing, for instance, the speeding up of photosynthesis.

Capturing motion at the atomic scale in general requires the use of “pump-probe” techniques, with one pulse of light initiating the movement and a second, delayed, pulse then taking a series of snapshots. This approach has already been used over timescales of just a few femtoseconds (10–15s) to follow the movement of atoms inside molecules or crystals, allowing the observation of the formation and rupture of chemical bonds in real-time. However, tracking the motion of valence electrons, the outermost electrons in an atom and those that affect an atom’s chemical properties, requires taking snapshots over even shorter periods of time – of the order of attoseconds, or 10–18s.

Research with krypton factor

This has now been achieved by a team led by Eleftherios Goulielmakis of the Max Planck Institute of Quantum Optics in Germany, who – adapting a technique previously used to study femtosecond atomic movement – fired femtosecond infrared laser pulses into a chamber containing krypton gas in order to ionize the atoms in the gas. They then probed the evolution of this ionization using ultraviolet pulses, each lasting around 100 attoseconds, which they directed along the same path through the krypton.

Each electron that is removed by the infrared pulses leaves a hole in the outer shell of its atom. This hole can then be filled when one of the ultraviolet pulses excites an electron from closer to the atom’s core into the vacancy. And it is this absorption of the ultraviolet pulse that provides the information about the motion of the valence electrons of the ion.

By progressively extending the delay between the infrared pump and the ultraviolet probe and recording how the absorption of the probe varies as a result, the researchers were able to track the evolution of the phase of the hole. Goulielmakis explained that tracking the motion of electrons around an atom means calculating the evolution of the wavefunction that describes the probability of those electrons being in a particular place at a particular time. This calculation requires knowledge of the energy of the electronic levels involved, their populations and their relative phase. With spectroscopy providing values for the first two quantities, the absorption of the attosecond pulses completes the picture.

Controlling plant growth

According to Goulielmakis, this technique could have a wide range of applications, including the study of systems that lose electronic coherence when they interact with their environment, such as condensed-matter systems. He also believes our understanding of photosynthesis could be improved, because the attosecond snapshots would provide a much higher resolution view of the way in which electrons are removed from water molecules and used to power reactions that turn carbon dioxide into organic compounds such as sugar. Indeed, he says it might be possible one day to steer electron motion with light and in so doing control the speed of photosynthesis. “This is speculation,” he adds, “but I’m sure that it will happen one day.”

Christoph Lienau of the University of Oldenburg in Germany, who was not part of the research team, describes the work as “an important breakthrough in our ability to observe microscopic processes”. He hopes that the technique can be developed for use in systems that are more complex than single atoms, such as molecules or the kind of solid-state nanostructures being proposed for use in solar cells.

This research is described in a paper in Nature.

Terahertz scanning acquires sense of direction

For several years terahertz scanners have been talked of as the great new hope in airport surveillance, but to date a number of technical hurdles have limited their application. In theory these scanners could be used from tens of metres away to detect illegal items, such as guns or contraband concealed within clothing, without exposing a passenger to harmful radiation or revealing their naked body on a TV monitor. Now, a breakthrough by a UK–US collaboration may enable terahertz scanners to finally live up to their potential.

Terahertz radiation lies between the microwave and mid-infrared regions of the electromagnetic spectrum and is emitted by almost all objects. Crucially, terahertz waves can pass through clothing and packaging, but they are strongly absorbed by metals and other inorganic substances.

The most common sources in terahertz applications are semiconductor lasers, but they tend to produce beams that are widely divergent – similar to how light is emitted from a lamp. So one of the challenges with building these scanners is to find a way of directing the terahertz radiation onto a specific target. Now, a team of researchers at Harvard University, led by Federico Capasso, along with colleagues at the University of Leeds in the UK, have developed a cover that can fit on the front of a laser to act as a waveguide.

A special cover

The device is made from metamaterials, which are specially engineered structures that can respond to light and other electromagnetic radiation very differently than conventional materials. While metamaterials have potential use in novel applications such as cloaking, negative refraction and high-resolution imaging, their use in semiconductor devices has been very limited to date.

Propagating plasmons
Propagating plasmons

In Capasso’s device, photons emitted from a quantum cascade laser – a type of semiconductor laser that can emit terahertz radiation – are made to pass through the metamaterial. At the metamaterial surface the photons interact with electrons to create temporary states known as surface plasmons, which subsequently break down with the re-emission of terahertz radiation.

With a careful design, consisting of a series of narrow grooves that can confine plasmons, the researchers were able to create a geometry that could re-emit terahertz radiation, outwards from the laser, in a tight beam. “In our case, the metamaterial serves a dual function: strongly confining the terahertz light emerging from the device to the laser facet and collimating the beam,” explains Nanfang Yu, a member of the Harvard team.

Although this research is still at the laboratory stage, Yu predicts that this type of terahertz scanning could become commercially available within the next five years. His team already has a broad patent on the technique and parts of the apparatus.

One of the challenges is that even the most practical quantum cascade lasers today operate at below 200 K, usually requiring liquid nitrogen as a coolant, which may not be feasible for widespread application.

This research is described in Nature Materials.

Supernova ejects material asymmetrically

A team of astronomers based in Europe has obtained a three-dimensional view of the innermost material released by a supernova – something never before seen. The researchers discovered a turbulent environment where stellar material is being ejected in a highly asymmetric fashion.

The subject of the study is Supernova 1987A (SN 1987A), located in the nearby Large Magellanic Cloud. Due to its proximity to the Milky Way, SN 1987A has caused a flurry of astronomical interest since it first appeared in 1987. It has been the basis for several remarkable observational “firsts”, including the detection of neutrinos released when its core collapsed, direct exploration of the radioactive elements present during the blast, and it revealed insights into how dust is formed during a supernova explosion.

This latest research, led by Karina Kjaer of Queen’s University, Belfast, looks at the geometry of the supernova blast. Kjaer worked with colleagues from the European Southern Observatory and Stockholm University, Sweden, to image the aftermath of the star’s explosion. This was done using the European Southern Observatory’s Very Large Telescope (VLT) fitted with SINFONI (Spectrograph for INtegral Field Observations in the Near Infrared). This equipment allowed the team to obtain detailed analysis of SN 1987A through use of its very high resolving ability and light filtering system.

Moving outwards freely

The researchers used a technique called Integral Field Spectroscopy, which enabled them to examine several parts of the moving gas ring simultaneously and obtain a 3D picture of the inner band of ejected material. “Because we know that time has passed since the explosion, and because the material is moving outwards freely, we can convert this velocity into a distance,” explains Kjaer. “There is currently no other way to obtain such a comprehensive picture of 87a’s inner ejecta.” The procedure was aided by adaptive optics systems, which make tiny adjustments to the shape of the telescope’s mirrors to counteract the distorting effect of atmospheric turbulence.

The new three-dimensional view shows the explosion to have been faster and stronger in certain directions, leading to an irregular shape – not the symmetrical distribution expected. The innermost material was not ejected as a sphere, but rather in two or more general directions. The significant asymmetry shown in the ejection of the material is a strong indication of the turbulent inner environment of the supernova.

“Although our observations are of one single supernova, the result has a huge impact, in that all supernovae are compared against SN 1987A,” says Kjaer. “Our observations have definitively changed the view of how 87a exploded.”

Massive stars have dramatic deaths

Supernovae result when massive stars reach the end of their lifetime and explode, causing an ejection of huge amounts of stellar material. However, the large interstellar distances involved make opportunities to accurately observe supernovae rare; SN1987A was the first supernova visible to the naked eye in 383 years.

“The new observations of SN 1987A may be very helpful in refining explosion models,” says Richard Scalzo, an astrophysicist at Yale University, who was not involved in this research. Scalzo believes that to improve our understanding of supernovae mechanics we will need to combine observations with 3D models.

The research team has been granted further use of the VLT to observe SN 1987A later this year. The astronomers hope to use brand new instruments to improve the resolution of the image, and include multicolour imaging. “The 3D view of SN 1987A was only possible because of a major advance in technology,” explains Kjaer. “We hope that these new observational techniques will help us continue pushing the knowledge of supernovae.”

This research is described in Astronomy & Astrophysics.

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