Mariana Trench Expedition Peers Deep into Earth's Mantle

Getting to the Mariana Trench, the deepest place on Earth, has captured humanity's imagination for decades — even more so in recent weeks, with news that director James Cameron is on the verge of taking only the second trip in history to the mysterious darkness nearly 7 miles (11 kilometers) beneath the ocean surface.

Last month, Doug Wiens, PhD, professor of earth and planetary science at Washington University in St. Louis, and two WUSTL students were cruising the tropical waters of the western Pacific above the Mariana trench aboard the research vessel Thomas G. Thompson. 

"This melt-rich layer is actually quite spotty under the Pacific Ocean basin and surrounding areas, as revealed by my analysis of seismometer data," says Dr. Nicholas Schmerr, a NASA Postdoctoral Program fellow. "Since it only exists in certain places, it can't be the only reason why rigid crustal plates carrying the continents can slide over softer rock below." Schmerr, who is stationed at NASA's Goddard Space Flight Center in Greenbelt, Md., is author of a paper on this research appearing in Science March 23.

The slow slide of Earth's continents results from plate tectonics. Our planet is more than four billion years old, and over this time, the forces of plate tectonics have carried continents many thousands of miles, forging mountain ranges when they collided and valleys that sometimes filled with oceans when they were torn apart. This continental drift could also have changed the climate by redirecting currents in the ocean and atmosphere.

The outermost layer of Earth, the lithosphere, is broken into numerous tectonic plates. The lithosphere consists of the crust and an underlying layer of cool and rigid mantle. Beneath the oceans, the lithosphere is relatively thin (about 65 miles), though beneath continents, it can be as thick as 200 miles. Lying beneath the lithosphere is the asthenosphere, a layer of rock that is slowly deforming and gradually flowing like taffy. Heat in Earth's core produced by the radioactive decay of elements escapes and warms mantle rocks above, making them softer and less viscous, and also causes them to convect. Like the circulating blobs in a lava lamp, rock in the mantle rises where it is warmer than its surroundings, and sinks where it's cooler. This churn moves the continental plates above, similar to the way a raft of froth gets pushed around the surface of a simmering pot of soup.

Although the basic process that drives plate tectonics is understood, many details remain a mystery. "Something has to decouple the crustal plates from the asthenosphere so they can slide over it," says Schmerr. "Numerous theories have been proposed, and one of those was that a melt-rich layer lubricates the boundary between the lithosphere and the asthenosphere, allowing the crustal plates to slide. However, since this layer is only present in certain regions under the Pacific plate, it can't be the only mechanism that allows plate tectonics to happen there. Something else must be letting the plate slide in areas where the melt doesn't exist."

Other possible mechanisms that would make the boundary between the lithosphere and the asthenosphere flow more easily include the addition of volatile material like water to the rock and differences in composition, temperature, or the grain size of minerals in this region. However, current data lacks the resolution to distinguish among them.

Schmerr made the discovery by analyzing the arrival times of earthquake waves at seismometers around the globe. Earthquakes generate various kinds of waves; one type has a back-and-forth motion and is called a shear wave, or S-wave. S-waves traveling through Earth will bounce or reflect off material interfaces inside Earth, arriving at different times depending on where they interact with these interfaces.

One type of S-wave reflects from Earth's surface halfway between an earthquake and a seismometer. An S-wave encountering a deeper melt layer at the lithosphere-asthenosphere boundary at this location will take a slightly shorter path to the seismometer and therefore arrive several tens of seconds earlier. By comparing the arrival times, heights, and shapes of the primary and the melt-layer-reflected waves at various locations, Schmerr could estimate the depth and seismic properties of melt layers under the Pacific Ocean basin.

"Most of the melt layers are where you would expect to find them, like under volcanic regions like Hawaii and various active undersea volcanoes, or around subduction zones -- areas at the edge of a continental plate where the oceanic plate is sinking into the deep interior and producing melt," said Schmerr. "However, the interesting result is that this layer does not exist everywhere, suggesting something other than melt is needed to explain the properties of the asthenosphere."

Understanding how plate tectonics works on Earth could help us figure out how other rocky planets evolved, according to Schmerr. For example, Venus has no oceans, and no evidence of plate tectonics, either. This might be a clue that water is needed for plate tectonics to work. One theory proposes that without water, the asthenosphere of Venus will be more rigid and unable to sustain plates, suggesting internal heat is released in some other way, maybe through periodic eruptions of global volcanism.

Schmerr plans to analyze data from other seismometer networks to see if the same patchy pattern of melt layers exists under other oceans and the continents as well. The research was supported by the NASA Postdoctoral Program and the Carnegie Institution of Washington Department of Terrestrial Magnetism Postdoctoral Fellowship.

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New Technique Lets Scientists Peer Within Nanoparticles, See Atomic Structure in 3-D

UCLA researchers are now able to peer deep within the world's tiniest structures to create three-dimensional images of individual atoms and their positions. Their research, published March 22 in the journal Nature, presents a new method for directly measuring the atomic structure of nanomaterials. 

"This is the first experiment where we can directly see local structures in three dimensions at atomic-scale resolution -- that's never been done before," said Jianwei (John) Miao, a professor of physics and astronomy and a researcher with the California NanoSystems Institute (CNSI) at UCLA.

Miao and his colleagues used a scanning transmission electron microscope to sweep a narrow beam of high-energy electrons over a tiny gold particle only 10 nanometers in diameter (almost 1,000 times smaller than a red blood cell). The nanoparticle contained tens of thousands of individual gold atoms, each about a million times smaller than the width of a human hair. These atoms interact with the electrons passing through the sample, casting shadows that hold information about the nanoparticle's interior structure onto a detector below the microscope.

Miao's team discovered that by taking measurements at 69 different angles, they could combine the data gleaned from each individual shadow into a 3-D reconstruction of the interior of the nanoparticle. Using this method, which is known as electron tomography, Miao's team was able to directly see individual atoms and how they were positioned inside the specific gold nanoparticle.

Presently, X-ray crystallography is the primary method for visualizing 3-D molecular structures at atomic resolutions. However, this method involves measuring many nearly identical samples and averaging the results. X-ray crystallography typically takes an average across trillions of molecules, which causes some information to get lost in the process, Miao said.

"It is like averaging together everyone on Earth to get an idea of what a human being looks like -- you completely miss the unique characteristics of each individual," he said.

X-ray crystallography is a powerful technique for revealing the structure of perfect crystals, which are materials with an unbroken honeycomb of perfectly spaced atoms lined up as neatly as books on a shelf. Yet most structures existing in nature are non-crystalline, with structures far less ordered than their crystalline counterparts -- picture a rock concert mosh pit rather than soldiers on parade.

"Our current technology is mainly based on crystal structures because we have ways to analyze them," Miao said. "But for non-crystalline structures, no direct experiments have seen atomic structures in three dimensions before."

Probing non-crystalline materials is important because even small variations in structure can greatly alter the electronic properties of a material, Miao noted. The ability to closely examine the inside of a semiconductor, for example, might reveal hidden internal flaws that could affect its performance.

"The three-dimensional atomic resolution of non-crystalline structures remains a major unresolved problem in the physical sciences," he said.

Miao and his colleagues haven't quite cracked the non-crystalline conundrum, but they have shown they can image a structure that isn't perfectly crystalline at a resolution of 2.4 angstroms (the average size of a gold atom is 2.8 angstroms). The gold nanoparticle they measured for their paper turned out to be composed of several different crystal grains, each forming a puzzle piece with atoms aligned in subtly different patterns. A nanostructure with hidden crystalline segments and boundaries inside will behave differently from one made of a single continuous crystal -- but other techniques would have been unable to visualize them in three dimensions, Miao said.

Miao's team also found that the small golden blob they studied was in fact shaped like a multi-faceted gem, though slightly squashed on one side from resting on a flat stage inside the gigantic microscope -- another small detail that might have been averaged away when using more traditional methods.

This project was inspired by Miao's earlier research, which involved finding ways to minimize the radiation dose administered to patients during CT scans. During a scan, patients must be X-rayed at a variety of angles, and those measurements are combined to give doctors a picture of what's inside the body. Miao found a mathematically more efficient way to obtain similar high-resolution images while taking scans at fewer angles. He later realized that this discovery could benefit scientists probing the insides of nanostructures, not just doctors on the lookout for tumors or fractures.

Nanostructures, like patients, can be damaged if too many scans are administered. A constant bombardment of high-energy electrons can cause the atoms in nanoparticles to be rearranged and the particle itself to change shape. By bringing his medical discovery to his work in materials science and nanoscience, Miao was able to invent a new way to peer inside the field's tiniest structures.

The discovery made by Miao's team may lead to improvements in resolution and image quality for tomography research across many fields, including the study of biological samples.

This research was conducted at CNSI's Electron Imaging Center for NanoMachines and funded by UC Discovery/Tomosoft Technologies. Tomosoft Technologies is a start-up company based on Miao's work.

Other UCLA co-authors included Chris Regan, an assistant professor of physics and astronomy and a CNSI researcher; graduate students Mary Scott, Chien-Chun Chen, Matthew Mecklenburg and Chun Zhu; and postdoctoral scholar Rui Xu. In particular, Chen and Scott played an important role in this work. Peter Ercius and Ulrich Dahmen from the National Center for Electron Microscopy at Lawrence Berkeley National Laboratory are also co-authors.

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Liquid-Like Thermoelectric Materials for New Generation Devices

In the continual quest for better thermoelectric materials -- which convert heat into electricity and vice versa -- researchers have identified a liquid-like compound whose properties give it the potential to be even more efficient than traditional thermoelectrics. 

Thermoelectric materials have been used to power spacecraft ranging from Apollo to the Curiosity rover now headed for Mars. Recently, however, scientists and engineers have been turning to these materials to use wasted heat -- released from automobiles or industrial machinery, for instance -- as an efficient energy source. They have also proposed using these materials to create more efficient heating systems in electric cars or even as new ways to exploit solar power.

In identifying this new type of thermoelectric material, the researchers studied a material made from copper and selenium. Although it is physically a solid, it exhibits liquid-like behaviors due to the way its copper atoms flow through the selenium's crystal lattice.

"It's like a wet sponge," explains Jeff Snyder, a faculty associate in applied physics and materials science in the Division of Engineering and Applied Science at the California Institute of Technology (Caltech) and a member of the research team. "If you have a sponge with very fine pores in it, it looks and acts like a solid. But inside, the water molecules are diffusing just as fast as they would if they were a regular liquid. That's how I imagine this material works. It has a solid framework of selenium atoms, but the copper atoms are diffusing around as fast as they would in a liquid."

The research, led by scientists from the Chinese Academy of Science's Shanghai Institute of Ceramics in collaboration with researchers from Brookhaven National Laboratory and the University of Michigan, as well as from Caltech, is described in a paper recently published in the journal Nature Materials.

A thermoelectric material generates electricity when there is a temperature difference between one end of the material and the other. For example, if you place a thermoelectric device right next to a heat source -- say a laptop battery -- then the side closest to the battery will be hotter. The electrons in the hot end will diffuse to the cool end, producing an electric current.

A good thermoelectric material must be good at conducting electricity but bad at conducting heat. If it were good at conducting heat, the heat from the hot end would move to the cool end so fast that the whole material would rapidly reach the same temperature. When that happens, the electrons stop flowing.

One way to improve thermoelectric efficiency, then, is to decrease a material's ability to conduct heat. To that end, researchers have been developing thermoelectric materials with a mix of crystalline and amorphous properties, Snyder says. A crystalline atomic structure allows electrons to flow easily, while an amorphous material, such as glass, has a more irregular atomic structure that hinders heat-carrying vibrations from traveling.

These heat-carrying vibrations travel via two types of waves. The first type is a longitudinal or pressure wave, in which the direction of displacement -- in this case, the jiggling of atoms -- is the same as the direction of the wave. The second type is a transverse wave, in which the direction of displacement is perpendicular to the direction of the wave, like when you shake a jump rope up and down, resulting in waves that travel horizontally along the rope.

In a solid material, a transverse wave travels because there is friction between the atoms, meaning that when one atom vibrates up and down, an adjacent atom moves with it, and the wave propagates. But in a liquid, there is minimal friction between the atoms, and a vibrating atom just slides up and down next to its neighbor. As a result, transverse waves cannot travel inside a liquid. Ocean waves are different because they have an interface between the liquid and the air.

The team found that because heat-carrying vibrations in a liquid can travel only via longitudinal waves, a material with liquid-like properties is less thermally conductive. Therefore, a liquid-like material that's also good at conducting electrically should be more thermoelectrically efficient than traditional amorphous materials, Snyder says.

In the case of the copper-selenium material that the researchers studied, the crystal structure of the selenium helps conduct electricity, while the free-flowing copper atoms behave like a liquid, damping down thermal conductivity. The efficiency of a thermoelectric material is quantified using a number called a "thermoelectric figure of merit." The copper-selenium material has a thermoelectric figure of merit of 1.5 at 1000 degrees Kelvin, one of the highest values in any bulk material, the researchers say.

NASA engineers first used this copper-selenium material roughly 40 years ago for spacecraft design, Snyder says. But its liquid-like properties -- which were not understood at the time -- made it difficult to work with. This new research, he says, has identified and explained why this copper-selenium material has such efficient thermoelectric properties, potentially opening up a whole new class of liquid-like thermoelectric materials for investigation.

"Hopefully, the scientific community now has another strategy to work with when looking for materials with a high thermoelectric figure of merit," Snyder says.

In addition to Snyder, the research group includes Caltech graduate student Tristan Day. The other authors on the Nature Materials paper, titled "Copper ion liquid-like thermoelectrics," are Huili Liu, Xun Shi, Lidong Chen, Fangfang Xu, Linlin Zhang, and Wenqing Zhang of the Chinese Academy of Science's Shanghai Institute of Ceramics; Qiang Li of Brookhaven National Laboratory; and Citrad Uher of the University of Michigan.

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Runaway Planets Zoom at a Fraction of Light Speed

Seven years ago, astronomers boggled when they found the first runaway star flying out of our galaxy at a speed of 1.5 million miles per hour. The discovery intrigued theorists, who wondered: If a star can get tossed outward at such an extreme velocity, could the same thing happen to planets? 

New research shows that the answer is yes. Not only do runaway planets exist, but some of them zoom through space at a few percent of the speed of light -- up to 30 million miles per hour.

"These warp-speed planets would be some of the fastest objects in our galaxy. If you lived on one of them, you'd be in for a wild ride from the center of the galaxy to the Universe at large," said astrophysicist Avi Loeb of the Harvard-Smithsonian Center for Astrophysics.

"Other than subatomic particles, I don't know of anything leaving our galaxy as fast as these runaway planets," added lead author Idan Ginsburg of Dartmouth College.

Such speedy worlds, called hypervelocity planets, are produced in the same way as hypervelocity stars. A double-star system wanders too close to the supermassive black hole at the galactic center. Strong gravitational forces rip the stars from each other, sending one away at high speed while the other is captured into orbit around the black hole.

For this study, the researchers simulated what would happen if each star had a planet or two orbiting nearby. They found that the star ejected outward could carry its planets along for the ride. The second star, as it's captured by the black hole, could have its planets torn away and flung into the icy blackness of interstellar space at tremendous speeds.

A typical hypervelocity planet would slingshot outward at 7 to 10 million miles per hour. However, a small fraction of them could gain much higher speeds under ideal conditions.

Current instruments can't detect a lone hypervelocity planet since they are dim, distant, and very rare. However, astronomers could spot a planet orbiting a hypervelocity star by watching for the star to dim slightly when the planet crosses its face in a transit.

For a hypervelocity star to carry a planet with it, that planet would have to be in a tight orbit. Therefore, the chances of seeing a transit would be relatively high, around 50 percent.

"With one-in-two odds of seeing a transit, if a hypervelocity star had a planet, it makes a lot of sense to watch for them," said Ginsburg.

Eventually, such worlds will escape the Milky Way and travel through the intergalactic void.

"Travel agencies advertising journeys on hypervelocity planets might appeal to particularly adventurous individuals," added Loeb.

The research will be published in the Monthly Notices of the Royal Astronomical Society, in a paper authored by Idan Ginsburg, Avi Loeb, and Gary Wegner (Dartmouth College).

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New Light Shed On Wandering Continents

A layer of partially molten rock about 22 to 75 miles underground can't be the only mechanism that allows continents to gradually shift their position over millions of years, according to a NASA-sponsored researcher. The result gives insight into what allows plate tectonics -- the movement of Earth's crustal plates -- to occur. 

"This melt-rich layer is actually quite spotty under the Pacific Ocean basin and surrounding areas, as revealed by my analysis of seismometer data," says Dr. Nicholas Schmerr, a NASA Postdoctoral Program fellow. "Since it only exists in certain places, it can't be the only reason why rigid crustal plates carrying the continents can slide over softer rock below." Schmerr, who is stationed at NASA's Goddard Space Flight Center in Greenbelt, Md., is author of a paper on this research appearing in Science March 23.

The slow slide of Earth's continents results from plate tectonics. Our planet is more than four billion years old, and over this time, the forces of plate tectonics have carried continents many thousands of miles, forging mountain ranges when they collided and valleys that sometimes filled with oceans when they were torn apart. This continental drift could also have changed the climate by redirecting currents in the ocean and atmosphere.

The outermost layer of Earth, the lithosphere, is broken into numerous tectonic plates. The lithosphere consists of the crust and an underlying layer of cool and rigid mantle. Beneath the oceans, the lithosphere is relatively thin (about 65 miles), though beneath continents, it can be as thick as 200 miles. Lying beneath the lithosphere is the asthenosphere, a layer of rock that is slowly deforming and gradually flowing like taffy. Heat in Earth's core produced by the radioactive decay of elements escapes and warms mantle rocks above, making them softer and less viscous, and also causes them to convect. Like the circulating blobs in a lava lamp, rock in the mantle rises where it is warmer than its surroundings, and sinks where it's cooler. This churn moves the continental plates above, similar to the way a raft of froth gets pushed around the surface of a simmering pot of soup.

Although the basic process that drives plate tectonics is understood, many details remain a mystery. "Something has to decouple the crustal plates from the asthenosphere so they can slide over it," says Schmerr. "Numerous theories have been proposed, and one of those was that a melt-rich layer lubricates the boundary between the lithosphere and the asthenosphere, allowing the crustal plates to slide. However, since this layer is only present in certain regions under the Pacific plate, it can't be the only mechanism that allows plate tectonics to happen there. Something else must be letting the plate slide in areas where the melt doesn't exist."

Other possible mechanisms that would make the boundary between the lithosphere and the asthenosphere flow more easily include the addition of volatile material like water to the rock and differences in composition, temperature, or the grain size of minerals in this region. However, current data lacks the resolution to distinguish among them.

Schmerr made the discovery by analyzing the arrival times of earthquake waves at seismometers around the globe. Earthquakes generate various kinds of waves; one type has a back-and-forth motion and is called a shear wave, or S-wave. S-waves traveling through Earth will bounce or reflect off material interfaces inside Earth, arriving at different times depending on where they interact with these interfaces.

One type of S-wave reflects from Earth's surface halfway between an earthquake and a seismometer. An S-wave encountering a deeper melt layer at the lithosphere-asthenosphere boundary at this location will take a slightly shorter path to the seismometer and therefore arrive several tens of seconds earlier. By comparing the arrival times, heights, and shapes of the primary and the melt-layer-reflected waves at various locations, Schmerr could estimate the depth and seismic properties of melt layers under the Pacific Ocean basin.

"Most of the melt layers are where you would expect to find them, like under volcanic regions like Hawaii and various active undersea volcanoes, or around subduction zones -- areas at the edge of a continental plate where the oceanic plate is sinking into the deep interior and producing melt," said Schmerr. "However, the interesting result is that this layer does not exist everywhere, suggesting something other than melt is needed to explain the properties of the asthenosphere."

Understanding how plate tectonics works on Earth could help us figure out how other rocky planets evolved, according to Schmerr. For example, Venus has no oceans, and no evidence of plate tectonics, either. This might be a clue that water is needed for plate tectonics to work. One theory proposes that without water, the asthenosphere of Venus will be more rigid and unable to sustain plates, suggesting internal heat is released in some other way, maybe through periodic eruptions of global volcanism.

Schmerr plans to analyze data from other seismometer networks to see if the same patchy pattern of melt layers exists under other oceans and the continents as well. The research was supported by the NASA Postdoctoral Program and the Carnegie Institution of Washington Department of Terrestrial Magnetism Postdoctoral Fellowship.

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Researchers Send 'Wireless' Message Using Neutrinos

A group of scientists led by researchers from the University of Rochester and North Carolina State University have for the first time sent a message using a beam of neutrinos – nearly massless particles that travel at almost the speed of light. The message was sent through 240 meters of stone and said simply, "Neutrino."

"Using neutrinos, it would be possible to communicate between any two points on Earth without using satellites or cables," said Dan Stancil, professor of electrical and computer engineering at NC State and lead author of a paper describing the research. "Neutrino communication systems would be much more complicated than today's systems, but may have important strategic uses."

Many have theorized about the possible uses of neutrinos in communication because of one particularly valuable property: they can penetrate almost anything they encounter. If this technology could be applied to submarines, for instance, then they could conceivably communicate over long distances through water, which is difficult, if not impossible, with present technology. And if we wanted to communicate with something in outer space that was on the far side of a moon or a planet, our message could travel straight through without impediment.

"Of course, our current technology takes massive amounts of high-tech equipment to communicate a message using neutrinos, so this isn't practical now," said Kevin McFarland, a University of Rochester physics professor who was involved in the experiment. "But the first step toward someday using neutrinos for communication in a practical application is a demonstration using today's technology."

The team of scientists that demonstrated that it was possible performed their test at the Fermi National Accelerator Lab (or Fermilab, for short), outside of Chicago. The group has submitted its findings to the journal Modern Physics Letters A.

At Fermilab the researchers had access to two crucial components. The first is one of the world's most powerful particle accelerators, which creates high-intensity beams of neutrinos by accelerating protons around a 2.5-mile-circumference track and then colliding them with a carbon target. The second is a multi-ton detector called MINERvA, located in a cavern 100 meters underground.

The fact that such a substantial setup is necessary to communicate using neutrinos means that much work will need to be done before the technology can be incorporated into a readily usable form.

The communication test was done during a two-hour period when the accelerator was running at half its full intensity due to an upcoming scheduled downtime. Regular MINERvA interaction data was collected at the same time the communication test was being carried out.

Today, most communication is carried out by sending and receiving electromagnetic waves. That is how our radios, cell phones, and televisions operate. But electromagnetic waves don't pass easily through most types of matter. They get blocked by water and mountains and many other liquids and solids. Neutrinos, on the other hand, regularly pass through entire planets without being disturbed. Because of their neutral electric charge and almost non-existent mass, neutrinos are not subject to magnetic attractions and are not significantly altered by gravity, so they are virtually free of impediments to their motion.

The message that the scientists sent using neutrinos was translated into binary code. In other words, the word "neutrino" was represented by a series of 1's and 0's, with the 1's corresponding to a group of neutrinos being fired and the 0's corresponding to no neutrinos being fired. The neutrinos were fired in large groups because they are so evasive that even with a multi-ton detector, only about one in ten billion neutrinos are detected. After the neutrinos were detected, a computer on the other end translated the binary code back into English, and the word "neutrino" was successfully received.

"Neutrinos have been an amazing tool to help us learn about the workings of both the nucleus and the universe," said Deborah Harris, Minerva project manager, "but neutrino communication has a long way to go before it will be as effective."

Minerva is an international collaboration of nuclear and particle physicists from 21 institutions that study neutrino behavior using a detector located at Fermi National Accelerator Laboratory near Chicago. This is the first neutrino experiment in the world to use a high-intensity beam to study neutrino reactions with nuclei of five different target materials, creating the first side-by-side comparison of interactions. This will help complete the picture of neutrinos and allow data to be more clearly interpreted in current and future experiments.

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New 'PENDULUM' For The Ytterbium Clock

The faster a clock ticks, the more precise it can be. Due to the fact that lightwaves vibrate faster than microwaves, optical clocks can be more precise than the caesium atomic clocks which presently determine time. The Physikalisch-Technische Bundesanstalt (PTB) is even working on several of such optical clocks simultaneously. The model with one single ytterbium ion caught in an ion trap is now experiencing another increase in accuracy. At PTB, scientists have succeeded in exciting a quantum-mechanically strongly "forbidden" transition of this ion and – in particular – in measuring it with extreme accuracy. The optical clock based on it is exact to 17 digits after the decimal point. The results are published in the current edition of the scientific journal Physical Review Letters.

Optical transitions are the modern counterpart of the pendulum of a mechanical clock. In atomic clocks, the "pendulum" is the radiation which excites the transition between two atomic states of different energy. In the case of caesium atomic clocks, it lies in the microwave range, in the case of optical clocks in the range of laser light so that their "pendulum" oscillates with higher velocity and optical clocks are – consequently – regarded as the atomic clocks of the future.

In the experiment performed at PTB, the scientists devoted themselves to a special forbidden transition. In quantum mechanics, "forbidden" means that the jump between the two energy states of the atoms is almost impossible due to the conservation of symmetry and angular momentum. The excited state can then be very persistent: In the case investigated here, the lifetime of the so-called F-state in the ytterbium ion Yb+ amounts to approx. 6 years. Due to this long lifetime, an extremely narrow resonance – whose linewidth only depends on the quality of the laser used – can be observed during the laser excitation of this state. A narrow resonance line is an important prerequisite for an exact optical clock. At the British National Physical Laboratory (NPL), the sister institute of PTB, the laser excitation of this Yb⁺-F state from the ground state was achieved for the first time in 1997. As the transition is, however, strongly forbidden, a relatively high laser intensity is required for its excitation. This disturbs the electron structure of the ion as a whole and leads to a shift of the resonance frequency so that an atomic clock based on it would exhibit a rate depending on the laser intensity.

At PTB it has now been possible to show that alternating excitation of the ion with two different laser intensities allows the unperturbed resonance frequency to be determined with high accuracy. Due to this, it has become possible to investigate other frequency shifts often occurring in atomic clocks– e.g. by electric fields or the thermal radiation of the environment. It has turned out that these are unexpectedly small in the case of the Yb⁺-F state, which can be attributed to the special electronic structure of the state. This is a decisive advantage for the further development of this atomic clock. In the experiments at PTB, the relative uncertainty of the Yb⁺ frequency was determined with 7 · 10^-17. This corresponds to an uncertainty of the atomic clock of only approx. 30 seconds over the age of the universe.

Both groups at NPL and PTB have measured the frequency of the Yb+ transition with their caesium clocks and the results agree within the scope of the uncertainties (1 · 10^-15 and 8 · 10^-16) which are mainly determined by the caesium clocks. In a research project recently approved within the scope of the European Metrology Research Programme, the two institutes will in future cooperate with other European partners even more intensively in the development of this optical clock. In the case of the Yb⁺ ion, it is of particular interest that it has two transitions which are suitable for optical clocks: Less strongly forbidden, but also very precise, the excitation of the D-level can be used at a wavelength of 436 nm. This opens up the possibility of investigating the accuracy of the optical clock by frequency comparisons of the two transitions in one ion, without having to refer to a caesium clock.

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Ultrafast Sonograms Shed New Light On Rapid Phase Transitions

An international team of physicists has developed a method for taking ultrafast 'sonograms' that can track the structural changes that take place within solid materials in trillionth-of-a-second intervals as they go through an important physical process called a phase transition. 

Common phase transitions include the melting of candle wax before it burns and dissolving sugar in water. They are purely structural changes that produce dramatic changes in a material's physical properties and they play a critical role both in nature and in industrial processes ranging from steel making to chip fabrication.

The researchers have applied this method to shed new light on the manner in which vanadium dioxide, the material that undergoes the fastest phase transition known, shifts between its transparent and reflective phases.

Many of these transitions, like that in vanadium dioxide, take place so rapidly that scientists have had difficulty catching them in the act. "This means that there is a lot that we still don't know about the dynamics of these critical processes," said Professor of Physics Richard Haglund, who directed the team of Vanderbilt researchers who were involved.

To build a more complete picture of this phenomenon in vanadium dioxide (VO2), one of the most unusual phase-change materials known, Vanderbilt researchers collaborated with physicists at the Fritz Haber Institute of the Max Planck Society in Berlin, who have developed the powerful new technique for obtaining a more complete picture of ultrafast phase changes. Details of the method, which can track the structural changes that take place within materials at intervals of less than a trillionth of a second, are reported in the Mar. 6 issue of the journal Nature Communications.

Vanadium dioxide shifts from a transparent, semiconducting phase to a reflective, metallic phase in the time it takes a beam of light to travel a tenth of a millimeter. This phase change can be caused by heating the material above 150 degrees Fahrenheit (65 degrees Celsius) or by hitting it with a pulse of laser light.

VO2 is one of a class of materials now being considered for use in faster computer memory. When mixed with suitable additives, it makes a window coating that blocks infrared transmission on hot days and reduces heat loss during cool periods. In addition, it has potential applications in optical shutters, sensors and cameras.

"With this new technique, we were able to see a lot of details that we've never seen before," said Haglund. These details include how the electrons in the material rearrange first and then are followed by the movement of the much more massive atoms as the material shifts from its semiconductor to metallic-phase orientation. These details provide new information that can be used to design high-speed optical switches using this unique material.

The new method is a variation on a standard method known as 'pump-and-probe.' It uses an infrared laser that can produce powerful pulses of light that only last for femtoseconds (millionths of a trillionth of a second). When these pump pulses strike the surface of the target material, they generate high-frequency atomic vibrations determined by the material's composition and phase. These vibrations change during a phase transition so they can be used to identify and track the transition in time.

At the same time, the physicists split off a small fraction of the infrared beam (the probe), convert it into white light and use it to illuminate the surface of the target. It turns out that these lattice vibrations produce changes in the material's surface reflectivity. As a result, the physicists can track what is happening inside the material by mapping the changes taking place on its surface.

The situation is analogous to hitting a gong with thousands of tiny microscopic hammers. The sound each hammer makes depends on the composition and arrangement of the atoms in the part of the gong where it hits. If the composition and arrangement of the atoms changes in one of these areas, then the sound the hammer makes also changes.

"The real power of this technique is that it is sensitive to atomic changes inside the material which are usually observed using expensive large-scale X-ray sources. Now we can do the experiment optically and in the lab on a tabletop," said Simon Wall, an Alexander von Humbolt fellow at the Fritz Haber Institute.

Vanderbilt graduate students Kannatassen Appavoo and Joyeeta Nag fabricated and characterized the vanadium dioxide thin films; Simon Wall, Daniel Wegkamp, Laura Foglia, Julia Stähler and Martin Wolf at the Fritz Haber Institute directed the laser experiments and subsequent data analysis.

Provided by Vanderbilt University (web)

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Diamonds Shatter Quantum Information Storage Record

Researchers show how to store quantum bits at room temperature using a less complex process for seconds at a time

BOSTON—The quantum world and the everyday world of human experience are supposed to be two different realms. Quantum effects, as demonstrated in the lab, are usually confined to the tiniest scales. They last for imperceptibly brief instants. And they appear mostly in highly controlled systems operating at cryogenic temperatures near absolute zero.

But experimental physicists are pushing across the assumed divide between the quantum and the ordinary by demonstrating quantum effects in more familiar environments. Now a group of researchers has furthered that cause by encoding quantum information into a room-temperature solid for time spans that can be ticked off on a stopwatch. The new quantum memory scheme can store information for more than a second, which extends by orders of magnitude the lifetime of information encoded as a quantum bit, or qubit, on a particle at ordinary temperatures. The American, German and British researchers have only just submitted the research to a peer-reviewed journal, but here in late February they presented their findings to a meeting of the American Physical Society.

A qubit, much like an ordinary bit in commonplace electronic devices, has a 0 state and a 1 state. But unlike a classical bit, a qubit can be in a so-called superposition of 0 and 1. That property, along with other phenomena such as quantum entanglement, means that quantum computers based on qubits would be phenomenally powerful—that is, if a practical machine could ever be built.

But that power comes at a price. A qubit can easily be corrupted by outside influences such as heat and magnetic fields. Physicists have produced long-lived qubits by all but eliminating such noise, confining individual atoms to vacuum traps or cooling them nearly to absolute zero. But some research groups have been trying to design qubits that can operate in solid-state systems at room temperature—to make a qubit, in short, that can survive in the world of the bit.

In the latest advance on that front, the research groups of Mikhail Lukin of Harvard University and Ignacio Cirac of the Max Planck Institute of Quantum Optics in Garching, Germany, and their colleagues encoded long-lived quantum information in the spin of a single-atom impurity in a synthetically produced diamond. Spin is a quantum property akin to the pointing of a particle's internal bar magnet, either up or down, representing 1 or 0.

The experimental quantum-grade diamond is 99.99 percent pure carbon 12, the most common isotope of the element. But the crystal also contains a small amount of the heavier isotope carbon 13 as well as implanted nitrogen ions that form defects in the diamond lattice known as nitrogen vacancy centers. Both impurities have certain quantum benefits.

Each, for example, features an intrinsic spin with a special talent. The nitrogen ion has an associated electron whose spin state is readily detectable by shining laser light on the nitrogen vacancy center. The carbon 13's nuclear spin state remains stable for long intervals.

The researchers figured out a way to combine these two attributes. Their approach uses the carbon 13 to store information for long periods of time and the nitrogen ion as a readout.

The scientists located an area in the diamond where a carbon 13 and a nitrogen ion are only about two nanometers apart. At that distance the spin of the nitrogen ion's electron and that of the nearby carbon nucleus couple together—the electron acts as a tiny magnetometer that reflects the carbon 13's nuclear spin state. By hitting the nitrogen vacancy center with laser light, the researchers can measure the electron's spin and, by extension, the spin of the carbon 13 nucleus.

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3D Displays Made From Flexible Fiber

Could new light-carrying fiber technology be the future of 3D display panels? A team of researchers associated with MIT’s Research Laboratory of Electronics (RLE) believe so.

A paper published recently on the Nature Photonics  website suggests that a new material, woven from specialist flexible fibers, could be used within future generations of 3D displays.

Each individual fiber is no thicker than a human hair, at 400 micrometers across. Whereas most light-emitting technology maintains a standard light intensity from any angle (for example, light bulbs), these fibers can vary light intensity and exposure in different directions.

The ore of the fiber is hollow. Cushioning the core, multiple layers of material with different optical properties assume the role of a mirror. The inside can then act as a conduit where drops of fluid can be inserted and activated. Once ‘activated’ by laser technology, the fluid emits light. This then bounces back and forth between the mirror-acting layers, creating a 360 degree laser beam emitted from the fiber’s core.

Surrounding the light-emitting core are four channels filled with liquid crystals, which vary the brightness of the generated light. Each individual channel is controlled by two electrodes running parallel to it.

As an advance in display technology, the researchers acknowledge the current drawback that each fiber can only produce one image pixel. However,they are investigating the idea that one pixel — one drop of fluid — could be moved back and forth to trick a viewer in to seeing a line rather than a single, colored point. Once the fibers are woven together, then this could be the first step in constructing a full display panel.

As light can be projected in to different areas without restriction, sensory output and information can be varied depending on a viewer’s eyes. Not only this, but the fiber could potentially be implemented within medical devices.

As an example, the fiber could be threaded into narrow areas to eradicate disease tissues while leaving healthy areas untouched. In the emerging field of photodynamic therapy, in which a photosensitizer, a light source and tissue oxygen are used to target and treat targeted locations. Photodynamic therapy is currently used to help treat acne, cancer, and some forms of degenerative disease.

“The coolest thing about this work, really, is the way it’s made,” says Marko Loncar , an associate professor at Harvard University. “The technology that they used to do it, basically, they can make kilometers of these things. It’s remarkable.”

Loncar adds, “And they envision this being used for surgeries and things like that, where it would be really hard to use any other laser approach.”

Seven researchers from MIT including including Yoel Fink, a professor of materials science and electrical engineering and the director of RLE John Joannopoulo, created the material with funding from the U.S. Army and the National Science Foundation.

[Via: MIT ]

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Metamaterials may advance with new femtosecond laser technique

Researchers in applied physics have cleared an important hurdle in the development of advanced materials, called metamaterials, that bend light in unusual ways. Working at a scale applicable to infrared light, the Harvard team has used extremely short and powerful laser pulses to create three-dimensional patterns of tiny silver dots within a material. Those suspended metal dots are essential for building futuristic devices like invisibility cloaks.

The new fabrication process, described in the journal Applied Physics Letters, advances nanoscale metal lithography into three dimensions -- and does it at a resolution high enough to be practical for metamaterials.

"If you want a bulk metamaterial for visible and infrared light, you need to embed particles of silver or gold inside a dielectric, and you need to do it in 3D, with high resolution," says lead author Kevin Vora, a graduate student at the Harvard School of Engineering and Applied Sciences (SEAS).

"This work demonstrates that we can create silver dots that are disconnected in x, y, and z," Vora says. "There's no other technique that feasibly allows you to do that. Being able to make patterns of nanostructures in 3D is a very big step towards the goal of making bulk metamaterials."

Vora works in the laboratory of Eric Mazur, Balkanski Professor of Physics and Applied Physics at SEAS. For decades, Mazur has been using a piece of equipment called a femtosecond laser to investigate how very tightly focused, powerful bursts of light can change the electrical, optical, and physical properties of a material.

When a conventional laser shines on a transparent material, the light passes straight through, with slight refraction. The femtosecond laser is special because it emits a burst of photons as bright as the surface of the sun in a flash lasting only 50 quadrillionths (5 × 10-14) of a second. Instead of shining through the material, that energy gets trapped within it, exciting the electrons within the material and achieving a phenomenon known as nonlinear absorption.

Inside the pocket where that energy is trapped, a chemical reaction can take place, permanently altering the internal structure of the material. The process has previously been exploited for 2D and simple 3D metal nanofabrication.

"Normally, when people use femtosecond lasers in fabrication, they're creating a wood pile structure: something stacked on something else, being supported by something else," explains Mazur.

"If you want to make an array of silver dots, however, they can't float in space."

In the new process, Vora, Mazur, and their colleagues combine silver nitrate, water, and a polymer called PVP into a solution, which they bake onto a glass slide. The solid polymer then contains ions of silver, which are photoreduced by the tightly focused laser pulses to form nanocrystals of silver metal, supported by the polymer matrix.

The need for this particular combination of chemicals, at the right concentrations, was not obvious in prior work. Researchers sometimes combine silver nitrate with water in order to create silver nanostructures, but that process provides no structural support for a 3D pattern. Another process combines silver nitrate, water, PVP, and ethanol, but the samples darken and degrade very quickly by producing silver crystals throughout the polymer.

With ethanol, the reaction happens too quickly and uncontrollably. Mazur's team needed nanoscale crystals, precisely distributed and isolated in 3D.

"It was just a question of removing that reagent, and we got lucky," Vora says. "What was most surprising about it was how simple it is. It was a matter of using less."

SeungYeon Kang, a graduate student at SEAS, and Shobha Shukla, a former postdoctoral fellow, coauthored the paper. The work was supported by the Air Force Office of Scientific Research.

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