Astronomers Solve Mystery of Vanishing Electrons in Earth's Outer Radiation Belt

UCLA researchers have explained the puzzling disappearing act of energetic electrons in Earth's outer radiation belt, using data collected from a fleet of orbiting spacecraft. (Credit: NASA Goddard Space Flight Center / Image by Reto Stöckli /Enhancements by Robert Simmon)

UCLA researchers have explained the puzzling disappearing act of energetic electrons in Earth's outer radiation belt, using data collected from a fleet of orbiting spacecraft.

In a paper published Jan. 29 in the advance online edition of the journalNature Physics, the team shows that the missing electrons are swept away from the planet by a tide of solar wind particles during periods of heightened solar activity.

"This is an important milestone in understanding Earth's space environment," said lead study author Drew Turner, an assistant researcher in the UCLA Department of Earth and Space Sciences and a member of UCLA's Institute for Geophysics and Planetary Physics (IGPP). "We are one step closer towards understanding and predicting space weather phenomena."

During powerful solar events such as coronal mass ejections, parts of the magnetized outer layers of sun's atmosphere crash onto Earth's magnetic field, triggering geomagnetic storms capable of damaging the electronics of orbiting spacecraft. These cosmic squalls have a peculiar effect on Earth's outer radiation belt, a doughnut-shaped region of space filled with electrons so energetic that they move at nearly the speed of light.

"During the onset of a geomagnetic storm, nearly all the electrons trapped within the radiation belt vanish, only to come back with a vengeance a few hours later," said Vassilis Angelopoulos, a UCLA professor of Earth and space sciences and IGPP researcher.

The missing electrons surprised scientists when the trend was first measured in the 1960s by instruments onboard the earliest spacecraft sent into orbit, said study co-author Yuri Shprits, a research geophysicist with the IGPP and the departments of Earth and space sciences, and atmospheric and oceanic sciences.

"It's a puzzling effect," he said. "Oceans on Earth do not suddenly lose most of their water, yet radiation belts filled with electrons can be rapidly depopulated."

Even stranger, the electrons go missing during the peak of a geomagnetic storm, a time when one might expect the radiation belt to be filled with energetic particles because of the extreme bombardment by the solar wind.

Where do the electrons go? This question has remained unresolved since the early 1960s. Some believed the electrons were lost to Earth's atmosphere, while others hypothesized that the electrons were not permanently lost at all but merely temporarily drained of energy so that they appeared absent.

"Our study in 2006 suggested that electrons may be, in fact, lost to the interplanetary medium and decelerated by moving outwards," Shprits said. "However, until recently, there was no definitive proof for this theory."

To resolve the mystery, Turner and his team used data from three networks of orbiting spacecraft positioned at different distances from Earth to catch the escaping electrons in the act. The data show that while a small amount of the missing energetic electrons did fall into the atmosphere, the vast majority were pushed away from the planet, stripped away from the radiation belt by the onslaught of solar wind particles during the heightened solar activity that generated the magnetic storm itself.

A greater understanding of Earth's radiation belts is vital for protecting the satellites we rely on for global positioning, communications and weather monitoring, Turner said. Earth's outer radiation belt is a harsh radiation environment for spacecraft and astronauts; the high-energy electrons can penetrate a spacecraft's shielding and wreak havoc on its delicate electronics. Geomagnetic storms triggered when the oncoming particles smash into Earth's magnetosphere can cause partial or total spacecraft failure.

"While most satellites are designed with some level of radiation protection in mind, spacecraft engineers must rely on approximations and statistics because they lack the data needed to model and predict the behavior of high-energy electrons in the outer radiation belt," Turner said.

During the 2003 "Halloween Storm," more than 30 satellites reported malfunctions, and one was a total loss, said Angelopoulos, a co-author of the current research. As the solar maximum approaches in 2013, marking the sun's peak activity over a roughly 11-year cycle, geomagnetic storms may occur as often as several times per month.

"High-energy electrons can cut down the lifetime of a spacecraft significantly," Turner said. "Satellites that spend a prolonged period within the active radiation belt might stop functioning years early."

While a mechanized spacecraft might include multiple redundant circuits to reduce the risk of total failure during a solar event, human explorers in orbit do not have the same luxury. High-energy electrons can punch through astronauts' spacesuits and pose serious health risks, Turner said.

"As a society, we've become incredibly dependent on space-based technology," he said. "Understanding this population of energetic electrons and their extreme variations will help create more accurate models to predict the effect of geomagnetic storms on the radiation belts."

Key observational data used in this study was collected by a network of NASA spacecraft known as THEMIS (Time History of Events and Macroscale Interactions during Substorms); Angelopoulos is the principal investigator of the THEMIS mission. Additional information was obtained from two groups of weather satellites called POES (Polar Operational Environmental Satellite) and GOES (Geostationary Operational Environmental Satellite).

A new collaboration between UCLA and Russia's Moscow State University promises to paint an even clearer picture of these vanishing electrons. Slated for launch in the spring of 2012, the Lomonosov spacecraft will fly in low Earth orbit to measure highly energetic particles with unprecedented accuracy, said Shprits, the principal investigator of the project. Several key instruments for the mission are being developed and assembled at UCLA.

Earth's radiation belts were discovered in 1958 by Explorer I, the first U.S. satellite that traveled to space.

"What we are studying was the first discovery of the space age," Shprits said. "People realized that launches of spacecraft didn't only make the news, they could also make scientific discoveries that were completely unexpected."

This project received federal funding from NASA and the National Science Foundation. Other co-authors include Michael Hartinger, a UCLA graduate student in Earth and space sciences.

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The above story is reprinted from materials provided by University of California - Los Angeles. The original article was written by Kim DeRose.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

---

Journal Reference:

1. Drew L. Turner, Yuri Shprits, Michael Hartinger, Vassilis Angelopoulos. Explaining sudden losses of outer radiation belt electrons during geomagnetic storms.Nature Physics, 2012; DOI: 10.1038/nphys2185

Secrets of tunneling through energy barriers: How massless electrons tunnel through energy barriers in a carbon sheet called graphene

ScienceDaily (Nov. 7, 2011) — Electrons moving in graphene behave in an unusual way, as demonstrated by 2010 Nobel Prize laureates for physics Andre Geim and Konstantin Novoselov, who performed transport experiments on this one-carbon-atom-thick material. A review article, just published in The European Physical Journal B, explores the theoretical and experimental results to date of electrons tunneling through energy barriers in graphene.

As good an electrical conductor at room temperature as copper graphene is, it also outperforms all other known materials as a heat conductor. It is both very dense due to its honeycomb lattice structure and almost completely transparent, making it suitable, among other applications, for touch screens and light panels.

What could partly explain graphene's properties is that electrons travelling inside the material behave as if they were massless. Their behavior is described by the so-called massless Dirac equation that is normally used for high-energy particles such as neutrinos nearing the speed of light. However, electrons in graphene move at a constant speed 300 times smaller than that of light.

In this review, P.E. Allain and J.N. Fuchs, both from the Université Paris-Sud, focus on the tunneling effect occurring when Dirac electrons found in graphene are transmitted through different types of energy barriers. Contrary to the laws of classical mechanics, which govern larger scale particles that cannot cross energy barriers, electron tunneling is possible in quantum mechanics -- though only under restricted conditions, depending on the width and energy height of the barrier.

However, the Dirac electrons found in graphene can tunnel through energy barriers regardless of their width and energy height; a phenomenon called Klein tunneling, described theoretically for 3D massive Dirac electrons by the Swedish physicist Oskar Klein in 1929. Graphene was the first material in which Klein tunneling was observed experimentally, as massive Dirac electrons required energy barriers too large to be observed.

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P. E. Allain, J. N. Fuchs. Klein tunneling in graphene: optics with massless electrons. The European Physical Journal B, 2011; 83 (3): 301 DOI: 10.1140/epjb/e2011-20351-3

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Secrets of tunneling through energy barriers: How massless electrons tunnel through energy barriers in a carbon sheet called graphene

ScienceDaily (Nov. 7, 2011) — Electrons moving in graphene behave in an unusual way, as demonstrated by 2010 Nobel Prize laureates for physics Andre Geim and Konstantin Novoselov, who performed transport experiments on this one-carbon-atom-thick material. A review article, just published in The European Physical Journal B, explores the theoretical and experimental results to date of electrons tunneling through energy barriers in graphene.

As good an electrical conductor at room temperature as copper graphene is, it also outperforms all other known materials as a heat conductor. It is both very dense due to its honeycomb lattice structure and almost completely transparent, making it suitable, among other applications, for touch screens and light panels.

What could partly explain graphene's properties is that electrons travelling inside the material behave as if they were massless. Their behavior is described by the so-called massless Dirac equation that is normally used for high-energy particles such as neutrinos nearing the speed of light. However, electrons in graphene move at a constant speed 300 times smaller than that of light.

In this review, P.E. Allain and J.N. Fuchs, both from the Université Paris-Sud, focus on the tunneling effect occurring when Dirac electrons found in graphene are transmitted through different types of energy barriers. Contrary to the laws of classical mechanics, which govern larger scale particles that cannot cross energy barriers, electron tunneling is possible in quantum mechanics -- though only under restricted conditions, depending on the width and energy height of the barrier.

However, the Dirac electrons found in graphene can tunnel through energy barriers regardless of their width and energy height; a phenomenon called Klein tunneling, described theoretically for 3D massive Dirac electrons by the Swedish physicist Oskar Klein in 1929. Graphene was the first material in which Klein tunneling was observed experimentally, as massive Dirac electrons required energy barriers too large to be observed.

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Journal Reference:

P. E. Allain, J. N. Fuchs. Klein tunneling in graphene: optics with massless electrons. The European Physical Journal B, 2011; 83 (3): 301 DOI: 10.1140/epjb/e2011-20351-3

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Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

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Watching motion of electrons in molecules during chemical reactions

ScienceDaily (Oct. 14, 2011) — A research group led by ETH Zurich has now, for the first time, visualized the motion of electrons during a chemical reaction. The new findings in the experiment are of fundamental importance for photochemistry and could also assist the design of more efficient solar cells.

In 1999, Ahmed Zewail was awarded the nobel prize in chemistry for his studies of chemical reactions using ultrashort laser pulses. Zewail was able to watch the motion of atoms and thus visualize transition states on the molecular level. Watching the dynamics of single electrons was still considered a dream at that time. Thanks to the latest developments in laser technology and intense research in the field of attosecond spectroscopy (1 attosecond = 10-18 s) the research has developed fast. For the first time, Prof. Hans Jakob Wörner from the Laboratory of Physical Chemistry at ETH Zurich, together with colleagues from Canada and France, was able to record electronic motion during a complete chemical reaction. The experiment is described in the latest issue of Science.

The research team irradiated nitrogen dioxide molecules (NO2) with a very short ultraviolet pulse. Subsequently, the molecule takes up the energy from the pulse which sets the electrons in motion. The electrons start rearranging themselves, which causes the electron cloud to oscillate between two different shapes for a very short time, before the molecule starts to vibrate and eventually decomposes into nitric oxide and an oxygen atom.

Conical intersections

Nitrogen dioxide has model character with respect to understanding electronic motion. In the NO2 molecule, two states of the electrons can have the same energy for a particular geometry -- commonly described as conical intersection. The conical intersection is very important for photochemistry and frequently occurs in natural chemical processes induced by light. The conical intersection works like a dip-switch. For example, if the retina of a human eye is irradiated by light, the electrons start moving, and the molecules of the retina (retinal) change their shape, which finally converts the information of light to electrical information for the human brain. The special aspect about conical intersections is that the motion of electrons is transferred to a motion of the atoms very efficiently.

Snapshot of an electron

In an earlier article, Hans Jakob Wörner has already published how attosecond spectroscopy can be used for watching the motion of electrons. The first weak ultraviolet pulse sets the electrons in motion. The second strong infrared pulse then removes an electron from the molecule, accelerates it and drives it back to the molecule. As a result, an attosecond light pulse is emitted, which carries a snapshot of the electron distribution in the molecule. Wörner illustrates the principle of attosecond spectroscopy: "The experiment can be compared to photographs, which, for example, image a bullet shot through an apple. The bullet would be too fast for the shutter of a camera, resulting in a blurred image. Therefore, the shutter is left open and the picture is illuminated with light flashes, which are faster than the bullet. That's how we get our snap-shot."

From the experiment to solar cells

When the electron returns to the molecule, it releases energy in the form of light. In the experiment, Wörner and his colleagues measured the light of the electrons and were therefore able to deduce detailed information on the electron distribution and its evolution with time. This information reveals details of chemical reaction mechanisms that were not accessible to most of previous experimental techniques. The experiment on NO2 helps understanding fundamental processes in molecules and is an ideal extension of computer simulations of photochemical processes: "What makes our experiment so important is that it verifies theoretical models," says Wörner. The immense interest in photochemical processes is not surprising, as this area of research aims at improving solar cells and making artificial photosynthesis possible.

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by ETH Zürich.

Journal Reference:

H. J. Worner, J. B. Bertrand, B. Fabre, J. Higuet, H. Ruf, A. Dubrouil, S. Patchkovskii, M. Spanner, Y. Mairesse, V. Blanchet, E. Mevel, E. Constant, P. B. Corkum, D. M. Villeneuve. Conical Intersection Dynamics in NO2 Probed by Homodyne High-Harmonic Spectroscopy. Science, 2011; 334 (6053): 208 DOI: 10.1126/science.1208664

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

View the original article here

Watching motion of electrons in molecules during chemical reactions

ScienceDaily (Oct. 14, 2011) — A research group led by ETH Zurich has now, for the first time, visualized the motion of electrons during a chemical reaction. The new findings in the experiment are of fundamental importance for photochemistry and could also assist the design of more efficient solar cells.

In 1999, Ahmed Zewail was awarded the nobel prize in chemistry for his studies of chemical reactions using ultrashort laser pulses. Zewail was able to watch the motion of atoms and thus visualize transition states on the molecular level. Watching the dynamics of single electrons was still considered a dream at that time. Thanks to the latest developments in laser technology and intense research in the field of attosecond spectroscopy (1 attosecond = 10-18 s) the research has developed fast. For the first time, Prof. Hans Jakob Wörner from the Laboratory of Physical Chemistry at ETH Zurich, together with colleagues from Canada and France, was able to record electronic motion during a complete chemical reaction. The experiment is described in the latest issue of Science.

The research team irradiated nitrogen dioxide molecules (NO2) with a very short ultraviolet pulse. Subsequently, the molecule takes up the energy from the pulse which sets the electrons in motion. The electrons start rearranging themselves, which causes the electron cloud to oscillate between two different shapes for a very short time, before the molecule starts to vibrate and eventually decomposes into nitric oxide and an oxygen atom.

Conical intersections

Nitrogen dioxide has model character with respect to understanding electronic motion. In the NO2 molecule, two states of the electrons can have the same energy for a particular geometry -- commonly described as conical intersection. The conical intersection is very important for photochemistry and frequently occurs in natural chemical processes induced by light. The conical intersection works like a dip-switch. For example, if the retina of a human eye is irradiated by light, the electrons start moving, and the molecules of the retina (retinal) change their shape, which finally converts the information of light to electrical information for the human brain. The special aspect about conical intersections is that the motion of electrons is transferred to a motion of the atoms very efficiently.

Snapshot of an electron

In an earlier article, Hans Jakob Wörner has already published how attosecond spectroscopy can be used for watching the motion of electrons. The first weak ultraviolet pulse sets the electrons in motion. The second strong infrared pulse then removes an electron from the molecule, accelerates it and drives it back to the molecule. As a result, an attosecond light pulse is emitted, which carries a snapshot of the electron distribution in the molecule. Wörner illustrates the principle of attosecond spectroscopy: "The experiment can be compared to photographs, which, for example, image a bullet shot through an apple. The bullet would be too fast for the shutter of a camera, resulting in a blurred image. Therefore, the shutter is left open and the picture is illuminated with light flashes, which are faster than the bullet. That's how we get our snap-shot."

From the experiment to solar cells

When the electron returns to the molecule, it releases energy in the form of light. In the experiment, Wörner and his colleagues measured the light of the electrons and were therefore able to deduce detailed information on the electron distribution and its evolution with time. This information reveals details of chemical reaction mechanisms that were not accessible to most of previous experimental techniques. The experiment on NO2 helps understanding fundamental processes in molecules and is an ideal extension of computer simulations of photochemical processes: "What makes our experiment so important is that it verifies theoretical models," says Wörner. The immense interest in photochemical processes is not surprising, as this area of research aims at improving solar cells and making artificial photosynthesis possible.

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Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by ETH Zürich.

Journal Reference:

H. J. Worner, J. B. Bertrand, B. Fabre, J. Higuet, H. Ruf, A. Dubrouil, S. Patchkovskii, M. Spanner, Y. Mairesse, V. Blanchet, E. Mevel, E. Constant, P. B. Corkum, D. M. Villeneuve. Conical Intersection Dynamics in NO2 Probed by Homodyne High-Harmonic Spectroscopy. Science, 2011; 334 (6053): 208 DOI: 10.1126/science.1208664

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

View the original article here

Scientists play ping-pong with single electrons

ScienceDaily (Sep. 22, 2011) — Scientists at Cambridge University have shown an amazing degree of control over the most fundamental aspect of an electronic circuit, how electrons move from one place to another.

Researchers from the University's Cavendish Laboratory have moved an individual electron along a wire, batting it back and forth over sixty times, rather like the ball in a game of ping-pong. The research findings, published September 22 in the journal Nature, may have applications in quantum computing, transferring a quantum 'bit' between processor and memory, for example.

Imagine you are at a party and you want to get to the other side of a crowded room to talk to someone. As you walk you have to weave around people who are walking, dancing or just standing in the way. You may also have to stop and greet friends along the way and by the time you reach the person you wanted to talk to you have forgotten what you were going to say. Wouldn't it be nice to be lifted up above the crowd, and pushed directly to your destination?

In a similar way, electrons carrying a current along a wire do not go directly from one end to the other but instead follow a complicated zigzag path. This is a problem if the electron is carrying information, as it tends to 'forget' it, or, more scientifically, the quantum state loses coherence.

In this work, a single electron can be trapped in a small well (called a quantum dot), just inside the surface of a piece of Gallium Arsenide (GaAs). A channel leads to another, empty, dot 4 microns (millionths of a metre) away. The channel is higher in energy than the surrounding electrons. A very short burst of sound (just a few billionths of a second long) is then sent along the surface, past the dot. The accompanying wave of electrical potential picks up the electron, which then surfs along the channel to the other dot, where it is captured. A burst of sound sent from the other direction returns the electron to the starting dot where the process can be repeated. The electron goes back and forth like a ping-pong ball. Rallies of up to 60 shots have been achieved before anything goes wrong.

"The movement of electrons by our 'surface acoustic wave' can also be likened to peristalsis in the esophagus, where food is propelled from the mouth to the stomach by a wave of muscle contraction," explains Rob McNeil, the PhD student who did most of the work, helped by postdoc Masaya Kataoka, both at the University of Cambridge's Department of Physics, the Cavendish Laboratory.

"This is an enabling technology for quantum computers," Chris Ford, team leader of the research from the Semiconductor Physics Group in the Cavendish, says. "There is a lot of work going on worldwide to make this new type of computer, which may solve certain complex problems much faster than classical computers. However, little effort has yet been put into connecting up different components, such as processor and memory. Although our experiments do not yet show that electrons 'remember' their quantum state, this is likely to be the case. This would make the method of transfer a candidate for moving quantum bits of information (qubits) around a quantum circuit, in a quantum computer. Indeed, our theorist, Crispin Barnes, proposed using this mechanism to make a whole quantum computer a long time ago, and this is an important step towards that goal."

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by University of Cambridge. The original story is licensed under a Creative Commons license.

Journal Reference:

R. P. G. McNeil, M. Kataoka, C. J. B. Ford, C. H. W. Barnes, D. Anderson, G. A. C. Jones, I. Farrer, D. A. Ritchie. On-demand single-electron transfer between distant quantum dots. Nature, 2011; 477 (7365): 439 DOI: 10.1038/nature10444

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Like fish on waves, electrons go surfing

ScienceDaily (Sep. 28, 2011) — Physicists at the RUB, working in collaboration with researchers from Grenoble and Tokyo, have succeeded in taking a decisive step towards the development of more powerful computers. They were able to define two little quantum dots (QDs), occupied with electrons, in a semiconductor and to select a single electron from one of them using a sound wave, and then to transport it to the neighbouring QD.

A single electron "surfs" thus from one quantum dot to the next like a fish on a wave. Such manipulation of a single electron will in the future also enable the combination of considerably more complex quantum bits instead of classical bits ("0" and "1" states). The researchers have reported their results in  the journal Nature.

Semiconductor physics: a fisherman's dream

Electrons can move as freely as fish in water in electric conductors (metals) and semiconductors such as silicon (Si) or gallium arsenide (GaAs), albeit not "swimming" of their own but moving owing to differences in voltage. Inside a metal, they are present as a huge number of fish that fill nearly the entire volume of water. In semiconductors, this "fish density" is not as high and so the distance between the electrons (fish) is much larger. The electrons can be concentrated in a thin layer near the surface by the application of an external voltage. The new method that the international team of researchers has developed now fulfils this "fisherman's dream" for semiconductor physicists. The electron "fish" are all in one layer close to the surface and easily, individually accessible from the surface.

Fishing one from the quantum dot

Prof. Andreas Wieck, physicist at the RUB, points out that there are, however no, "big fish," all electrons being similar and even always identical, undistinguishable objects. The method that the researchers from Germany, France and Japan used, nevertheless enables the "emission" of individual electrons from the QD, moving them over a specific distance and then detecting them at the neighbouring QD. A distance of four micrometres (µm) was used in the experiment -- this is twenty times larger than a highly integrated transistor. Targeted transport of individual electrons is possible in the following way: First, a QD is defined between the tips of four electrodes to form this zero-dimensional object, containing some hundred electrons. The scientists subsequently send a sound wave along the semiconductor surface using interdigital (like two combs fitted together without touching each other) electrodes to which they apply a radio frequency voltage. This method functions in the opposite way as the electrical discharge of a piezo ignition system in which a crystal is deformed to attain a voltage. The researchers applied voltage to the crystal and thus deform it, and the alternating voltage leads to the formation of a sound wave.

The fish surfs on the wave

In a sample, this wave moves, for example, from left to right through the quantum dot at the velocity of sound -- inside the crystal at three kilometres per second. Its height is adjusted so that it extracts exactly one "fish" from it. The latter subsequently surfs on the wave in a one-dimensional channel. The "fish" arrives at the neighbouring quantum dot 4 µm to the right thereof. The researchers were able to attain good statistics by repetition of the waves and measurements and thus capable of determining the reliability of the method. During the first experiments, the probability of emission and detection of a single electron with the wave was 96 and 92%, respectively.

The innovation: aligning the fish

It is not possible to differentiate between the electrons "fish," but they can be differently aligned because they rotate like little spinning tops. This is called the "spin" of the electron. For example one can align a fish with "its head upwards," let it be transported with the wave, and then detect it again at the target quantum dot still having "its head upwards." The time for the spin to change is longer than the surfing time on the wave, so the probability of this occurring is very high. The quantum bits of the future will also consist of such spin-polarized electrons.

The researchers attained their results with samples prepared by so-called molecular beam epitaxy at the chair of Applied Solid State Physics at the Ruhr University Bochum. They were structured in Tokyo and subsequently measured in Grenoble. But not only the samples, also a further development of this concept originates from Bochum: Prof. Wieck already published his vision of an electron directional coupler with two parallel one-dimensional channels, in which the electrons can skip from one to the other channel, 21 years ago. The research team has in the meantime realized this vision based on the results presented here. A further publication is therefore to follow shortly.

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Ruhr-Universitaet-Bochum.

Journal Reference:

Sylvain Hermelin, Shintaro Takada, Michihisa Yamamoto, Seigo Tarucha, Andreas D. Wieck, Laurent Saminadayar, Christopher Bäuerle, Tristan Meunier. Electrons surfing on a sound wave as a platform for quantum optics with flying electrons. Nature, 2011; 477 (7365): 435 DOI: 10.1038/nature10416

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

View the original article here

Like fish on waves, electrons go surfing

ScienceDaily (Sep. 28, 2011) — Physicists at the RUB, working in collaboration with researchers from Grenoble and Tokyo, have succeeded in taking a decisive step towards the development of more powerful computers. They were able to define two little quantum dots (QDs), occupied with electrons, in a semiconductor and to select a single electron from one of them using a sound wave, and then to transport it to the neighbouring QD.

A single electron "surfs" thus from one quantum dot to the next like a fish on a wave. Such manipulation of a single electron will in the future also enable the combination of considerably more complex quantum bits instead of classical bits ("0" and "1" states). The researchers have reported their results in  the journal Nature.

Semiconductor physics: a fisherman's dream

Electrons can move as freely as fish in water in electric conductors (metals) and semiconductors such as silicon (Si) or gallium arsenide (GaAs), albeit not "swimming" of their own but moving owing to differences in voltage. Inside a metal, they are present as a huge number of fish that fill nearly the entire volume of water. In semiconductors, this "fish density" is not as high and so the distance between the electrons (fish) is much larger. The electrons can be concentrated in a thin layer near the surface by the application of an external voltage. The new method that the international team of researchers has developed now fulfils this "fisherman's dream" for semiconductor physicists. The electron "fish" are all in one layer close to the surface and easily, individually accessible from the surface.

Fishing one from the quantum dot

Prof. Andreas Wieck, physicist at the RUB, points out that there are, however no, "big fish," all electrons being similar and even always identical, undistinguishable objects. The method that the researchers from Germany, France and Japan used, nevertheless enables the "emission" of individual electrons from the QD, moving them over a specific distance and then detecting them at the neighbouring QD. A distance of four micrometres (µm) was used in the experiment -- this is twenty times larger than a highly integrated transistor. Targeted transport of individual electrons is possible in the following way: First, a QD is defined between the tips of four electrodes to form this zero-dimensional object, containing some hundred electrons. The scientists subsequently send a sound wave along the semiconductor surface using interdigital (like two combs fitted together without touching each other) electrodes to which they apply a radio frequency voltage. This method functions in the opposite way as the electrical discharge of a piezo ignition system in which a crystal is deformed to attain a voltage. The researchers applied voltage to the crystal and thus deform it, and the alternating voltage leads to the formation of a sound wave.

The fish surfs on the wave

In a sample, this wave moves, for example, from left to right through the quantum dot at the velocity of sound -- inside the crystal at three kilometres per second. Its height is adjusted so that it extracts exactly one "fish" from it. The latter subsequently surfs on the wave in a one-dimensional channel. The "fish" arrives at the neighbouring quantum dot 4 µm to the right thereof. The researchers were able to attain good statistics by repetition of the waves and measurements and thus capable of determining the reliability of the method. During the first experiments, the probability of emission and detection of a single electron with the wave was 96 and 92%, respectively.

The innovation: aligning the fish

It is not possible to differentiate between the electrons "fish," but they can be differently aligned because they rotate like little spinning tops. This is called the "spin" of the electron. For example one can align a fish with "its head upwards," let it be transported with the wave, and then detect it again at the target quantum dot still having "its head upwards." The time for the spin to change is longer than the surfing time on the wave, so the probability of this occurring is very high. The quantum bits of the future will also consist of such spin-polarized electrons.

The researchers attained their results with samples prepared by so-called molecular beam epitaxy at the chair of Applied Solid State Physics at the Ruhr University Bochum. They were structured in Tokyo and subsequently measured in Grenoble. But not only the samples, also a further development of this concept originates from Bochum: Prof. Wieck already published his vision of an electron directional coupler with two parallel one-dimensional channels, in which the electrons can skip from one to the other channel, 21 years ago. The research team has in the meantime realized this vision based on the results presented here. A further publication is therefore to follow shortly.

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Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Ruhr-Universitaet-Bochum.

Journal Reference:

Sylvain Hermelin, Shintaro Takada, Michihisa Yamamoto, Seigo Tarucha, Andreas D. Wieck, Laurent Saminadayar, Christopher Bäuerle, Tristan Meunier. Electrons surfing on a sound wave as a platform for quantum optics with flying electrons. Nature, 2011; 477 (7365): 435 DOI: 10.1038/nature10416

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

View the original article here

Scientists play ping-pong with single electrons

ScienceDaily (Sep. 22, 2011) — Scientists at Cambridge University have shown an amazing degree of control over the most fundamental aspect of an electronic circuit, how electrons move from one place to another.

Researchers from the University's Cavendish Laboratory have moved an individual electron along a wire, batting it back and forth over sixty times, rather like the ball in a game of ping-pong. The research findings, published September 22 in the journal Nature, may have applications in quantum computing, transferring a quantum 'bit' between processor and memory, for example.

Imagine you are at a party and you want to get to the other side of a crowded room to talk to someone. As you walk you have to weave around people who are walking, dancing or just standing in the way. You may also have to stop and greet friends along the way and by the time you reach the person you wanted to talk to you have forgotten what you were going to say. Wouldn't it be nice to be lifted up above the crowd, and pushed directly to your destination?

In a similar way, electrons carrying a current along a wire do not go directly from one end to the other but instead follow a complicated zigzag path. This is a problem if the electron is carrying information, as it tends to 'forget' it, or, more scientifically, the quantum state loses coherence.

In this work, a single electron can be trapped in a small well (called a quantum dot), just inside the surface of a piece of Gallium Arsenide (GaAs). A channel leads to another, empty, dot 4 microns (millionths of a metre) away. The channel is higher in energy than the surrounding electrons. A very short burst of sound (just a few billionths of a second long) is then sent along the surface, past the dot. The accompanying wave of electrical potential picks up the electron, which then surfs along the channel to the other dot, where it is captured. A burst of sound sent from the other direction returns the electron to the starting dot where the process can be repeated. The electron goes back and forth like a ping-pong ball. Rallies of up to 60 shots have been achieved before anything goes wrong.

"The movement of electrons by our 'surface acoustic wave' can also be likened to peristalsis in the esophagus, where food is propelled from the mouth to the stomach by a wave of muscle contraction," explains Rob McNeil, the PhD student who did most of the work, helped by postdoc Masaya Kataoka, both at the University of Cambridge's Department of Physics, the Cavendish Laboratory.

"This is an enabling technology for quantum computers," Chris Ford, team leader of the research from the Semiconductor Physics Group in the Cavendish, says. "There is a lot of work going on worldwide to make this new type of computer, which may solve certain complex problems much faster than classical computers. However, little effort has yet been put into connecting up different components, such as processor and memory. Although our experiments do not yet show that electrons 'remember' their quantum state, this is likely to be the case. This would make the method of transfer a candidate for moving quantum bits of information (qubits) around a quantum circuit, in a quantum computer. Indeed, our theorist, Crispin Barnes, proposed using this mechanism to make a whole quantum computer a long time ago, and this is an important step towards that goal."

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Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by University of Cambridge. The original story is licensed under a Creative Commons license.

Journal Reference:

R. P. G. McNeil, M. Kataoka, C. J. B. Ford, C. H. W. Barnes, D. Anderson, G. A. C. Jones, I. Farrer, D. A. Ritchie. On-demand single-electron transfer between distant quantum dots. Nature, 2011; 477 (7365): 439 DOI: 10.1038/nature10444

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New light shed on the private lives of electrons: Lasers allow scientists to observe how electrons become entangled

ScienceDaily (July 3, 2011) — A Princeton researcher and his international collaborators have used lasers to peek into the complex relationship between a single electron and its environment, a breakthrough that could aid the development of quantum computers.

The technique reveals how an isolated electron and its surroundings develop a relationship known as a Kondo state -- a state of matter that is of great interest to physicists and engineers. The results not only yield insights into a long-standing quandary in theoretical physics, but also may help scientists understand how to store information at the smallest possible scales, which would open vast new realms of computing power.

"What we've done is illuminate the private life of a single electron," said Hakan Tureci, an assistant professor of electrical engineering at Princeton and a lead researcher on the project. "It's taken nearly a century to isolate, control and probe a single electron in this way -- an extraordinary feat enabled by quantum theory, cryogenics and nanotechnology."

The research was conducted by an international team of scientists from the United States, Germany and Switzerland. The researchers on the project included Tureci, Atac Imamoglu, a professor at Swiss Federal Institute of Technology Zurich in Switzerland, Jan von Delft, a professor at LMU Munich, and Leonid Glazman, a professor at Yale University.

The key theoretical results and a proposal for testing the ideas experimentally were published March 11 in the journal Physical Review Letters.

These theoretical projections were recently confirmed in experiments led by Imamoglu, which were published June 29 in the journal Nature.

The research brings fresh insight to the study of the Kondo problem, a phenomenon first observed in the 1930s, when researchers were surprised to find that resistance to electricity flowing through certain metals increases at very low temperatures. Normally, resistance through metals decreases as temperature is lowered, but that was not the case with these metals.

The phenomenon was explained 30 years later by Japanese scientist, Jun Kondo, as resulting from the presence of cobalt or other magnetic impurities in the metals.

Scientists have further realized that the Kondo effect results from a relationship between electrons known as "entanglement" in which the quantum state of one electron is tied to those of neighboring electrons, even if the particles are later separated by considerable distances. In the case of Kondo effect, a trapped electron is entangled in a complex manner with a cloud of surrounding electrons.

Researchers have been intrigued by the Kondo effect in part because understanding how a trapped electron becomes entangled with its environment could help overcome barriers to quantum computing, which could lead to far more powerful computers than currently exist.

Previous observation methods allowed scientists to make measurements of the Kondo state, but could not provide information on how electrons developed such a relationship with their surroundings.

To better understand how an electron gradually becomes entangled in this manner with its environment, Tureci and his collaborators investigated the idea of using a laser to probe electrons evolving into the Kondo state. They first developed a theory about how laser light scattered off electrons could carry information about this process.

Depending on the state of the electron, they surmised, it should absorb different colors of laser light to varying degrees. The light reflected back would carry a signature of the entangled quantum state, offering a window into the relationship between the trapped electron and its environment.

To isolate the electrons, they proposed using nanostructured devices, small machines built one atom at a time that trap the electrons in small wells. The particles are only provided limited isolation in the wells and so eventually become entangled with a cloud of surrounding electrons in the device.

Tureci's collaborators in Switzerland tested the idea by projecting a laser beam on the device and measuring the light that was transmitted.

The light signature matched theoretical predictions. The researchers also found that they could use the light signatures to confirm when they turned the Kondo state off using a magnetic field.

"By doing this experiment," Tureci said, "we showed that you can extract this information that was previously unavailable in earlier experiments on the Kondo effect."

He said the finding could provide insight into quantum computing because entanglement, depending on its nature, could allow new ways of storing and processing information or could threaten to destabilize the computing process.

Whereas current computers use transistors to store "bits" of information as ones or zeros, scientists believe quantum computers might one day use trapped electrons that are entangled with one another as "qubits," the basic information units of quantum computing, which can have the odd quality of representing a blend of "one" and "zero" simultaneously.

A series of qubits could thus store exponentially more information than the 0 and 1 combination of classical bits.

While quantum computers could theoretically be far smaller and faster than transistor-based machines, using electrons or other sub-atomic particles as storage devices is no trivial feat.

The undesirable entangled relationship between electrons and their environment, such as that seen in the Kondo effect, can destabilize the desired relationship between trapped electrons that form the qubits and gradually destroy the information they store.

"Our technique offers a window into the Kondo state, allowing us a chance to study electrons that are highly entangled with their environments and understand how they got that way," Tureci said.

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Princeton University, Engineering School, via EurekAlert!, a service of AAAS.

Journal Reference:

C. Latta, F. Haupt, M. Hanl, A. Weichselbaum, M. Claassen, W. Wuester, P. Fallahi, S. Faelt, L. Glazman, J. von Delft, H. E. Türeci, A. Imamoglu. Quantum quench of Kondo correlations in optical absorption. Nature, 2011; 474 (7353): 627 DOI: 10.1038/nature10204

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Scientists achieve guiding of electrons by purely electric fields

ScienceDaily (May 10, 2011) — The investigation of the properties of electrons plays a key role for the understanding of the fundamental laws of nature. However, being extremely small and quick, electrons are difficult to control. Physicists around Dr. Peter Hommelhoff, head of the Max Planck Research Group "Ultrafast Quantum Optics" at the Max Planck Institute of Quantum Optics (Garching near Munich), have now demonstrated efficient guiding of slow electrons by applying a microwave voltage to electrodes fabricated on a planar substrate.

The research is published online in Physical Review Letters.

This new technique of electron guiding -- which resembles the guiding of light waves in optical fibres -- promises a variety of applications, from guided matter-wave experiments to non-invasive electron microscopy.

Electrons have been the first elementary particles revealing their wave-like properties and have therefore been of great importance in the development of the theory of quantum mechanics. Even now the observation of electrons leads to new insight into the fundamental laws of physics. Measurements involving confined electrons have so far mainly been performed in so-called Penning traps, which combine a static magnetic field with an oscillating electric field.

For a number of experiments with propagating electrons, like interferometry with slow electrons, it would be advantageous to confine the electrons by a purely electric field. This can be done in an alternating quadrupole potential similar to the standard technique that is used for ion trapping. These so-called Paul traps are based on four electrodes to which a radiofrequency voltage is applied. The resulting field evokes a driving force which keeps the particle in the centre of the trap. Wolfgang Paul received the Nobel Prize in physics for the invention of these traps in 1989.

For several years by now scientists realize Paul traps with micro structured electrodes on planar substrates, using standard microelectronic chip technology. Dr. Hommelhoff and his group have now applied this method for the first time to electrons. Since the mass of these point-like particles is only a tenth of a thousandth of the mass of an ion, electrons react much faster to electric fields than the rather heavy ions. Hence, in order to guide electrons, the frequency of the alternating voltage applied to the electrodes has to be much higher than for the confinement of ions and is in the microwave range, at around 1 GHz.

In the experiment electrons are generated in a thermal source (in which a tungsten wire is heated like in a light bulb) and the emitted electrons are collimated to a parallel beam of a few electron volts. From there the electrons are injected into the "wave-guide." It is being generated by five electrodes on a planar substrate to which an alternating voltage with a frequency of about 1 GHz is applied. This introduces an oscillating quadrupole field in a distance of half a millimetre above the electrodes, which confines the electrons in the radial direction. In the longitudinal direction there is no force acting on the particles so that they are free to travel along the "guide tube." As the confinement in the radial direction is very strong the electrons are forced to follow even small directional changes of the electrodes.

In order to make this effect more visible the 37mm long electrodes are bent to a curve of 30 degrees opening angle and with a bending radius of 40mm. At the end of the structure the guided electrons are ejected and registered by a detector. A bright spot caused by guided electrons appears on the detector right at the exit of the guide tube, which is situated in the left part of the picture. When the alternating field is switched off a more diffusively illuminated area shows up on the right side. It is caused by electrons spreading out from the source and propagating on straight trajectories over the substrate.

"With this fundamental experiment we were able to show that electrons can be efficiently guided be purely electric fields," says Dr. Hommelhoff. "However, as our electron source yields a rather poorly collimated electron beam we still lose many electrons." In the future the researchers plan to combine the new microwave guide with an electron source based on field emission from an atomically sharp metal tip. These devices deliver electron beams with such a strong collimation that their transverse component is limited by the Heisenberg uncertainty principle only.

Under these conditions it should be feasible to investigate the individual quantum mechanical oscillations of the electrons in the radial potential of the guide. "The strong confinement of electrons observed in our experiment means that a "jump" from one quantum state to the neighbouring higher state requires a lot of energy and is therefore not very likely to happen," explains Johannes Hoffrogge, doctoral student at the experiment. "Once a single quantum state is populated it will remain so for an extended period of time and can be used for quantum experiments." This would make it possible to conduct quantum physics experiments such as interferometry with guided slow electrons. Here the wave function of an electron is first split up; later on, its two components are brought together again whereby characteristic superpositions of quantum states of the electron can be generated. But the new method could also be applied for a new form of electron microscopy.

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Max Planck Institute of Quantum Optics.

Journal Reference:

J. Hoffrogge, R. Fröhlich, M. Kasevich, P. Hommelhoff. Microwave Guiding of Electrons on a Chip. Physical Review Letters, 2011; 106 (19) DOI: 10.1103/PhysRevLett.106.193001

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Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

View the original article here

Measurement of 'hot' electrons could have solar energy payoff; Nanoantennas hold promise for infrared photovoltaics

ScienceDaily (May 9, 2011) — Basic scientific curiosity paid off in unexpected ways when Rice University researchers investigating the fundamental physics of nanomaterials discovered a new technology that could dramatically improve solar energy panels.

The research is described in a new paper in the journal Science.

"We're merging the optics of nanoscale antennas with the electronics of semiconductors," said lead researcher Naomi Halas, Rice's Stanley C. Moore Professor in Electrical and Computer Engineering. "There's no practical way to directly detect infrared light with silicon, but we've shown that it is possible if you marry the semiconductor to a nanoantenna. We expect this technique will be used in new scientific instruments for infrared-light detection and for higher-efficiency solar cells."

More than a third of the solar energy on Earth arrives in the form of infrared light. But silicon -- the material that's used to convert sunlight into electricity in the vast majority of today's solar panels -- cannot capture infrared light's energy. Every semiconductor, including silicon, has a "bandgap" where light below a certain frequency passes directly through the material and is unable to generate an electrical current. By attaching a metal nanoantenna to the silicon, where the tiny antenna is specially tuned to interact with infrared light, the Rice team showed they could extend the frequency range for electricity generation into the infrared. When infrared light hits the antenna, it creates a "plasmon," a wave of energy that sloshes through the antenna's ocean of free electrons. The study of plasmons is one of Halas' specialties, and the new paper resulted from basic research into the physics of plasmons that began in her lab years ago.

It has been known that plasmons decay and give up their energy in two ways; they either emit a photon of light or they convert the light energy into heat. The heating process begins when the plasmon transfers its energy to a single electron -- a 'hot' electron. Rice graduate student Mark Knight, lead author on the paper, together with Rice theoretical physicist Peter Nordlander, his graduate student Heidar Sobhani, and Halas set out to design an experiment to directly detect the hot electrons resulting from plasmon decay.

Patterning a metallic nanoantenna directly onto a semiconductor to create a "Schottky barrier," Knight showed that the infrared light striking the antenna would result in a hot electron that could jump the barrier, which creates an electrical current. This works for infrared light at frequencies that would otherwise pass directly through the device.

"The nanoantenna-diodes we created to detect plasmon-generated hot electrons are already pretty good at harvesting infrared light and turning it directly into electricity," Knight said. "We are eager to see whether this expansion of light-harvesting to infrared frequencies will directly result in higher-efficiency solar cells."

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Rice University, via EurekAlert!, a service of AAAS.

Journal Reference:

Mark W. Knight, Heidar Sobhani, Peter Nordlander, and Naomi J. Halas. Photodetection with Active Optical Antennas. Science, 2011; 332 (6030): 702-704 DOI: 10.1126/science.1203056

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

View the original article here

Scientists achieve guiding of electrons by purely electric fields

ScienceDaily (May 10, 2011) — The investigation of the properties of electrons plays a key role for the understanding of the fundamental laws of nature. However, being extremely small and quick, electrons are difficult to control. Physicists around Dr. Peter Hommelhoff, head of the Max Planck Research Group "Ultrafast Quantum Optics" at the Max Planck Institute of Quantum Optics (Garching near Munich), have now demonstrated efficient guiding of slow electrons by applying a microwave voltage to electrodes fabricated on a planar substrate.

The research is published online in Physical Review Letters.

This new technique of electron guiding -- which resembles the guiding of light waves in optical fibres -- promises a variety of applications, from guided matter-wave experiments to non-invasive electron microscopy.

Electrons have been the first elementary particles revealing their wave-like properties and have therefore been of great importance in the development of the theory of quantum mechanics. Even now the observation of electrons leads to new insight into the fundamental laws of physics. Measurements involving confined electrons have so far mainly been performed in so-called Penning traps, which combine a static magnetic field with an oscillating electric field.

For a number of experiments with propagating electrons, like interferometry with slow electrons, it would be advantageous to confine the electrons by a purely electric field. This can be done in an alternating quadrupole potential similar to the standard technique that is used for ion trapping. These so-called Paul traps are based on four electrodes to which a radiofrequency voltage is applied. The resulting field evokes a driving force which keeps the particle in the centre of the trap. Wolfgang Paul received the Nobel Prize in physics for the invention of these traps in 1989.

For several years by now scientists realize Paul traps with micro structured electrodes on planar substrates, using standard microelectronic chip technology. Dr. Hommelhoff and his group have now applied this method for the first time to electrons. Since the mass of these point-like particles is only a tenth of a thousandth of the mass of an ion, electrons react much faster to electric fields than the rather heavy ions. Hence, in order to guide electrons, the frequency of the alternating voltage applied to the electrodes has to be much higher than for the confinement of ions and is in the microwave range, at around 1 GHz.

In the experiment electrons are generated in a thermal source (in which a tungsten wire is heated like in a light bulb) and the emitted electrons are collimated to a parallel beam of a few electron volts. From there the electrons are injected into the "wave-guide." It is being generated by five electrodes on a planar substrate to which an alternating voltage with a frequency of about 1 GHz is applied. This introduces an oscillating quadrupole field in a distance of half a millimetre above the electrodes, which confines the electrons in the radial direction. In the longitudinal direction there is no force acting on the particles so that they are free to travel along the "guide tube." As the confinement in the radial direction is very strong the electrons are forced to follow even small directional changes of the electrodes.

In order to make this effect more visible the 37mm long electrodes are bent to a curve of 30 degrees opening angle and with a bending radius of 40mm. At the end of the structure the guided electrons are ejected and registered by a detector. A bright spot caused by guided electrons appears on the detector right at the exit of the guide tube, which is situated in the left part of the picture. When the alternating field is switched off a more diffusively illuminated area shows up on the right side. It is caused by electrons spreading out from the source and propagating on straight trajectories over the substrate.

"With this fundamental experiment we were able to show that electrons can be efficiently guided be purely electric fields," says Dr. Hommelhoff. "However, as our electron source yields a rather poorly collimated electron beam we still lose many electrons." In the future the researchers plan to combine the new microwave guide with an electron source based on field emission from an atomically sharp metal tip. These devices deliver electron beams with such a strong collimation that their transverse component is limited by the Heisenberg uncertainty principle only.

Under these conditions it should be feasible to investigate the individual quantum mechanical oscillations of the electrons in the radial potential of the guide. "The strong confinement of electrons observed in our experiment means that a "jump" from one quantum state to the neighbouring higher state requires a lot of energy and is therefore not very likely to happen," explains Johannes Hoffrogge, doctoral student at the experiment. "Once a single quantum state is populated it will remain so for an extended period of time and can be used for quantum experiments." This would make it possible to conduct quantum physics experiments such as interferometry with guided slow electrons. Here the wave function of an electron is first split up; later on, its two components are brought together again whereby characteristic superpositions of quantum states of the electron can be generated. But the new method could also be applied for a new form of electron microscopy.

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Max Planck Institute of Quantum Optics.

Journal Reference:

J. Hoffrogge, R. Fröhlich, M. Kasevich, P. Hommelhoff. Microwave Guiding of Electrons on a Chip. Physical Review Letters, 2011; 106 (19) DOI: 10.1103/PhysRevLett.106.193001

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

View the original article here