domingo, 25 de julio de 2010

Entanglement Achieved in Solid-State Circuitry

Entanglement Achieved in Solid-State Circuitry


This is an SEM image of a typical Cooper pair splitter. The bar is 1 micrometer. A central superconducting electrode (blue) is connected to two quantum dots engineered in the same single wall carbon nanotube (in purple). Entangled electrons inside the superconductor can be coaxed to move in opposite directions in the nanotube, ending up at separate quantum dots, while remaining entangled. (Credit: L.G. Herrmann, F. Portier, P. Roche, A. Levy Yeyati, T. Kontos, and C. Strunk)


Entanglement Achieved in Solid-State Circuitry
ScienceDaily (Jan. 12, 2010) — For the first time, physicists have convincingly demonstrated that physically separated particles in solid-state devices can be quantum-mechanically entangled. The achievement is analogous to the quantum entanglement of light, except that it involves particles in circuitry instead of photons in optical systems.


Both optical and solid-state entanglement offer potential routes to quantum computing and secure communications, but solid-state versions may ultimately be easier to incorporate into electronic devices.

In optical entanglement experiments, a pair of entangled photons may be separated via a beam splitter. Despite their physical separation, the entangled photons continue to act as a single quantum object. A team of physicists from France, Germany and Spain has now performed a solid-state entanglement experiment that uses electrons in a superconductor in place of photons in an optical system.

As conventional superconducting materials are cooled, the electrons they conduct entangle to form what are known as Cooper pairs. In the new experiment, Cooper pairs flow through a superconducting bridge until they reach a carbon nanotube that acts as the electronic equivalent of a beam splitter. Occasionally, the electrons part ways and are directed to separate quantum dots -- but remain entangled. Although the quantum dots are only a micron or so apart, the distance is large enough to demonstrate entanglement comparable to that seen in optical systems.

In addition to the possibility of using entangled electrons in solid-state devices for computing and secure communications, the breakthrough opens a whole new vista on the study of quantum mechanically entangled systems in solid materials.

The experiment is reported in an upcoming issue of Physical Review Letters and highlighted with a Viewpoint in the January 11 issue of Physics.

LUIS RINCON
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Scientists Create Multi-Particle Entanglement of Atoms in a Bose-Einstein Condensate on a Microchip

Scientists Create Multi-Particle Entanglement of Atoms in a Bose-Einstein Condensate on a Microchip


Scientists Create Multi-Particle Entanglement of Atoms in a Bose-Einstein Condensate on a Microchip
ScienceDaily (Apr. 6, 2010) — The microcosm, the realm of quantum physics, is ruled by probability and chance. The behaviour of quantum particles cannot be predicted with certainty but only with certain probabilities given by quantum physics. This results in a so-called quantum noise, which fundamentally limits the precision of the most refined atomic clocks and interferometers. The solution to this problem is the use of entangled atomic systems.


A breakthrough has now been achieved by a team led by Professor Theodor W. Hänsch and Professor Philipp Treutlein (Ludwig-Maximilians-Universität Munich and Max Planck Institute of Quantum Optics in Garching, Philipp Treutlein is Professor at the Universität Basel since February 2010). For the first time, the scientists succeeded in generating multi-particle entanglement on an atom-chip, according to a report published in the journal Nature. This technique opens a way to significantly enhance the precision of chip-based atomic clocks or interferometers and could also form the basis for quantum computers on microchips. The Munich experiments have been carried out in cooperation with theoretical physicists around Dr. Alice Sinatra from the Ecole Normale Supérieure (ENS) in Paris.

Entanglement is one of the most fascinating phenomena of physics. Once two particles are prepared in an entangled state, they loose their individuality and have to be treated as one single system. Whatever happens to one of the particles it will have an instantaneous impact on the other one, independent of the distance between the particles. Already 80 years ago Albert Einstein dubbed this phenomenon which contradicts every intuition 'spooky action at a distance'. Entanglement is a strict consequence of quantum theory, yet it was not before the last decade of the twentieth century that entangled states of atoms could be experimentally generated and verified. This opened up the possibility to not only get a better understanding of this mysterious phenomenon but also to make use of it for technical applications such as communication, metrology and computing.

In the experiment described here the Munich group succeeded for the first time to generate entanglement on an atom chip. An atom chip is a microstructured chip that is able to store and manipulate single atoms or atomic clouds. Atom chips have already shown to be versatile tools, both for the study of fundamental problems of quantum physics and for a number of interesting applications. For instance, a chip-based atomic clock, which is suitable for portable use, has been developed using this technology. However, up to now no method existed to generate entanglement on a chip. And as long as atomic clocks run with atoms that are independent of each other, their precision will be limited by the fundamental quantum noise.

Two years ago the theoretical physicists Alice Sinatra and Li Yun developed, in cooperation with the group of Philipp Treutlein, a concept how to suppress this quantum noise. The experiment starts with trapping a cloud of rubidium atoms on the chip and cooling it down to less than a millionth of a degree above absolute zero. At these temperatures the atoms form a Bose-Einstein condensate (BEC), a new state, in which all the atoms are in the same well-defined quantum state. The rubidium atoms can be described by a so-called spin, which can be oriented either upwards or downwards. The ground state of the atoms in the BEC corresponds to a downwards oriented spin. A microwave pulse which is applied to the BEC now rotates the spins such that each atom is in a superposition of both spin states.

The BEC is then exposed to a state-dependent potential which is exerted by a second microwave field. "Under the influence of this field the atoms are only allowed to collide with atoms of the same spin state. Therefore the dynamic evolution of their states depends on the states of all other atoms. This effect leads to an entanglement of the atoms," explains Max Riedel, doctoral student at the experiment.

In a measurement on a BEC of non-entangled atoms, on average half of the atoms are found in the ground state (spin downwards), the other half in the excited state (spin upwards). "Deviations from this mean value that occur from measurement to measurement, lead to a quantum noise that is evenly distributed among the spin components orthogonal to the mean spin," adds Pascal Böhi, another doctoral student.

In order to investigate the influence of the state-dependent potential on the quantum noise the scientists determined the noise for each spin component using yet another microwave pulse. As they could clearly demonstrate, for one spin component the noise could be "squeezed" below the limit given by the Heisenberg uncertainty relation. From the observed noise reduction the scientists concluded that inside the BEC clusters of at least four atoms are entangled.

Using entangled ensembles of atoms the precision of atomic clocks could be increased significantly. Further applications include highly sensitive atom interferometers for the detection of extremely weak forces and the realisation of a quantum gate, a key element in future quantum computers. But the scientists also hope to get a deeper understanding of the processes that lead to quantum correlations in quantum many body systems.

The experiments were carried out with support from the Deutsche Forschungsgemeinschaft in the framework of the cluster of excellence "Nanosystems Initiative Munich (NIM)" and with support from the European Union in the framework of the project "Atomic Quantum Technologies (AQUTE)."

LUIS RINCON
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'Butterfly Effect' in the Brain Makes the Brain Intrinsically Unreliable

'Butterfly Effect' in the Brain Makes the Brain Intrinsically Unreliable


'Butterfly Effect' in the Brain Makes the Brain Intrinsically Unreliable
ScienceDaily (July 1, 2010) — Next time your brain plays tricks on you, you have an excuse: according to new research by UCL scientists published June 30 in the journal Nature, the brain is intrinsically unreliable.


This may not seem surprising to most of us, but it has puzzled neuroscientists for decades. Given that the brain is the most powerful computing device known, how can it perform so well even though the behaviour of its circuits is variable?

A long-standing hypothesis is that the brain's circuitry actually is reliable -- and the apparently high variability is because your brain is engaged in many tasks simultaneously, which affect each other.

It is this hypothesis that the researchers at UCL tested directly. The team -- a collaboration between experimentalists at the Wolfson Institute for Biomedical Research and a theorist, Peter Latham, at the Gatsby Computational Neuroscience Unit -- took inspiration from the celebrated butterfly effect -- from the fact that the flap of a butterfly's wings in Brazil could set off a tornado in Texas. Their idea was to introduce a small perturbation into the brain, the neural equivalent of butterfly wings, and ask what would happen to the activity in the circuit. Would the perturbation grow and have a knock-on effect, thus affecting the rest of the brain, or immediately die out?

It turned out to have a huge knock-on effect. The perturbation was a single extra 'spike', or nerve impulse, introduced to a single neuron in the brain of a rat. That single extra spike caused about thirty new extra spikes in nearby neurons in the brain, most of which caused another thirty extra spikes, and so on. This may not seem like much, given that the brain produces millions of spikes every second. However, the researchers estimated that eventually, that one extra spike affected millions of neurons in the brain.

"This result indicates that the variability we see in the brain may actually be due to noise, and represents a fundamental feature of normal brain function," said lead author Dr. Mickey London, of the Wolfson Institute for Biomedical Research, UCL.

This rapid amplification of spikes means that the brain is extremely 'noisy' -- much, much noisier than computers. Nevertheless, the brain can perform very complicated tasks with enormous speed and accuracy, far faster and more accurately than the most powerful computer ever built (and likely to be built in the foreseeable future). The UCL researchers suggest that for the brain to perform so well in the face of high levels of noise, it must be using a strategy called a rate code. In a rate code, neurons consider the activity of an ensemble of many neurons, and ignore the individual variability, or noise, produced by each of them.

So now we know that the brain is truly noisy, but we still don't know why. The UCL researchers suggest that one possibility is that it's the price the brain pays for high connectivity among neurons (each neuron connects to about 10,000 others, resulting in over 8 million kilometres of wiring in the human brain). Presumably, that high connectivity is at least in part responsible for the brain's computational power. However, as the research shows, the higher the connectivity, the noisier the brain. Therefore, while noise may not be a useful feature, it is at least a by-product of a useful feature.

LUIS RINCON
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Redefining Electrical Current Law With the Transistor Laser

Redefining Electrical Current Law With the Transistor Laser


Redefining Electrical Current Law With the Transistor Laser
ScienceDaily (May 17, 2010) — While the laws of physics weren't made to be broken, sometimes they need revision. A major current law has been rewritten thanks to the three-port transistor laser, developed by Milton Feng and Nick Holonyak Jr. at the University of Illinois.




With the transistor laser, researchers can explore the behavior of photons, electrons and semiconductors. The device could shape the future of high-speed signal processing, integrated circuits, optical communications, supercomputing and other applications. However, harnessing these capabilities hinges on a clear understanding of the physics of the device, and data the transistor laser generated did not fit neatly within established circuit laws governing electrical currents.

"We were puzzled," said Feng, the Holonyak Chair Professor of Electrical and Computer Engineering. "How did that work? Is it violating Kirchhoff's law? How can the law accommodate a further output signal, a photon or optical signal?"

Kirchhoff's current law, described by Gustav Kirchhoff in 1845, states charge input at a node is equal to the charge output. In other words, all the electrical energy going in must go out again. On a basic bipolar transistor, with ports for electrical input and output, the law applies straightforwardly. The transistor laser adds a third port for optical output, emitting light.

This posed a conundrum for researchers working with the laser: How were they to apply the laws of conservation of charge and conservation of energy with two forms of energy output?

"The optical signal is connected and related to the electrical signals, but until now it's been dismissed in a transistor," said Holonyak, the John Bardeen Chair Professor of Electrical and Computer Engineering and Physics at the U. of I. "Kirchhoff's law takes care of balancing the charge, but it doesn't take care of balancing the energies. The question is, how do you put it all together, and represent it in circuit language?"

The unique properties of the transistor laser required Holonyak, Feng and graduate student Han Wui Then to re-examine and modify the law to account for photon particles as well as electrons, effectively expanding it from a current law to a current-energy law. They published their model and supporting data in the Journal of Applied Physics, available online May 10.

"The previous law had to do with the particles -- electrons coming out at a given point. But it was never about energy conservation as it was normally known and used," Feng said. "This is the first time we see how energy is involved in the conservation process."

Simulations based on the modified law fit data collected from the transistor laser, allowing researchers to predict the bandwidth, speed and other properties for integrated circuits, according to Feng. With accurate simulations, the team can continue exploring applications in integrated circuits and supercomputing.

"This fits so well, it's amazing," Feng said. "The microwave transistor laser model is very accurate for predicting frequency-dependent electrical and optical properties. The experimental data are very convincing."

The Army Research Office supported this work.

LUIS RINCON
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IXYS Introduces Super Low Noise pHEMT Technology Devices For Microwave Applications Up To 38 GHz

IXYS Introduces Super Low Noise pHEMT Technology Devices For Microwave Applications Up To 38 GHz


IXYS Introduces Super Low Noise pHEMT Technology Devices For Microwave Applications Up To 38 GHz
FREMONT, Calif., Jul 01, 2010 (BUSINESS WIRE) -- MicroWave Technology, Inc. (MwT), a wholly owned subsidiary of IXYS Corporation , announced that it has introduced a family of three AlGaAs/InGaAs based low noise pHEMT devices with extremely low noise figures with operational frequency up to 38 GHz.

These low noise devices are MwT-LN240, MwT-LN300 and MwT-LN600. The super low noise devices are fabricated using high reliability AlGaAs/InGaAs pHEMT (pseudomorphic High Electron Mobility Transistor) process with a nominal 0.15 micron gate length and gate widths of 240 um, 300 um, and 600 um, respectively. These devices are equally effective for wideband (e.g. 6-18 GHz or 18-26 GHz) and narrow band applications up to 38 GHz. With minimum noise figure as low as 0.5 dB at 12 GHz with 2.5V drain bias, these low noise devices are ideally suited for commercial wireless and military applications requiring very low noise figure and high associated gain. These devices are targeted at wide range applications including broadband military EW and defense communications, wireless communication infrastructures, point-to-point microwave radios, space/high rel, instrumentation and medical equipment.

This new family of MwT low noise device is an ideal choice to replace low noise pHEMT devices from NEC, Eudyna/Sumitomo, Mitsubishi, etc. The wafer can be screened to meet high quality and reliability requirements for military and space applications. These devices are also available in surface mount packages such as MwT-71 package. The complete noise models such as "gamma opt" and noise parameters over frequency range are available for these devices to aid circuit design simulations. An application note on active bias circuitry for setting and stabilizing the gate bias is also available.

As an application support vehicle, MwT has developed 11 to 13 GHz hybrid modules using an MwT-LN240 (a 240 micrometer device) with a noise figure as low as 0.7 dB. A 6 to 18 GHz balanced amplifier module using a pair of MwT-LN240 devices has achieved noise figure between 1.5 and 1.7 dB across the band. The exceptionally good RF performances from these hybrid amplifiers have convincingly demonstrated the state-of-the-art noise performance for the MwT-LN series low noise devices, as well as the superior design capability for low noise amplifiers at MwT.


LUIS RINCON
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Cell-inspired electronics

Cell-inspired electronics


Cell-inspired electronics
By mimicking cells, MIT researcher designs electronic circuits for ultra-low-power and biomedical applications.
A single cell in the human body is approximately 10,000 times more energy-efficient than any nanoscale digital transistor, the fundamental building block of electronic chips. In one second, a cell performs about 10 million energy-consuming chemical reactions, which altogether require about one picowatt (one millionth millionth of a watt) of power.

MIT's Rahul Sarpeshkar is now applying architectural principles from these ultra-energy-efficient cells to the design of low-power, highly parallel, hybrid analog-digital electronic circuits. Such circuits could one day be used to create ultra-fast supercomputers that predict complex cell responses to drugs. They may also help researchers to design synthetic genetic circuits in cells.

In his new book, Ultra Low Power Bioelectronics (Cambridge University Press, 2010), Sarpeshkar outlines the deep underlying similarities between chemical reactions that occur in a cell and the flow of current through an analog electronic circuit. He discusses how biological cells perform reliable computation with unreliable components and noise (which refers to random variations in signals — whether electronic or genetic). Circuits built with similar design principles in the future can be made robust to electronic noise and unreliable electronic components while remaining highly energy efficient. Promising applications include image processors in cell phones or brain implants for the blind.

"Circuits are a language for representing and trying to understand almost anything, whether it be networks in biology or cars," says Sarpeshkar, an associate professor of electrical engineering and computer science. "There's a unified way of looking at the biological world through circuits that is very powerful."

Circuit designers already know hundreds of strategies to run analog circuits at low power, amplify signals, and reduce noise, which have helped them design low-power electronics such as mobile phones, mp3 players and laptop computers.

"Here's a field that has devoted 50 years to studying the design of complex systems," says Sarpeshkar, referring to electrical engineering. "We can now start to think of biology in the same way." He hopes that physicists, engineers, biologists and biological engineers will work together to pioneer this new field, which he has dubbed "cytomorphic" (cell-inspired or cell-transforming) electronics.

Finding connections

Sarpeshkar, an electrical engineer with many years of experience in designing low-power and biomedical circuits, has frequently turned his attention to finding and exploiting links between electronics and biology. In 2009, he designed a low-power radio chip that mimics the structure of the human cochlea to separate and process cell phone, Internet, radio and television signals more rapidly and with more energy efficiency than had been believed possible.

That chip, known as the RF (radio frequency) cochlea, is an example of "neuromorphic electronics," a 20-year-old field founded by Carver Mead, Sarpeshkar's thesis advisor at Caltech. Neuromorphic circuits mimic biological structures found in the nervous system, such as the cochlea, retina and brain cells.

Sarpeshkar's expansion from neuromorphic to cytomorphic electronics is based on his analysis of the equations that govern the dynamics of chemical reactions and the flow of electrons through analog circuits. He has found that those equations, which predict the reaction's (or circuit's) behavior, are astonishingly similar, even in their noise properties.

Chemical reactions (for example, the formation of water from hydrogen and oxygen) only occur at a reasonable rate if enough energy is available to lower the barriers that prevent such reactions from occurring. A catalyst such as an enzyme can lower such barriers. Similarly, electrons flowing through a circuit in a transistor exploit input voltage energy to allow them to reduce the barrier for electrons to flow from the transistor's source to the transistor's drain. Changes in the input voltage lower the barrier and increase current flow in transistors, just as adding an enzyme to a chemical reaction speeds it up.

Essentially, cells may be viewed as circuits that use molecules, ions, proteins and DNA instead of electrons and transistors. That analogy suggests that it should be possible to build electronic chips — what Sarpeshkar calls "cellular chemical computers" — that mimic chemical reactions very efficiently and on a very fast timescale.

One potentially powerful application of such circuits is in modeling genetic network — the interplay of genes and proteins that controls a cell's function and fate. In a paper presented at the 2009 IEEE Symposium on Biological Circuits and Systems, Sarpeshkar designed a circuit that allows any genetic network reaction to be simulated on a chip. For example, circuits can simulate the interactions between genes involved in lactose metabolism and the transcription factors that regulate their expression in bacterial cells.

In the long term, Sarpeshkar plans to develop circuits that mimic interactions within entire cellular genomes, which are important in enabling scientists to understand and treat complex diseases such as cancer and diabetes. Eventually, researchers may be able to use such chips to simulate the entire human body, he believes. Such chips would be much faster than computer simulations now, which are highly inefficient at modeling the effects of noise in the large-scale nonlinear circuits within cells.

He is also investigating how circuit design principles can help genetically engineer cells to perform useful functions, for example, the robust and sensitive detection of toxins in the environment.

Sarpeshkar's focus on modeling cells as analog rather than digital circuits offers a new approach that will expand the frontiers of synthetic biology, says James Collins, professor of biomedical engineering at Boston University. "Rahul has nicely laid a foundation that many of us in synthetic biology will be able to build on," he says.

LUIS RINCON
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Molekularer Maschendraht für Chips der Zukunft

Molekularer Maschendraht für Chips der Zukunft


Molekularer Maschendraht für Chips der Zukunft
Halbleitende Graphen-Bänder lassen sich gezielt aus Molekülen zusammensetzen

Graphen-Bänder in Zickzack-Struktur (Grafik)
© Empa Mit Kohlenstoff statt Silizium könnten die Transistoren der Zukunft weiter schrumpfen und nur noch mit wenigen Elektronen zwischen "0" und "1" schalten. Galten dafür bislang die vielseitigen Nanoröhrchen als viel versprechendes Material, könnten sie nun von winzigen Bänder aus Graphen – das sind nur eine Atomlage dünne Kohlenstoff-Strukturen – verdrängt werden. Deutsche und schweizerrische Wissenschaftler entwickelten eine elegante Methode, um solche nur Millionstel Millimeter schmalen Bänder mit halbleitenden Eigenschaften herzustellen. In der Zeitschrift "Nature" erläutern sie ihr "Bottom-Up"-Verfahren, das Graphen-Bändern den Weg in die Mikroelektronik ebnen könnte.
"Mit unserer Methode können wir die Bandlücke – die zentrale Voraussetzung für Halbleiter – gezielt einstellen", sagt Roman Fasel vom Schweizer Materialforschungsinstitut Empa in Dübendorf. Zusammen mit Kollegen des Max-Planck-Instituts für Polymerforschung in Mainz sowie der ETH Zürich und von den Universitäten Zürich und Bern setzte er kleinere Moleküle aus Kohlenstoff, Wasserstoff und Halogenen auf hochreine Gold- oder Silberoberflächen. Hier verknüpften sie sich zu längeren Polymerketten. Aufgeheizt auf etwa 440 Grad Celsius, lösten sich Wasserstoff- und Halogenatome ab. Zurück blieben Nanobänder aus Graphen mit genau definierter Struktur.

Je nach Art der zu Beginn verwendeten Monomere entstanden entweder gradlinige oder zickzackförmige Graphenbänder. Diese Strukturen sind verantwortlich für die Größe der elektronischen Bandlücke, die die Schalteigenschaften in einem Transistor oder die Lichtfarbe in Leucht- und Laserdioden beeinflussen. Ein wesentlicher Vorteil im Vergleich zu halbleitenden Nanoröhrchen aus Kohlenstoff liegt in der Sortenreinheit dieser Graphen-Bänder.

Um erste Transistoren aus den Nanobändern herstellen zu können, wollen die Forscher ihre Graphen-Strukturen nun auf Siliziumdioxid statt auf Metallflächen wachsen lassen. Auch an einer Methode zur zerstörungsfreien Ablösung der Graphen-Bänder von der Metalloberfläche wird derzeit gearbeitet.

Erst 2004 erstmals entdeckt, scheinen sich die hauchdünnen Graphen-Strukturen zu einem neuen Schlüsselmaterial für elektronische Anwendungen zu entwickeln. So berichteten koreanische Forscher vor wenigen Wochen, dass sie erfolgreich große Flächen aus Graphen mit einem einfachen Verfahren produzieren konnten. Mit diesen fertigten sie einen flexiblen und zugleich berührungsempfindlichen Flachbildschirm. "Die Graphen-Forschung boomt derzeit und das Material hat in vielen Bereichen den Nanoröhrchen schon die Show gestohlen", sagt Fasel.


LUIS RINCON
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Graphene Grows Up

Graphene Grows Up


Graphene Grows Up



Whether as used for circuitry for displays and detectors or oxidized to produce emissive materials, graphene promises to advance photonics
22 July 2010, SPIE Newsroom. DOI: 10.1117/2.2201007.02
Nanoscale carbon has been making headlines for more than two decades. Whether furled up in carbon nanotubes or assembled into cagelike fullerenes, carbon at the atomic scale shows great potential in everything from photovoltaics to ultra-strong composite materials. Now, graphene -- thin, planar sheets of carbon -- promises to advance photonics with improved detectors, display circuitry, and even emitters.

Graphene is a semi-metal that consists of a hexagonal lattice of carbon atoms, typically in a single atomic layer. It interacts very strongly with light, absorbing approximately 2.3% of incident photons at any wavelength. This unusual broadband behavior makes it enormously appealing for optoelectronic applications such as saturable absorbers for modelocking lasers.

At Cambridge University (Cambridge, UK), researchers led by Nanomaterials and Spectroscopy Group head Andrea Ferrari have used graphene-based saturable absorbers to mode lock tunable lasers. "We first created a laser working at the telecom window at 1550 nm and now we have lasers based on graphene saturable absorbers that can produce 200-fs pulses as well as work over a broad wavelength range."

Another intriguing graphene application is as a more economical substitute for indium tin oxide (ITO), the transparent conductor typically used for contacts in displays. Indium is a relatively scarce material. As displays -- and the demand for ITO -- rise, the price of ITO promises to increase, driving manufacturers to seek out alternatives. Graphene not only consists of plentiful, cheap carbon, it is also less brittle and more flexible than ITO.

"Graphene may not be as good compared to ITO if you just look at the transparency and conductivity, but if you can find applications where the properties of graphene are good enough or you get some other distinct advantages over ITO, then you can justify the use of graphene oxide or reduced graphene oxides for these applications," says Manish Chhowalla, associate professor at Rutgers University (New Brunswick, NJ).

Chhowalla's group collaborated with groups at Umeå University (Umeå, Sweden) and Linköping University (Linköping, Sweden) to produce light-emitting electrochemical cells (LECs) enabled by graphene contacts.1 LECs use electrolytes as the active media. They offer benefits for light emission but the electrolytes tend to react with ITO, reducing the electro-chemical stability of the conductor. For the technology to compete with organic light-emitting diodes (OLEDs), it requires a more suitable transparent conductor like the graphene.

The researchers were able to deposit the electrodes on the LEC from solution, producing light from an essentially all-organic device. "The key thing is that we have an electronically active material that can be processed from a solution," says Chhowalla. "Once you have that, there are techniques readily available to do the patterning and device fabrication." Methods include inkjet printing or stamping or roll-to-roll types of processes.

Watching the Detectors
Graphene can exhibit ballistic transport and in diffusive transport features extremely high electron mobilities. Although researchers have engineered bilayer graphene structures with bandgaps, natural graphene is a zero-band-gap material. Unlike for typical III-V semiconductors, that band structure is symmetric. These characteristics can be particularly useful for photodetectors in which graphene is the active medium. Because the work functions of graphene and metal contacts are generally different, the graphene valence and conduction bands undergo a gradual distortion near the metal-graphene interface, which creates an electric field. That field acts to separate electron- hole pairs generated within a few hundred nanometers of the interface. As a result of the symmetric band structure, the electrons and holes feature comparable mobilities. That ensures a very fast photo response -- as high as 200,000 cm2/Vs in the suspended condition, according to Phaedon Avouris, manager, Nanometer Scale Science & Technology and IBM Fellow and IBM's T.J. Watson Research Center (Yorktown Heights, NY). "Also the Fermi velocity of carriers in graphene is an order of magnitude higher than in any other semiconductor," he says. "If you irradiate a graphene device near the contacts, you're going to get a very fast photocurrent."

In most photodetectors, a bias must be applied between the contacts to collect photocurrent. That bias generates dark current, which contributes to higher shot noise. A graphene-based photodetector requires no bias for photo current collection, so all the current generated in the device results from incident light. Taken together with the broadband response, these characteristics raised the prospect of an ultralow noise, ultra high-speed universal detector.

Of course, this is the real world where nothing comes easily. In general, the electric field generated at the graphene/source interface is the opposite of the electric field generated at graphene/drain interface. As a result, if incident radiation shines on both source and drain, the photo currents generated cancel each other out. To eliminate this problem, the IBM group first tried the obvious solution, masking the detector to irradiate only one set of contacts at a time. The device demonstrated a flat response over speeds from DC level to 40 GHz. In more recent work, the group developed interdigitated electrodes consisting of palladium, a high-work-function metal and titanium, a low-work-function metal (see Figure 1).2 Because the work functions differ, so do the electric fields they generate, which means that the photocurrents do not cancel. In tests at 10 Gb/s, the device demonstrated error-free detection.



Figure 1. Electrodes formed of interdigitated high- and low-work-function metals keeps the electric fields generated at the contacts graphene detectors from canceling, opening the way for high-speed, broadband, low noise detectors. (Courtesy of IBM)
Go For the Glow
Converting graphene to graphene oxide yields a material with a relaxation time long enough to allow photoluminescence. Not only can graphene oxide generate output when optically pumped, it can do so at multiple wavelengths. Chhowalla's group, for example, used a pulsed UV source to obtain emission at both blue and red wavelengths, depending on how the material was processed.3

There is currently a debate underway regarding the exact mechanism that produces the luminescence. "There can be several different reasons for what we think is happening," says Chhowalla. "Graphene oxide contains carbon atoms bonded with oxygen functional groups but that carbon-oxygen bonding is not necessarily uniform. You might have four-or five-membered carbon rings where you don't have any oxygen bonding, so you have small molecular conducting regions within an insulating C-O matrix. These very tiny clusters of carbon have optical and electronic bandgaps associated with them. A larger cluster will have a smaller bandgap and a smaller cluster will have a larger bandgap. By tuning the size of the clusters, we can get red emission from clusters that are large and blue emission from clusters that are small."

Other researchers believe that the photoluminescence is an artifact of the oxygen. At Cambridge University, Ferrari's group has coaxed photoluminescence from graphene oxide produced using oxygen plasma.4 "We wanted to have individual graphene flakes produced by micromechanical exfoliation and in a controlled way cut them down to quantum dots," he says. "If you use a plasma, you can work at room temperature and it is much easier to control the process." He theorizes that the luminescence is due to oxygen related states at the oxidation sites, rather than confinement in small clusters.

Broadband non-linear photoluminescence is also possible in pristine graphene, says Ferrari, with several groups having observed this recently. In much the same way that an ultrafast laser can create a non-equilibrium carrier population and, thus, saturable absorption, these carriers can also recombine, giving rise to luminescence with no need to modify the pristine structure of graphene. Although the target of global interest, the research is in its infancy and gives rise to spirited debate about the physics, the materials characteristics, and even the practicality. Only time will tell whether graphene will eclipse graphene oxide in this area.With the constant global advances and applications such as ITO replacement under aggressive development by display manufacturers, it appears the chances are good we will see graphene products within a three to five year time horizon. "The progress the field has made in the last six years is incredible," says Ferrari. "Graphene optoelectronics and photonics has become a real technology."


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Miniature Energy Harvesting Technology Could Power Wireless Electronics

Miniature Energy Harvesting Technology Could Power Wireless Electronics


Miniature Energy Harvesting Technology Could Power Wireless Electronics
ScienceDaily (July 9, 2010) — The journal NanoLetters recently published an article highlighting the fascinating nanogenerators developed by Dr. Yong Shi, a professor in the Mechanical Engineering Department at Stevens Institute of Technology.


Dr. Shi's work focuses on miniature energy harvesting technologies that could potentially power wireless electronics, portable devices, stretchable electronics, and implantable biosensors. The concept involves piezoelectric nanowire- and nanofiber-based generators that would power such devices through a conversion of mechanical energy into electrical energy. Dr. Shi uses a piezoelectric nanogenerator based on PZT nanofibers. The PZT nanofibers, with a diameter and length of approximately 60 nm and 500 ìm, are aligned on interdigitated electrodes of platinum fine wires and packaged using a soft polymer on a silicon substrate. The measured output voltage and power under periodic stress application to the soft polymer was 1.63 V and 0.03 MicroWatts, respectively.

This amazing breakthrough in piezoelectric nanofiber research has incredible potential to enable new technology development across a multitude of science and engineering industries and related research.

"One of the major limitations of current active implantable biomedical devices is that they are battery powered. This means that they either have to be recharged or replaced periodically. Dr. Shi's group has demonstrated a technology that will allow implantable devices to recover some of the mechanical energy in flowing blood or peristaltic fluid movement in the GI tract to power smart implanable biomedical devices," says, Dr. Arthur Ritter, Director of Biomedical Engineering at Stevens. "The fact that his technology is based on nano-structures makes possible power supplies for nano-robots that can exist in the blood stream for extended periods of time and transmit diagnostic data, take samples for biopsy and/or send images wirelessly to external data bases for analysis."

Dr. Shi's groundbreaking work is part of a rich Institute-wide research community that investigates Nanotechnology and Multiscale Systems in a collaborative entrepreneurial environment.


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Two Brain Circuits Involved With Habitual Learning

Two Brain Circuits Involved With Habitual Learning

Two Brain Circuits Involved With Habitual Learning
ScienceDaily (June 10, 2010) — Driving to and from work is a habit for most commuters - we do it without really thinking. But before our commutes became routine, we had to learn our way through trial-and-error exploration. A new study out of MIT has found that there are two brain circuits involved with this kind of learning and that the patterns of activity in these circuits evolve as our behaviors become more habitual.



These different functions are thought to reside in different parts of basal ganglia. The dorsolateral part of the striatum (the input side of the basal ganglia) controls movement and is connected to the sensorimotor cortex, while the dorsomedial striatum controls flexible behavior and is connected to higher areas known as association cortex. But it has not been clear how these distinct circuits contribute to the learning of new behaviors.

Now for first time, researchers at MIT have recorded the activity of these two circuits in rats as they learned to navigate a maze, and found that the circuits have distinct patterns of activity that evolve during the course of learning.

The team led by Ann Graybiel, a MIT Institute Professor and member of the McGovern Institute for Brain Research, recorded the activity of thousands of neurons in the striatum as rats learned to find a cache of chocolate sprinkles at the end of a maze. As they approached a T-junction in the maze, the rats had to decide whether to turn right or left. The correct direction was indicated by a sound or a touch cue, the meaning of which the rats had to discover through trial-and-error. And just like human commuters, the rats performed this over and over again until the correct choice became routine.

As the rats' performance improved with repetition, the two different striatal circuits showed distinct patterns of activity. The dorsolateral striatal neurons were most active at the specific action points within the maze (start, stop, turn etc) and this pattern became steadily stronger with practice. The dorsomedial neurons, by contrast, showed highest activity around the decision period -- when the rat experienced the cue and had to decide which way to turn. These neurons were also most active as the rats were learning, and their activity declined in later trials once the rats had mastered the task.

"We think the two basal ganglionic circuits must work in parallel," said Catherine Thorn, first author of the study. "We see what looks like competition between the two circuits until the learned behavior becomes ingrained as a habit."

"These brain circuits are affected in Parkinson's disease, substance abuse and many psychiatric disorders," says Graybiel. "If we can learn how to tilt the competition in one direction or the other, we might help bring new focus to existing therapies, and possibly aid in the development of new therapies." But in terms of every day life, Graybiel adds, "it is good to know that we can train our brains to develop good habits and avoid bad ones."


Rats exhibit different patterns of neural activity in the dorsolateral and dorsomedial parts of the striatum while learning to navigate a maze. Dorsolateral striatal neurons are most active (red) when the rat performs specific actions like starting, turning, and stopping. Dorsomedial striatal neurons are most active when the rat is deciding which way to turn, but this activity declines over time as the rat masters the task. (Credit: Catherine Thorn/MIT)



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Quantum-dot comb laser with low relative-intensity noise for each mode

 


Quantum-dot comb laser with low relative-intensity noise for each mode

Quantum-dot comb laser with low relative-intensity noise for each mode


Multiple wavelengths emitted by a semiconductor laser can be used as independent channels for simple and affordable high-bandwidth parallel optical interconnects in photonic integrated circuits.
9 June 2008, SPIE Newsroom. DOI: 10.1117/2.1200806.1143
The field of silicon-based photonics is currently enjoying an explosion of research interest.1 Future progress in information processing and increased computer speed depend both on integrating micro- and optoelectronic devices and developing optical integrated circuits. Electrical interconnects have inherent bandwidth limitations that are spurring the quest for higher-bandwidth optical alternatives. The dense wavelength division multiplexing (DWDM) approach appears to be especially attractive for cost-efficient high-bandwidth chip-to-chip and board-to-board communications. In contrast to parallel optical solutions, DWDM relies on a single, simple, and cheap laser capable of emitting many spectrally separated channels. The use of 100 channels at 10Gb/s modulation would give 1Tb/s total bandwidth in one link. Because efficient lasing in silicon-based devices remains a challenge,2 future optical integrated circuits will likely include a III-V semiconductor laser integrated into a silicon chip.




Figure 1. Lasing spectra. (a) Overall spectrum around the mode of the maximum intensity. (b) Spectrally filtered mode. SMSR: Single-mode suppression ratio.
An attractive candidate for such a multiwavelength emitter is an edge-emitting (or Fabry–Perot, abbreviated FP) semiconductor laser. Wavelengths that correspond to different longitudinal modes of the FP resonator can be filtered out, separately modulated, and used as independent optical channels. In this case, channel separation is naturally determined by resonator length. All channels can be stabilized and tracked simultaneously.

A prerequisite for application of a multiwavelength light source in DWDM systems is low relative-intensity noise of each longitudinal mode (modal RIN). In single-frequency lasers with a large side mode suppression ratio (>40dB), the values of RIN are rather low. These lasers are suitable for telecommunications applications. In case of the conventional quantum well (QW) multimode (multifrequency) lasers, the value of RIN rises significantly due to random redistribution of mode intensity between longitudinal laser modes. In contrast, in quantum dot (QD) lasers, the nonlinear gain saturation effect is pronounced, and a strong decrease in RIN for individual FP modes can be achieved.





Figure 2. Spectrum of relative intensity noise (RIN) of filtered modal intensity at 1265.2nm.
We grew a QD laser structure by molecular beam epitaxy.3 The active area consisted of 10 planes of InAs/In0.15Ga0.85As QDs. Waveguide FP lasers with a 3μm-wide ridge were fabricated, with a cavity length of roughly 1mm. For eye-diagram and bit-error-rate (BER) evaluations, an individual longitudinal mode of the optically isolated laser was sifted out using a fiber FP tunable filter. The spectrally filtered mode was amplified by passing it through optical amplifiers and then externally modulated.

Figure 1(a) shows an overall emission spectrum of the QD laser taken at a continuous wave current of 85mA. The output power is 50mW per two laser facets. The spectrum comprises a series of longitudinal FP modes separated by 0.22nm. The zeroth-order mode, i.e., the mode of the maximum intensity, is centered at 1265.5nm. Its full width at half maximum is about 2nm. An external FP etalon can be adjusted to transmit only one longitudinal mode. We experimented with longitudinal modes that fall into a wavelength interval from 1263.3 to 1266.4nm. These modes are indicated in Figure 1(a) by an index ranging from −10 to +4. Figure 1(b) shows a filtered intensity of the −10th longitudinal mode.

Modal RIN for the spectrally filtered FP mode is shown in Figure 2. At low frequencies, the RIN spectrum is nearly flat at a level of −105dB/Hz. With higher frequencies, the modal RIN drops from −120 to −145dB/Hz in the 0.1–10GHz range. If a received power is sufficiently high, the BER is governed by the total RIN, i.e., an integral over the relative intensity noise spectrum. Our calculation has shown that the total RIN of 0.4% would be acceptable for error-free transmission (BER<10−15). Using the data in Figure 2, the total RIN was calculated to be 0.21% over the full frequency range of analysis (0.001–10GHz).





Figure 3. Eye diagram for the zeroth mode at –3dBm received power at the photodetector.
An individual longitudinal mode after spectral filtering was modulated at 10Gb/s by a 231−1 pseudorandom binary non-return-to-zero sequence (see Figure 3) and shows an eye pattern. We obtained error-free operation with a BER of less than 10−13. A similar BER was achieved for the modes with the highest intensity (mode order ranges from −7 to +2): see Figure 1(a).

The results demonstrate that we can reduce the requirements for precise lasing wavelengths because channel separation is naturally pre-determined by only one parameter, which is cavity length. All channels can be stabilized and tracked simultaneously. Sufficiently long wavelengths make such lasers compatible with optical and optoelectronic components based on silica fiber or silicon-based planar waveguides. The multimode QD laser shows promise for the development of simple and cost-effective high-bandwidth optical interconnects for silicon-based photonic integrated circuits.

Future optimization of the comb laser is aimed at increasing the intensity of individual FP modes and further improving their RIN level, as well as increasing the number of channels suitable for optical transmission. In cooperation with academic and industrial partners, we have also started to develop transmitters on photonic integrated circuits in III-V materials and on silicon photonic chips. In each case, the transmitter comprises a laser fiber-coupled to a demultiplexer that separates the comb input channels, modulators for each of the channels, a multiplexer that combines the modulated channels, and a coupler into the output fiber. We expect to demonstrate the feasibility of our comb laser's for short- to medium-reach WDM transmission, such as 100Gb/s Ethernet and passive optical networks.


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domingo, 27 de junio de 2010

Precise Trace Gas Analysis, Without the Noise

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Scientists can analyze atmospheric gas concentrations with laser-based sensors that use systems similar to the one illustrated above. The sensor instrument directs a laser through gas and, based on how much laser light is absorbed by a sample, scientists can determine the specific gases present and their concentrations


Precise Trace Gas Analysis, Without the Noise
ScienceDaily ) — Analyzing trace atmospheric gases can now be considerably more precise with the help of a device that delivers stable and reliable power to the lasers used in gas sensors.


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The low-noise current controller was developed at the Department of Energy's Pacific Northwest National Laboratory. The technology was recently licensed to Bozeman, Montana-based Wavelength Electronics Inc. by Battelle, which operates PNNL for DOE.

"Low-noise current controllers open up new ways for us to analyze trace gases," said Matthew Taubman, a PNNL scientist who developed the device. "Now we can evaluate significantly smaller gas concentrations."

Scientists often analyze atmospheric gas concentrations with laser-based sensors. Researchers sample air at sites of interest, such as on the ground near power plants or at high altitudes from airplanes. The sensor instrument then directs a laser through the sample. Based on how much laser light is absorbed by the sample, scientists can determine the specific gases present and their concentrations.

But smaller concentrations of certain gases can be challenging to analyze. One particular problem occurs when "noises," or random fluctuations, exist in a laser's wavelength and line width. Such noise prevents researchers from making precise readings.

PNNL scientists reduced this problem by developing a low-noise current controller. The device reduces the noise on the laser's power source, allowing scientists to detect smaller levels of trace gases. PNNL's controller is the lowest noise controller on the market that was specifically designed for extra-sensitive sensors that use quantum cascade lasers, also called QCLs. Sensors made with QCLs emit light in a wavelength region that many trace gases strongly absorb. QCL-based sensors become even more sensitive when they are powered with low-noise current controllers like PNNL's.

Wavelength Electronics already develops compact current controllers and related components for a variety of semiconductor lasers. Wavelength CEO Mary Johnson also recognized there was a strong potential for her company to make similar QCL-friendly components for Wavelength's customers -- original equipment manufacturers, or OEMs.

That prompted Wavelength to request support through PNNL's Technology Assistance Program, which pairs small businesses looking to overcome specific technology challenges with PNNL scientists. Through the program, Taubman -- the researcher who developed PNNL's low-noise current controller -- compared Wavelength controllers to PNNL low-noise controllers.

At the same time, PNNL and Wavelength brought PNNL's low-noise controllers to one of Wavelength's customers, sensor systems manufacturer Aerodyne Research Inc. of Billerica, Massachusetts. Aerodyne tested the low-noise controllers and learned the devices provided the sensitivity their QCL-based sensors needed. This convinced Johnson that licensing PNNL's controllers would speed up Wavelength's entry into the QCL-based sensor market.

"There's a tremendous need for this technology right now and Wavelength Electronics is pleased to be able to offer it to trace gas researchers and analytical instrument manufacturers," Johnson said.

Wavelength is looking to launch products that incorporate PNNL's low-noise current controller technology by the end of this year. The products will specifically target the QCL market. But other models will be available to work with laser diodes that could be used in microbial detection, skin cancer scanning, DNA sequencing, remote measurements and testing with VCSELs, or vertical cavity surface-emitting lasers, a kind of laser diode.

But Aerodyne didn't want to wait until Wavelength released its new products. Soon after the license went through, Wavelength made the sensor system manufacturer a copy of the low-noise controller for immediate use. Aerodyne is now using these controllers with their most demanding sensor applications, those with continuous-wave QCLs.
http://www.sciencedaily.com/releases/2010/05/100524130844.htm

Researchers Identify the Source of 'Noise' in HIV

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Researchers Identify the Source of 'Noise' in HIV
ScienceDaily (Apr. 20, 2010) — New research identifies a molecular mechanism that the human immunodeficiency virus (HIV) appears to utilize for generating random fluctuations called "noise" in its gene expression. The study, published by Cell Press in the April 20th issue of the Biophysical Journal, pinpoints the likely source of HIV gene-expression noise and provides intriguing insight into the role of this noise in driving HIV's fate decision between active replication and latency.


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After infecting a human cell, HIV integrates into the genome and typically begins to actively replicate. However, the virus can also enter a long-lived latent state, which remains the greatest barrier to eradicating virus from the patient. Senior study author, Dr. Leor S. Weinberger, a molecular virologist and systems biologist from the Department of Chemistry and Biochemistry at the University of California, San Diego, recently showed that noise in HIV gene-expression critically influences the viral decision to enter either active replication or latency. However, the source of the noise was not clear.

To probe the source of this inherent noise in HIV gene expression, Dr. Abhyudai Singh working in Dr. Weinberger's laboratory exploited a technique from electrical engineering that analyzes how noise changes across different levels of expression. The researchers examined cells carrying a single integrated copy of HIV engineered to produce a quantifiable protein, and measured HIV-1 expression noise at dozens of different viral integration sites which act as distinct genetic environments for viral gene expression.

Surprisingly, the authors find that HIV noise levels are substantially higher than measured in other organisms, and that HIV gene expression occurs in randomly timed bursts. During these expression bursts multiple copies of HIV gene products are produced which leads to the high noise levels in HIV gene expression. The bursting model argues that during active expression HIV cycles between periods of silence and bursting and provides insight into how HIV may be activated by host signaling molecules.

"We know that noise in gene-expression can critically influence HIV's entry to proviral latency. These new results point to transcriptional bursting as a major source of the noise" says Dr. Weinberger. "This finding that transcriptional bursting generates an exceptionally noisy HIV promoter, noisier than almost all other measured promoters, supports the theory that latency may be fundamental to the HIV life cycle and that HIV evolved for probabilistic entry into latency."

Researchers include Abhyudai Singh, University of California, San Diego, La Jolla, CA; Brandon Razooky, University of California, San Diego, La Jolla, CA; Chris D. Cox, University of Tennessee, Knoxville, TN; Michael L. Simpson, Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN; and Leor S. Weinberger, University of California, San Diego, La Jolla, CA, Whitaker Institute for Biomedical Engineering University of California, San Diego, La Jolla, CA.
http://www.sciencedaily.com/releases/2010/04/100420132828.htm

Molecules As Electric Conductors

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Molecules As Electric Conductors
ScienceDaily (July 19, 2009) — Researchers from Graz University of Technology, Humboldt University in Berlin, M.I.T., Montan University in Leoben and Georgia Institute of Technology report an important advance in the understanding of electrical conduction through single molecules.


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Minimum size, maximum efficiency: The use of molecules as elements in electronic circuits shows great potential. One of the central challenges up until now has been that most molecules only start to conduct once a large voltage has been applied. An international research team with participation of the Graz University of Technology has shown that molecules containing an odd number of electrons are much more conductive at low bias voltages. These fundamental findings in the highly dynamic research field of nanotechnology open up a diverse array of possible applications: More efficient microchips and components with considerably increased storage densities are conceivable.

One electron instead of two: Most stable molecules have a closed shell configuration with an even number of electrons. Molecules with an odd number of electrons tend to be harder for chemists to synthesize but they conduct much better at low bias voltages. Although using an odd rather than an even number of electrons may seem simple, it is a fundamental realization in the field of nanotechnology – because as a result of this, metal elements in molecular electronic circuits can now be replaced by single molecules. "This brings us a considerable step closer to the ultimate minitiurization of electronic components", explains Egbert Zojer from the Institute for Solid State Physics of the Graz University of Technology.

Molecules instead of metal

The motivation for this basic research is the vision of circuits that only consist of a few molecules. "If it is possible to get molecular components to completely assume the functions of a circuit's various elements, this would open up a wide array of possible applications, the full potential of which will only become apparent over time. In our work we show a path to realizing the highly electrically conductive elements", Zojer excitedly reports the momentous consequences of the discovery.

Specific new perspectives are opened up in the field of molecular electronics, sensor technology or the development of bio-compatible interfaces between inorganic and organic materials: The latter refers to the contact with biological systems such as human cells, for instance, which can be connected to electronic circuits in a bio-compatible fashion via the conductive molecules.

When Noise Becomes the Signal

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When Noise Becomes the Signal
ScienceDaily (Apr. 21, 2010) — European researchers have developed a new class of electronics that uses noise -- normally a problem -- as part of the signal. It means better, faster electronics.



"It is a tale, Told by an idiot, full of sound and fury, Signifying nothing," according to Shakespeare's Macbeth. Of course Shakespeare was speaking about the brevity of life, but his words apply equally to noise in a signal.

A signal -- any signal -- inevitably has some noise, a degree of imprecision that carries no information, literally signifying nothing, and even confusing the underlying data. Most of the time engineers work very hard to ensure a high signal to noise ratio -- lots of signal for very little noise.

"That is why in commonly used electronics, the signals are boosted well above noise margins," explains Lukas Worschech, coordinator of the SUBTLE project (http://subtle.fisica.unipg.it/objectives) and a professor at the University of Wurzburg.

That is a fine and worthy goal in most applications, but as devices become smaller, and more complex, the noise becomes a greater and greater problem. It is a bit like dots of ink becoming less sharp as they get smaller, as with impressionist paintings.

"Electronics is based on switches, which can turn on and off signals. The smaller the switches are, the more complex circuits can be realised," notes Worschech.

"However, with increasing miniaturisation of electronic circuits, an increasing fraction of the applied power is converted into nondeterministic signals that add to the ambient noise. It is sometimes referred to as the thermal death of electronics."

The love of limits

It is a problem, and it is setting limits on the miniaturisation, complexity and power of small circuits. Engineers and scientists love limits; it gives them a chance to employ some cunning insight to overcome them.

The scientists and engineers at the SUBTLE project have been very cunning, and their solution is subtle indeed. The project sought to use the noise -- normally a problem -- to boost the signal.

The idea relies on a concept called 'stochastic resonance' (SR), a phenomena in physics first identified in climatology to explain a pronounced variability within the climatic system, where very small changes can lead to profound impacts, such as a mini-ice age.

Stochastic resonance is where a very small variation in a cycle can 'tune in' with other periodical variations to create a massive impact. Its application depends on nonlinear systems, where linear inputs do not equate to linear outputs -- where the sum is greater than the parts.

"This is the SUBTLE solution: do not avoid noise, but exploit noise by SR. This is [made] possible by utilising the feedback action between switches and conducting channels," reveals Worschech. "That is what naturally happens in nanoelectronics. The conductors are very closely spaced to each other. The interaction can even be tailored by the shaping of the conductors."

Forced clarity

Applied to signals, the phenomena opens up the opportunity for engineers to 'force' signal clarity by tuning noise within it. It offers a possible solution to the problem of noise in extremely small components. The idea is not new and was first mooted some years ago, but making it work is an enormous engineering and scientific challenge.

A challenge greeted with gusto by the EU-funded SUBTLE project. The work programme was ambitious; they sought to conceive and create a new class of electronics that uses noise to enhance the signal.

"SUBTLE is a STREP project associated with nanoelectronic devices in which quantum-confined electron channels are so closely spaced to each other that tailored feedback action exists," Worschech notes.

This tailored feedback enhances the signal. The devices employ two, allied, phenomena: back action on the channel gate and noise-induced switching. A channel gate is used to route a signal, and back action is like feedback in an audio system. The subsequent noise can be used to switch the circuit from one channel to another.

Both required very sophisticated circuit design and fabrication and SUBTLE took full advantage of its partners, who are European leaders in the field.

They developed highly novel techniques and pushed the state of the art in a number of domains. They proved selective etching of independent contacts in a double quantum well structure, creating a quantum gate transistor. It is an extremely complex element at nano-scale dimensions and required molecular beam precision to be realised.

These are very sophisticated achievements that enable smaller, cheaper, more power efficient and complex circuits, and they may have application in other fields, too. For example, the SUBTLE team, as part of its work, developed submicron arrays of resonant tunneling diodes that can act as an artificial neuron.

Artificial neurons

These are simple computing, logic gates. Their actions resemble the firing of signals as they are observed between neurons. The SUBTLE team believes that its devices can be thus used in the future to mimic neuron action in artificial networks and to serve as sensors for signals usually hidden under the noise.

This work demonstrates starkly the amazing potential of these new sensors. They can operate at less than millivolts, significantly less that the current state of the art. In combination, they could be used to create neural networks, where actions cascade based in part on the noise of individual spiking neurons. This incredible sensitivity makes the devices an ideal candidate for quantum computing.

And there's more. Typical computers focus on output, the result of a sum: 5 + 2 = 7. Conventional computers discard the input -- the individual transistor states that give us the binary digits. However, the quantum transistors developed by SUBTLE can morph, or reverse, opening the prospect of maintaining the inputs.

"This could be a reversible computer, where you could return to the inputs from the output. It will probably be essential for quantum computing because there will be instances where you need the input," explains Worschech.

These are areas for further research, and the consortium is hoping to set up another project. In the meantime, the project has filed a patent and has been approached by companies that want to build sensors from SUBTLE technology.

"These gates work at such low voltages and with so little noise that they are far ahead of the current state of the art in terms of the sensitivity," Worschech reveals.

The wide variety of applications and the intensity of excitement generated by the SUBTLE project demonstrate the importance of its work. It has led to a new class of electronics and a whole host of potential new solutions to old problems. And all from some very subtle insights.

The SUBTLE project received funding from the FET- Open strand of the EU's Sixth Framework Programme for research.
http://www.sciencedaily.com/releases/2010/04/100422085553.htm

New Material Can Keep Electronics Cool: Few Atomic Layers of Graphene Reveal Unique Thermal Properties

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New Material Can Keep Electronics Cool: Few Atomic Layers of Graphene Reveal Unique Thermal Properties
ScienceDaily (May 10, 2010) — Professor Alexander Balandin and a team of UC Riverside researchers, including Chun Ning Lau, an associate professor of physics, have taken another step toward new technology that could keep laptops and other electronic devices from overheating.



Balandin, a professor of electrical engineering in the Bourns College of Engineering, experimentally showed in 2008 that graphene, a recently discovered single-atom-thick carbon crystal, is a strong heat conductor. The problem for practical applications was that it is difficult to produce large, high quality single atomic layers of the material.

Now, in a paper published in Nature Materials, Balandin and co-workers found that multiple layers of graphene, which are easier to make, retain the strong heat conducting properties.

That's also a significant discovery in fundamental physics. Balandin's group, in addition to measurements, explained theoretically how the materials' ability to conduct heat evolves when one goes from conventional three-dimensional bulk materials to two-dimensional atomically-thin films, such as graphene.

The results published in Nature Materials may have important practical applications in removal of dissipated hear from electronic devices.

Heat is an unavoidable by-product when operating electronic devices. Electronic circuits contain many sources of heat, including millions of transistors and interconnecting wiring. In the past, bigger and bigger fans have been used to keep computer chips cool, which improved performance and extended their life span. However, as computers have become faster and gadgets have gotten smaller and more portable the big-fan solution no longer works.

New approaches to managing heat in electronics include incorporating materials with superior thermal properties, such as graphene, into silicon computer chips. In addition, proposed three-dimension electronics, which use vertical integration of computer chips, would depend on heat removal even more, Balandin said.

Silicon, the most common electronic material, has good electronic properties but not so good thermal properties, particularly when structured at the nanometer scale, Balandin said. As Balandin's research shows, graphene has excellent thermal properties in addition to unique electronic characteristics.

"Graphene is one of the hottest materials right now," said Balandin, who is also chair of the Material Sciences and Engineering program. "Everyone is talking about it."

Graphene is not a replacement for silicon, but, instead could be used in conjunction with silicon, Balandin said. At this point, there is no reliable way to synthesize large quantities of graphene. However, progress is being made and it could be possible in a year or two, Balandin said.

Initially, graphene would likely be used in some niche applications such as thermal interface materials for chip packaging or transparent electrodes in photovoltaic solar cells, Balandin said. But, in five years, he said, it could be used with silicon in computer chips, for example as interconnect wiring or heat spreaders. It may also find applications in ultra-fast transistors for radio frequency communications. Low-noise graphene transistors have already been demonstrated in Balandin's lab.

Balandin published the Nature Materials paper with two of his graduate students Suchismita Ghosh, who is now at Intel Corporation, and Samia Subrina, Lau. one of her graduate students, Wenzhong Bao, and Denis L. Nika and Evghenii P. Pokatilov, visting researchers in Balandin's lab who are based at the State University of Moldova.
http://www.sciencedaily.com/releases/2010/05/100510132211.htm

Single-Atom Transistor Discovered

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(a) Colored scanning electron microscope image of the measured device. Aluminum top gate is used to induce a two-dimensional electron layer at the silicon-silicon oxide interface below the metallization. The barrier gate is partially below the top gate and depletes the electron layer in the vicinity of the phosphorus donors (the red spheres added to the original image). The barrier gate can also be used to control the conductivity of the device. All the barrier gates in the figure form their own individual transistors. (b) Measured differential conductance through the device at 4 Tesla magnetic field. The red and the yellow spheres illustrate the spin-down and -up states of a donor electron which induce the lines of high conductivity clearly visible in the figure. (Credit: American Chemical Society)

Single-Atom Transistor Discovered
ScienceDaily — Researchers from Helsinki University of Technology (Finland), University of New South Wales (Australia), and University of Melbourne (Australia) have succeeded in building a working transistor, whose active region composes only of a single phosphorus atom in silicon.


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The results have just been published in Nano Letters, a journal of the American Chemical Society.

The working principles of the device are based on sequential tunneling of single electrons between the phosphorus atom and the source and drain leads of the transistor. The tunneling can be suppressed or allowed by controlling the voltage on a nearby metal electrode with a width of a few tens of nanometers.

The rapid development of computers, which created the present information society, has been mainly based on the reduction of the size of transistors. Scientists have known for a long time that this development has to slow down critically during the future decades when the even tighter inexpensive packing of transistors would require them to shrink down to the atomic length scales. In the recently developed transistor, all the electric current passes through the same single atom. This allows researchers to study the effects arising in the extreme limit of the transistor size.

"About half a year ago, I and one of the leaders of this research, Prof. Andrew Dzurak, were asked when we expect a single-atom transistor to be fabricated. We looked at each other, smiled, and said that we have already done that," says Dr. Mikko Möttönen. "In fact, our purpose was not to build the tiniest transistor for a classical computer, but a quantum bit which would be the heart of a quantum computer that is being developed worldwide," he continues.

Problems arising when the size of a transistor is shrunk towards the ultimate limit are due to the emergence of so-called quantum mechanical effects. On one hand, these phenomena are expected to challenge the usual transistor operation. On the other hand, they allow classically irrational behavior which can, in principle, be harnessed for conceptually more efficient computing, quantum computing.

The driving force behind the measurements reported now is the idea to utilize the spin degree of freedom of an electron of the phosphorus donor as a quantum bit, a qubit. The researchers were able to observe in their experiments spin up and down states for a single phosphorus donor for the first time. This is a crucial step towards the control of these states, that is, the realization of a qubit.
http://www.sciencedaily.com/releases/2009/12/091206085833.htm

Nanoelectronic Transistor Combined With Biological Machine Could Lead To Better Electronics

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Nanoelectronic Transistor Combined With Biological Machine Could Lead To Better Electronics


Nanoelectronic Transistor Combined With Biological Machine Could Lead To Better Electronics
ScienceDaily (Sep. 7, 2009) — If artificial devices could be combined with biological machines, laptops and other electronic devices could get a boost in operating efficiency.

An artist's representation of a nanobioelectronic device incorporating alamethycin biological pore. In the core of the device is a silicon nanowire (grey), covered with a lipid bilayer (blue). The bilayer incorporates bundles of alamethicin molecules (purple) that form pore channels in the membrane. Transport of protons though these pore channels changes the current through the nanowire. (Credit: Image by Scott Dougherty, LLNL)
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Lawrence Livermore National Laboratory researchers have devised a versatile hybrid platform that uses lipid-coated nanowires to build prototype bionanoelectronic devices.

Mingling biological components in electronic circuits could enhance biosensing and diagnostic tools, advance neural prosthetics such as cochlear implants, and could even increase the efficiency of future computers.

While modern communication devices rely on electric fields and currents to carry the flow of information, biological systems are much more complex. They use an arsenal of membrane receptors, channels and pumps to control signal transduction that is unmatched by even the most powerful computers. For example, conversion of sound waves into nerve impulses is a very complicated process, yet the human ear has no trouble performing it.

"Electronic circuits that use these complex biological components could become much more efficient," said Aleksandr Noy, the LLNL lead scientist on the project.

While earlier research has attempted to integrate biological systems with microelectronics, none have gotten to the point of seamless material-level incorporation.

"But with the creation of even smaller nanomaterials that are comparable to the size of biological molecules, we can integrate the systems at an even more localized level," Noy said.

To create the bionanoelectronic platform the LLNL team turned to lipid membranes, which are ubiquitous in biological cells. These membranes form a stable, self-healing,and virtually impenetrable barrier to ions and small molecules.

"That's not to mention that these lipid membranes also can house an unlimited number of protein machines that perform a large number of critical recognition, transport and signal transduction functions in the cell," said Nipun Misra, a UC Berkeley graduate student and a co-author on the paper.

Julio Martinez, a UC Davis graduate student and another co-author added: "Besides some preliminary work, using lipid membranes in nanoelectronic devices remains virtually untapped."

The researchers incorporated lipid bilayer membranes into silicon nanowire transistors by covering the nanowire with a continuous lipid bilayer shell that forms a barrier between the nanowire surface and solution species.

"This 'shielded wire' configuration allows us to use membrane pores as the only pathway for the ions to reach the nanowire," Noy said. "This is how we can use the nanowire device to monitor specific transport and also to control the membrane protein."

The team showed that by changing the gate voltage of the device, they can open and close the membrane pore electronically.

The research appears Aug. 10 in the online version of the Proceedings of the National Academy of Sciences.

Photography: Blur's Noise And Distortion Reversed

LUIS RINCON EES 18257927


Photography: Blur's Noise And Distortion Reversed
ScienceDaily (July 9, 2009) — Errant pixels and blurry regions in a photo, whether digital or scanned, are the bane of photographers everywhere. Moreover, in vision processing research degraded photos are common and require restoration to a high-quality undegraded state. Research published in the International Journal of Signal and Imaging Systems Engineering could provide new insights.


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There are countless examples of image editors and photo cleanup software that have built-in tools designed to remove noise and sharpen up edges. Some of these are very powerful others less so. Any "cleanup" process that works by changing individual pixels leads to overall degradation of the image and loss of information. However, a delicate touch with the most subtle tools can produce acceptable quality results.

Now, S. Uma of the Department of Electronics and Communication Engineering, at Coimbatore Institute of Technology, and S. Annadurai of the Government College of Technology, Coimbatore, India, have turned to neural networks to help them clean up their image. The approach could significantly reduce information loss while reversing blurring caused by lens aberrations and faults and reducing noise that distorts the appearance of an image. The team suggests that distortions in an image due to atmospheric disturbances between camera and distant subjects could be unraveled and a photo taken on a hot, hazy day made acceptable.

The researchers point out that earlier attempts at this kind of inverse filtering of an image rely on the image having a high signal-to-noise (SNR) ratio. Other approaches require huge amounts of computing power and are generally untenable. This is especially true in the fledgling field of artificial vision, whether robotic or prosthetic. However, some success with neural networks has been achieved.

Now, Uma and Annadurai have developed a modified recurrent Hopfield neural network that builds and extends the work of others to allow them to quickly process an image reducing distortion, noise and blurring. The team has tested their approach on square grayscale images just 256 pixels across. They were able to reverse severe blurring and noise deliberately added to the original photographic sample to much more acceptable levels in a short time using limited computing resources than was possible with previous neural network approaches or any other inverse filtering techniques.

An analysis of the before and after quality shows that quality is improved by between 39% and 67% using the team's approach and results take half the time of other methods that produce lesser improvements. The success bodes well for image processing, in various fields including vision research, art, homeland security, and science.

http://www.sciencedaily.com/releases/2009/07/090708094827.htm

Electrosmog on the circuit board

LUIS RINCON EES 18257927



Electrosmog On The Circuit Board
http://www.sciencedaily.com/releases/2009/04/090406102630.htm
ScienceDaily (Apr. 9, 2009) — The smaller the components in electronic circuits, the more interference-prone they are. If the components are too densely packed, they can interfere with one another. A near-field scanner can accurately detect weak fields and help to protect bank cards against fraud.


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Their miniature size is their strength – and also their weakness. Be it in cell phones, cars or computers, electronic components are getting smaller and smaller and increasingly powerful. The smaller they are, the faster they can switch and the less energy they need for each switching operation. However, as energy requirements shrink, so do signal-to-noise ratios. "Circuits are becoming more and more susceptible with each generation," explains Thomas Mager of the Fraunhofer Research Institution for Electronic Nano Systems ENAS in Paderborn.

"Only a few years ago, it still took several volts to destabilize processors. Today, a few hundred millivolts are sometimes enough to disrupt millions of transistors." This means that designers of electronic circuits need to give greater consideration to electromagnetic compatibility. It is no longer just a question of protecting complete electronic packages such as cell phones or MP3 players against external influences, or shielding the environment against their electromagnetic emissions, but is also about how each individual component on the circuit board behaves.

In a collaborative project carried out with Continental and Infineon Technologies, the Fraunhofer ENAS has developed a measuring system that can locate even the weakest electrical and magnetic fields to an accuracy of a few hundredths of a millimeter. Where are there areas of conspicuously high electromagnetic radiation? How do the components influence one another? The near-field scanner can scan not only individual chips and processors but also complete laptops, cell phones or aircraft control units, and can reveal which types of field the test object is radiating.

"We are also working with our French project partner CEA-Leti on a function that applies targeted electromagnetic fields to the test object. In this way, we can test for areas that respond sensitively to external fields," says Mager. This makes the system particularly interesting for developers of smart cards. Fraudsters elicit confidential information from bank cards by bombarding them with pulses of laser light, electrical current or voltage. The resulting field patterns can reveal details about the chip card, such as its PIN number. The near-field scanner provides time- and space-resolved images of the radiated fields of the card, allowing their weak points to be identified and helping card developers to better protect their products against fraud.

Discovery Promises To Improve Semiconductor-Based Sensors


Publicado por Luis Rincon CI 18257927 EES

 


Illustration of a Coulomb glass system: Electrons (red) in a random landscape, interacting with each other (yellow-orange lines). Noise of the resistance of the system is created by collective "hopping" of the electrons (green arrow). (Credit: Image courtesy of DOE/Argonne National Laboratory)

Discovery Promises To Improve Semiconductor-Based Sensors
ScienceDaily (May 10, 2007) — More sensitive sensors and detectors based on semiconductor electronics could result from new findings by researchers from the United States, Norway and Russia.

http://www.sciencedaily.com/releases/2007/05/070509161026.htm

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Their research has yielded a decisive step in identifying the origin of the universal "one-over-f" (1/f) noise phenomenon; "f" stands for "frequency."

"One-over-f noise appears almost everywhere, from electronic devices and fatigue in materials to traffic on roads, the distribution of stars in galaxies, and DNA sequences," said Valerii Vinokour or Argonne's Materials Science Division. "Finding the common origin of one-over-f noise in its many forms is one of the grand challenges of materials physics. Our theory establishes the origin and lower limit to one-over-f noise in semiconductor electronics, helping to optimize detectors for commercial application."

Noise is a fluctuation in time, a deviation from the average. Humans and other animals carry a common example in their heartbeats, where 1/f noise can be detected as a deviation from normal pulse. In nanomaterials, such as the tiny circuits in semiconductor electronics, the noise generated by the random motion of a single electron can be devastating, since there are so few electrons in the system.

Vinokur and his team showed that the 1/f noise in doped semiconductors, the platform for all modern electronics, originates in the random distribution of impurities and the mutual interaction of the many electrons surrounding them. These two ingredients — randomness and interaction — trap electrons in the Coulomb glass, a state like window glass where electrons move by hopping from one random location to another. 1/f noise arises from the electrons; hopping motion. After discovering the theoretical connection between 1/f noise and formation of the Coulomb glass, Vinokur and his collaborators confirmed it with large-scale computer simulations: suppression of the interactions was found to remove the Coulomb glass behavior and 1/f noise.

"Our results," Vinokour said, "establish that one-over-f noise is a generic property of Coulomb glasses and, moreover, of a wide class of random interacting systems and phenomena ranging from mechanical properties of real materials and electric properties of electronic devices to fluctuations in the traffic of computer networks and the Internet."

These research findings were published in the May 11 issue of Physical Review Letters.

Collaborators on this research were Vinokur and Andreas Glatz, at Argonne, Y.M. Galperin from University of Oslo, Oslo, Norway, and A.F. Ioffe of the Physico-Technical Institute of the Russian Academy of Sciences, St. Petersburg, Russia.