Saturday, July 30, 2011

Discovery may overcome obstacle for quantum computing: Researchers find a way to quash decoherence

 Researchers have made a major advance in predicting and quashing environmental decoherence, a phenomenon that has proven to be one of the most formidable obstacles standing in the way of quantum computing.

The findings -- based on theoretical work conducted at the University of British Columbia and confirmed by experiments at the University of California Santa Barbara -- are published online in the July 20 issue of the journal Nature.

Quantum mechanics states that matter can be in more than one physical state at the same time -- like a coin simultaneously showing heads and tails. In small objects like electrons, physicists have had success in observing and controlling these simultaneous states, called "state superposition."

Larger, more complex physical systems appear to be in one consistent physical state because they interact and "entangle" with other objects in their environment. This entanglement makes these complex objects "decay" into a single state -- a process called decoherence.

Quantum computing's potential to be exponentially faster and more powerful than any conventional computer technology depends on switches that are capable of state superposition -- that is, being in the "on" and "off" positions at the same time. Until now, all efforts to achieve such superposition with many molecules at once were blocked by decoherence.

"For the first time we've been able to predict and control all the environmental decoherence mechanisms in a very complex system, in this case a large magnetic molecule called the 'Iron-8 molecule,'" said Phil Stamp, UBC professor of physics and astronomy and director of the Pacific Institute of Theoretical Physics. "Our theory also predicted that we could suppress the decoherence, and push the decoherence rate in the experiment to levels far below the threshold necessary for quantum information processing, by applying high magnetic fields."

In the experiment, the California researchers prepared a crystalline array of Iron-8 molecules in a quantum superposition, where the net magnetization of each molecule was simultaneously oriented up and down. The decay of this superposition by decoherence was then observed in time -- and the decay was spectacularly slow, behaving exactly as the UBC researchers predicted.

"Magnetic molecules now suddenly appear to have serious potential as candidates for quantum computing hardware," said Susumu Takahashi, assistant professor of chemistry and physics at the University of Southern California. "This opens up a whole new area of experimental investigation with sizeable potential in applications, as well as for fundamental work."

Takahashi conducted the experiments while at UC Santa Barbara and analyzed the data while at UC Santa Barbara and the University of Southern California.

"Decoherence helps bridge the quantum universe of the atom and the classical universe of the everyday objects we interact with," Stamp said. "Our ability to understand everything from the atom to the Big Bang depends on understanding decoherence, and advances in quantum computing depend on our ability to control it."

The research was supported by the Pacific Institute of Theoretical Physics at UBC, the Natural Sciences and Engineering Research Council of Canada, the Canadian Institute for Advanced Research, the Keck Foundation, and the National Science Foundation.

Story Source:

The above story is reprinted (with editorial adaptations ) from materials provided by University of British Columbia.

Journal Reference:

S. Takahashi, I. S. Tupitsyn, J. van Tol, C. C. Beedle, D. N. Hendrickson, P. C. E. Stamp. Decoherence in crystals of quantum molecular magnets. Nature, 2011; DOI: 10.1038/nature10314

New photonic crystals have both electronic and optical properties

 In an advance that could open new avenues for solar cells, lasers, metamaterials and more, researchers at the University of Illinois have demonstrated the first optoelectronically active 3-D photonic crystal.

"We've discovered a way to change the three-dimensional structure of a well-established semiconductor material to enable new optical properties while maintaining its very attractive electrical properties," said Paul Braun, a professor of materials science and engineering and of chemistry who led the research effort.

The team published its advance in the journal Nature Materials.

Photonic crystals are materials that can control or manipulate light in unexpected ways thanks to their unique physical structures. Photonic crystals can induce unusual phenomena and affect photon behavior in ways that traditional optical materials and devices can't. They are popular materials of study for applications in lasers, solar energy, LEDs, metamaterials and more.

However, previous attempts at making 3-D photonic crystals have resulted in devices that are only optically active that is, they can direct light but not electronically active, so they can't turn electricity to light or vice versa.

The Illinois team's photonic crystal has both properties.

"With our approach to fabricating photonic crystals, there's a lot of potential to optimize electronic and optical properties simultaneously," said Erik Nelson, a former graduate student in Braun's lab who now is a postdoctoral researcher at Harvard University. "It gives you the opportunity to control light in ways that are very unique to control the way it's emitted and absorbed or how it propagates."

To create a 3-D photonic crystal that is both electronically and optically active, the researchers started with a template of tiny spheres packed together. Then, they deposit gallium arsenide (GaAs), a widely used semiconductor, through the template, filling in the gaps between the spheres.

The GaAs grows as a single crystal from the bottom up, a process called epitaxy. Epitaxy is common in industry to create flat, two-dimensional films of single-crystal semiconductors, but Braun's group developed a way to apply it to an intricate three-dimensional structure.

"The key discovery here was that we grew single-crystal semiconductor through this complex template," said Braun, who also is affiliated with the Beckman Institute for Advanced Science and Technology and with the Frederick Seitz Materials Research Laboratory at Illinois. "Gallium arsenide wants to grow as a film on the substrate from the bottom up, but it runs into the template and goes around it. It's almost as though the template is filling up with water. As long as you keep growing GaAs, it keeps filling the template from the bottom up until you reach the top surface."

The epitaxial approach eliminates many of the defects introduced by top-down fabrication methods, a popular pathway for creating 3-D photonic structures. Another advantage is the ease of creating layered heterostructures. For example, a quantum well layer could be introduced into the photonic crystal by partially filling the template with GaAs and then briefly switching the vapor stream to another material.

Once the template is full, the researchers remove the spheres, leaving a complex, porous 3-D structure of single-crystal semiconductor. Then they coat the entire structure with a very thin layer of a semiconductor with a wider bandgap to improve performance and prevent surface recombination.

To test their technique, the group built a 3-D photonic crystal LED the first such working device.

Now, Braun's group is working to optimize the structure for specific applications. The LED demonstrates that the concept produces functional devices, but by tweaking the structure or using other semiconductor materials, researchers can improve solar collection or target specific wavelengths for metamaterials applications or low-threshold lasers.

"From this point on, it's a matter of changing the device geometry to achieve whatever properties you want," Nelson said. It really opens up a whole new area of research into extremely efficient or novel energy devices.

Story Source:

The above story is reprinted (with editorial adaptations) from materials provided by University of Illinois at Urbana-Champaign, via EurekAlert!, a service of AAAS.

Journal Reference:

Erik C. Nelson, Neville L. Dias, Kevin P. Bassett, Simon N. Dunham, Varun Verma, Masao Miyake, Pierre Wiltzius, John A. Rogers, James J. Coleman, Xiuling Li, Paul V. Braun. Epitaxial growth of three-dimensionally architectured optoelectronic devices. Nature Materials, 2011; DOI: 10.1038/nmat3071

Chemists create molecular flasks: Researchers design a self-assembling material that can house other molecules

Chemical reactions happen all of the time: some things burn or rust, others react to light exposure--even batteries use chemical reactions to supply electricity. One of the big challenges chemists continually face is finding new ways to control these reactions or create conditions that promote desirable reactions and limit undesirable ones.

Recently, researchers at New York University demonstrated an ability to make new materials with empty space on the inside, which could potentially control desired and unwanted chemical reactions.

Mike Ward, of NYU's Department of Chemistry and a team of researchers, essentially created a "molecular flask," self-assembling cages capable of housing other compounds inside of them. These "flasks" may eventually allow researchers to isolate certain chemical reactions within or outside the cage.

The research is published in the July 22, 2011 issue of the journal Science.

"We wanted to create frameworks to serve as the 'hotel' for 'guest' molecules, which can deliver the function independent of framework design," said Ward. "This makes it possible to separate chemicals based on size or perform reactions inside well-defined cages, which could potentially give you more control over chemical reactivity and reaction products. Moreover, these frameworks may prove ideal for encapsulating a wide range of guest molecules, producing materials with new optical or magnetic properties."

The molecular "hotels" described by Ward and his collaborators take the shape of a truncated octahedron, one of 13 shapes described as an Archimedean solid, discovered by the Greek mathematician Archimedes. Archimedean solids are characterized by a specific number of sides that meet at corners which are all identical. The regularity of these shapes often means they are of particular interest to chemists and materials researchers looking to create complex materials that assemble themselves.

The extraordinary aspect of this work, supported by the National Science Foundation (NSF), is the self-assembly of the molecular tiles into a polyhedron, a well-defined, three-dimensional, geometric solid. The individual polyhedra assemble themselves using the attractive interactions associated with hydrogen bonds. They then further organize into a crystal lattice that resembles a porous structure called zeolite, an absorbent material with many industrial uses.

The new material differs from zeolite because it is constructed from organic building blocks rather than inorganic ones, which make it more versatile and easier to engineer. In general, inorganic compounds are considered mineral in origin, while organic compounds are considered biological in origin.

This discovery paves the way towards development of a new class of solids with properties that may prove useful for a range of industrial and consumer products.

"By using geometric design principles and very simple chemical precursors, the Ward group has been able to construct relatively sturdy materials which contain many identically sized and shaped cavities," explained Michael Scott, program director in the Division of Materials Research at NSF. "The hollow space inside these materials offers many exciting opportunities for chemists to do things such as isolate unstable molecules, catalyze unknown reactions and separate important chemical compounds."

Future research projects will try to create other types of Archimedean solids or use the truncated octahedron to house different types of functional molecules.

Story Source:

The above story is reprinted (with editorial adaptations) from materials provided by National Science Foundation.

Journal Reference:

Yuzhou Liu, Chunhua Hu, Angiolina Comotti, Michael D. Ward. Supramolecular Archimedean Cages Assembled with 72 Hydrogen Bonds. Science, 2011; 333 (6041): 436-440 DOI: 10.1126/science.1204369

New graphene discovery boosts oil exploration efforts, could enable self-powered microsensors

Researchers at Rensselaer Polytechnic Institute have developed a new method to harvest energy from flowing water. This discovery aims to hasten the creation of self-powered microsensors for more accurate and cost-efficient oil exploration.

Led by Rensselaer Professor Nikhil Koratkar, the researchers investigated how the flow of water over surfaces coated with the nanomaterial graphene could generate small amounts of electricity. The research team demonstrated the creation of 85 nanowatts of power from a sheet of graphene measuring .03 millimeters by .015 millimeters.

This amount of energy should be sufficient to power tiny sensors that are introduced into water or other fluids and pumped down into a potential oil well, Koratkar said. As the injected water moves through naturally occurring cracks and crevices deep in the earth, the devices detect the presence of hydrocarbons and can help uncover hidden pockets of oil and natural gas. As long as water is flowing over the graphene-coated devices, they should be able to provide a reliable source of power. This power is necessary for the sensors to relay collected data and information back to the surface.

"It's impossible to power these microsensors with conventional batteries, as the sensors are just too small. So we created a graphene coating that allows us to capture energy from the movement of water over the sensors," said Koratkar, professor in the Department of Mechanical, Aerospace, and Nuclear Engineering and the Department of Materials Science and Engineering in the Rensselaer School of Engineering. "While a similar effect has been observed for carbon nanotubes, this is the first such study with graphene. The energy-harvesting capability of graphene was at least an order of magnitude superior to nanotubes. Moreover, the advantage of the flexible graphene sheets is that they can be wrapped around almost any geometry or shape."

Details of the study were published online last week by the journal Nano Letters. The study also will appear in a future print edition of the journal.

It is the first research paper to result from the $1 million grant awarded to Koratkar's group in March 2010 by the Advanced Energy Consortium.

Hydrocarbon exploration is an expensive process that involves drilling deep down in the earth to detect the presence of oil or natural gas. Koratkar said oil and gas companies would like to augment this process by sending out large numbers of microscale or nanoscale sensors into new and existing drill wells. These sensors would travel laterally through the earth, carried by pressurized water pumped into these wells, and into the network of cracks that exist underneath Earth's surface. Oil companies would no longer be limited to vertical exploration, and the data collected from the sensors would arm these firms with more information for deciding the best locations to drill.

The team's discovery is a potential solution for a key challenge to realizing these autonomous microsensors, which will need to be self-powered. By covering the microsensors with a graphene coating, the sensors can harvest energy as water flows over the coating.

"We'll wrap the graphene coating around the sensor, and it will act as a 'smart skin' that serves as a nanofluidic power generator," Koratkar said.

Graphene is a single-atom-thick sheet of carbon atoms, which are arranged like a chain-link fence. For this study, Koratkar's team used graphene that was grown by chemical vapor deposition on a copper substrate and transferred onto silicon dioxide. The researchers created an experimental water tunnel apparatus to test the generation of power as water flows over the graphene at different velocities.

Along with physically demonstrating the ability to generate 85 nanowatts of power from a small fragment of graphene, the researchers used molecular dynamics simulations to better understand the physics of this phenomenon. They discovered that chloride ions present in the water stick to the surface of graphene. As water flows over the graphene, the friction force between the water flow and the layer of adsorbed chloride ions causes the ions to drift along the flow direction. The motion of these ions drags the free charges present in graphene along the flow direction -- creating an internal current.

This means the graphene coating requires ions to be present in water to function properly. Therefore, oil exploration companies would need to add chemicals to the water that is injected into the well. Koratkar said this is an easy, inexpensive solution.

For the study, Koratkar's team also tested the energy harvested from water flowing over a film of carbon nanotubes. However, the energy generation and performance was far inferior to those attained using graphene, he said.

Looking at potential future applications of this new technology, Koratkar said he could envision self-powered microrobots or microsubmarines. Another possibility is harvesting power from a graphene coating on the underside of a boat.

Story Source:

The above story is reprinted (with editorial adaptations) from materials provided by Rensselaer Polytechnic Institute.

Journal Reference:

Prashant Dhiman, Fazel Yavari, Xi Mi, Hemtej Gullapalli, Yunfeng Shi, Pulickel M. Ajayan, Nikhil Koratkar. Harvesting Energy from Water Flow over Graphene. Nano Letters, 2011; 110714113416089 DOI: 10.1021/nl2011559

Wiedemann-Franz Law: Physicists break 150-year-old empirical laws of physics

A violation of one of the oldest empirical laws of physics has been observed by scientists at the University of Bristol. Their experiments on purple bronze, a metal with unique one-dimensional electronic properties, indicate that it breaks the Wiedemann-Franz Law. This historic discovery is described in a paper published July 20 in Nature Communications.

In 1853, two German physicists, Gustav Wiedemann and Rudolf Franz, studied the thermal conductivity (a measure of a system's ability to transfer heat) of a number of elemental metals and found that the ratio of the thermal to electrical conductivities was approximately the same for different metals at the same temperature.

The origin of this empirical observation did not become clear however until the discovery of the electron and the advent of quantum physics in the early twentieth century. Electrons have a spin and a charge. When they move through a metal they cause an electrical current because of the moving charge. In addition, the moving electrons also carry heat through the metal but now it is via both the charge and the spin. So a moving electron must carry both heat and charge: that is why the ratio does not vary from metal to metal.

For the past 150-plus years, the Wiedemann-Franz law has proved to be remarkably robust, the ratio varying at most by around 50 per cent amongst the thousands of metallic systems studied.

In 1996, American physicists C. L. Kane and Matthew Fisher made a theoretical prediction that if you confine electrons to individual atomic chains, the Wiedemann-Franz law could be strongly violated. In this one-dimensional world, the electrons split into two distinct components or excitations, one carrying spin but not charge (the spinon), the other carrying charge but not spin (the holon). When the holon encounters an impurity in the chain of atoms it has no choice but for its motion to be reflected. The spinon, on the other hand, has the ability to tunnel through the impurity and then continue along the chain. This means that heat is conducted easily along the chain but charge is not. This gives rise to a violation of the Wiedemann-Franz law that grows with decreasing temperature.

The experimental group, led by Professor Nigel Hussey of the Correlated Electron Systems Group at the University of Bristol, tested this prediction on a purple bronze material comprising atomic chains along which the electrons prefer to travel.

Remarkably, the researchers found that the material conducted heat 100,000 times better than would have been expected if it had obeyed the Wiedemann-Franz law like other metals. Not only does this remarkable capability of this compound to conduct heat have potential from a technological perspective, such unprecedented violation of the Wiedemann-Franz law provides striking evidence for this unusual separation of the spin and charge of an electron in the one-dimensional world.

Professor Hussey said: "One can create purely one-dimensional atomic chains on substrates, or free-standing two-dimensional sheets, like graphene, but in a three-dimensional complex solid, there will always be some residual coupling between individual chains of atoms within the complex that allow the electrons to move in three-dimensional space.

"In this purple bronze, however, nature has conspired to limit this coupling to such an extent that the electrons are effectively confined to individual chains and thus creating a one-dimensional world inside the three-dimensional complex. The goal now is to find a way, for example, using pressure or chemical substitution, to increase the ability of the electrons to hop between adjacent chains and to study the evolution of the spin and charge states as the three-dimensional world is restored within the material."

Story Source:

The above story is reprinted (with editorial adaptations ) from materials provided by University of Bristol.

Journal Reference:

Nicholas Wakeham, Alimamy F. Bangura, Xiaofeng Xu, Jean-Francois Mercure, Martha Greenblatt, Nigel E. Hussey. Gross violation of the Wiedemann–Franz law in a quasi-one-dimensional conductor. Nature Communications, 2011; 2: 396 DOI: 10.1038/ncomms1406