Point defects in super-chilled diamonds may offer stable candidates for quantum computing bits

ScienceDaily (Oct. 13, 2011) — Diamond, nature's hardest known substance, is essential for our modern mechanical world -- drills, cutters, and grinding wheels exploit the durability of diamonds to power a variety of industries. But diamonds have properties that may also make them excellent materials to enable the next generation of solid-state quantum computers and electrical and magnetic sensors.

To further explore diamonds' quantum computing potential, researchers from the University of Science and Technology of China tested the properties of a common defect found in diamond: the nitrogen-vacancy (NV) center.

Consisting of a nitrogen atom impurity paired with a 'hole' where a carbon atom is absent from the matrix structure, the NV center has the potential to store information because of the predictable way in which electrons confined in the center interact with electromagnetic waves. The research team probed the energy level properties of the trapped electrons by cooling the diamonds to an extremely chilly 5.6 degrees Kelvin and then measuring the magnetic resonance and fluorescent emission spectra. The team also measured the same spectra at gradually warmer increments, up to 295 degrees Kelvin.

The results, as reported in the AIP's journal Applied Physics Letters, show that at temperatures below 100 Kelvin the electrons' transition energies, or the energies required to get from one energy level to the next, were stable. Shifting transition energies could make quantum mechanical manipulations tricky, so cooler temperatures may aid the study and development of diamonds for quantum computation and ultra-sensitive detectors, the authors write.

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

Journal Reference:

X.-D. Chen, C.-H. Dong, F.-W. Sun, C.-L. Zou, J.-M. Cui, Z.-F. Han, and G.-C Guo. Temperature dependent energy level shifts of nitrogen-vacancy centers in diamond. Applied Physics Letters, 2011

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Computing building blocks created from bacteria and DNA

ScienceDaily (Oct. 18, 2011) — Scientists have successfully demonstrated that they can build some of the basic components for digital devices out of bacteria and DNA, which could pave the way for a new generation of biological computing devices, in research published October 18 in the journal Nature Communications.

The researchers, from Imperial College London, have demonstrated that they can build logic gates, which are used for processing information in devices such as computers and microprocessors, out of harmless gut bacteria and DNA. These are the most advanced biological logic gates ever created by scientists.

Professor Richard Kitney, co-author of the paper from the Centre for Synthetic Biology and Innovation and the Department of Bioengineering at Imperial College London, says: "Logic gates are the fundamental building blocks in silicon circuitry that our entire digital age is based on. Without them, we could not process digital information. Now that we have demonstrated that we can replicate these parts using bacteria and DNA, we hope that our work could lead to a new generation of biological processors, whose applications in information processing could be as important as their electronic equivalents."

Although still a long way off, the team suggest that these biological logic gates could one day form the building blocks in microscopic biological computers. Devices may include sensors that swim inside arteries, detecting the build up of harmful plaque and rapidly delivering medications to the affected zone. Other applications may include sensors that detect and destroy cancer cells inside the body and pollution monitors that can be deployed in the environment, detecting and neutralising dangerous toxins such as arsenic.

Previous research only proved that biological logic gates could be made. The team say that the advantage of their biological logic gates over previous attempts is that they behave more like their electronic counterparts. The new biological gates are also modular, which means that they can be fitted together to make different types of logic gates, paving the way for more complex biological processors to be built in the future.

In the new study, the researchers demonstrated how these biological logic gates worked. In one experiment, they showed how biological logic gates can replicate the way that electronic logic gates process information by either switching "on" or "off."

The scientists constructed a type of logic gate called an "AND Gate" from bacteria called Escherichia coli (E.Coli), which is normally found in the lower intestine. The team altered the E.Coli with modified DNA, which reprogrammed it to perform the same switching on and off process as its electronic equivalent when stimulated by chemicals.

The researchers were also able to demonstrate that the biological logic gates could be connected together to form more complex components in a similar way that electronic components are made. In another experiment, the researchers created a "NOT gate" and combined it with the AND gate to produce the more complex "NAND gate."

The next stage of the research will see the team trying to develop more complex circuitry that comprises multiple logic gates. One of challenges faced by the team is finding a way to link multiple biological logic gates together, similar to the way in which electronic logic gates are linked together, to enable complex processing to be carried out.

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The above story is reprinted from materials provided by Imperial College London.

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Journal Reference:

Baojun Wang, Richard I Kitney, Nicolas Joly, Martin Buck. Engineering modular and orthogonal genetic logic gates for robust digital-like synthetic biology. Nature Communications, 2011; 2: 508 DOI: 10.1038/ncomms1516

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Progress in quantum computing: Researchers control rate of photon emission from luminescent imperfections in diamond

ScienceDaily (Oct. 12, 2011) — Engineers and physicists at Harvard have managed to capture light in tiny diamond pillars embedded in silver, releasing a stream of single photons at a controllable rate.

The advance represents a milestone on the road to quantum networks in which information can be encoded in spins of electrons and carried through a network via light, one photon at a time.

The finding was published in Nature Photonics, appearing online Oct. 9.

"We can make the emission of photons faster, which will allow us to do more processing per second -- for example, more computations -- in the future quantum network," explains principal investigator Marko Loncar, Associate Professor of Electrical Engineering at the Harvard School of Engineering and Applied Sciences (SEAS).

The device Loncar's research team has built consists of parallel rows of tiny, nanofabricated diamond posts, embedded in a layer of silver, that can each act as a single photon source.

By removing the silver wrapping from their nanostructures, the team was also able to achieve a slower release of photons, which is of interest for probing the dynamics of the quantum system.

The breakthrough takes advantage of imperfections in the diamond's crystal lattice, where carbon atoms are replaced by other elements. To the naked eye, these imperfections can appear as discolorations in the diamond, turning it yellow in the case of nitrogen. Occasionally, there is also a vacancy (missing carbon atom) next to the nitrogen atom.

Each nitrogen-vacancy imperfection can serve as a nearly perfect quantum emitter, capable of emitting red photons one by one, even at room temperature. The technology is a promising candidate for realization of scalable, on-chip quantum networks.

"The color centers in diamond are very interesting as qubits for quantum information processing, where they can be used as memory to store information," says Loncar. "More importantly, they can be interrogated -- they can be written into and read out -- with light."

Loncar's team fabricates diamond posts that contain negatively charged nitrogen vacancy centers, which can absorb light and hold its energy for a given amount of time, finally releasing it in the form of photons.

"The rate at which photons are emitted can be controlled by carefully nano-engineering the center's surrounding," says co-author Irfan Bulu, a research associate in the Loncar group. Attaining fine control of that release, however, has been difficult.

"One of the main challenges has been the efficiency with which you can write information into the spin of these color centers, as well as the efficiency with which you can collect photons emitted from the color centers," explains co-author Jennifer Choy, a graduate student in Loncar's lab at SEAS. "The other challenge has been the rate -- how quickly you can perform these processes."

Previous work from Loncar's group solved the collection efficiency problem by using diamond nanowires to channel and direct the flow of photons. The new research manipulates the radius of diamond pillars and adds the silver coating. The diamond-silver construction acts as an optical nanoresonator, creating a strong electromagnetic field around the emitter and offering a new level of control over the rate of emission.

Moreover, the device functions at room temperature -- an essential requirement for practical computing applications -- and the nanostructured chips are fully scalable.

"We've designed everything in parallel in a massive system, which allows us to make thousands or millions of devices with more or less the same properties, and we use conventional microfabrication and nanofabrication techniques, unlike what has been done in this field before," says Birgit Hausmann, a graduate student in Loncar's lab at SEAS and one of the co-authors.

In addition to Loncar, Choy, Hausmann, and Bulu, co-authors included Tom Babinec, a graduate student at SEAS; Mughees Khan, a staff scientist at the Wyss Institute for Biologically Inspired Engineering at Harvard; Patrick Maletinsky, a fellow of the Department of Physics at Harvard; and Amir Jacoby, Professor of Physics in the Harvard Faculty of Arts and Sciences.

The work was supported by grants and fellowships from the U.S. Department of Defense, the Defense Advanced Research Projects Agency (DARPA) QuEST program, the National Science Foundation (NSF), the King Abdullah University of Science and Technology (KAUST), the Sloan Foundation, and the NSF-supported Nanoscale Science and Engineering Center (NSEC) at Harvard. Fabrication took place at the NSF-supported Center for Nanoscale Systems (CNS) at Harvard.

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

Journal Reference:

Jennifer T. Choy, Birgit J. M. Hausmann, Thomas M. Babinec, Irfan Bulu, Mughees Khan, Patrick Maletinsky, Amir Yacoby, Marko Loncar. Enhanced single-photon emission from a diamond–silver aperture. Nature Photonics, 2011; DOI: 10.1038/nphoton.2011.249

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New scheme for photonic quantum computing

ScienceDaily (Oct. 13, 2011) — The concepts of quantum technology promise to achieve more powerful information processing than is possible with even the best possible classical computers. To actually build efficient quantum computers remains a significant challenge in practice. A new scheme, called "coherent photon conversion," could potentially overcome all of the currently unresolved problems for optical implementations of quantum computing.

The international team of scientists led by researchers from the Vienna Center for Quantum Science and Technology (VCQ) at the University of Vienna (group of Anton Zeilinger) introduces this new scheme this week in Nature.

A new way for photons

Quantum technology derives its potential by exploiting uniquely quantum features such as superposition and entanglement. Single photons are excellent quantum information carriers, because they are naturally almost perfectly isolated from their environment. Also, quantum computers based on photons promise to be extremely fast. But current schemes for preparing, processing and measuring photons are inefficient.

The new scheme provides a method of coherent conversion between different photon states and is based on enhancing the nonlinearity of a medium by a strong laser field. The method paves a road to solving all open challenges for optical quantum computation. For example, deterministically doubling single photons solves the preparation and measuring problems, and a novel type of photon-photon interaction gives efficient quantum gates. This new quantum optics toolbox opened up by "coherent photon conversion" promises to lead to a nonlinear optical quantum computer.

First experiments

In a first series of experiments the group uses photons and highly non-linear glass fibers for a proof-of-principle demonstration of a process suitable for implementing the scheme. While deterministic operation has yet to be achieved, the authors' results suggest a line for development how this might be possible with sophisticated optical technologies, such as using highly nonlinear glasses and stronger lasers. Interestingly, the general idea of "coherent photon conversion" can also be applied to various other physical systems like atoms or nanomechanical devices.

This work was carried out as a joint effort by researchers of the Vienna Center for Quantum Science and Technology (VCQ) of the University of Vienna, the Institute for Quantum Optics and Quantum Information Vienna (IQOQI) of the Austrian Academy of Sciences, the National Institute of Informatics and the NTT Basic Research Laboratories (NTT Corporation) in Japan, as well as the Centre for Engineered Quantum Systems at the University of Queensland in Australia. It was supported by the European Research Council (Advanced Grant QIT4QAD), the Austrian Science Fund (grant F4007, an Erwin Schroedinger Fellowship, the Vienna Doctoral Program CoQuS FWF W121), the European Commission (QU-ESSENCE and QAP), the John Templeton Foundation and in part by the Japanese FIRST programme and the Ontario Ministry of Research and Innovation.

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

Journal Reference:

N. K. Langford, S. Ramelow, R. Prevedel, W. J. Munro, G. J. Milburn, A. Zeilinger. Efficient quantum computing using coherent photon conversion. Nature, 2011; DOI: 10.1038/nature10463

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New scheme for photonic quantum computing

ScienceDaily (Oct. 13, 2011) — The concepts of quantum technology promise to achieve more powerful information processing than is possible with even the best possible classical computers. To actually build efficient quantum computers remains a significant challenge in practice. A new scheme, called "coherent photon conversion," could potentially overcome all of the currently unresolved problems for optical implementations of quantum computing.

The international team of scientists led by researchers from the Vienna Center for Quantum Science and Technology (VCQ) at the University of Vienna (group of Anton Zeilinger) introduces this new scheme this week in Nature.

A new way for photons

Quantum technology derives its potential by exploiting uniquely quantum features such as superposition and entanglement. Single photons are excellent quantum information carriers, because they are naturally almost perfectly isolated from their environment. Also, quantum computers based on photons promise to be extremely fast. But current schemes for preparing, processing and measuring photons are inefficient.

The new scheme provides a method of coherent conversion between different photon states and is based on enhancing the nonlinearity of a medium by a strong laser field. The method paves a road to solving all open challenges for optical quantum computation. For example, deterministically doubling single photons solves the preparation and measuring problems, and a novel type of photon-photon interaction gives efficient quantum gates. This new quantum optics toolbox opened up by "coherent photon conversion" promises to lead to a nonlinear optical quantum computer.

First experiments

In a first series of experiments the group uses photons and highly non-linear glass fibers for a proof-of-principle demonstration of a process suitable for implementing the scheme. While deterministic operation has yet to be achieved, the authors' results suggest a line for development how this might be possible with sophisticated optical technologies, such as using highly nonlinear glasses and stronger lasers. Interestingly, the general idea of "coherent photon conversion" can also be applied to various other physical systems like atoms or nanomechanical devices.

This work was carried out as a joint effort by researchers of the Vienna Center for Quantum Science and Technology (VCQ) of the University of Vienna, the Institute for Quantum Optics and Quantum Information Vienna (IQOQI) of the Austrian Academy of Sciences, the National Institute of Informatics and the NTT Basic Research Laboratories (NTT Corporation) in Japan, as well as the Centre for Engineered Quantum Systems at the University of Queensland in Australia. It was supported by the European Research Council (Advanced Grant QIT4QAD), the Austrian Science Fund (grant F4007, an Erwin Schroedinger Fellowship, the Vienna Doctoral Program CoQuS FWF W121), the European Commission (QU-ESSENCE and QAP), the John Templeton Foundation and in part by the Japanese FIRST programme and the Ontario Ministry of Research and Innovation.

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

Journal Reference:

N. K. Langford, S. Ramelow, R. Prevedel, W. J. Munro, G. J. Milburn, A. Zeilinger. Efficient quantum computing using coherent photon conversion. Nature, 2011; DOI: 10.1038/nature10463

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Progress in quantum computing: Researchers control rate of photon emission from luminescent imperfections in diamond

ScienceDaily (Oct. 12, 2011) — Engineers and physicists at Harvard have managed to capture light in tiny diamond pillars embedded in silver, releasing a stream of single photons at a controllable rate.

The advance represents a milestone on the road to quantum networks in which information can be encoded in spins of electrons and carried through a network via light, one photon at a time.

The finding was published in Nature Photonics, appearing online Oct. 9.

"We can make the emission of photons faster, which will allow us to do more processing per second -- for example, more computations -- in the future quantum network," explains principal investigator Marko Loncar, Associate Professor of Electrical Engineering at the Harvard School of Engineering and Applied Sciences (SEAS).

The device Loncar's research team has built consists of parallel rows of tiny, nanofabricated diamond posts, embedded in a layer of silver, that can each act as a single photon source.

By removing the silver wrapping from their nanostructures, the team was also able to achieve a slower release of photons, which is of interest for probing the dynamics of the quantum system.

The breakthrough takes advantage of imperfections in the diamond's crystal lattice, where carbon atoms are replaced by other elements. To the naked eye, these imperfections can appear as discolorations in the diamond, turning it yellow in the case of nitrogen. Occasionally, there is also a vacancy (missing carbon atom) next to the nitrogen atom.

Each nitrogen-vacancy imperfection can serve as a nearly perfect quantum emitter, capable of emitting red photons one by one, even at room temperature. The technology is a promising candidate for realization of scalable, on-chip quantum networks.

"The color centers in diamond are very interesting as qubits for quantum information processing, where they can be used as memory to store information," says Loncar. "More importantly, they can be interrogated -- they can be written into and read out -- with light."

Loncar's team fabricates diamond posts that contain negatively charged nitrogen vacancy centers, which can absorb light and hold its energy for a given amount of time, finally releasing it in the form of photons.

"The rate at which photons are emitted can be controlled by carefully nano-engineering the center's surrounding," says co-author Irfan Bulu, a research associate in the Loncar group. Attaining fine control of that release, however, has been difficult.

"One of the main challenges has been the efficiency with which you can write information into the spin of these color centers, as well as the efficiency with which you can collect photons emitted from the color centers," explains co-author Jennifer Choy, a graduate student in Loncar's lab at SEAS. "The other challenge has been the rate -- how quickly you can perform these processes."

Previous work from Loncar's group solved the collection efficiency problem by using diamond nanowires to channel and direct the flow of photons. The new research manipulates the radius of diamond pillars and adds the silver coating. The diamond-silver construction acts as an optical nanoresonator, creating a strong electromagnetic field around the emitter and offering a new level of control over the rate of emission.

Moreover, the device functions at room temperature -- an essential requirement for practical computing applications -- and the nanostructured chips are fully scalable.

"We've designed everything in parallel in a massive system, which allows us to make thousands or millions of devices with more or less the same properties, and we use conventional microfabrication and nanofabrication techniques, unlike what has been done in this field before," says Birgit Hausmann, a graduate student in Loncar's lab at SEAS and one of the co-authors.

In addition to Loncar, Choy, Hausmann, and Bulu, co-authors included Tom Babinec, a graduate student at SEAS; Mughees Khan, a staff scientist at the Wyss Institute for Biologically Inspired Engineering at Harvard; Patrick Maletinsky, a fellow of the Department of Physics at Harvard; and Amir Jacoby, Professor of Physics in the Harvard Faculty of Arts and Sciences.

The work was supported by grants and fellowships from the U.S. Department of Defense, the Defense Advanced Research Projects Agency (DARPA) QuEST program, the National Science Foundation (NSF), the King Abdullah University of Science and Technology (KAUST), the Sloan Foundation, and the NSF-supported Nanoscale Science and Engineering Center (NSEC) at Harvard. Fabrication took place at the NSF-supported Center for Nanoscale Systems (CNS) at Harvard.

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

Journal Reference:

Jennifer T. Choy, Birgit J. M. Hausmann, Thomas M. Babinec, Irfan Bulu, Mughees Khan, Patrick Maletinsky, Amir Yacoby, Marko Loncar. Enhanced single-photon emission from a diamond–silver aperture. Nature Photonics, 2011; DOI: 10.1038/nphoton.2011.249

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New technique offers enhanced security for sensitive data in cloud computing

ScienceDaily (Oct. 13, 2011) — Researchers from North Carolina State University and IBM have developed a new, experimental technique to better protect sensitive information in cloud computing -- without significantly affecting the system's overall performance.

Under the cloud-computing paradigm, the computational power and storage of multiple computers is pooled, and can be shared by multiple users. Hypervisors are programs that create the virtual workspace that allows different operating systems to run in isolation from one another -- even though each of these systems is using computing power and storage capability on the same computer. A longstanding concern in cloud computing is that attackers could take advantage of vulnerabilities in a hypervisor to steal or corrupt confidential data from other users in the cloud.

The NC State research team has developed a new approach to cloud security, which builds upon existing hardware and firmware functionality to isolate sensitive information and workload from the rest of the functions performed by a hypervisor. The new technique, called "Strongly Isolated Computing Environment" (SICE), demonstrates the introduction of a different layer of protection.

"We have significantly reduced the 'surface' that can be attacked by malicious software," says Dr. Peng Ning, a professor of computer science at NC State and co-author of a paper describing the research. "For example, our approach relies on a software foundation called the Trusted Computing Base, or TCB, that has approximately 300 lines of code, meaning that only these 300 lines of code need to be trusted in order to ensure the isolation offered by our approach. Previous techniques have exposed thousands of lines of code to potential attacks. We have a smaller attack surface to protect."

SICE also lets programmers dedicate specific cores on widely-available multi-core processors to the sensitive workload -- allowing the other cores to perform all other functions normally. A core is the brain of a computer chip, and many computers now use chips that have between two and eight cores. By confining the sensitive workload to one or a few cores with strong isolation, and allowing other functions to operate separately, SICE is able to provide both high assurance for the sensitive workload and efficient resource sharing in a cloud.

In testing, the SICE framework generally took up approximately 3 percent of the system's performance overhead on multi-core processors for workloads that do not require direct network access. "That is a fairly modest price to pay for the enhanced security," Ning says. "However, more research is needed to further speed up the workloads that require interactions with the network."

The paper, "SICE: A Hardware-Level Strongly Isolated Computing Environment for x86 Multi-core Platforms," was co-authored by Ning; NC State Ph.D. student Ahmed Azab; and Dr. Xiaolan Zhang of IBM's T.J. Watson Research Center. The paper will be presented at the 18th ACM Conference on Computer and Communications Security, Oct. 17-21 in Chicago, Ill. The research was funded by the National Science Foundation, U.S. Army Research Office and IBM.

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by North Carolina State University.

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Point defects in super-chilled diamonds may offer stable candidates for quantum computing bits

ScienceDaily (Oct. 13, 2011) — Diamond, nature's hardest known substance, is essential for our modern mechanical world -- drills, cutters, and grinding wheels exploit the durability of diamonds to power a variety of industries. But diamonds have properties that may also make them excellent materials to enable the next generation of solid-state quantum computers and electrical and magnetic sensors.

To further explore diamonds' quantum computing potential, researchers from the University of Science and Technology of China tested the properties of a common defect found in diamond: the nitrogen-vacancy (NV) center.

Consisting of a nitrogen atom impurity paired with a 'hole' where a carbon atom is absent from the matrix structure, the NV center has the potential to store information because of the predictable way in which electrons confined in the center interact with electromagnetic waves. The research team probed the energy level properties of the trapped electrons by cooling the diamonds to an extremely chilly 5.6 degrees Kelvin and then measuring the magnetic resonance and fluorescent emission spectra. The team also measured the same spectra at gradually warmer increments, up to 295 degrees Kelvin.

The results, as reported in the AIP's journal Applied Physics Letters, show that at temperatures below 100 Kelvin the electrons' transition energies, or the energies required to get from one energy level to the next, were stable. Shifting transition energies could make quantum mechanical manipulations tricky, so cooler temperatures may aid the study and development of diamonds for quantum computation and ultra-sensitive detectors, the authors write.

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

Journal Reference:

X.-D. Chen, C.-H. Dong, F.-W. Sun, C.-L. Zou, J.-M. Cui, Z.-F. Han, and G.-C Guo. Temperature dependent energy level shifts of nitrogen-vacancy centers in diamond. Applied Physics Letters, 2011

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Brain-like computing a step closer to reality

ScienceDaily (June 24, 2011) — The development of 'brain-like' computers has taken a major step forward with the publication of research led by the University of Exeter.

Published in the journal Advanced Materials, the study involved the first ever demonstration of simultaneous information processing and storage using phase-change materials. This new technique could revolutionize computing by making computers faster and more energy-efficient, as well as making them more closely resemble biological systems.

Computers currently deal with processing and memory separately, resulting in a speed and power 'bottleneck' caused by the need to continually move data around. This is totally unlike anything in biology, for example in human brains, where no real distinction is made between memory and computation. To perform these two functions simultaneously the University of Exeter research team used phase-change materials, a kind of semi-conductor that exhibits remarkable properties.

Their study demonstrates conclusively that phase-change materials can store and process information simultaneously. It also shows experimentally for the first time that they can perform general-purpose computing operations, such as addition, subtraction, multiplication and division. More strikingly perhaps it shows that phase-change materials can be used to make artificial neurons and synapses. This means that an artificial system made entirely from phase-change devices could potentially learn and process information in a similar way to our own brains.

Lead author Professor David Wright of the University of Exeter said: "Our findings have major implications for the development of entirely new forms of computing, including 'brain-like' computers. We have uncovered a technique for potentially developing new forms of 'brain-like' computer systems that could learn, adapt and change over time. This is something that researchers have been striving for over many years."

This study focused on the performance of a single phase-change cell. The next stage in Exeter's research will be to build systems of interconnected cells that can learn to perform simple tasks, such as identification of certain objects and patterns.

This research was funded by the Engineering and Physical Sciences Research Council.

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

Journal Reference:

C. David Wright, Yanwei Liu, Krisztian I. Kohary, Mustafa M. Aziz, Robert J. Hicken. Arithmetic and Biologically-Inspired Computing Using Phase-Change Materials. Advanced Materials, 2011; DOI: 10.1002/adma.201101060

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Chameleon magnets: Ability to switch magnets 'on' or 'off' could revolutionize computing

ScienceDaily (May 28, 2011) — What causes a magnet to be a magnet, and how can we control a magnet's behavior? These are the questions that University at Buffalo researcher Igor Zutic, a theoretical physicist, has been exploring over many years.

He is one of many scientists who believe that magnets could revolutionize computing, forming the basis of high-capacity and low-energy memory, data storage and data transfer devices.

In a recent commentary in Science, Zutic and fellow UB physicist John Cerne, who studies magnetism experimentally, discuss an exciting advancement: A study by Japanese scientists showing that it is possible to turn a material's magnetism on and off at room temperature.

A material's magnetism is determined by a property all electrons possess: something called "spin." Electrons can have an "up" or "down" spin, and a material is magnetic when most of its electrons possess the same spin. Individual spins are akin to tiny bar magnets, which have north and south poles.

In the Japanese study, which also appears in the current issue of Science, a team led by researchers at Tohoku University added cobalt to titanium dioxide, a nonmagnetic semiconductor, to create a new material that, like a chameleon, can transform from a paramagnet (a nonmagnetic material) to a ferromagnet (a magnetic material) at room temperature.

To achieve change, the researchers applied an electric voltage to the material, exposing the material to extra electrons. As Zutic and Cerne explain in their commentary, these additional electrons -- called "carriers" -- are mobile and convey information between fixed cobalt ions that causes the spins of the cobalt electrons to align in one direction.

In an interview, Zutic calls the ability to switch a magnet "on" or "off" revolutionary. He explains the promise of magnet- or spin-based computing technology -- called "spintronics" -- by contrasting it with conventional electronics.

Modern, electronic gadgets record and read data as a blueprint of ones and zeros that are represented, in circuits, by the presence or absence of electrons. Processing information requires moving electrons, which consumes energy and produces heat.

Spintronic gadgets, in contrast, store and process data by exploiting electrons' "up" and "down" spins, which can stand for the ones and zeros devices read. Future energy-saving improvements in data processing could include devices that process information by "flipping" spin instead of shuttling electrons around.

In their Science commentary, Zutic and Cerne write that chameleon magnets could "help us make more versatile transistors and bring us closer to the seamless integration of memory and logic by providing smart hardware that can be dynamically reprogrammed for optimal performance of a specific task."

"Large applied magnetic fields can enforce the spin alignment in semiconductor transistors," they write. "With chameleon magnets, such alignment would be tunable and would require no magnetic field and could revolutionize the role ferromagnets play in technology."

In an interview, Zutic says that applying an electric voltage to a semiconductor injected with cobalt or other magnetic impurities may be just one way of creating a chameleon magnet.

Applying heat or light to such a material could have a similar effect, freeing electrons that can then convey information about spin alignment between ions, he says.

The so-far elusive heat-based chameleon magnets were first proposed by Zutic in 2002. With his colleagues, Andre Petukhov of the South Dakota School of Mines and Technology, and Steven Erwin of the Naval Research Laboratory, he elucidated the behavior of such magnets in a 2007 paper.

The concept of nonmagnetic materials becoming magnetic as they heat up is counterintuitive, Zutic says. Scientists had long assumed that orderly, magnetic materials would lose their neat, spin alignments when heated -- just as orderly, crystalline ice melts into disorderly water as temperatures rise.

The carrier electrons, however, are the key. Because heating a material introduces additional carriers that can cause nearby electrons to adopt aligned spins, heating chameleon materials -- up to a certain temperature -- should actually cause them to become magnetic, Zutic explains. His research on magnetism is funded by the Department of Energy, Office of Naval Research, Air Force Office of Scientific Research and the National Science Foundation.

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by University at Buffalo.

Journal Reference:

I. Zutic, J. Cerne. Chameleon Magnets. Science, 2011; 332 (6033): 1040 DOI: 10.1126/science.1205775

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Toward optical computing in handheld electronics: Graphene optical modulators could lead to ultrafast communications

ScienceDaily (May 9, 2011) — Scientists at the University of California, Berkeley, have demonstrated a new technology for graphene that could break the current speed limits in digital communications.

The team of researchers, led by UC Berkeley engineering professor Xiang Zhang, built a tiny optical device that uses graphene, a one-atom-thick layer of crystallized carbon, to switch light on and off. This switching ability is the fundamental characteristic of a network modulator, which controls the speed at which data packets are transmitted. The faster the data pulses are sent out, the greater the volume of information that can be sent. Graphene-based modulators could soon allow consumers to stream full-length, high-definition, 3-D movies onto a smartphone in a matter of seconds, the researchers said.

"This is the world's smallest optical modulator, and the modulator in data communications is the heart of speed control," said Zhang, who directs a National Science Foundation (NSF) Nanoscale Science and Engineering Center at UC Berkeley. "Graphene enables us to make modulators that are incredibly compact and that potentially perform at speeds up to ten times faster than current technology allows. This new technology will significantly enhance our capabilities in ultrafast optical communication and computing."

In this latest work, described in the May 8 advanced online publication of the journal Nature, researchers were able to tune the graphene electrically to absorb light in wavelengths used in data communication. This advance adds yet another advantage to graphene, which has gained a reputation as a wonder material since 2004 when it was first extracted from graphite, the same element in pencil lead. That achievement earned University of Manchester scientists Andre Geim and Konstantin Novoselov the Nobel Prize in Physics last year.

Zhang worked with fellow faculty member Feng Wang, an assistant professor of physics and head of the Ultrafast Nano-Optics Group at UC Berkeley. Both Zhang and Wang are faculty scientists at Lawrence Berkeley National Laboratory's Materials Science Division.

"The impact of this technology will be far-reaching," said Wang. "In addition to high-speed operations, graphene-based modulators could lead to unconventional applications due to graphene's flexibility and ease in integration with different kinds of materials. Graphene can also be used to modulate new frequency ranges, such as mid-infrared light, that are widely used in molecular sensing."

Graphene is the thinnest, strongest crystalline material yet known. It can be stretched like rubber, and it has the added benefit of being an excellent conductor of heat and electricity. This last quality of graphene makes it a particularly attractive material for electronics.

"Graphene is compatible with silicon technology and is very cheap to make," said Ming Liu, post-doctoral researcher in Zhang's lab and co-lead author of the study. "Researchers in Korea last year have already produced 30-inch sheets of it. Moreover, very little graphene is required for use as a modulator. The graphite in a pencil can provide enough graphene to fabricate 1 billion optical modulators."

It is the behavior of photons and electrons in graphene that first caught the attention of the UC Berkeley researchers.

The researchers found that the energy of the electrons, referred to as its Fermi level, can be easily altered depending upon the voltage applied to the material. The graphene's Fermi level in turn determines if the light is absorbed or not.

When a sufficient negative voltage is applied, electrons are drawn out of the graphene and are no longer available to absorb photons. The light is "switched on" because the graphene becomes totally transparent as the photons pass through.

Graphene is also transparent at certain positive voltages because, in that situation, the electrons become packed so tightly that they cannot absorb the photons.

The researchers found a sweet spot in the middle where there is just enough voltage applied so the electrons can prevent the photons from passing, effectively switching the light "off."

"If graphene were a hallway, and electrons were people, you could say that, when the hall is empty, there's no one around to stop the photons," said Xiaobo Yin, co-lead author of the Nature paper and a research scientist in Zhang's lab. "In the other extreme, when the hall is too crowded, people can't move and are ineffective in blocking the photons. It's in between these two scenarios that the electrons are allowed to interact with and absorb the photons, and the graphene becomes opaque."

In their experiment, the researchers layered graphene on top of a silicon waveguide to fabricate optical modulators. The researchers were able to achieve a modulation speed of 1 gigahertz, but they noted that the speed could theoretically reach as high as 500 gigahertz for a single modulator.

While components based upon optics have many advantages over those that use electricity, including the ability to carry denser packets of data more quickly, attempts to create optical interconnects that fit neatly onto a computer chip have been hampered by the relatively large amount of space required in photonics.

Light waves are less agile in tight spaces than their electrical counterparts, the researchers noted, so photon-based applications have been primarily confined to large-scale devices, such as fiber optic lines.

"Electrons can easily make an L-shaped turn because the wavelengths in which they operate are small," said Zhang. "Light wavelengths are generally bigger, so they need more space to maneuver. It's like turning a long, stretch limo instead of a motorcycle around a corner. That's why optics require bulky mirrors to control their movements. Scaling down the optical device also makes it faster because the single atomic layer of graphene can significantly reduce the capacitance -- the ability to hold an electric charge -- which often hinders device speed."

Graphene-based modulators could overcome the space barrier of optical devices, the researchers said. They successfully shrunk a graphene-based optical modulator down to a relatively tiny 25 square microns, a size roughly 400 times smaller than a human hair. The footprint of a typical commercial modulator can be as large as a few square millimeters.

Even at such a small size, graphene packs a punch in bandwidth capability. Graphene can absorb a broad spectrum of light, ranging over thousands of nanometers from ultraviolet to infrared wavelengths. This allows graphene to carry more data than current state-of-the-art modulators, which operate at a bandwidth of up to 10 nanometers, the researchers said.

"Graphene-based modulators not only offer an increase in modulation speed, they can enable greater amounts of data packed into each pulse," said Zhang. "Instead of broadband, we will have 'extremeband.' What we see here and going forward with graphene-based modulators are tremendous improvements, not only in consumer electronics, but in any field that is now limited by data transmission speeds, including bioinformatics and weather forecasting. We hope to see industrial applications of this new device in the next few years."

Other UC Berkeley co-authors of this paper are graduate student Erick Ulin-Avila and post-doctoral researcher Thomas Zentgraf in Zhang's lab; and visiting scholar Baisong Geng and graduate student Long Ju in Wang's lab.

This work was supported through the Center for Scalable and Integrated Nano-Manufacturing (SINAM), an NSF Nanoscale Science and Engineering Center. Funding from the Department of Energy's Basic Energy Science program at Lawrence Berkeley National Laboratory also helped support this research.

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by University of California - Berkeley. The original article was written by Sarah Yang, Media Relations.

Journal Reference:

Ming Liu, Xiaobo Yin, Erick Ulin-Avila, Baisong Geng, Thomas Zentgraf, Long Ju, Feng Wang, Xiang Zhang. A graphene-based broadband optical modulator. Nature, 2011; DOI: 10.1038/nature10067

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

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