Templated Growth Technique Produces Graphene Nanoribbons With Metallic Properties

"We can now make very narrow, conductive nanoribbons that have quantum ballistic properties," said Walt de Heer, a professor in the School of Physics at the Georgia Institute of Technology."These narrow ribbons become almost like a perfect metal. Electrons can move through them without scattering, just like they do in carbon nanotubes."

De Heer was scheduled to discuss recent results of this graphene growth process March 21st at the American Physical Society's March 2011 Meeting in Dallas. The research was sponsored by the National Science Foundation-supported Materials Research Science and Engineering Center (MRSEC).

First reported Oct. 3 in the advance online edition of the journal Nature Nanotechnology, the new fabrication technique allows production of epitaxial graphene structures with smooth edges. Earlier fabrication techniques that used electron beams to cut graphene sheets produced nanoribbon structures with rough edges that scattered electrons, causing interference. The resulting nanoribbons had properties more like insulators than conductors.

"In our templated growth approach, we have essentially eliminated the edges that take away from the desirable properties of graphene," de Heer explained."The edges of the epitaxial graphene merge into the silicon carbide, producing properties that are really quite interesting."

The"templated growth" technique begins with etching patterns into the silicon carbide surfaces on which epitaxial graphene is grown. The patterns serve as templates directing the growth of graphene structures, allowing the formation of nanoribbons and other structures of specific widths and shapes without the use of cutting techniques that produce the rough edges.

In creating these graphene nanostructures, de Heer and his research team first use conventional microelectronics techniques to etch tiny"steps" -- or contours -- into a silicon carbide wafer whose surface has been made extremely flat. They then heat the contoured wafer to approximately 1,500 degrees Celsius, which initiates melting that polishes any rough edges left by the etching process.

Established techniques are then used for growing graphene from silicon carbide by driving off the silicon atoms from the surface. Instead of producing a consistent layer of graphene across the entire surface of the wafer, however, the researchers limit the heating time so that graphene grows only on portions of the contours.

The width of the resulting nanoribbons is proportional to the depth of the contours, providing a mechanism for precisely controlling the nanoribbon structures. To form complex structures, multiple etching steps can be carried out to create complex templates.

"This technique allows us to avoid the complicated e-beam lithography steps that people have been using to create structures in epitaxial graphene," de Heer noted."We are seeing very good properties that show these structures can be used for real electronic applications."

Since publication of the Nature Nanotechnology paper, de Heer's team has been refining its technique."We have taken this to an extreme -- the cleanest and narrowest ribbons we can make," he said."We expect to be able to do everything we need with the size ribbons that we are able to make right now, though we probably could reduce the width to 10 nanometers or less."

While the Georgia Tech team is continuing to develop high-frequency transistors -- perhaps even at the terahertz range -- its primary effort now focuses on developing quantum devices, de Heer said. Such devices were envisioned in the patents Georgia Tech holds on various epitaxial graphene processes.

"This means that the way we will be doing graphene electronics will be different," he explained."We will not be following the model of using standard field-effect transistors (FETs), but will pursue devices that use ballistic conductors and quantum interference. We are headed straight into using the electron wave effects in graphene."

Taking advantage of the wave properties will allow electrons to be manipulated with techniques similar to those used by optical engineers. For instance, switching may be carried out using interference effects -- separating beams of electrons and then recombining them in opposite phases to extinguish the signals.

Quantum devices would be smaller than conventional transistors and operate at lower power. Because of its ability to transport electrons with virtually no resistance, epitaxial graphene may be the ideal material for such devices, de Heer said.

"Using the quantum properties of electrons rather than the standard charged-particle properties means opening up new ways of looking at electronics," he predicted."This is probably the way that electronics will evolve, and it appears that graphene is the ideal material for making this transition."

De Heer's research team hopes to demonstrate a rudimentary switch operating on the quantum interference principle within a year.

Epitaxial graphene may be the basis for a new generation of high-performance devices that will take advantage of the material's unique properties in applications where higher costs can be justified. Silicon, today's electronic material of choice, will continue to be used in applications where high-performance is not required, de Heer said.

"This is an important step in the process," he added."There are going to be a lot of surprises as we move into these quantum devices and find out how they work. We have good reason to believe that this can be the basis for a new generation of transistors based on quantum interference."

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Electromechanical Circuit Sets Record Beating Microscopic 'Drum'

Described in the March 10 issue ofNature, the NIST experiments created strong interactions between microwave light oscillating 7.5 billion times per second and a"micro drum" vibrating at radio frequencies 11 million times per second. Compared to previously reported experiments combining microscopic machines and electromagnetic radiation, the rate of energy exchange in the NIST device -- the"coupling" that reflects the strength of the connection -- is much stronger, the mechanical vibrations last longer, and the apparatus is much easier to make.

Similar in appearance to an Irish percussion instrument called a bodhrán, the NIST drum is a round aluminum membrane 100 nanometers thick and 15 micrometers wide, lightweight and flexible enough to vibrate freely yet larger and heavier than the nanowires typically used in similar experiments.

"The drum is so much larger than nanowires physically that you can make this coupling strength go through the roof," says first author John Teufel, a NIST research affiliate who designed the drum."The drum hits a perfect compromise where it's still microscale but you can couple to it strongly."

The NIST experiments shifted the microwave energy by 56 megahertz (MHz, or million cycles per second) per nanometer of drum motion, 1,000 times more than the previous state of the art.

"We turned up the rate at which these two things talk to each other," Teufel says.

The drum is incorporated into a superconducting cavity cooled to 40 milliKelvin, a temperature at which aluminum allows electric current to flow without resistance -- a quantum property. Scientists apply microwaves to the cavity. Then, by applying a drive tone set at the difference between the frequencies of the microwave radiation particles (photons) and the drum, researchers dramatically increase the overall coupling strength to make the two systems communicate faster than their energy dissipates. The microwaves can be used to measure and control the drum vibrations, and vice versa. The drum motion will persist for hundreds of microseconds, according to the paper, a relatively long time in the fast-paced quantum world.

In engineering terms, the drum acts as a capacitor -- a device that holds electric charge. Its capacitance, or ability to hold charge, depends on the position of the drum about 50 nanometers above an aluminum electrode. When the drum vibrates, the capacitance changes and the mechanical motion modulates the properties of the electrical circuit. The same principle is at work with a microphone and FM radio, but here the natural drum motion, mostly at one frequency, is transmitted to the listener in the lab.

The experiment is a step towards entanglement -- a curious quantum state linking the properties of objects -- between the microwave photons and the drum motion, Teufel says. The apparatus has the high coupling strength and low energy losses needed to generate entanglement, he says. Further experiments will address whether the mechanical drumbeats obey the rules of quantum mechanics, which govern the behavior of light and atoms.

The drum is a key achievement in NIST's effort to develop components for superconducting quantum computers and quantum simulations, while also working toward the widely sought scientific goal of making the most precise measurements possible of mechanical motion.

Quantum computers, if they can be built, could solve certain problems that are intractable today. The microwave and radiofrequency signals in the new electromechanical circuit could be used to represent quantum information. NIST scientists plan to combine the new circuit with superconducting quantum bits to create and manipulate motion of relatively large objects on the smallest (quantum) scales.

The experiment reported inNatureis a prelude to cooling the drum to its"ground state," or lowest-energy state. Starting from the ground state, the drum could be manipulated for the applications mentioned above. In addition, such control would enable tests of the boundary between the everyday classical and quantum worlds. The drum also has possible practical applications such as measuring length and force with sensitivities at levels of attometers (billionths of a billionth of a meter) and attonewtons (billionths of a billionth of a newton), respectively.

As a non-regulatory agency, NIST promotes U.S. innovation and industrial competitiveness by advancing measurement science, standards and technology in ways that enhance economic security and improve our quality of life.

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New Developments in Quantum Computing

At the Association for Computing Machinery's 43rd Symposium on Theory of Computing in June, associate professor of computer science Scott Aaronson and his graduate student Alex Arkhipov will present a paper describing an experiment that, if it worked, would offer strong evidence that quantum computers can do things that classical computers can't. Although building the experimental apparatus would be difficult, it shouldn't be as difficult as building a fully functional quantum computer.

Aaronson and Arkhipov's proposal is a variation on an experiment conducted by physicists at the University of Rochester in 1987, which relied on a device called a beam splitter, which takes an incoming beam of light and splits it into two beams traveling in different directions. The Rochester researchers demonstrated that if two identical light particles -- photons -- reach the beam splitter at exactly the same time, they will both go either right or left; they won't take different paths. It's another quantum behavior of fundamental particles that defies our physical intuitions.

The MIT researchers' experiment would use a larger number of photons, which would pass through a network of beam splitters and eventually strike photon detectors. The number of detectors would be somewhere in the vicinity of the square of the number of photons -- about 36 detectors for six photons, 100 detectors for 10 photons.

For any run of the MIT experiment, it would be impossible to predict how many photons would strike any given detector. But over successive runs, statistical patterns would begin to build up. In the six-photon version of the experiment, for instance, it could turn out that there's an 8 percent chance that photons will strike detectors 1, 3, 5, 7, 9 and 11, a 4 percent chance that they'll strike detectors 2, 4, 6, 8, 10 and 12, and so on, for any conceivable combination of detectors.

Calculating that distribution -- the likelihood of photons striking a given combination of detectors -- is a hard problem. The researchers' experiment doesn't solve it outright, but every successful execution of the experiment does take a sample from the solution set. One of the key findings in Aaronson and Arkhipov's paper is that, not only is calculating the distribution a hard problem, but so is simulating the sampling of it. For an experiment with more than, say, 100 photons, it would probably be beyond the computational capacity of all the computers in the world.

The question, then, is whether the experiment can be successfully executed. The Rochester researchers performed it with two photons, but getting multiple photons to arrive at a whole sequence of beam splitters at exactly the right time is more complicated. Barry Sanders, director of the University of Calgary's Institute for Quantum Information Science, points out that in 1987, when the Rochester researchers performed their initial experiment, they were using lasers mounted on lab tables and getting photons to arrive at the beam splitter simultaneously by sending them down fiber-optic cables of different lengths. But recent years have seen the advent of optical chips, in which all the optical components are etched into a silicon substrate, which makes it much easier to control the photons' trajectories.

The biggest problem, Sanders believes, is generating individual photons at predictable enough intervals to synchronize their arrival at the beam splitters."People have been working on it for a decade, making great things," Sanders says."But getting a train of single photons is still a challenge."

Sanders points out that even if the problem of getting single photons onto the chip is solved, photon detectors still have inefficiencies that could make their measurements inexact: in engineering parlance, there would be noise in the system. But Aaronson says that he and Arkhipov explicitly consider the question of whether simulating even a noisy version of their optical experiment would be an intractably hard problem. Although they were unable to prove that it was, Aaronson says that"most of our paper is devoted to giving evidence that the answer to that is yes." He's hopeful that a proof is forthcoming, whether from his research group or others'.

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New Kinds of Superconductivity? Physicists Demonstrate Coveted 'Spin-Orbit Coupling' in Atomic Gases

In the researchers' demonstration of spin-orbit coupling, two lasers allow an atom's motion to flip it between a pair of energy states. The new work, published inNature, demonstrates this effect for the first time in bosons, which make up one of the two major classes of particles. The same technique could be applied to fermions, the other major class of particles, according to the researchers. The special properties of fermions would make them ideal for studying new kinds of interactions between two particles -- for example those leading to novel"p-wave" superconductivity, which may enable a long-sought form of quantum computing known as topological quantum computation.

In an unexpected development, the team also discovered that the lasers modified how the atoms interacted with each other and caused atoms in one energy state to separate in space from atoms in the other energy state.

One of the most important phenomena in quantum physics, spin-orbit coupling describes the interplay that can occur between a particle's internal properties and its external properties. In atoms, it usually describes interactions that only occur within an atom: how an electron's orbit around an atom's core (nucleus) affects the orientation of the electron's internal bar-magnet-like"spin." In semiconductor materials such as gallium arsenide, spin-orbit coupling is an interaction between an electron's spin and its linear motion in a material.

"Spin-orbit coupling is often a bad thing," said JQI's Ian Spielman, senior author of the paper."Researchers make 'spintronic' devices out of gallium arsenide, and if you've prepared a spin in some desired orientation, the last thing you'd want it to do is to flip to some other spin when it's moving."

"But from the point of view of fundamental physics, spin-orbit coupling is really interesting," he said."It's what drives these new kinds of materials called 'topological insulators.'"

One of the hottest topics in physics right now, topological insulators are special materials in which location is everything: the ability of electrons to flow depends on where they are located within the material. Most regions of such a material are insulating, and electric current does not flow freely. But in a flat, two-dimensional topological insulator, current can flow freely along the edge in one direction for one type of spin, and the opposite direction for the opposite kind of spin. In 3-D topological insulators, electrons would flow freely on the surface but be inhibited inside the material. While researchers have been making higher and higher quality versions of this special class of material in solids, spin-orbit coupling in trapped ultracold gases of atoms could help realize topological insulators in their purest, most pristine form, as gases are free of impurity atoms and the other complexities of solid materials.

Usually, atoms do not exhibit the same kind of spin-orbit coupling as electrons exhibit in gallium-arsenide crystals. While each individual atom has its own spin-orbit coupling going on between its internal components (electrons and nucleus), the atom's overall motion generally is not affected by its internal energy state.

But the researchers were able to change that. In their experiment, researchers trapped and cooled a gas of about 200,000 rubidium-87 atoms down to 100 nanokelvins, 3 billion times colder than room temperature. The researchers selected a pair of energy states, analogous to the"spin-up" and"spin-down" states in an electron, from the available atomic energy levels. An atom could occupy either of these"pseudospin" states. Then researchers shined a pair of lasers on the atoms so as to change the relationship between the atom's energy and its momentum (its mass times velocity), and therefore its motion. This created spin-orbit coupling in the atom: the moving atom flipped between its two"spin" states at a rate that depended upon its velocity.

"This demonstrates that the idea of using laser light to create spin-orbit coupling in atoms works. This is all we expected to see," Spielman said."But something else really neat happened."

They turned up the intensity of their lasers, and atoms of one spin state began to repel the atoms in the other spin state, causing them to separate.

"We changed fundamentally how these atoms interacted with one another," Spielman said."We hadn't anticipated that and got lucky."

The rubidium atoms in the researchers' experiment were bosons, sociable particles that can all crowd into the same space even if they possess identical values in their properties including spin. But Spielman's calculations show that they could also create this same effect in ultracold gases of fermions. Fermions, the more antisocial type of atoms, cannot occupy the same space when they are in an identical state. And compared to other methods for creating new interactions between fermions, the spin states would be easier to control and longer lived.

A spin-orbit-coupled Fermi gas could interact with itself because the lasers effectively split each atom into two distinct components, each with its own spin state, and two such atoms with different velocities could then interact and pair up with one other. This kind of pairing opens up possibilities, Spielman said, for studying novel forms of superconductivity, particularly"p-wave" superconductivity, in which two paired atoms have a quantum-mechanical phase that depends on their relative orientation. Such p-wave superconductors may enable a form of quantum computing known as topological quantum computation.

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New Generation of Optical Integrated Devices for Future Quantum Computers

Quantum computers, holding the great promise of tremendous computational power for particular tasks, have been the goal of worldwide efforts by scientists for several years. Tremendous advances have been made but there is still a long way to go.

Building a quantum computer will require a large number of interconnected components -- gates -- which work in a similar way to the microprocessors in current personal computers. Currently, most quantum gates are large structures and the bulky nature of these devices prevents scalability to the large and complex circuits required for practical applications.

Recently, the researchers from the University of Bristol's Centre for Quantum Photonics showed, in several important breakthroughs, that quantum information can be manipulated with integrated photonic circuits. Such circuits are compact (enabling scalability) and stable (with low noise) and could lead in the near future to mass production of chips for quantum computers.

Now the team, in collaboration with Dr Terry Rudolph at Imperial College, London, shows a new class of integrated divides that promise further reduction in the number of components that will be used for building future quantum circuits.

These devices, based on optical multimode interference (and therefore often called MMIs) have been widely employed in classical optics as they are compact and very robust to fabrication tolerances."While building a complex quantum network requires a large number of basic components, MMIs can often enable the implementation with much fewer resources," said Alberto Peruzzo, PhD student working on the experiment.

Until now it was not clear how these devices would work in the quantum regime. Bristol researchers have demonstrated that MMIs can perform quantum interference at the high fidelity required.

Scientists will now be able to implement more compact photonics circuits for quantum computing. MMIs can generate large entangled states, at the heart of the exponential speedup promised by quantum computing.

"Applications will range from new circuits for quantum computation to ultra precise measurement and secure quantum communication," said Professor Jeremy O'Brien, director of the Centre for Quantum Photonics.

The team now plans to build new sophisticated circuits for quantum computation and quantum metrology using MMI devices.

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