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The First Two-Qubit Logic Gate in Silicon

Qubits come in several flavors: atoms suspended by laser beams, photons trapped in microwave cavities, superconducting rings in which currents can run in two directions simultaneously.  David DiVincenzo (a theoretical physicist at RWTH Aachen University in Germany, recently interviewed by Spectrum) and Daniel Loss proposed in 1998 using the spin state of electrons trapped in quantum dots to store quantum bits, and many view this as the most promising approach to quantum computing

Last week researchers at the University of New South Wales (UNSW) in Sidney, Australia, reported in the journal Nature that such trapped electrons can be integrated with existing CMOS technology, and that it might be possible to create quantum computer chips that could store thousands, even millions of qubits on a single silicon processor chip.

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Nanoscale Photodetector Promises Next Generation Photonic Circuits

Last year, we covered joint research between the University of Rochester and the Swiss Federal Institute of Technology in Zurich in which a primitive circuit consisting of a silver nanowire and single-layer flake of molybdenum disulfide (MoS2) was developed that could guide both electricity and light along the same wire.

Now researchers at the University of Rochester are continuing their work with nanowires and MoS2 to create a nanoscale photodetector that the researchers believe could be a step towards a new type of photonic circuit.

"Our devices are a step towards miniaturization below the diffraction limit," said Kenneth Goodfellow, a graduate student at the University of Rochester, in a press release. "It is a step towards using light to drive, or, at least complement electronic circuitry for faster information transfer."

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Disappearing Circuits Move From Spy Thrillers to Reality

Generally speaking, the issue that most electronic circuit research is aimed at is making them smaller yet still functional. It would seem that creating circuits that change over time, or even disappear entirely, is an endeavor that has been largely neglected, outside of a TV spy show from the 1960s that gets periodically rebooted into films.

Now researchers from the Georgia Institute of Technology have taken up the challenge of creating circuits than change over time, and may have come up with a technology that could have some attractive biomedical applications.

In research published in the journal Nanoscale, the Georgia Tech researchers deposited carbon atoms onto graphene using a focused electron beam process to create patterns that evolve over time on the graphene.

“We will now be able to draw electronic circuits that evolve over time,” said Andrei Fedorov, a professor at Georgia Tech, in a press release. “You could design a circuit that operates one way now, but after waiting a day for the carbon to diffuse over the graphene surface, you would no longer have an electronic device. Today the device would do one thing; tomorrow it would do something entirely different.”

Providing further evidence that intentionally setting out to create disappearing circuits is rare at best, the Georgia Tech researchers admit that they were initially just trying to see how they could remove hydrocarbon contaminants from graphene.

What they soon discovered was that when they deposited the carbon atoms on the graphene, they could create patterns and these patterns served to create negatively charged areas in the graphene.

But what really grabbed their attention was the discovery that these patterns would change over time as the carbon atoms moved around the surface of the graphene until they were spread uniformly across the entire surface. This change, which occurs over tens of hours, converts positively charged (p-doped) surface regions into surfaces with a uniformly negative charge (n-doped). It also manages to form an intermediate p-n junction domain during this transformation.

“The electronic structures continuously change over time,” Fedorov explained. “That gives you a reconfigurable device, especially since our carbon deposition is done not using bulk films, but rather an electron beam that is used to draw where you want a negatively-doped domain to exist.”

While the security applications for such a capability have been demonstrated vividly in the TV and movie series Mission: Impossible, the researchers have suggested that such a capability could prove useful in biomedicine.

“Perhaps there could be certain activated, triggered processes that could benefit from this type of behavior in which the electronic state changes continuously over time,” said Fedorov in the press release.

In further research, the Georgia Tech team will be aiming to find an application that could not be achievable without this capability.


Novel Nanostructures Could Usher in Touchless Displays

In a world where the “swipe” has become a dominant computer interface method along with moving and clicking the mouse, the question becomes what’s next? For researchers at Stuttgart’s Max Planck Institute for Solid State Research and LMU Munich, Germany, the answer continues to be a swipe, but one in which you don’t actually need to touch the screen with your finger. Researchers call these no-contact computer screens touchless positioning interfaces (TPI).

In research published in the journal Advanced Materials, the Germany-based researchers have developed nanostructures capable of changing their electrical and optical properties when a finger passes by them. The resulting device could usher in a new generation of touchless displays.

While touchless displays raise the question of whether every finger that passes by a display’s surface is really intended to interface with the computer, the researchers believe this new interface will address the problems of mechanical wear suffered by today’s touch screens over time, as well as concerns over screens, especially at ATMs, being transmission vectors for viruses and bacteria.

Computer hardware analysts aren’t completely sold on whether touchless displays are really next step in computer interfaces. That debate notwithstanding, the technology that enables this approach is impressive. The researchers have developed what amounts to a humidity sensor that reacts to the minute amount of sweat on a finger and converts it to an electrical signal or a change in color of the nanostructured material.

The nanostructured material is made up of something called phosphatoantimonic acid, which takes up water molecules and swells considerably in the process. Not only does the material swell, but its electrical conductivity increases with each water molecule it absorbs.

While this clearly makes for a pretty dependable humidity sensor, the researchers were not looking to create just another moisture gauge, but a new approach to computer interfaces.

“Because these sensors react in a very local manner to any increase in moisture, it is quite conceivable that this sort of material with moisture-dependent properties could also be used for touchless displays and monitors,” said Pirmin Ganter, doctoral student at the Max Planck Institute and the Chemistry Department, in a press release.

To get the material to be more than just react to humidity, they took nanosheets of the material and combined them with a photonic nanostructure that reacts to the water by changing color. If this material were to be used in a display, the change in color would let the user know that the screen is recognizing the finger and its movement.

“The color of the nanostructure turns from blue to red when a finger gets near, for example. In this way, the color can be tuned through the whole of the visible spectrum depending on the amount of water vapor taken up,” explained Bettina Lotsch, one of the researchers, in the press release..

You can see how the color of the material changes as a finger passes near it in the video below.

While the color change is a helpful indicator that the user is, in fact, interacting with the display, the real merit of this technology over others like it is how quickly it reacts. Previous touchless interfaces could take seconds to respond to the near-miss finger swipe, but this technology responds in mere milliseconds.

The researchers are continuing to look at how they can improve the process for producing the material in order to bring down its costs. They’re also looking for a way to give it a protective coating to reduce wear that will occur because of incidental contact, while still maintaining its special properties.


IBM Solves Nanotube Transistor's Big Shrinking Problem

Carbon nanotubes could allow transistors on computer chips to shrink beyond the physical limits of today’s silicon switches. But to get there, nanotube transistors would need to get past a major hurdle, the resistance between the nanotube and the metal contacts that inject current into them. Now IBM researchers have broken through that barrier, paving the way to devices that could be the heart the next decade’s chips.

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Optical Rectenna Could Double Solar Cell Efficiency

Researchers at the Georgia Institute of Technology have developed a first of its kind: an optical rectenna, which combines the qualities of an antenna with a rectifier diode. If further refined, the researchers believe the device could lead to a new generation of highly efficient solar cells.

While rectennas have been around since the 1960s, they have not been able to operate at optical wavelengths. The challenge in achieving this goal was to make the antenna portion of the devices small enough as well as fabricating some kind of rectifier into them.

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Reseachers Create First Integrated Circularly Polarized Light Detector on a Silicon Chip

What do you get when you combine some biomimicry, metamaterials and nanowires? It turns out to be the first integrated circularly polarized light detector on a silicon chip. Its development could usher in a new generation of portable sensors that can use polarized light for applications ranging from drug screening to quantum computing.

Researchers at Vanderbilt University have used silver nanowires to fabricate a metamaterial that is capable of detecting polarized light in a way not unlike the way cuttlefish, bees, or mantis shrimp do it.

“Although it is largely invisible to human vision, the polarization state of light can provide a lot of valuable information,” said assistant professor Jason Valentine in a press release. “However, the traditional way of detecting it requires several optical elements that are quite bulky and difficult to miniaturize. We have managed to get around this limitation by the use of ‘metamaterials’—materials engineered to have properties that are not found in nature.”

Polarized light comes in basically two forms—linear or circular. In contrast to non-polarized light, in which the electric fields of the photons are oriented in random directions, polarized light, whether linear or circular, features electric fields oriented in a single plane. (With circularly polarized light, the plane is continually rotating through 360 degrees.)

One of the distinguishing capabilities of circularly polarized light (CPL) is that it can discern the difference between right-handed and left-handed versions of molecules—a property known as chirality. Chirality is critically important in drugs because whether they are left handed or right handed determines their biological activity. For instance, there is the famous case of thalidomide, which in one chirality alleviates morning sickness in pregnant women and in the other causes birth defects. Having a portable sensor capable of detecting a drug’s chirality could be a game changer.

“Inexpensive CPL detectors could be integrated into the drug production process to provide real time sensing of drugs,” said Vanderbilt University doctoral student Wei Li, in a press release. “Portable detectors could be used to determine drug chirality in hospitals and in the field.”

In research published in the journal Nature Communications, the researchers fabricated the portable CPL sensors by laying down nanowires in a zig-zag pattern over a thin sheet of acrylic affixed to a thick silver plate. This material is affixed to the bottom of a silicon wafer with the nanowire side up.

The nanowires create a sea of electrons that produces “plasmon” density waves, the oscillations in the density of electrons that are generated when photons hit a metal surface. These plasmon density waves absorb energy from the photons that pass through the silicon wafer. The absorption of the energy produces “hot” or energetic electrons, which generate a detectable electrical current.

The researchers found that they could make the zig-zag pattern of nanowires with a right- or left-handed orientation. When they arranged the nanowires in right-handed pattern, the surface absorbed right circularly polarized light and reflected left circularly polarized light. When arranged in a left-handed pattern, the opposite effect occurred. And when they arranged the nanowires to have both left- and right-handed patterns, the sensor could discern between left and right circularly polarized light.

The researchers concede that their current prototype is not efficient enough to be commercially viable. However, they have a few tricks up their sleeves that they believe will improve that efficiency in the next generation of their devices.


Graphene Has a Place on the Hype Cycle, Says European Flagship Director

Two years ago, the European Commission announced the Graphene Flagship, a 10-year, €1 billion effort to help move graphene out of research labs and into commercial applications. The massive effort, which celebrates its second anniversary this week, now includes groups from 23 countries. How has it fared? IEEE Spectrum senior associate editor Rachel Courtland catches up with flagship director Jari Kinaret, a theoretical physicist based at Chalmers University in Sweden, to talk about the program’s progress, graphene hype, and the role of other 2-D materials

To start off, how has the flagship program performed so far?

We are on track to do what we had promised. The first year, we produced more than 300 publications and nearly 600 conference talks, and filed a number of patent applications and some invention disclosures.

One thing that has happened is that we have increased the number of industrial partners. Two years ago, when the flagship started, we were about 75 partners. Something like 20 percent of them were industrial. Then we went through our competitive call, and we added another dozen partners last spring. So, right now we are about 140 partners and the industrial portion is maybe 25 percent. Next April, we will be about 150 partners, and about one-third of them will be industries. This increase in the industrial nature of the program is quite visible, and it’s very much according to plan.

Do you have a sense of where the research community and industry would be without this funding? How do you measure the impact of the program?

Of course as a scientist you would like to do a controlled measurement. You have one sample where you do something and then you have another sample where you don’t do something and you can see the difference. You can’t do that with this kind of one-time event. You can’t have Europe with the flagship and Europe without the flagship and compare those two.

What we can see is that the flagship, through its visibility, has engaged many new industrial branches and many new companies. We cover a very broad range of activities, from say, basic chemical companies like BASF, to component manufacturers—ST Microelectronics would be an example—to systems integrators such as Airbus. Without the flagship, it would be very difficult for actors in different parts of the value chain to find each other. Now we can bring them all together, and they can work together saying, “to make my components better, I need this kind of material” or “to make a new system if would be really great if we had a component like this.”

So people can work along the entire chain vertically, but also horizontally. People working with high-frequency electronics or photonics or sensors see that, while the applications are rather different, the manufacturing techniques that need to be developed to commercialize them are similar. We are now working across work packages to focus more on the manufacturing issues. That would be very difficult, if not impossible, without the flagship.

Then there is the great catalyst effect. Many partners have used their participation [to attract] additional funding from their own governments or private funders. Membership is taken as a token of quality.

Are there any accomplishments so far that you’re particularly proud of?

The one that has had perhaps the most media interest is something called [the] shear exfoliation technique, work that was done by Jonathan Coleman and his group at Trinity College Dublin. It is basically a way that you can make graphene in your own kitchen. This shear exfoliation technique has been commercialized by Thomas Swan & Co, Ltd. in the U.K.

Another example, is that we have a group of [researchers] who got together to make a very fast graphene-based photodetector that functions at 50 GHz. It was slightly embarrassing, because they know it functions at least at 50 GHz, but that’s where their measurement equipment finished. They estimated that it functions probably at 200 GHz. That kind of technology has raised a lot of interest among companies that are interested in optical fiber communication.

The third one I would like to pick shows the benefit of the flagship to organizations that are not partners. In the Graphene Week conference last summer in Manchester, which the Graphene Flagship organizes, [Robert Roelver at Bosch] reported on work that they had done with a flagship partner, the Max Planck Institute in Stuttgart, developing a graphene-based magnetic field sensor that was 100 times more efficient than any other existing magnetic field sensor. These magnetic field sensors may sound esoteric, but we have them in cell phones as a backup to GPS. That’s three examples that I can give you right off the top of my head.

Where do you hope to be when the flagship is complete?

At the end of 10 years, we hope that graphene and related layered materials have left the academic laboratories and entered society as new products in many areas. This is a very ambitious goal if you think of other new materials, like carbon fiber composites. They were developed in the 70’s, roughly speaking, and now they are starting to make an appearance in things like [family] cars. We want to get to that point, and we realize 10 years is too short to make it all the way. But in 10 years we can certainly make it a good part of the way there, so that in some areas, people are no longer surprised or raising eyebrows if they hear, ‘This is made of graphene.’”

There’s already a lot of discussion about graphene’s applications, and it seems like a lot of people are asking whether the material is overly hyped.

Yes, a very good point. One of the things I always show in my presentations these days is something called the Gartner hype cycle.

It’s clear that graphene has probably passed the high peak, the peak of inflated expectations, and we are in the downward-going slope. That downward-going slope of the hype occurs when people realize that this isn’t the solution to all the world’s problems. Typically it’s associated with negative press. People are starting to wonder if everything that has been promised is only hot air or if something is really coming out of it.

What we hope to do is make this dip less dramatic. One way it’s probably going to be less dramatic is that different graphene technologies mature at different rates. If you are in the sports field, you would have to say that graphene has already reached the plateau of productivity. The plateau of productivity is where your reaction if you hear that something contains graphene is, “so what?” It basically has no news value. I can take a ten-minute walk from my office and buy both alpine skis and tennis racquets made of graphene composites. It’s becoming, if not commonplace, at least within reach. Other areas like graphene-based electronics are going to take a much longer time to get to the plateau of productivity.

How can you push a graphene-based technology toward that plateau of productivity?

You need to identify the applications that are worth pursuing. Those could be applications where the replacement of an existing material by graphene is relatively simple or applications where the benefit of using graphene is worth the extra effort.

One thing that seems to be common is the gap between the stage where it works in the lab and the place where it works well enough that you can be pretty sure that it will be a product. External programs such as the flagship, where many partners collaborate and share the risk, are very helpful.

Graphene lacks a natural band gap, the energy barrier in a semiconductor that gives the material a natural “off” and “on” state. Is this a big issue?

This is a non-issue in many areas—take composite materials or using graphene in electrodes in batteries. In photonics, that means you can absorb light regardless of wavelength, so it’s a very positive aspect. In digital electronics it is close to being a game stopper. You can’t just take your silicon MOSFET design and replace the silicon with graphene, because you would find great difficulty in turning your transistor off. For high-frequency applications, the issue is not nearly as bad but it is still a challenge.

For digital electronics, there is still quite a lot of promise, because the electrons are so fast. You need to come up with some other kind of design than the standard MOSFET structure. People are working on that. Tunneling transistors or other vertical structures show a respectable on-off ratio, but have challenges when it comes to manufacturing. So yes, lack of bandgap is an issue. You probably should not take the hexagonal piece of graphene and try to fit it into the circular hole that is digital electronics.

What about other two-dimensional materials?

Indeed, some of them have bandgaps and therefore they would be easier substitutes for silicon. There are challenges regarding the materials’ quality, production, and integration with existing technologies, but as far as existence of a bandgap is concerned, they certainly offer an easier solution than graphene does.

Other materials have been in the flagship from the very beginning, and they still are in the flagship. If one was pedantic, it should be called “Graphene and Related Materials Flagship”, but that just does not work. Your editor would not allow you to have a title like that.

Is there anything else you’d like to add?

When physicists talk about graphene, we think first about the electronics applications, because they tend to be closest to our minds. The real first applications may be in areas we perhaps don’t prioritize so much, things like different kinds of composite materials.

Now people are getting excited about the fact that graphene is an impermeable membrane—it doesn’t let anything to go through. If you have an impermeable membrane, you can make holes in it, and you can choose what goes through and what doesn’t. So you may be able to separate carbon dioxide from other gases, and that would be of great interest for, say, carbon sequestration. The fact that there is more to the world than electronics is, I think, what we are learning to appreciate more and more.


Graphene Keeping It Cool In Electronics

Cooling fans and other system-level solutions are reaching their limits as circuit densities continue to grow. It’s no wonder then that graphene’s remarkable heat conductivity has led to a lot of research into using it to for thermal management in electronics.

Now an international team of researchers, organized by a team at the University of Michigan, has found that layered graphene can be an important tool for thermal management because of its ability to release heat efficiently.

In research published in the journal Nature Communications, the scientists demonstrated that the electrostatic interactions between electrically charged particles—known as Coulomb interactions—in  different layers of multi-layered graphene offers a key mechanism for dispersing heat. This occurs despite the fact that all electronic states are strongly confined within individual 2D layers.

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Thin Is in for Invisibility Cloaks

When Xiang Zhang first invented an invisibility cloak that worked at optical wavelengths, his son complained that it wouldn’t allow him to easily stroll around unseen, like Harry Potter sneaking through the library.

“My son always joked that, ‘Daddy, if I want to make myself disappear I have to carry a huge cylinder around myself,’” says Zhang, a professor of mechanical engineering at the University of California, Berkeley. The boy wondered why he couldn’t have something more like a sunblock, a spray-on material that rendered him invisible. That may not be such a far-fetched vision, Zhang says.

He and his colleagues have created an extremely thin cloak that works in visible light, made of a metasurface, a special type of metamaterial that is thin enough to be considered two-dimensional. They described the device in last week’s Science.

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IEEE Spectrum’s nanotechnology blog, featuring news and analysis about the development, applications, and future of science and technology at the nanoscale.

Dexter Johnson
Madrid, Spain
Rachel Courtland
Associate Editor, IEEE Spectrum
New York, NY
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