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Researchers Create Quantum Dots that Transmit Identical Single Photons

Single photons will play an important role in quantum communication. This will require quantum repeaters, in much the same way that optical amplifiers are required for the transmission of digital data through optical fibers. However, these quantum repeaters will work only if all the single photons that they receive have the exact same wavelength. A team of researchers at universities in Switzerland, Germany, and France reported on 8 September in Nature Communications that they’ve developed quantum dots that can produce streams of photons with identical wavelengths. 

To create the photons, the researchers used self-assembled indium gallium arsenide (InGaAs) quantum dots embedded in gallium arsenide. Each quantum dot, although it consists of about a hundred thousand atoms, traps a single electron that can occupy two energy levels. By illuminating the quantum dot with laser light, the electron is moved into the higher energy state. When the electron drops down to its lower energy level, it emits a single photon whose wavelength is determined by the difference in the energy of the two levels. “In many ways it behaves like a single atom, and this is why it is often called an 'artificial atom,’” says Andreas Kuhlmann a post-doctoral researcher at the University of Basel who was the paper’s lead author. “But because it is inside a semiconductor it is quite robust, and that is, of course, nice if you want at one point to develop a product.”  

Still, unlike a single atom, the atoms in the quantum dot are an unruly bunch.  The fluctuating nuclear spins of the atoms in the quantum dot interact with the electron spin, and fluctuating electric fields created by electrons hopping around cause the two energy levels of the quantum dot to wobble. This results in the emission of photons with wavelengths that vary, if ever so slightly. “In order to get indistinguishable single photons, which all have exactly the same color, we needed to find a way to suppress the noise,” explains Kuhlmann.

How did they do it? They cooled the quantum dot to 4.2 Kelvin. They could reduce the noise even further by cooling it even more, an option that is not that attractive.  Despite what one might think at first blush, “4.2 Kelvin is in some ways quite warm; at some point you want to develop a product, and it makes sense to stay as warm as possible,” says Kuhlmann.

That wasn’t the only trick they employed to reduce the noise, says Kuhlmann. “The samples are grown layer by layer using molecular beam epitaxy, and when you grow these samples you will get some defects.” Crucial to limiting noise, he says, is limiting these defects because they become populated by electric charges that fluctuate. “It is extremely important that you have high-quality material and that the people who grow these samples know what they are doing,” says Kuhlmann.

The researchers overcame several hurdles. Though nuclear spins are an intrinsic property of the InGaAs semiconductor, the researchers found a way to reduce the influence of the nuclear spins.  Because of its indirect band gap, it can’t emit photons when irradiated by light, explains Kuhlmann. But he and his collaborators created a quantum dot that transmits photons. And because they were able to precisely control the number of electrons trapped in the quantum dot by applying a voltage to a gate, they were able to reduce this number to one. Over a broad voltage range, the quantum dot is empty. However, at a particular voltage, the dot’s sweet-spot, the noise from the nuclear spins is strongly suppressed. “We don’t know why yet”, says Kuhlmann, “but the improvement in the photons is immediately noticeable.” 

The result, says Kuhlmann:

If we switch on our laser, and there is a single electron inside this quantum dot, then this permanent excitation suppresses the fluctuations of the nuclear spins in the sample. When no electron is trapped in the quantum dot, we found that by changing the electrical field that we apply to the device, we also can suppress the spin noise. 

One of the problems that still has to resolved is that a number of photons are lost when the photons exit the device via the gallium arsenide layer that covers the quantum dot. “There are ways to engineer these devices; for example, you put a quantum dot inside a nanowire, and you can send almost all the photons along the waveguide,” says Kuhlmann.  There should be applications galore for the device in quantum communication and computing.  “A single photon is an ideal flying qubit,” says Kuhlmann.


Peering Into Nanoparticles One at a Time Reveals Hidden World

Imagine you could single out individuals in a large group and see what each was doing instead of observing a large number of them all grouped together. You could detect how each is distinct within the group.

This is essentially what researchers at Chalmers University in Sweden have been able to achieve with a new microscopy technique that is capable of looking at a single nanoparticle rather than just a mass of them all clumped together.

“We were able to show that you gain deeper insights into the physics of how nanomaterials interact with molecules in their environment by looking at the individual nanoparticle as opposed to looking at many of them at the same time, which is what is usually done,” said Associate Professor Christoph Langhammer, who led the project, in a press release.

In their findings, published in the journal Nature Materials, the researchers leveraged an imaging technique known as plasmonic  nanospectroscopy, which involves exploiting the oscillations in the density of electrons that are generated when photons hit a metal surface.

The researchers applied the experimental spectroscopy technique to examine hydrogen absorption in single palladium nanoparticles. The observations were surprising. They discovered that despite various nanoparticles having the same size and shape, they would absorb hydrogen at pressures as different as 40 millibars.

In real world applications, this observation could help lead to more sensitive hydrogen sensors for detecting leaks in fuel-cell-powered vehicles.

“One main challenge when working on hydrogen sensors is to design materials whose response to hydrogen is as linear and reversible as possible. In that way, the gained fundamental understanding of the reasons underlying the differences between seemingly identical individual particles and how this makes the response irreversible in a certain hydrogen concentration range can be helpful,” added Langhammer in the release.

While others have been able to image single nanoparticles previously, those efforts came at a rather high cost of heating the nanoparticles up, or impacting them in some other way that eliminates the ability to observe them accurately.

“When studying individual nanoparticles you have to send some kind of probe to ask the particle ‘what are you doing?’,” said Langhammer. “This usually means focusing a beam of high-energy electrons or photons or a mechanical probe onto a very tiny volume. You then quickly get very high energy densities, which might perturb the process you want to look at.”

Not only is this effect is minimized in their new approach, according to Langhammer, but it is also compatible with ambient conditions, so that it is possible to study nanoparticles one at a time in their actual environments. This ability to observe nanoparticles outside the lab could prove to be a key development for studies on the impact of nanoparticles in the environment.


Graphene's Killer App? Measuring Electrical Resistance

Graphene’s merits in electronic devices and as a light bulb coating are still being debated. But new results suggest the atom-thick carbon sheet has one clear advantage: precise but practical calibrations of electrical resistance.

This might seem like a minor use for the world’s most celebrated wonder material, but it’s one that sits at the very base of the electrical engineering pyramid. A more practical calibration could help national standards laboratories and industries that depend on those standards. It may also help disseminate the International System of Units (SI), which could be overhauled as early as 2018.

The most exacting metrology laboratories calibrate their electrical units based on quantum mechanical phenomena. The ohm, the SI unit of electrical resistance, is calibrated by taking advantage of the quantum Hall effect. The Hall effect occurs when a magnetic field is applied perpendicular to the flow of current. The resulting force on electrons causes them to migrate to the side, which in turn raises a voltage perpendicular to the flow of current.

In the quantum version of the Hall effect, which occurs in a thin layer of material, the voltage and resulting resistance are “quantized” and so take on discrete integer values. Conveniently, the quantum Hall resistance is expected to be completely independent of the kind of device that’s built. Instead it depends only on two unvarying constants of nature: the fundamental charge of the electron and a quantum mechanical measure dubbed the Planck constant. 

Unfortunately, the physical conditions required to take advantage of the quantum resistance standard are exacting. State-of-the-art measurements, which are taken using a device made of thin layers of gallium arsenide and aluminum gallium arsenide, can require a 10-Tesla magnetic field (and so a massive superconducting magnet) and temperatures within a few degrees of absolute zero. 

Researchers have long suspected that the unique behavior of electrons in graphene, namely the big spacing between electron energy levels when the material is exposed to a magnetic field, could be exploited to produce precise measurements of resistance under less extreme physical conditions. 

Several recent results support that idea. In August, Jan-Theodoor Janssen  at the UK’s National Physical Laboratory and colleagues reported a way to build a small system for a graphene resistance standard that can operate at a higher temperature and lower magnetic field. This week, a team based at France’s National Metrology and Testing Laboratory and various departments at the National Center for Scientific Research showed that graphene can indeed be used to calibrate resistance with great accuracy, rivalling that of gallium devices, and that it can do so over an even wider range of operational conditions.

The French team constructed its resistance device from a high-quality sheet of graphene grown on a silicon carbide wafer. The resulting 100-by-420-micrometer “Hall bar” contained a source and drain to raise a voltage across the device, as well as a set of metallic pads along the sides that were used to measure resistance.

The team found they could measure resistance with a level of accuracy rivaling those yielded by gallium arsenide devices, but with a magnetic field one-third as strong, with a temperature as high as 10 Kelvin, or with measurement currents 10 times as high (though not all three at the same time).

The temperature the graphene device operates at is high enough that a lab could accurately measure resistance without needing liquid helium as a refrigerant. “These results support graphene as the material of choice for the next generation of easy-to-use, helium-free, and affordable quantum electrical standards, approaching an ideal standard that would be invariant, available to anyone, at any place and any time,” the French team wrote this week in Nature Nanotechnology.

Graphene could also help bring about the realization of a simplified ampere, one of the seven SI base units. In the new SI, this unit of current will be redefined in terms of the fundamental charge of the electron, and quantum electrical standards will play a closer, more integrated role.

The kilogram will also be redefined. Instead of being measured against a physical artifact locked in a vault, it will be realized directly through the watt balance, which measures the weight of an object against an opposing electromagnetic force. That force is calibrated against quantum electrical standards. (Read the strange story of the kilogram change here.)

In the long term, it could be possible for any national metrology laboratory to measure the kilogram from scratch, says Jon Pratt of the U.S. National Institute for Standards and Technology, which is also investigating graphene’s potential as a resistance standard. “If we are to make watt balances available to everyone, we will need quantum standards that are low cost and easy to use,” he says. “Graphene appears to move us towards this, at least for resistance.”

This article was updated on 14 September 2015 to clarify the NPL and LNE/CNRS findings.


Graphene and Perovskite Lead to Inexpensive and Highly Efficient Solar Cells

Perovskite is the new buzzword in photovoltaics. And graphene is the buzzword for just about every other high-tech application, including photovoltaics.

Now researchers at Hong Kong Polytechnic University have combined these two materials to make a semi-transparent solar cell capable of power conversion efficiencies around 12 percent, a significant improvement over the roughly 7-percent efficiency of traditional semi-transparent solar cells.

The semi-transparent design of these solar cells means that they can absorb light from both sides and could allow them to be used as windows that serve the dual function of letting light into a building and generating electricity.

In the design of the Hong Kong researchers’ solar cell, the perovskite serves as active layer for harvesting the light, and the graphene acts as the transparent electrode material. Graphene has long been pursued as a potential replacement for indium tin oxide (ITO) as a transparent electrode material for displays.

Here again, graphene’s transparency, high conductivity, and potentially low cost seemed attractive to the researchers. The researchers improved on the conductivity of the graphene by coating it with a thin layer of a polymer that also served as an adhesion layer to the perovskite active layer during the lamination process.

The researchers were able to improve the energy conversion capability of the solar cells by employing a multi-layer chemical vapor deposition process in which the graphene formed the top transparent electrodes. This approach maintained the transparency of the electrodes while increasing their sheet resistance.

A big concern for the researchers was lowering costs. They claim that their solar cells cost less than US$.06/watt, which they reckon is more than a 50 percent reduction in the costs of silicon solar cells. They believe that the whole process is ripe for scaling up because the mechanical flexibility of the graphene enables the possibility of roll-to-roll processing.


One-step Fabrication Method Produces Nanofibers Ideal for Energy Storage

Researchers at Case Western Reserve University have tackled the problem of getting carbon nanotubes and graphene to exhibit their extraordinary thermal, electrical and mechanical properties as three-dimensional nanostructures. As a result of their efforts, they have walked away with an improved one-step fabrication method for producing these nanostructures and carbon fibers that could lead to highly efficient energy storage systems.

In research published in the journal Science Advances, the Case Western researchers employed an anodic oxidation of aluminum (AAO) template to directly grow hollow fibers made of carbon nanotubes in a simple one-step process without the need for metal catalysts.

The researchers believe that the flexible hollow fibers could improve energy storage devices and could even be woven into textiles as power sources for wearable electronics. Exploiting the flexibility of carbon nanomaterials by using them as a power source in wearable electronics has been the focus of a fair amount of research.

“The porous 3-D carbon structure provides a huge surface/interface area for charge storage,” explained Liming Dai, a professor at Case Western and one of the lead researchers, told IEEE Spectrum via e-mail. “Furthermore, the presence of holes facilitates the electrolyte diffusion while the seamless nodal junction allows fast electron transport, and hence the fast charging and discharging rates.”

In the research, Dai and his colleagues fashioned wire capacitors from the 3-D carbon nanotubes; these energy storage devices could potentially be woven into different fabrics.

“We will try to make textiles from the fibers, but we also don't mind others to work along this line as long as our publications are properly cited,” said Dai.

In future research, Dai and his colleagues plan to develop 3-D multilayered structures.


New Layering Process Brings Graphene's 2D Properties to the 3D World

Researchers at Rensselaer Polytechnic Institute (RPI) have taken a significant step towards transforming high quality 2D graphene sheets into 3D macroscopic structures that could be used for applications such as thermal management for high power electronics, structural composites, flexible and stretchable electrodes for energy storagesensors, and membranes.

In research published in the journal Science, the RPI researchers developed a new layered structure for graphene that addresses the problem of achieving the mechanical strength of graphene in its 3D form while maintaining its attractive thermal and electrical properties in its 2D form.

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Graphene "Decorated" With Lithium Becomes a Superconductor

Graphene is a conductor unlike anything seen before. Electrons are able to travel though it without resistance at room temperature, promising a new approach to electronics.  It’s even been engineered to act like a semiconductor with a band gap for stopping and starting the flow of electrons, thus offering an alternative to silicon in electronics.

Despite these properties, and a host of others that seem to sprout up regularly, nobody had been able to make graphene behave as a superconductor, until now.

An international research team from Canada and Germany has been able to demonstrate that graphene can be made to behave as a superconductor when it’s doped with lithium atoms. The researchers believe that this new property could lead to a new generation of superconducting nanoscale devices.

Superconductors are materials that conduct electricity without resistance and without dissipating energy. In ordinary materials, electrons repel each other, but in superconductors the electrons form pairs known as Cooper pairs, which together flow through the material without resistance. Phonons, the mechanism that facilitates these electrons’ alliances are vibrations in lattice crystalline structures.

Graphene is not naturally a superconductor, and neither is its three-dimensional source—graphite. However, it was demonstrated a decade ago that graphite could be induced into behaving like a superconductor. If it’s possible with graphite, it should be with graphene, right?

Other research groups believed so and developed computer models demonstrating that combining graphene with lithium might do the trick. Lithium, they predicted, could contribute a lot of phonons to the graphene’s electrons.

In a research paper available on arXiv, the researchers demonstrated in physical experiments that the computer models were indeed correct in their predictions. Andrea Damascelli at the University of British Colombia in Vancouver, together with collaborators in Europe, grew layers of graphene on silicon-carbide substrates, then deposited lithium atoms onto the graphene in a vacuum at 8 K, creating a version of graphene known as “decorated” graphene.

In the testing and measuring of their material, the researchers found that the electrons slowed down as they travelled through the lattice, which they believe to be the result of enhanced electron–phonon coupling. The key observation was that this increased number of coupled pairs led to superconductivity, which the researchers measured by identifying an energy gap between the material's conducting and non-conducting electrons. That energy gap is equal to the amount of energy needed to break Cooper pairs.

The researchers who demonstrated last year the role phonons played in the superconductivity of graphite and calcium, Patrick Kirchmann and Shuolong Yang of the SLAC National Accelerator Laboratory, believe this latest work could usher in the fabrication of nanoscale superconducting quantum interference devices and single-electron superconductor quantum dots.

A young woman looks out of a large office window towards a group of neoclassical buildings. The scene is tinted yellow by light from a sun low above the horizon.

A Clearer Outlook for Quantum Dot-Enabled Solar Windows

Quantum dots have been knocking on the door of photovoltaics for a while now. But it turns out that maybe they should have been tapping on the window instead.

In joint research between the Department of Energy’s Los Alamos National Laboratory (LANL) and the University of Milan-Bicocca (UNIMIB) in Italy, researchers have spent the last 16 months perfecting a technique that makes it possible to embed quantum dots into windows so that the window itself becomes a solar panel.

In research published in the journal Nature Nanotechnology, the international team were able to improve upon their previous iteration of the technology by making the quantum dots from non-toxic materials while having the window be largely tint free and transparent.

“In these devices, a fraction of light transmitted through the window is absorbed by nanosized particles (semiconductor quantum dots) dispersed in a glass window, re-emitted at [an] infrared wavelength invisible to the human eye, and wave-guided to a solar cell at the edge of the window,” said Victor Klimov, lead researcher on the project at LANL, in a press release. 

Of course, this is not the first time someone thought that it would be a good idea to make windows into solar collectors. But this latest iteration marks a significant development in the evolution of the technology. Previous technologies used organic emitters that limited the size of the concentrators to just a few centimeters.

The energy conversion efficiency the researchers were able to acheive with the solar windows was around 3.2 percent, which stands up pretty well when compared with state-of-the-art quantum dot-based solar cells that have reached 9 percent conversion efficiency.

The US-Italy team’s first breakthrough last year was to make large-area solar concentrators that were free from reabsorption losses that had plagued previous attempts. This lastest breakthrough was in finding a way to toss out the cadmium quantum dots for a more environmentally friendly compound.

“Our new devices use quantum dots of a complex composition which includes copper (Cu), indium (In), selenium (Se) and sulfur (S),” said Klimov in the release: “these particles do not contain any toxic metals that are typically present” in similar systems, he added. And Klimov claims that because their quantum dots absorb light from throughout the spectrum of incoming sunlight, it doesn’t distort colors.  

The researchers acknowledge that we won’t see solar windows for sale in the near term, as bringing down production costs still remains an issue. But they believe this latest development should bring solar windows closer to the market, as their new quantum dots are cheaper than traditional dots.


The Universe of 2-D Materials Expands in the Real World

While research and its media coverage are quick to extol the virtues of the array of two-dimensional (2-D) materials, what is sometimes neglected is that they often come with some fairly significant obstacles that keep them from replacing silicon. Among these difficulties is the lack of a band gap. However, the potential showstoppers don’t end there. Merely exposing these materials to real-world environments with air can make these materials unstable and begin their decomposition.

Recent research has shown that encapsulation techniques in which a protective shell covers the 2-D material have proven effective at isolating materials like silicene from real-world environments, albeit briefly.

Now researchers at the University of Manchester in the U.K. have combined encapsulation with a number of other manufacturing techniques—including a different means by which the 2-D materials are cleaved away from their 3-D versions and a special method for transferring them onto a substrate and aligning them—to produce 2-D materials that can survive in real-world environments.

“This is an important breakthrough in the area of 2-D materials research, as it allows us to dramatically increase the variety of materials that we can experiment with using our expanding 2-D crystal toolbox,” said Roman Gorbachev, who led the research, in a press release.

In research published in the journal NanoLetters, the Manchester team demonstrated their production technique on two 2-D materials: niobium diselenide and black phosphorus, which has been building quite a reputation for itself over the last 18 months. The researchers fabricated field-effect devices using these materials and their technique. The results showed that the devices remained conductive and fully stable under ambient conditions.

The niobium dieselenide maintained its superconducting properties right down to a monolayer. Meanwhile, the black phosphorus, in the form of a trilayer, had such high electron mobility that it achieved Landau quantization, which is responsible for changes in the electronic properties of materials when exposed to a magnetic field.

The main impact of the research is that it shows a way towards making the full gamut of 2-D materials accessible to real-world applications.

As Andre Geim, the Nobel laureate at Manchester who, with his colleague Kostya Novoselov, first synthesized graphene, noted in the release: “The more materials we have to play with, the greater potential there is for creating applications that could revolutionize the way we live.”


Microwave Oven Is Key to Safer Quantum Dot Manufacturing

As we noted earlier this month, quantum dots have finally made a broad commercial impact on LED technology. So now it’s time to refine the processes for making them and to increase profit margins.

Now researchers at Oregon State University (OSU) have thrown their hat into the ring, presenting a manufacturing method for quantum dots that promises a new era in LED lighting.

In research published in the Journal of Nanoparticle Research, the OSU researchers employed a so-called “continuous flow” chemical reactor. Chemicals were continuously put into the reactor and came out as a continuous stream of product. This continuous flow setup makes the process cheaper, faster, and highly scalable. Meanwhile, using microwaves to heat the reagents—with a machine that operates on more or less the same principle as your microwave oven at home—addresses the issue of how to maintain precise temperature control during the chemical process.

The researchers claim that this method leads to precisely sized and shaped quantum dots that are consistent in their composition. They believe that this development will mark a big change in LED lighting.

“We may finally be able to produce low-cost, energy-efficient LED lighting with the soft quality of white light that people really want,” said Greg Herman, an associate professor of chemical engineering at OSU, in a press release. “At the same time, this technology will use nontoxic materials and dramatically reduce the waste of the materials that are used, which translates to lower cost and environmental protection.”

The environmental benefit of the quantum dots stems from the use of copper indium diselenide, which is a more environmentally benign material than the cadmium that is typically used in LED lighting systems.

Because the process will give manufacturers the ability to fine-tune the size and shape of the quantum dots, that flexibility should make them able to produce dots for a variety of applications. Smaller dots emit green light, while the larger dots emit light in the orange to red range.

The OSU researchers believe that their precision manufacturing method will yield better color control than other quantum-dot-making techniques. So much so that they foresee quantum dots produced via this method providing a cheaper alternative in an array of applications including optics, electronics, and biomedicine.



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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