Aurelia Butler

So-called quantum dots are a new class of materials with many applications. Quantum dots are realized by tiny semiconductor crystals with dimensions in the nanometre range. The optical and electrical properties can be controlled through the size of these crystals. As QLEDs, they are already on the market in the latest generations of TV flat screens, where they ensure particularly brilliant and high-resolution colour reproduction. However, quantum dots are not only used as “dyes,” they are also used in solar cells or as semiconductor devices, right up to computational building blocks, the qubits, of a quantum computer.
Now, a team led by Dr. Annika Bande at HZB has extended the understanding of the interaction between several quantum dots with an atomistic view in a theoretical publication.
Annika Bande heads the “Theory of Electron Dynamics and Spectroscopy” group at HZB and is particularly interested in the origins of quantum physical phenomena. Although quantum dots are extremely tiny nanocrystals, they consist of thousands of atoms with, in turn, multiples of electrons. Even with supercomputers, the electronic structure of such a semiconductor crystal could hardly be calculated, emphasises the theoretical chemist, who recently completed her habilitation at Freie Universität. “But we are developing methods that describe the problem approximately,” Bande explains. “In this case, we worked with scaled-down quantum dot versions of only about a hundred atoms, which nonetheless feature the characteristic properties of real nanocrystals.”
With this approach, after a year and a half of development and in collaboration with Prof. Jean Christophe Tremblay from the CNRS-Université de Lorraine in Metz, we succeeded in simulating the interaction of two quantum dots, each made of hundreds of atoms, which exchange energy with each other. Specifically, we have investigated how these two quantum dots can absorb, exchange and permanently store the energy controlled by light. A first light pulse is used for excitation, while the second light pulse induces the storage.
In total, we investigated three different pairs of quantum dots to capture the effect of size and geometry. We calculated the electronic structure with highest precision and simulated the electronic motion in real time at femtosecond resolution (10-15 s).
The results are also very useful for experimental research and development in many fields of application, for example for the development of qubits or to support photocatalysis, to produce green hydrogen gas by sunlight. “We are constantly working on extending our models towards even more realistic descriptions of quantum dots,” says Bande, “e.g. to capture the influence of temperature and environment.”
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Materials provided by Helmholtz-Zentrum Berlin für Materialien und Energie. Note: Content may be edited for style and length. More

Quantum computers with their promises of creating new materials and solving intractable mathematical problems are a dream of many physicists. Now, they are slowly approaching viable realizations in many laboratories all over the world. But there are still enormous challenges to master. A central one is the construction of stable quantum bits — the fundamental unit of quantum computation called qubit for short — that can be networked together.
In a study published in Nature Materials and led by Daniel Jirovec from the Katsaros group at IST Austria in close collaboration with researchers from the L-NESS Inter-university Centre in Como, Italy, scientists now have created a new and promising candidate system for reliable qubits.
Spinning Absence
The researchers created the qubit using the spin of so-called holes. Each hole is just the absence of an electron in a solid material. Amazingly, a missing negatively charged particle can physically be treated as if it were a positively charged particle. It can even move around in the solid when a neighboring electron fills the hole. Thus, effectively the hole described as positively charged particle is moving forward.
These holes even carry the quantum-mechanical property of spin and can interact if they come close to each other. “Our colleagues at L-NESS layered several different mixtures of silicon and germanium just a few nanometers thick on top of each other. That allows us to confine the holes to the germanium-rich layer in the middle,” Jirovec explains. “On top, we added tiny electrical wires — so-called gates — to control the movement of holes by applying voltage to them. The electrically positively charged holes react to the voltage and can be extremely precisely moved around within their layer.”
Using this nano-scale control, the scientists moved two holes close to each other to create a qubit out of their interacting spins. But to make this work, they needed to apply a magnetic field to the whole setup. Here, their innovative approach comes into play.
Linking Qubits
In their setup, Jirovec and his colleagues cannot only move holes around but also alter their properties. By engineering different hole properties, they created the qubit out of the two interacting hole spins using less than ten millitesla of magnetic field strength. This is a weak magnetic field compared to other similar qubit setups, which employ at least ten times stronger fields.
But why is that relevant? “By using our layered germanium setup we can reduce the required magnetic field strength and therefore allow the combination of our qubit with superconductors, usually inhibited by strong magnetic fields,” Jirovec says. Superconductors — materials without any electrical resistance — support the linking of several qubits due to their quantum-mechanical nature. This could enable scientists to build new kinds of quantum computers combining semiconductors and superconductors.
In addition to the new technical possibilities, these hole spin qubits look promising because of their processing speed. With up to one hundred million operations per second as well as their long lifetime of up to 150 microseconds they seem particularly viable for quantum computing. Usually, there is a tradeoff between these properties, but this new design brings both advantages together.
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Materials provided by Institute of Science and Technology Austria. Note: Content may be edited for style and length. More

The impact of deploying Artificial Intelligence (AI) for radiation cancer therapy in a real-world clinical setting has been tested by Princess Margaret researchers in a unique study involving physicians and their patients.
A team of researchers directly compared physician evaluations of radiation treatments generated by an AI machine learning (ML) algorithm to conventional radiation treatments generated by humans.
They found that in the majority of the 100 patients studied, treatments generated using ML were deemed to be clinically acceptable for patient treatments by physicians.
Overall, 89% of ML-generated treatments were considered clinically acceptable for treatments, and 72% were selected over human-generated treatments in head-to-head comparisons to conventional human-generated treatments.
Moreover, the ML radiation treatment process was faster than the conventional human-driven process by 60%, reducing the overall time from 118 hours to 47 hours. In the long term this could represent a substantial cost savings through improved efficiency, while at the same time improving quality of clinical care, a rare win-win.
The study also has broader implications for AI in medicine. More

Researchers have demonstrated “giant flexoelectricity” in soft elastomers that could improve robot movement range and make self-powered pacemakers a real possibility. In a paper published this month in the Proceedings of the National Academy of Sciences, scientists from the University of Houston and Air Force Research Laboratory explain how to engineer ostensibly ordinary substances like silicone rubber into an electric powerhouse.
What do the following have in common: a self-powered implanted medical device, a soft human-like robot and how we hear sound? The answer as to why these two disparate technologies and biological phenomena are similar lies in how the materials they are made of can significantly change in size and shape — or deform — like a rubber band, when an electrical signal is sent.
Some materials in nature can perform this function, acting as an energy converter that deforms when an electrical signal is sent through or supplies electricity when manipulated. This is called piezoelectricity and is useful in creating sensors and laser electronics, among several other end uses. However, these naturally occurring materials are rare and consist of stiff crystalline structures that are often toxic, three distinct drawbacks for human applications.
Human-made polymers offer steps toward alleviating these pain points by eliminating material scarcity and creating soft polymers capable of bending and stretching, known as soft elastomers, but previously those soft elastomers lacked significant piezoelectric attributes.
In a paper published this month in the Proceedings of the National Academy of Sciences, Kosar Mozaffari, graduate student at the Cullen College of Engineering at the University of Houston; Pradeep Sharma, M.D. Anderson Chair Professor & Department Chair of Mechanical Engineering at the University of Houston and Matthew Grasinger, LUCI Postdoctoral Fellow at the Air Force Research Laboratory, offer a solution.
“This theory engineers a connection between electricity and mechanical motion in soft rubber-like materials,” said Sharma. “While some polymers are weakly piezoelectric, there are no really soft rubber like materials that are piezoelectric.”
The term for these multifunctional soft elastomers with increased capability is “giant flexoelectricity.” In other words, these scientists demonstrate how to boost flexoelectric performance in soft materials. More

A new formula from Army scientists is leading to new insights on how to build an energy-efficient legged teammate for dismounted warfighters.
In a recent peer-reviewed PLOS One paper, the U.S. Army Combat Capabilities Development Command, known as DEVCOM, Army Research Laboratory’s Drs. Alexander Kott, Sean Gart and Jason Pusey offer new insights on building autonomous military robotic legged platforms to operate as efficiently as any other ground mobile systems.
Its use could lead to potentially important changes to Army vehicle development. Scientists said they may not know exactly why legged, wheeled and tracked systems fit the same curve yet, but they are convinced their findings drive further inquiry.
“If vehicle developers find a certain design would require more power than is currently possible given a variety of real-world constraints, the new formula could point to specific needs for improved power transmission and generation, or to rethink the mass and speed requirements of the vehicle,” Gart said.
Inspired by a 1980s formula that shows relationships between the mass, speed and power expenditure of animals, the team developed a new formula that applied to a very broad range of legged, wheeled and tracked systems — such as motor vehicles and ground robots.
Although much of the data has been available for 30 years, this team believes they are the first to actually assemble it and study the relationships that emerge from this data. Their findings show that legged systems are as efficient as wheeled and tracked platforms. More

Scientists have created a cybersecurity technology called Shadow Figment that is designed to lure hackers into an artificial world, then stop them from doing damage by feeding them illusory tidbits of success.
The aim is to sequester bad actors by captivating them with an attractive — but imaginary — world.
The technology is aimed at protecting physical targets — infrastructure such as buildings, the electric grid, water and sewage systems, and even pipelines. The technology was developed by scientists at the U.S. Department of Energy’s Pacific Northwest National Laboratory.
The starting point for Shadow Figment is an oft-deployed technology called a honeypot — something attractive to lure an attacker, perhaps a desirable target with the appearance of easy access.
But while most honeypots are used to lure attackers and study their methods, Shadow Figment goes much further. The technology uses artificial intelligence to deploy elaborate deception to keep attackers engaged in a pretend world — the figment — that mirrors the real world. The decoy interacts with users in real time, responding in realistic ways to commands.
“Our intention is to make interactions seem realistic, so that if someone is interacting with our decoy, we keep them involved, giving our defenders extra time to respond,” said Thomas Edgar, a PNNL cybersecurity researcher who led the development of Shadow Figment. More

The myriad processes occurring in biological cells may seem unbelievably complex at first glance. And yet, in principle, they are merely a logical succession of events, and could even be used to form digital circuits. Researchers have now developed a molecular switching circuit made of DNA, which can be used to mechanically alter gels, depending on the pH. DNA-based switching circuits could have applications in soft robotics, say the researchers in their article in Angewandte Chemie.
DNA is a long molecule that can be folded and twisted in various ways. It has a backbone and bases that stick out from the backbone and pair up with counterparts in other DNA strands. When a series of these matching pairs comes together, they form a twisted, ladder-like double strand — the familiar DNA double helix. The flexibility of DNA, which makes it possible to produce bends, loops, and a wide variety of other shapes, has inspired researchers to build DNA switches. These switches change shape after receiving an input, and can then affect their surroundings.
Hao Pei from Shanghai Key Laboratory of Green Chemistry and Chemical Processes at the East China Normal University in Shanghai, China, and colleagues have now developed a configurable, multi-mode logic switching network that reacts differently with its surroundings depending on pH and DNA input. All the components of the switching circuit were produced from DNA.
The team developed a series of four DNA switches, each with slightly different lengths and combinations of bases. These variations meant they reacted differently with a single DNA strand depending on the pH of their surroundings. For example, at a slightly alkaline pH of 8, two of the switches formed triple-stranded DNA (triplexes), while the others remained loosely stretched out. These reactions and folds led to secondary reactions, which were utilized by the researchers as logic functions in the switching circuit. The result was, for example, a fluorescent signal that could be read as an output.
To demonstrate the use of the switching circuit in a real mechanical system, the team incorporated the DNA switches into polyacrylamide gels. The DNA acted as a crosslinker, joining the polymer molecules in the gel together. The shorter the crosslinker, or the more folded the DNA, the denser the gel became. Once a piece of DNA with matching bases was added as an input, a logic circuit was set in place, causing the DNA switches to unfold, form triplexes, or relax. The reaction circuit was also dependent on the pH. As a result, certain combinations of DNA input and pH range caused the DNA crosslinker to grow longer and the gel to swell up, in some cases nearly doubling in size.
As DNA switches have almost infinite possibilities for combinations of twists and folds, the researchers consider their switching circuits to be a vital step toward soft matter robotics, where controllable, miniaturized logic functional networks are important.
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What makes cheetah the fastest land mammal? Why aren’t other animals, such as horses, as fast? While we haven’t yet figured out why, we have some idea about how — cheetahs, as it turns out, make use of a “galloping” gait at their fastest speeds, involving two different types of “flight”: one with the forelimbs and hind limbs beneath their body following a forelimb liftoff, called “gathered flight,” while another with the forelimbs and hind limbs stretched out after a hind limb liftoff, called “extended flight”. Of these, the extended flight is what enables cheetahs to accelerate to high speeds, and it depends on ground reaction forces satisfying specific conditions; in the case of horses, the extended flight is absent.
Additionally, cheetahs show appreciable spine movement during flight, alternating between flexing and stretching in gathered and extended modes, respectively, which contributes to its high-speed locomotion. However, little is understood about the dynamics governing these abilities.
“All animal running constitutes a flight phase and a stance phase, with different dynamics governing each phase,” explains Dr. Tomoya Kamimura from Nagoya Institute of Technology, Japan, who specializes in intelligent mechanics and locomotion. During the flight phase, all feet are in the air and the center of mass (COM) of the whole body exhibits ballistic motion. Conversely, during the stance phase, the body receives ground reaction forces through the feet. “Due to such complex and hybrid dynamics, observations can only get us so far in unraveling the mechanisms underlying the running dynamics of animals,” Dr. Kamimura says.
Consequently, researchers have turned to computer modeling to gain a better dynamic perspective of the animal gait and spine movement during running and have had remarkable success using fairly simple models. However, few studies so far have explored the types of flight and spine motion during galloping (as seen in a cheetah). Against this backdrop, Dr. Kamimura and his colleagues from Japan have now addressed this issue in a recent study published in Scientific Reports, using a simple model emulating vertical and spine movement.
The team, in their study, employed a two-dimensional model comprising two rigid bodies and two massless bars (representing the cheetah’s legs), with the bodies connected by a joint to replicate the bending motion of the spine and a torsional spring. Additionally, they assumed an anterior-posterior symmetry, assigning identical dynamical roles to the fore and hind legs.
By solving the simplified equations of motion governing this model, the team obtained six possible periodic solutions, with two of them resembling two different flight types (like cheetah galloping) and four, only one flight type (unlike cheetah galloping), based on the criteria related to the ground reaction forces provided by the solutions themselves. Researchers then verified these criteria with measured cheetah data, revealing that cheetah galloping in the real world indeed satisfied the criterion for two flight types through spine bending .
Additionally, the periodic solutions also revealed that horse galloping only involves gathered flight due to restricted spine motion, suggesting that the additional extended flight in cheetahs combined with spine bending allowed them to achieve such great speeds!
“While the mechanism underlying this difference in flight types between animal species still remains unclear, our findings extend the understanding of the dynamic mechanisms underlying high-speed locomotion in cheetahs. Furthermore, they can be applied to the mechanical and control design of legged robots in the future,” speculates an optimistic Dr. Kamimura.
Cheetahs inspiring legged robots! Who would’ve thought?
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Materials provided by Nagoya Institute of Technology. Note: Content may be edited for style and length. More