Computers Math

Topological insulators, or TIs, have two faces: Electrons flow freely along their surface edges, like cars on a superhighway, but can’t flow through the interior of the material at all. It takes a special set of conditions to create this unique quantum state — part electrical conductor, part insulator — which researchers hope to someday exploit for things like spintronics, quantum computing and quantum sensing. For now, they’re just trying to understand what makes TIs tick.
In the latest advance along those lines, researchers at the Department of Energy’s SLAC National Accelerator Laboratory and Stanford University systematically probed the “phase transition” in which a TI loses its quantum properties and becomes just another ordinary insulator.
They did this by using spiraling beams of laser light to generate harmonics — much like the vibrations of a plucked guitar string — from the material they were examining. Those harmonics make it easy to distinguish what’s happening in the superhighway layer from what’s happening in the interior and see how one state gives way to the other, they reported in Nature Photonics today.
“The harmonics generated by the material amplify the effects we want to measure, making this a very sensitive way to see what’s going on in a TI,” said Christian Heide, a postdoctoral researcher with the Stanford PULSE Institute at SLAC, who led the experiments.
“And since this light-based approach can be done in a lab with tabletop equipment, it makes exploring these materials easier and more accessible than some previous methods.”
These results are exciting, added PULSE principal investigator Shambhu Ghimire, because they show the new method has potential for watching TIs flip back and forth between superhighway and insulating states as it happens and in fine detail — much like a using camera with a very fast shutter speed. More

Swarms of microrobots injected into the human body could unblock internal medical devices and avoid the need for further surgery, according to new research from the University of Essex.
The study is the first-time scientists have developed magnetic microrobotics to remove deposits in shunts — common internal medical devices used to treat a variety of conditions by draining excess fluid from organs.
Shunts are prone to malfunctioning, often caused by blockages due to a build-up of sediment. The sediment not only narrows and obstructs liquid passing through the shunt, but it also affects the shunt’s flexibility. This leads to patients needing repeated, invasive surgeries throughout their lives either to replace the shunt or use a catheter to remove the blockage.
However, this new research, led by microrobotics expert Dr Ali Hoshiar, from Essex’s School of Computer Science and Electronic Engineering, has shown there could be a wireless, non-invasive alternative to clearing the blockage in a shunt.
Published in the IEEE Transaction on Biomedical Engineering journal, Dr Hoshiar and his team have shown that a swarm of hundreds of microrobots — made of nano size magnetic nanoparticles — injected into the shunt could remove the sediment instead.
“Once the magnetic microrobots are injected into the shunt they can be moved along the tube to the affected area using a magnetic field, generated by a powerful magnet on the body’s surface,” explained Dr Hoshiar. “The swarm of microrobots can then be moved so they scrape away the sediment, clearing the tube.
“The non-invasive nature of this method is a considerable advantage to existing methods as it will potentially eliminate the risk of surgery and a surgery-related infection, thereby decreasing recovery time.”
With each microrobot smaller than the width of a human hair, once the swarm has done its job, it can either be guided to the stomach via a magnetic field or bodily fluid, so they leave the body naturally. Because the microrobots have very high biocompatibility they will not cause toxicity.
The research also found a direct relation between the strength of the magnetic field and the success of scraping away the sediment in the shunt.
This is the first proof-of-concept experiment using microswarms for opening a blockage in a shunt. The next stage of this research is to work with clinicians to carry out trials. The researchers are also looking at how the concept can be used to other applications.
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Researchers report an experimental demonstration of a space-to-ground quantum key distribution (QKD) network using a compact QKD terminal aboard the Chinese Space Lab Tiangong-2 and four ground stations. The new QKD system is less than half the weight of the system the researchers developed for the Micius satellite, which was used to perform the world’s first quantum-encrypted virtual teleconference.
The demonstration represents an important step toward practical QKD based on constellations of small satellites, a setup considered one of the most promising routes to creating a global quantum communication network.
“QKD offers unconditional security by using single photons to encode information between two distant terminals,” said research team member Cheng-Zhi Peng from the University of Science and Technology of China. “The compact system we developed can reduce the cost of implementing QKD by making it possible to use small satellites.”
Peng and researchers from other institutions in China describe their new system and experimental results in Optica, Optica Publishing Group’s journal for high-impact research. They also found that QKD performance can be boosted by building a network of satellites orbiting at different angles, or inclinations, in relation to the equator.
“Our new work demonstrates the feasibility of a space-ground QKD network based on a compact satellite payload combined with constellations of satellites with different orbit types,” said Peng. “In the near future, this type of QKD system could be used in applications that require high security such as government affairs, diplomacy and finance.”
Shrinking the QKD system
QKD uses the quantum properties of light to generate secure random keys for encrypting and decrypting data. In previous work, the research group demonstrated satellite-to-ground QKD and satellite-relayed intercontinental quantum networks using the Micius satellite. However, the QKD system used aboard that satellite was bulky and expensive. About the size of a large refrigerator, the system weighed around 130 kg and required 130 W of power. More

An international team of researchers has designed and built a chip that runs computations directly in memory and can run a wide variety of AI applications-all at a fraction of the energy consumed by computing platforms for general-purpose AI computing.
The NeuRRAM neuromorphic chip brings AI a step closer to running on a broad range of edge devices, disconnected from the cloud, where they can perform sophisticated cognitive tasks anywhere and anytime without relying on a network connection to a centralized server. Applications abound in every corner of the world and every facet of our lives, and range from smart watches, to VR headsets, smart earbuds, smart sensors in factories and rovers for space exploration.
The NeuRRAM chip is not only twice as energy efficient as the state-of-the-art “compute-in-memory” chips, an innovative class of hybrid chips that runs computations in memory, it also delivers results that are just as accurate as conventional digital chips. Conventional AI platforms are a lot bulkier and typically are constrained to using large data servers operating in the cloud.
In addition, the NeuRRAM chip is highly versatile and supports many different neural network models and architectures. As a result, the chip can be used for many different applications, including image recognition and reconstruction as well as voice recognition.
“The conventional wisdom is that the higher efficiency of compute-in-memory is at the cost of versatility, but our NeuRRAM chip obtains efficiency while not sacrificing versatility,” said Weier Wan, the paper’s first corresponding author and a recent Ph.D. graduate of Stanford University who worked on the chip while at UC San Diego, where he was co-advised by Gert Cauwenberghs in the Department of Bioengineering.
The research team, co-led by bioengineers at the University of California San Diego, presents their results in the Aug. 17 issue of Nature. More

Researchers from Göttingen (Germany) and Lausanne (Switzerland) have successfully created electron-photon pairs in a controlled way in an electron microscope for the first time. Using a new method, they could precisely detect the involved particles. The findings of the study expand the toolbox of quantum technology.
Faster computers, tap-proof communication, better car sensors — quantum technologies have the potential to revolutionize our lives just as once the invention of computers or the internet. Experts worldwide are trying to implement findings from basic research into quantum technologies. To this end, they often require individual particles, such as photons — the elementary particles of light — with tailored properties. However, obtaining individual particles is complicated and requires intricate methods. In a study recently published in the journal Science, researchers now present a new method that simultaneously generates two individual particles in form of a pair.
The international team from the Göttingen Max Planck Institute (MPI) for Multidisciplinary Sciences, the University of Göttingen, and the Swiss Federal Institute of Technology in Lausanne (EPFL) succeeded in coupling single free electrons and photons in an electron microscope. In the Göttingen experiment, the beam from an electron microscope passes through an integrated optical chip, fabricated by the Swiss team. The chip consists of a fiber-optic coupling and a ring-shaped resonator that stores light by keeping moving photons on a circular path. “When an electron scatters at the initially empty resonator, a photon is generated,” explains Armin Feist, scientist at the MPI and one of the study’s first authors. “In the process, the electron loses exactly the amount of energy that the photon requires to be created virtually from nothing in the resonator. As a result, the two particles are coupled through their interaction and form a pair.” With an improved measurement method, the physicists could precisely detect the individual particles involved and their simultaneous manifestation.
Future quantum technology with free electrons
“With the electron-photon pair, we only need to measure one particle to obtain information about the energy content and temporal appearance of the second one,” emphasizes Germaine Arend, a PhD candidate at the MPI and also first author of the study. This allows researchers to use one quantum particle in an experiment while, at the same time, confirming its presence by detecting the other particle, in a so-called heralding scheme. Such a feature is necessary for many applications in quantum technology.
Max Planck Director Claus Ropers sees electron-photon pairs as a new opportunity for quantum research: “The method opens up fascinating new possibilities in electron microscopy. In the field of quantum optics, entangled photon pairs already improve imaging. With our work, such concepts can now be explored with electrons.” Tobias Kippenberg, professor at EPFL adds, “For the first time, we bring free electrons into the toolbox of quantum information science. More broadly, coupling free electrons and light using integrated photonics could open the way to a new class of hybrid quantum technologies.”
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Superconductors are the key to lossless current flow. However, the realization of superconducting diodes has only recently become an important topic of fundamental research. An international research team involving the theoretical physicist Mathias Scheurer from the University of Innsbruck have now succeeded in reaching a milestone: the realization of a superconducting diode effect without an external magnetic field, thus proving the assumption that superconductivity and magnetism coexist. They report on this in Nature Physics.
One speaks of a superconducting diode effect when a material behaves like a superconductor in one direction of current flow and like a resistor in the other. In contrast to a conventional diode, such a superconducting diode exhibits a completely vanishing resistance and thus no losses in the forward direction. This could form the basis for future lossless quantum electronics. Physicists first succeeded in creating the diode effect about two years ago, but with some fundamental limitations. “At that time, the effect was very weak and it was generated by an external magnetic field, which is very disadvantageous in potential technological applications,” explains Mathias Scheurer from the Institute of Theoretical Physics at the University of Innsbruck.
The new experiments carried out by experimental physicists at Brown University, described in the current issue of Nature Physics, do not require an external magnetic field. In addition to the aforementioned application-relevant advantages, the experiments confirm a thesis previously theorized by Mathias Scheurer: Namely, that superconductivity and magnetism coexist in a system consisting of three graphene layers twisted against each other. The system thus virtually generates its own internal magnetic field, creating a diode effect. “The diode effect observed by colleagues at Brown University was additionally very strong. Moreover, the diode direction can be reversed by a simple electric field. Together, this makes trilayer graphene such a promising platform for the superconducting diode effect,” clarifies Mathias Scheurer, who received an ERC Starting Grant this year for his research on two-dimensional materials, especially graphene.
Promising material graphene
The diode effect described in Nature Physics was also produced with graphene, a material consisting of a single layer of carbon atoms arranged in a honeycomb pattern. Stacking several layers of graphene leads to completely new properties, including the ability of three graphene layers twisted against each other to conduct electric current without loss. The fact that a superconducting diode effect exists without an external magnetic field in this system has great implications for the study of the complex physical behavior of twisted trilayer graphene, as it demonstrates the coexistence of superconductivity and magnetism. This shows that the diode effect not only has technological relevance, but also has the potential to improve our understanding of fundamental processes in many-body physics. The theoretical basis for this has already been published in another high-ranking publication.
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Engineers have created intelligent 3D printers that can quickly detect and correct errors, even in previously unseen designs, or unfamiliar materials like ketchup and mayonnaise, by learning from the experiences of other machines.
The engineers, from the University of Cambridge, developed a machine learning algorithm that can detect and correct a wide variety of different errors in real time, and can be easily added to new or existing machines to enhance their capabilities. 3D printers using the algorithm could also learn how to print new materials by themselves. Details of their low-cost approach are reported in the journal Nature Communications.
3D printing has the potential to revolutionise the production of complex and customised parts, such as aircraft components, personalised medical implants, or even intricate sweets, and could also transform manufacturing supply chains. However, it is also vulnerable to production errors, from small-scale inaccuracies and mechanical weaknesses through to total build failures.
Currently, the way to prevent or correct these errors is for a skilled worker to observe the process. The worker must recognise an error (a challenge even for the trained eye), stop the print, remove the part, and adjust settings for a new part. If a new material or printer is used, the process takes more time as the worker learns the new setup. Even then, errors may be missed as workers cannot continuously observe multiple printers at the same time, especially for long prints.
“3D printing is challenging because there’s a lot that can go wrong, and so quite often 3D prints will fail,” said Dr Sebastian Pattinson from Cambridge’s Department of Engineering, the paper’s senior author. “When that happens, all of the material and time and energy that you used is lost.”
Engineers have been developing automated 3D printing monitoring, but existing systems can only detect a limited range of errors in one part, one material and one printing system. More

There’s still plenty of room at the bottom to generate piezoelectricity. Engineers at Rice University and their colleagues are showing the way.
A new study describes the discovery of piezoelectricity — the phenomenon by which mechanical energy turns into electrical energy — across phase boundaries of two-dimensional materials.
The work led by Rice materials scientists Pulickel Ajayan and Hanyu Zhu and their colleagues at Rice’s George R. Brown School of Engineering, the University of Southern California, the University of Houston, Wright-Patterson Air Force Base Research Laboratory and Pennsylvania State University appears in Advanced Materials.
The discovery could aid in the development of ever-smaller nanoelectromechanical systems, devices that could be used, for example, to power tiny actuators and implantable biosensors, and ultrasensitive temperature or pressure sensors.
The researchers show the atomically thin system of a metallic domain surrounding semiconducting islands creates a mechanical response in the material’s crystal lattice when subjected to an applied voltage.
The presence of piezoelectricity in 2D materials often depends on the number of layers, but synthesizing the materials with a precise number of layers has been a formidable challenge, said Rice research scientist Anand Puthirath, co-lead author of the paper. More