Computers Math

The laws of quantum physics rule the microcosm. They determine, for example, how easily electrons move through a crystal and thus whether the material is a metal, a semiconductor or an insulator. Quantum physics may lead to exotic properties in certain materials: In so-called topological insulators, only the electrons that can occupy some specific quantum states are free to move like massless particles on the surface, while this mobility is completely absent for electrons in the bulk. What’s more, the conduction electrons in the “skin” of the material are necessarily spin polarized, and form robust, metallic surface states that could be utilized as channels in which to drive pure spin currents on femtosecond time scales (1 fs= 10-15 s).
These properties open up exciting opportunities to develop new information technologies based on topological materials, such as ultrafast spintronics, by exploiting the spin of the electrons on their surfaces rather than the charge. In particular, optical excitation by femtosecond laser pulses in these materials represents a promising alternative to realize highly efficient, lossless transfer of spin information. Spintronic devices utilizing these properties have the potential of a superior performance, as they would allow to increase the speed of information transport up to frequencies a thousand times faster than in modern electronics.
However, many questions still need to be answered before spintronic devices can be developed. For example, the details of exactly how the bulk and surface electrons from a topological material respond to the external stimulus i.e., the laser pulse, and the degree of overlap in their collective behaviors on ultrashort time scales.
A team led by HZB physicist Dr. Jaime Sánchez-Barriga has now brought new insights into such mechanisms. The team, which has also established a Helmholtz-RSF Joint Research Group in collaboration with colleagues from Lomonosov State University, Moscow, examined single crystals of elemental antimony (Sb), previously suggested to be a topological material. “It is a good strategy to study interesting physics in a simple system, because that’s where we can hope to understand the fundamental principles,” Sánchez-Barriga explains. “The experimental verification of the topological property of this material required us to directly observe its electronic structure in a highly excited state with time, spin, energy and momentum resolutions, and in this way we accessed an unusual electron dynamics,” adds Sánchez-Barriga.
The aim was to understand how fast excited electrons in the bulk and on the surface of Sb react to the external energy input, and to explore the mechanisms governing their response. “By controlling the time delay between the initial laser excitation and the second pulse that allows us to probe the electronic structure, we were able to build up a full time-resolved picture of how excited states leave and return to equilibrium on ultrafast time scales. The unique combination of time and spin-resolved capabilities also allowed us to directly probe the spin-polarization of excited states far out-of-equilibrium,” says Dr. Oliver J. Clark.
The data show a “kink” structure in transiently occupied energy-momentum dispersion of surface states, which can be interpreted as an increase in effective electron mass. The authors were able to show that this mass enhancement plays a decisive role in determining the complex interplay in the dynamical behaviors of electrons from the bulk and the surface, also depending on their spin, following the ultrafast optical excitation.
“Our research reveals which essential properties of this class of materials are the key to systematically control the relevant time scales in which lossless spin-polarised currents could be generated and manipulated,” explains Sánchez-Barriga. These are important steps on the way to spintronic devices which based on topological materials possess advanced functionalities for ultrafast information processing.
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A team of researchers from Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), the University of Liège and the Helmholtz Institute Erlangen-Nürnberg for Renewable Energy have developed a microswimmer that appears to defy the laws of fluid dynamics: their model, consisting of two beads that are connected by a linear spring, is propelled by completely symmetrical oscillations. The Scallop theorem states that this cannot be achieved in fluid microsystems. The findings have now been published in the academic journal Physical Review Letters.
Scallops can swim in water by quickly clapping their shells together. They are large enough to still be able to move forwards through the moment of inertia while the scallop is opening its shell for the next stroke. However, the Scallop theorem applies more or less depending on the density and viscosity of the fluid: A swimmer that makes symmetrical or reciprocal forward or backward motions similar to the opening and closing of the scallop shell will likely not move an inch. ‘Swimming through water is as tough for microscopic organisms as swimming through tar would be for humans,’ says Dr. Maxime Hubert. ‘This is why single-cell organisms have comparatively complex means of propulsion such as vibrating hairs or rotating flagella.’
Swimming at the mesoscale
Dr. Hubert is a postdoctoral researcher in Prof. Dr. Ana-Suncana Smith’s group at the Institute of Theoretical Physics at FAU. Together with researchers at the University of Liège and the Helmholtz Institute Erlangen-Nürnberg for Renewable Energy, the FAU team has developed a swimmer which does not seem to be limited by the Scallop theorem: The simple model consists of a linear spring that connects two beads of different sizes. Although the spring expands and contracts symmetrically under time reversal, the microswimmer is still able to move through the fluid.
‘We originally tested this principle using computer simulations,’ says Maxime Hubert. ‘We then built a functioning model’. In the practical experiment, the scientists placed two steel beads measuring just a few hundred micrometres in diameter on the surface of water contained in a Petri dish. The surface tension of the water represented the contraction of the spring and expansion in the opposite direction was achieved with a magnetic field which caused the microbeads to periodically repel other.
Vision: Swimming robots for transporting drugs
The swimmer is able to propel itself because the beads are of different sizes. Maxime Hubert says, ‘The smaller bead reacts much faster to the spring force than the larger bead. This causes asymmetrical motion and the larger bead is pulled along with the smaller bead. We are therefore using the principle of inertia, with the difference that here we are concerned with the interaction between the bodies rather than the interaction between the bodies and water.’
Although the system won’t win any prizes for speed — it moves forwards about a thousandth of its body length during each oscillation cycle — the sheer simplicity of its construction and mechanism is an important development. ‘The principle that we have discovered could help us to construct tiny swimming robots,’ says Maxime Hubert. ‘One day they might be used to transport drugs through the blood to a precise location.’
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Scientists on the hunt for an unconventional kind of superconductor have produced the most compelling evidence to date that they’ve found one. In a pair of papers, researchers at the University of Maryland’s (UMD) Quantum Materials Center (QMC) and colleagues have shown that uranium ditelluride (or UTe2 for short) displays many of the hallmarks of a topological superconductor — a material that may unlock new ways to build quantum computers and other futuristic devices.
“Nature can be wicked,” says Johnpierre Paglione, a professor of physics at UMD, the director of QMC and senior author on one of the papers. “There could be other reasons we’re seeing all this wacky stuff, but honestly, in my career, I’ve never seen anything like it.”
All superconductors carry electrical currents without any resistance. It’s kind of their thing. The wiring behind your walls can’t rival this feat, which is one of many reasons that large coils of superconducting wires and not normal copper wires have been used in MRI machines and other scientific equipment for decades.
But superconductors achieve their super-conductance in different ways. Since the early 2000s, scientists have been looking for a special kind of superconductor, one that relies on an intricate choreography of the subatomic particles that actually carry its current.
This choreography has a surprising director: a branch of mathematics called topology. Topology is a way of grouping together shapes that can be gently transformed into one another through pushing and pulling. For example, a ball of dough can be shaped into a loaf of bread or a pizza pie, but you can’t make it into a donut without poking a hole in it. The upshot is that, topologically speaking, a loaf and a pie are identical, while a donut is different. In a topological superconductor, electrons perform a dance around each other while circling something akin to the hole in the center of a donut.
Unfortunately, there’s no good way to slice a superconductor open and zoom in on these electronic dance moves. At the moment, the best way to tell whether or not electrons are boogieing on an abstract donut is to observe how a material behaves in experiments. Until now, no superconductor has been conclusively shown to be topological, but the new papers show that UTe2 looks, swims and quacks like the right kind of topological duck. More

Electronic circuits that compute and store information contain millions of tiny switches that control the flow of electric current. A deeper understanding of how these tiny switches work could help researchers push the frontiers of modern computing.
Now scientists have made the first snapshots of atoms moving inside one of those switches as it turns on and off. Among other things, they discovered a short-lived state within the switch that might someday be exploited for faster and more energy-efficient computing devices.
The research team from the Department of Energy’s SLAC National Accelerator Laboratory, Stanford University, Hewlett Packard Labs, Penn State University and Purdue University described their work in a paper published in Science today.
“This research is a breakthrough in ultrafast technology and science,” says SLAC scientist and collaborator Xijie Wang. “It marks the first time that researchers used ultrafast electron diffraction, which can detect tiny atomic movements in a material by scattering a powerful beam of electrons off a sample, to observe an electronic device as it operates.”
Capturing the cycle
For this experiment, the team custom-designed miniature electronic switches made of vanadium dioxide, a prototypical quantum material whose ability to change back and forth between insulating and electrically conducting states near room temperature could be harnessed as a switch for future computing. The material also has applications in brain-inspired computing because of its ability to create electronic pulses that mimic the neural impulses fired in the human brain. More

By combining oil drops with water containing a detergent-like substance, the scientists found they could produce artificial swimmers that are able to swim independently and even harvest energy to recharge.
The oil droplets use fluctuating temperature changes in their surrounding environment to store energy and to swim. When cooled, the droplets release thin ‘tail-like’ threads into the environment. The friction generated between the tails and surrounding fluid, pushes the droplet causing them to move. On heating, the droplets then retract their tails returning to their original state, and harness the heat from their environment to recharge.
The researchers show that the droplets recharge multiple times and are able to swim for periods of up to 12 minutes at a time.
Dr Stoyan Smoukov, Reader in Chemical Engineering at Queen Mary University of London and author of the study, said: “In biology, research shows that to create even the simplest artificial cells we need over 470 genes. However, through this international collaboration, we show that just by using a few simple and inexpensive components we can create a new type of active matter that can change shape and move just like a living thing.”
“We hope that this study will open up the opportunity for people to engage in cutting-edge science. As the only equipment needed is a simple optical microscope, people could create these microswimmers with the most basic laboratory set-ups, or even at home. With thousands of swimmers per drop of water, it’s a world in a drop situation. And when it costs 7p per teaspoon, there’s plenty for everyone.”
Other types of artificial swimmers exist however their movements are either driven by chemical reactions, which create bubbles that propel the swimmers through fluids, or by physical forces such as magnetic or electric fields. Instead, this new class of swimmers, which are around the size of a red blood cell, are able to spontaneously assemble and move without using external forces.
As the swimmers are not harmful to other living things, the scientists hope they could be used to study the basic interactions between living organisms such as bacteria and algae.
“In nature we often see large numbers of organisms such as bacteria, grouping together but our understanding of how these organisms interact with each other is incomplete. By mixing our simple artificial swimmers with groups of living organisms we could develop a clearer picture of how biological microswimmers communicate with each other. For example, do they only communicate due to the physical act of ‘bumping’ into each other, or are there other chemicals or signals released into the environment essential for their interaction.”
Video: https://www.youtube.com/watch?v=OKe54UucXD8
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Researchers at UC San Francisco have successfully developed a “speech neuroprosthesis” that has enabled a man with severe paralysis to communicate in sentences, translating signals from his brain to the vocal tract directly into words that appear as text on a screen.
The achievement, which was developed in collaboration with the first participant of a clinical research trial, builds on more than a decade of effort by UCSF neurosurgeon Edward Chang, MD, to develop a technology that allows people with paralysis to communicate even if they are unable to speak on their own. The study appears July 15 in the New England Journal of Medicine.
“To our knowledge, this is the first successful demonstration of direct decoding of full words from the brain activity of someone who is paralyzed and cannot speak,” said Chang, the Joan and Sanford Weill Chair of Neurological Surgery at UCSF, Jeanne Robertson Distinguished Professor, and senior author on the study. “It shows strong promise to restore communication by tapping into the brain’s natural speech machinery.”
Each year, thousands of people lose the ability to speak due to stroke, accident, or disease. With further development, the approach described in this study could one day enable these people to fully communicate.
Translating Brain Signals into Speech
Previously, work in the field of communication neuroprosthetics has focused on restoring communication through spelling-based approaches to type out letters one-by-one in text. Chang’s study differs from these efforts in a critical way: his team is translating signals intended to control muscles of the vocal system for speaking words, rather than signals to move the arm or hand to enable typing. Chang said this approach taps into the natural and fluid aspects of speech and promises more rapid and organic communication. More

A football is not a quantum particle. There are crucial differences between the things we know from everyday life and tiny quantum objects. Quantum phenomena are usually very fragile. To study them, one normally uses only a small number of particles, well shielded from the environment, at the lowest possible temperatures.
Through a collaboration between the University of Vienna, the Austrian Academy of Sciences and TU Wien, however, it has now been possible to measure a hot glass sphere consisting of about one billion atoms with unprecedented precision and to control it at the quantum level. Its movement was deliberately slowed down until it assumed the ground state of lowest possible energy. The measurement method almost reached the limit set by Heisenberg’s uncertainty principle — physics just does not allow for any more precision than that. This was made possible by applying special methods from control engineering to quantum systems. The results have now been published in the scientific journal Nature.
Perfect precision is impossible
The measurement influences the measured object — this is one of the most basic principles of quantum theory. “Werner Heisenberg came up with a famous thought experiment — the so-called Heisenberg microscope” explains physicist Lorenzo Magrini, the first author of the study from the University of Vienna. “If you want to measure the position of an object very precisely under a microscope, you have to use light with the shortest possible wavelength. But short wavelength means higher energy, so the movement of the particle is disturbed more strongly.” You just cannot accurately measure the location and the state of motion of a particle at the same time. The product of their uncertainties is always limited by Planck’s constant — this is the so-called Heisenberg uncertainty principle. However, it is possible to find out how close one can get to this limit set by nature.
Prof. Markus Aspelmeyer’s team at the University of Vienna is investigating this using a glass sphere with a diameter of less than 200 nanometres, consisting of about one billion particles — very small by our everyday standards, but still very large compared to objects usually studied in quantum physics.
The glass sphere can be kept in place with a laser beam. The atoms of the sphere are heated up by the laser, and the internal temperature of the sphere rises to several hundred degrees Celsius. This means that the atoms of the glass sphere are wobbling around violently. In the experiment, however, it was not the wobbling movements of the individual atoms that were studied, but the collective motion of the sphere in the laser trap. “These are two completely different things, just as the movement of a pendulum in a pendulum clock is something different from the movement of the individual atoms inside the pendulum,” says Markus Aspelmeyer. More

More and more companies are using chatbots in customer services. Due to advances in artificial intelligence and natural language processing, chatbots are often indistinguishable from humans when it comes to communication. But should companies let their customers know that they are communicating with machines and not with humans? Researchers at the University of Göttingen investigated. Their research found that consumers tend to react negatively when they learn that the person they are talking to is, in fact, a chatbot. However, if the chatbot makes mistakes and cannot solve a customer’s problem, the disclosure triggers a positive reaction. The results of the study were published in the Journal of Service Management.
Previous studies have shown that consumers have a negative reaction when they learn that they are communicating with chatbots — it seems that consumers are inherently averse to the technology. In two experimental studies, the Göttingen University team investigated whether this is always the case. Each study had 200 participants, each of whom was put into the scenario where they had to contact their energy provider via online chat to update their address on their electricity contract following a move. In the chat, they encountered a chatbot — but only half of them were informed that they were chatting online with a non-human contact. The first study investigated the impact of making this disclosure depending on how important the customer perceives the resolution of their service query to be. In a second study, the team investigated the impact of making this disclosure depending on whether the chatbot was able to resolve the customer’s query or not. To investigate the effects, the team used statistical analyses such as covariance and mediation analysis.
The result: most noticeably, if service issues are perceived as particularly important or critical, there is a negative reaction when it is revealed that the conversation partner is a chatbot. This scenario weakens customer trust. Interestingly, however, the results also show that disclosing that the contact was a chatbot leads to positive customer reactions in cases where the chatbot cannot resolve the customer’s issue. “If their issue isn’t resolved, disclosing that they were talking with a chatbot, makes it easier for the consumer to understand the root cause of the error,” says first author Nika Mozafari from the University of Göttingen. “A chatbot is more likely to be forgiven for making a mistake than a human.” In this scenario, customer loyalty can even improve.
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