Aurelia Butler

The double-slit experiment is the most famous and probably the most important experiment in quantum physics: individual particles are shot at a wall with two openings, behind which a detector measures where the particles arrive. This shows that the particles do not move along a very specific path, as is known from classical objects, but along several paths simultaneously: each individual particle passes through both the left and the right opening.
Normally, however, this can only be proven by carrying out the experiment over and over again and evaluating the results of many particle detections at the end. At TU Wien, it has now been possible to develop a new variant of such a two-way interference experiment that can correct this flaw: A single neutron is measured at a specific position — and due to the sophisticated measurement setup, this single measurement proofs already that the particle moved along two different paths at the same time. It is even possible to determine the ratio in which the neutron was distributed between the two paths. Thus, the phenomenon of quantum superposition can be proven without having to resort to statistical arguments. The results have now been published in the journal “Physical Review Research.”
The double-slit experiment
“In the classical double-slit experiment, an interference pattern is created behind the double slit,” explains Stephan Sponar from the Atomic Institute at TU Wien. “The particles move as a wave through both openings at the same time, and the two partial waves then interfere with each other. In some places they reinforce each other, in other places they cancel each other out.”
The probability of measuring the particle behind the double slit at a very specific location depends on this interference pattern: where the quantum wave is amplified, the probability of measuring the particle is high. Where the quantum wave is cancelled out, the probability is low. Of course, this wave distribution cannot be seen by looking at a single particle. Only when the experiment is repeated many times does the wave pattern become increasingly recognisable point by point and particle by particle.
“So, the behaviour of individual particles is explained based on results that only become visible through the statistical investigation of many particles,” says Holger Hofmann from Hiroshima University, who developed the theory behind the experiment. “Of course, this is not entirely satisfactory. We have therefore considered how the phenomenon of two-way interference can be proven based on the detection of a single particle.”
Rotating the neutron More

A long-standing quest for science and technology has been to develop electronics and information processing that operate near the fastest timescales allowed by the laws of nature.
A promising way to achieve this goal involves using laser light to guide the motion of electrons in matter, and then using this control to develop electronic circuit elements — a concept known as lightwave electronics.
Remarkably, lasers currently allow us to generate bursts of electricity on femtosecond timescales — that is, in a millionth of a billionth of a second. Yet our ability to process information in these ultrafast timescales has remained elusive.
Now, researchers at the University of Rochester and the Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) have made a decisive step in this direction by demonstrating a logic gate — the building block of computation and information processing — that operates at femtosecond timescales. The feat, reported in the journal Nature, was accomplished by harnessing and independently controlling, for the first time, the real and virtual charge carriers that compose these ultrafast bursts of electricity.
The researchers’ advances have opened the door to information processing at the petahertz limit, where one quadrillion computational operations can be processed per second. That is almost a million times faster than today’s computers operating with gigahertz clock rates, where 1 petahertz is 1 million gigahertz.
“This is a great example of how fundamental science can lead to new technologies,” says Ignacio Franco, an associate professor of chemistry and physics at Rochester who, in collaboration with doctoral student Antonio José Garzón-Ramírez ’21 (PhD), performed the theoretical studies that lead to this discovery. More

Our electronics can no longer shrink and are on the verge of overheating. But in a new discovery from the University of Copenhagen, researchers have uncovered a fundamental property of magnetism, which may become relevant for the development of a new generation of more powerful and less hot computers.
The ongoing miniaturization of components for computers which have electrons as their vehicles for information transfer has become challenged. Instead, it could be possible to use magnetism and thereby keep up the development of both cheaper and more powerful computers. This is one of the perspectives as scientists from the Niels Bohr Institute (NBI), University of Copenhagen, today publish a new discovery in the journal Nature Communications.
“The function of a computer involves sending electric current through a microchip. While the amount is tiny, the current will not only transport information but also contribute to heating up the chip. When you have a huge number of components tightly packed, the heat becomes a problem. This is one of the reasons why we have reached the limit for how much you can shrink the components. A computer based on magnetism would avoid the problem of overheating,” says Professor Kim Lefmann, Condensed Matter Physics, NBI.
“Our discovery is not a direct recipe for making a computer based on magnetism. Rather we have disclosed a fundamental magnetic property which you need to control, if you want to design a such computer.”
Quantum mechanics halt acceleration
To grasp the discovery, one needs to know that magnetic materials are not necessarily uniformly oriented. In other words, areas with magnetic north and south poles may exist side by side. These areas are termed domains, and the border between a north and south pole domain is the domain wall. While the domain wall is not a physical object it nevertheless has several particle-like properties. Thereby, it is an example of what physicists refer to as quasi-particles, meaning virtual phenomena which resemble particles. More

A group of researchers from Pisa, Jyväskylä, San Sebastian and MIT have demonstrated how a heterostructure consisting of superconductors and magnets can be used to create uni-directional current like that found in semiconductor diodes.
These novel superconductor diodes, however, operate at much lower temperatures than their semiconductor counterparts and are therefore useful in quantum technologies.
Electronics for quantum technology
Most of our everyday electronic appliances, such as radios, logic components or solar panels, rely on diodes where current can flow primarily in one direction. Such diodes rely on the electronic properties of semiconductor systems which cease to work at the ultralow sub-Kelvin temperatures required in tomorrow’s quantum technology. Superconductors are metals whose electrical resistivity is usually zero but, when contacted with other metals, can exhibit high contact resistance.
This can be understood from the energy gap, which indicates a forbidden region for electronic excitations that form in superconductors. It resembles the energy gap in semiconductors but is typically much smaller. While the presence of such a gap has been known for decades, the diode-like feature has not been previously observed, because it requires breaking the usually robust symmetry of the contact’s current-voltage characteristics.
The new work demonstrates how this symmetry can be broken with the help of a ferromagnetic insulator suitably placed in the junction. Since a big part of today’s research on quantum technologies is based on superconducting materials operating at ultralow temperatures, this innovation is readily available for them.
Power of collaboration
The research finding was made as part of the SUPERTED project, which is being funded under the EU’s Future and Emerging Technologies (FET Open). This project aims at creating the world’s first superconducting thermoelectric detector of electromagnetic radiation, based on superconductor/magnet heterostructures.
“Actually, finding the diode functionality was a pleasant surprise, a consequence of the thorough characterization of SUPERTED samples,” explains Elia Strambini, from Istituto Nanoscienze — CNR and Scuola Normale Superiore (SNS) in Pisa, who made the initial discovery.
Francesco Giazotto, from Istituto Nanoscienze — CNR and SNS and who led the experimental efforts, says:
“I believe this finding is promising for several tasks in quantum technology, such as current rectification or current limiting.”
SUPERTED coordinator; Professor Tero Heikkilä, from the University of Jyväskylä, worked on the theory behind the effect: “This finding showed the power of collaboration between different types of researchers, from materials science to superconducting electronics and theory. Without European support such collaboration would not take place.” More

The best things in life are unlikely to occur. In many situations, taking at least moderate risks yields higher expected rewards. Yet many people struggle with taking such risks: they are overly cautious and forego high payoffs. “However, we are not alone in this struggle, but we can observe and learn from others,” says Wataru Toyokawa. “We therefore wanted to find out whether social learning can also rescue us from adverse risk aversion.” The answer is yes, as the authors from the Cluster of Excellence Centre for the Advanced Study of Collective Behaviour showed in a just published study in the journal eLife.
Collective rescue occurs even among a biased collective
It is a long-established finding that collectives achieve better decisions by aggregating information or judgments, known as the wisdom of crowds. Individual errors cancel each other out, so that collectives do the right thing even if many individuals err. However, the wisdom of crowds does not work directly here, because the crowd is not wise; rather, the collective is biased towards undue risk aversion. “I wondered how social learning could still be beneficial in such a situation,” states Toyokawa. “Simply copying the majority would not help us at all, it would even yield more extreme risk aversion. So, if social learning helps at all, it must be by a different mechanism.”
To uncover these mechanisms, Toyokawa developed a dynamical mathematical model, which predicted that social learning can indeed promote favourable risk taking. He then proceeded to review the predictions from his model in large-scale online experiments with human subjects. Each participant played a browser-based game where they could choose between a variety of options — which might turn out good or bad, and with different probabilities. Toyokawa observed: “When the subjects played individually without any information from other participants, they predominantly preferred safe options with lower rewards. However, when social learning was possible, that is, when participants could see what others chose — but not know how successful others’ choices were -, it became more and more likely that they choose riskier options with higher expected rewards.” In other words, social learners made riskier choices that were more rewarding in the long run.
Occasionally copying others increases exploration and persistence
“By observing others’ choices, we could make smarter decisions, even though every single individual’s own decisions might be unduly risk averse,” Toyokawa summarizes. “Herewith, we identified a key mechanism underlying this counter-intuitive result: risk-aversion was mitigated not because the majority chose the risky option, nor were individuals simply attracted towards the majority. Rather, participants’ choices became risker even though the majority chose the safer alternative at the outset, by striking a right balance between what they experienced themselves and what they observed from others.”
Wolfgang Gaissmaier stresses that this is a striking demonstration of the power of social learning: “Under social influence, individuals became more explorative and more persistent in trying out the risky, more profitable option, even if that option might sometimes disappoint them in the short run. And once individual risk aversion was reduced, this process perpetuated itself, as there were more and more risk takers around to be copied.”
“The finding that adverse risk aversion is mitigated under social influence will help us better understand the evolution of learning under social interaction,” concludes Wataru Toyokawa. “The study suggests that social learning is advantageous in wider environmental conditions than previously assumed.”
The study was funded by the Cluster of Excellence Centre for the Advanced Study of Collective Behaviour
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Ever smaller and more intricate — without miniaturization, we wouldn’t have the components today that are required for high-performance laptops, compact smartphones or high-resolution endoscopes. Research is now being carried out in the nanoscale on switches, rotors or motors that comprise of only a few atoms in order to build what are known as molecular machines. A research team at Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) has successfully built the world’s smallest energy powered gear wheel with corresponding counterpart. The nano gear unit is the first that can also be actively controlled and driven. The researchers’ findings have recently been published in the journal Nature Chemistry.
Miniaturization plays a key role in the further development of modern technologies and makes it possible to manufacture smaller devices that have more power. It also plays a significant role in manufacturing, since it allows materials and functional materials or medication to be produced at previously unprecedented levels of precision. Now, research has entered the nanoscale — which is invisible to the naked eye — focusing on individual atoms and molecules. The significance of this new field of research is demonstrated by the Nobel Prize for Chemistry, which was awarded for research into molecular machines in 2016.
Some important components used in molecular machines such as switches, rotors, forceps, robot arms or even motors already exist in the nanoscale. A further essential component for any machine is the gear wheel, which allows changes in direction and speed and enables movements to be connected to each other. Molecular counterparts also exist for gear wheels, however, up to now, they have only moved passively back and forth, which is not extremely useful for a molecular machine.
The molecular gear wheel developed by the research team led by Prof. Dr. Henry Dube, Chair of Organic Chemistry I at FAU and previously head of a junior research group at LMU in Munich, measures only 1.6 nm, which corresponds to around 50,000ths of the thickness of a human hair — the smallest of its kind. But that’s not all. The research team has succeeded in actively powering a molecular gear wheel and its counterpart and has thus solved a fundamental problem in the construction of machines on the nanoscale.
The gear unit comprises two components that are interlocked with each other and are made up of only 71 atoms. One component is a triptycene molecule whose structure is similar to a propeller or bucket wheel (shown in light gray in the animation). The second component is a flat fragment of a thioindigo molecule, similar to a small plate (shown in gold in the animation). If the plate rotates 180 degrees, the propeller rotates by only 120 degrees. The result is a 2:3 transmission ratio.
The nano gear unit is controlled by light, making it a molecular photogear. As they are directly driven by the light energy, the plate and the triptycene propeller move in locked synchronous rotation. Heat alone was not sufficient in order to make the gear unit rotate, as the FAU team discovered. When the researchers heated the solution around the gear unit in the dark, the propeller turned, but the plate did not — the gear “slipped.” The researchers thus came to the conclusion that the nano gear unit can be activated and controlled using a light source.
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Chemists at the U.S. Department of Energy’s Brookhaven National Laboratory have developed a new machine-learning (ML) framework that can zero in on which steps of a multistep chemical conversion should be tweaked to improve productivity. The approach could help guide the design of catalysts — chemical “dealmakers” that speed up reactions.
The team developed the method to analyze the conversion of carbon monoxide (CO) to methanol using a copper-based catalyst. The reaction consists of seven fairly straightforward elementary steps.
“Our goal was to identify which elementary step in the reaction network or which subset of steps controls the catalytic activity,” said Wenjie Liao, the first author on a paper describing the method just published in the journal Catalysis Science & Technology. Liao is a graduate student at Stony Brook University who has been working with scientists in the Catalysis Reactivity and Structure (CRS) group in Brookhaven Lab’s Chemistry Division.
Ping Liu, the CRS chemist who led the work, said, “We used this reaction as an example of our ML framework method, but you can put any reaction into this framework in general.”
Targeting activation energies
Picture a multistep chemical reaction as a rollercoaster with hills of different heights. The height of each hill represents the energy needed to get from one step to the next. Catalysts lower these “activation barriers” by making it easier for reactants to come together or allowing them to do so at lower temperatures or pressures. To speed up the overall reaction, a catalyst must target the step or steps that have the biggest impact. More

Computational detective work by U.S. and German physicists has confirmed cerium zirconium pyrochlore is a 3D quantum spin liquid.
Despite the name, quantum spin liquids are solid materials in which quantum entanglement and the geometric arrangement of atoms frustrate the natural tendency of electrons to magnetically order themselves in relation to one another. The geometric frustration in a quantum spin liquid is so severe that electrons fluctuate between quantum magnetic states no matter how cold they become.
Theoretical physicists routinely work with quantum mechanical models that manifest quantum spin liquids, but finding convincing evidence that they exist in actual physical materials has been a decadeslong challenge. While a number of 2D or 3D materials have been proposed as possible quantum spin liquids, Rice University physicist Andriy Nevidomskyy said there’s no established consensus among physicists that any of them qualify.
Nevidomskyy is hoping that will change based on the computational sleuthing he and colleagues from Rice, Florida State University and the Max Planck Institute for Physics of Complex Systems in Dresden, Germany, published this month in the open-access journal npj Quantum Materials.
“Based on all the evidence we have today, this work confirms that the single crystals of the cerium pyrochlore identified as candidate 3D quantum spin liquids in 2019 are indeed quantum spin liquids with fractionalized spin excitations,” he said.
The inherent property of electrons that leads to magnetism is spin. Each electron behaves like a tiny bar magnet with a north and south pole, and when measured, individual electron spins always point up or down. In most everyday materials, spins point up or down at random. But electrons are antisocial by nature, and this can cause them to arrange their spins in relation to their neighbors in some circumstances. In magnets, for example, spins are collectively arranged in the same direction, and in antiferromagnets they are arranged in an up-down, up-down pattern. More