TL;DR

• Graphene nanoribbons have outstanding properties that can be precisely controlled. Researchers have succeeded in attaching electrodes to individual atomically precise nanoribbons, paving the way for precise characterization of the fascinating ribbons and their possible use in quantum technology.
• Using scaffolds of folded DNA, engineers assembled arrays of quantum rods with desirable photonic properties that could enable them to be used as highly efficient micro-LEDs for televisions or virtual reality devices.
• A team of scientists developed a new quantum-mechanics-based approach to predict metal ductility. The team demonstrated its effectiveness on refractory multi-principal-element alloys.
• Random Telegraph Noise (RTN), a type of unwanted electronic noise, has long been a nuisance in electronic systems, causing fluctuations and errors in signal processing. However, a team of researchers has made an intriguing breakthrough that can potentially harness these fluctuations in semiconductors. They reported that magnetic fluctuations and their gigantic RTN signals can be generated in a vdW-layered semiconductor by introducing vanadium in tungsten diselenide (V-WSe2) as a minute magnetic dopant.
• Researchers have found a way to control the interaction of light and quantum ‘spin’ in organic semiconductors, that works even at room temperature.
• Bringing protons up to speed with strong laser pulses — this still-young concept promises many advantages over conventional accelerators. For instance, it seems possible to build much more compact facilities. Prototypes to date, however, in which laser pulses are fired at ultra-thin metal foils, show weaknesses — especially in the frequency with which they can accelerate protons. An international working group has tested a new technique: In this approach, frozen hydrogen acts as a ‘target’ for the laser pulses.
• Substances transform properties when cooled below a critical temperature, as seen in water freezing. Quantum mechanics causes unique phase shifts in some metals, diverging from macro laws. Recent research directly validates this, offering insights into quantum physics.
• Using ultra-high-precision laser spectroscopy on a simple molecule, a group of physicists has measured the wave-like vibration of atomic nuclei with an unprecedented level of precision. The physicists report that they can thus confirm the wave-like movement of nuclear material more precisely than ever before and that they have found no evidence of any deviation from the established force between atomic nuclei.
• In a potential boon for quantum computing, physicists have shown that topologically protected quantum states can be entangled with other, highly manipulable quantum states in some electronic materials.
• The study takes a new approach to answer a long-standing mystery about insulator-to-metal transitions.
• And more!

Quantum Computing Market

According to the recent market research report ‘Quantum Computing Market with COVID-19 impact by Offering (Systems and Services), Deployment (On Premises and Cloud Based), Application, Technology, End-use Industry and Region — Global Forecast to 2026’, published by MarketsandMarkets, the Quantum Computing market is expected to grow from USD 472 million in 2021 to USD 1,765 million by 2026, at a CAGR of 30.2%. The early adoption of quantum computing in the banking and finance sector is expected to fuel the growth of the market globally. Other key factors contributing to the growth of the quantum computing market include rising investments by governments of different countries to carry out research and development activities related to quantum computing technology. Several companies are focusing on the adoption of QCaaS post-COVID-19. This, in turn, is expected to contribute to the growth of the quantum computing market. However, stability and error correction issues are expected to restrain the growth of the market.

According to ‘Quantum Computing Market Research Report: By Offering, Deployment Type, Application, Technology, Industry — Industry Share, Growth, Drivers, Trends and Demand Forecast to 2030’ report, the quantum computing market is projected to reach $64,988 million by 2030. Machine learning (ML) is expected to progress at the highest CAGR, during the forecast period, among all application categories, owing to the fact that quantum computing is being integrated in ML for improving the latter’s use case.

Latest Research

Contacting individual graphene nanoribbons using carbon nanotube electrodes

by Jian Zhang, Liu Qian, Gabriela Borin Barin, Abdalghani H. S. Daaoub, Peipei Chen, Klaus Müllen, Sara Sangtarash, Pascal Ruffieux, Roman Fasel, Hatef Sadeghi, Jin Zhang, Michel Calame, Mickael L. Perrin in Nature Electronics

Quantum technology is promising, but also perplexing. In the coming decades, it is expected to provide us with various technological breakthroughs: smaller and more precise sensors, highly secure communication networks, and powerful computers that can help develop new drugs and materials, control financial markets, and predict the weather much faster than current computing technology ever could.

To achieve this, we need so-called quantum materials: substances that exhibit pronounced quantum physical effects. One such material is graphene. This two-dimensional structural form of carbon has unusual physical properties, such as extraordinarily high tensile strength, thermal and electrical conductivity — as well as certain quantum effects. Restricting the already two-dimensional material even further, for instance, by giving it a ribbon-like shape, gives rise to a range of controllable quantum effects.

This is precisely what Mickael Perrin’s team leverage in their work: For several years now, scientists in Empa’s Transport at Nanoscale Interfaces laboratory, headed by Michel Calame, have been conducting research on graphene nanoribbons under Perrin’s leadership.

“Graphene nanoribbons are even more fascinating than graphene itself,” explains Perrin. “By varying their length and width, as well as the shape of their edges, and by adding other atoms to them, you can give them all kinds of electrical, magnetic, and optical properties.”

Size scaling in bottom-up GNR-based transistors with various geometries.

Research on the promising ribbons isn’t easy. The narrower the ribbon, the more pronounced its quantum properties are — but it also becomes more difficult to access a single ribbon at a time. This is precisely what must be done in order to understand the unique characteristics and possible applications of this quantum material and distinguish them from collective effects.

In a new study, Perrin and Empa researcher Jian Zhang, together with an international team, have succeeded for the first time in contacting individual long and atomically precise graphene nanoribbons. Not a trivial task: “A graphene nanoribbon that is just nine carbon atoms wide measures as little as 1 nanometer in width,” Zhang says. To ensure that only a single nanoribbon is contacted, the researchers employed electrodes of a similar size: They used carbon nanotubes that were also only 1 nanometer in diameter.

Precision is key for such a delicate experiment. It begins with the source materials. The researchers obtained the graphene nanoribbons via a strong and long-standing collaboration with Empa’s nanotech surfaces laboratory, headed by Roman Fasel. “Roman Fasel and his team have been working on graphene nanoribbons for a long time and can synthesize many different types with atomic precision from individual precursor molecules,” Perrin explains. The precursor molecules came from the Max Planck Institute for Polymer Research in Mainz.

As is often required for advancing the state of the art, interdisciplinarity is key, and different international research groups were involved, each bringing in their own specialty to the table: The carbon nanotubes were grown by a research group at Peking University, and to interpret the results of the study, the Empa researchers collaborated with computational scientists at the University of Warwick. “A project like this would not be possible without collaboration,” Zhang emphasizes.

Contacting individual ribbons by nanotubes posed a considerable challenge for the researchers.

“The carbon nanotubes and the graphene nanoribbons are grown on separate substrates,” Zhang explains. “First, the nanotubes need to be transferred to the device substrate and contacted by metal electrodes. Then we cut them with high-resolution electron-beam lithography to separate them into two electrodes.”

Finally, the ribbons are transferred onto the same substrate. Precision is key: Even the slightest rotation of the substrates can significantly reduce the probability of successful contact.

“Having access to high-quality infrastructure at the Binnig and Roher Nanotechnology Center at IBM Research in Rüschlikon was essential to test and implement this technology,” Perrin says.

Electron transport in 9-AGNR transistors (D7) with S-SWNT leads.

The scientists confirmed the success of their experiment through charge transport measurements. “Because quantum effects are usually more pronounced at low temperature, we performed the measurements at temperatures close to absolute zero in a high vacuum,” Perrin explains. But he is quick to add yet another particularly promising quality of graphene nanoribbons: “Due to the extremely small size of these nanoribbons, we expect their quantum effects to be so robust that they are observable even at room temperature.” This, the researcher says, could allow us to design and operate chips that actively harness quantum effects without the need for an elaborate cooling infrastructure.

“This project enables the realization of single nanoribbon devices, not only to study fundamental quantum effects such as how electrons and phonons behave at the nanoscale, but also to exploit such effects for applications in quantum switching, quantum sensing, and quantum energy conversion,” adds Hatef Sadeghi, a professor at the Univeristy of Warwick who collaborated on the project.

Graphene nanoribbons are not ready for commercial applications just yet, and there is still a lot of research to be done. In a follow-up study, Zhang and Perrin aim to manipulate different quantum states on a single nanoribbon. In addition, they plan on creating devices based on two ribbons connected in series, forming a so-called double quantum dot. Such a circuit could serve as a qubit — the smallest unit of information in a quantum computer. Moreover, Perrin, in the context of his recently obtained ERC Starting Grant and an SNSF Eccellenza Professorial Fellowship, plans to explore the use of nanoribbons as highly-efficient energy converters. In his inaugural lecture at ETH Zurich, he paints a picture of a world, in which we can harness electricity from temperature difference, while hardly losing any energy as heat — this would indeed be a real quantum leap.

Ultrafast dense DNA functionalization of quantum dots and rods for scalable 2D array fabrication with nanoscale precision

by Chi Chen, Xin Luo, Alexander E. K. Kaplan, Moungi G. Bawendi, Robert J. Macfarlane, Mark Bathe in Science Advances

Flat screen TVs that incorporate quantum dots are now commercially available, but it has been more difficult to create arrays of their elongated cousins, quantum rods, for commercial devices. Quantum rods can control both the polarization and color of light, to generate 3D images for virtual reality devices.

Using scaffolds made of folded DNA, MIT engineers have come up with a new way to precisely assemble arrays of quantum rods. By depositing quantum rods onto a DNA scaffold in a highly controlled way, the researchers can regulate their orientation, which is a key factor in determining the polarization of light emitted by the array. This makes it easier to add depth and dimensionality to a virtual scene.

“One of the challenges with quantum rods is: How do you align them all at the nanoscale so they’re all pointing in the same direction?” says Mark Bathe, an MIT professor of biological engineering and the senior author of the new study. “When they’re all pointing in the same direction on a 2D surface, then they all have the same properties of how they interact with light and control its polarization.”

MIT postdocs Chi Chen and Xin Luo are the lead authors of the paper, an associate professor of materials science and engineering; Alexander Kaplan PhD ’23; and Moungi Bawendi, the Lester Wolfe Professor of Chemistry, are also authors of the study.

Strategy to fabricate scalable QD/QR 2D array with nanoscale precision using dehydration-assisted DNA conjugation and SALSA.

Over the past 15 years, Bathe and others have led in the design and fabrication of nanoscale structures made of DNA, also known as DNA origami. DNA, a highly stable and programmable molecule, is an ideal building material for tiny structures that could be used for a variety of applications, including delivering drugs, acting as biosensors, or forming scaffolds for light-harvesting materials.

Bathe’s lab has developed computational methods that allow researchers to simply enter a target nanoscale shape they want to create, and the program will calculate the sequences of DNA that will self-assemble into the right shape. They also developed scalable fabrication methods that incorporate quantum dots into these DNA-based materials. In a 2022 paper, Bathe and Chen showed that they could use DNA to scaffold quantum dots in precise positions using scalable biological fabrication. Building on that work, they teamed up with Macfarlane’s lab to tackle the challenge of arranging quantum rods into 2D arrays, which is more difficult because the rods need to be aligned in the same direction.

Existing approaches that create aligned arrays of quantum rods using mechanical rubbing with a fabric or an electric field to sweep the rods into one direction have had only limited success. This is because high-efficiency light-emission requires the rods to be kept at least 10 nanometers from each other, so that they won’t “quench,” or suppress, their neighbors’ light-emitting activity. To achieve that, the researchers devised a way to attach quantum rods to diamond-shaped DNA origami structures, which can be built at the right size to maintain that distance. These DNA structures are then attached to a surface, where they fit together like puzzle pieces.

“The quantum rods sit on the origami in the same direction, so now you have patterned all these quantum rods through self-assembly on 2D surfaces, and you can do that over the micron scale needed for different applications like microLEDs,” Bathe says. “You can orient them in specific directions that are controllable and keep them well-separated because the origamis are packed and naturally fit together, as puzzle pieces would.”

Loading efficiency of QDs/QRs to 2D Rh.

As the first step in getting this approach to work, the researchers had to come up with a way to attach DNA strands to the quantum rods. To do that, Chen developed a process that involves emulsifying DNA into a mixture with the quantum rods, then rapidly dehydrating the mixture, which allows the DNA molecules to form a dense layer on the surface of the rods. This process takes only a few minutes, much faster than any existing method for attaching DNA to nanoscale particles, which may be key to enabling commercial applications.

“The unique aspect of this method lies in its near-universal applicability to any water-loving ligand with affinity to the nanoparticle surface, allowing them to be instantly pushed onto the surface of the nanoscale particles. By harnessing this method, we achieved a significant reduction in manufacturing time from several days to just a few minutes,” Chen says.

These DNA strands then act like Velcro, helping the quantum rods stick to a DNA origami template, which forms a thin film that coats a silicate surface. This thin film of DNA is first formed via self-assembly by joining neighboring DNA templates together via overhanging strands of DNA along their edges. The researchers now hope to create wafer-scale surfaces with etched patterns, which could allow them to scale their design to device-scale arrangements of quantum rods for numerous applications, beyond only microLEDs or augmented reality/virtual reality.

“The method that we describe in this paper is great because it provides good spatial and orientational control of how the quantum rods are positioned. The next steps are going to be making arrays that are more hierarchical, with programmed structure at many different length scales. The ability to control the sizes, shapes, and placement of these quantum rod arrays is a gateway to all sorts of different electronics applications,” Macfarlane says.

“DNA is particularly attractive as a manufacturing material because it can be biologically produced, which is both scalable and sustainable, in line with the emerging U.S. bioeconomy. Translating this work towards commercial devices by solving several remaining bottlenecks, including switching to environmentally safe quantum rods, is what we’re focused on next,” Bathe adds.

A ductility metric for refractory-based multi-principal-element alloys

by Prashant Singh, Brent Vela, Gaoyuan Ouyang, Nicolas Argibay, Jun Cui, Raymundo Arroyave, Duane D. Johnson in Acta Materialia

A team of scientists from Ames National Laboratory and Texas A&M University developed a new way to predict metal ductility. This quantum-mechanics-based approach fills a need for an inexpensive, efficient, high-throughput way to predict ductility. The team demonstrated its effectiveness on refractory multi-principal-element alloys. These are materials of interest for use in high-temperature conditions, however, they frequently lack necessary ductility for potential applications in aerospace, fusion reactors, and land-based turbines.

Ductility describes how well a material can withstand physical strain without cracking or breaking. According to Prashant Singh, a scientist at Ames Lab and leader of the theoretical design efforts, there are currently no robust ways to predict metal ductility. Additionally, trial-and-error experimentation is expensive and time-consuming, especially in extreme conditions.

A typical way to model atoms is with rigid spheres that are symmetrical. However, Singh explained that in real materials, the atoms are different sizes and have shapes. When mixing elements with different sized atoms, the atoms continually adjust to fit within the fixed space. This behavior creates local atomic distortion.

The new analysis incorporates local atomic distortion in determining whether a material is brittle or ductile. It also expands on the capabilities of current approaches.

“They [current approaches] are not very efficient at distinguishing between ductile and brittle systems for small compositional changes. But the new approach can capture such non-trivial details, because now we have added a quantum mechanical feature in the approach that was missing,” Singh said.

Another advantage to this new high-throughput testing method is its efficiency. Singh explained that it can test thousands of materials rapidly. The speed and capacity make it possible to predict which material combinations are worth taking to the experimental level. This minimizes the time and resources needed to discover these materials through experimental methods.

To determine how well their ductility test worked, Gaoyuan Ouyang, an Ames Lab Scientist, led the team’s experimental efforts. They performed validation tests on a set of predicted refractory multi-principal-element alloys (RMPEAs). RMPEAs are materials that have potential for use in high temperature environments, such as aerospace propulsion systems, nuclear reactors, turbines, and other energy applications.

Through their validation testing, the team found that, “The predicted ductile metals underwent significant deformation under high stress, while the brittle metal cracked under similar loads, confirming the robustness of new quantum mechanical method,” Ouyang said.

Electrically tunable magnetic fluctuations in multilayered vanadium-doped tungsten diselenide

by Lan-Anh T. Nguyen, Jinbao Jiang, Tuan Dung Nguyen, Philip Kim, Min-Kyu Joo, Dinh Loc Duong, Young Hee Lee in Nature Electronics

Random Telegraph Noise (RTN), a type of unwanted electronic noise, has long been a nuisance in electronic systems, causing fluctuations and errors in signal processing. However, a team of researchers from the Center for Integrated Nanostructure Physics within the Institute for Basic Science (IBS), South Korea has made an intriguing breakthrough that can potentially harness these fluctuations in semiconductors. Led by Professor LEE Young Hee, the team reported that magnetic fluctuations and their gigantic RTN signals can be generated in a vdW-layered semiconductor by introducing vanadium in tungsten diselenide (V-WSe2) as a minute magnetic dopant.

High contact resistance in lateral devices usually limits the manifestation of inherent quantum states and further degrades the device’s performance. To overcome these limitations, the researchers introduced a vertical magnetic tunneling junction device by sandwiching a few layers of V-WSe2, a magnetic material, between the top and bottom graphene electrodes. This device was able to manifest inherent quantum states such as magnetic fluctuations and achieve high-amplitude RTN signals, even with a small vanadium doping concentration of just ~0.2%.

Dr. Lan-Anh T. NGUYEN, the first author of the study said, “The key to success is to realize large magnetic fluctuations in resistance by constructing vertical magnetic tunneling junction devices with low contact resistance.”

RTN signals after FC with out-of-plane and in-plane 1 T magnetic fields.

Through the resistance measurement experiments using these devices, the researchers observed RTNs with a high amplitude of up to 80% between well-defined two-stable states. In the bistable state, the magnetic fluctuations in resistance prevail with temperature through the competition between intralayer and interlayer coupling among the magnetic domains. They were able to identify this bistable magnetic state through discrete Gaussian peaks in the RTN histogram with distinctive features in the noise power spectrum.

Most importantly the researchers discovered the ability to switch the bistable magnetic state and the cut-off frequency of the RTN simply by changing the voltage polarity. This exciting discovery paves the way for the application of 1/f2 noise spectroscopy in magnetic semiconductors and offers magnetic switching capability in spintronics.

“This is a first step to observe the bistable magnetic state from large resistance fluctuations in magnetic semiconductors and offers the magnetic switching capability with 1/f2 noises by means of simple voltage polarity in spintronics,” explained Professor Lee.

Reversible spin-optical interface in luminescent organic radicals

by Sebastian Gorgon, Kuo Lv, Jeannine Grüne, Bluebell H. Drummond, et al in Nature

Researchers have found a way to control the interaction of light and quantum ‘spin’ in organic semiconductors, that works even at room temperature.

Spin is the term for the intrinsic angular momentum of electrons, which is referred to as up or down. Using the up/down spin states of electrons instead of the 0 and 1 in conventional computer logic could transform the way in which computers process information. And sensors based on quantum principles could vastly improve our abilities to measure and study the world around us.

An international team of researchers, led by the University of Cambridge, has found a way to use particles of light as a ‘switch’ that can connect and control the spin of electrons, making them behave like tiny magnets that could be used for quantum applications. The researchers designed modular molecular units connected by tiny ‘bridges’. Shining a light on these bridges allowed electrons on opposite ends of the structure to connect to each other by aligning their spin states. Even after the bridge was removed, the electrons stayed connected through their aligned spins. This level of control over quantum properties can normally only be achieved at ultra-low temperatures. However, the Cambridge-led team has been able to control the quantum behaviour of these materials at room temperature, which opens up a new world of potential quantum applications by reliably coupling spins to photons.

Almost all types of quantum technology — based on the strange behaviour of particles at the subatomic level — involve spin. As they move, electrons usually form stable pairs, with one electron spin up and one spin down. However, it is possible to make molecules with unpaired electrons, called radicals. Most radicals are very reactive, but with careful design of the molecule, they can be made chemically stable.

ESR on high-spin states.

“These unpaired spins change the rules for what happens when a photon is absorbed and electrons are moved up to a higher energy level,” said first author Sebastian Gorgon, from Cambridge’s Cavendish Laboratory. “We’ve been working with systems where there is one net spin, which makes them good for light emission and making LEDs.”

Gorgon is a member of Professor Sir Richard Friend’s research group, where they have been studying radicals in organic semiconductors for light generation, and identified a stable and bright family of materials a few years ago. These materials can beat the best conventional OLEDs for red light generation.

“Using tricks developed by different fields was important,” said Dr Emrys Evans from Swansea University, who co-led the research. “The team has significant expertise from a number of areas in physics and chemistry, such as the spin properties of electrons and how to make organic semiconductors work in LEDs. This was critical for knowing how to prepare and study these molecules in the solid state, enabling our demonstration of quantum effects at room temperature.”

Organic semiconductors are the current state-of-the-art for lighting and commercial displays, and they could be a more sustainable alternative to silicon for solar cells. However, they have not yet been widely studied for quantum applications, such as quantum computing or quantum sensing.

“We’ve now taken the next big step and linked the optical and magnetic properties of radicals in an organic semiconductor,” said Gorgon. “These new materials hold great promise for completely new applications, since we’ve been able to remove the need for ultra-cold temperatures.”

“Knowing what electron spins are doing, let alone controlling them, is not straightforward, especially at room temperature,” said Friend, who co-led the research. “But if we can control the spins, we can build some interesting and useful quantum objects.”

The researchers designed a new family of materials by first determining how they wanted the electron spins to behave. Using this bottom-up approach, they were able to control the properties of the end material by using a building block method and changing the ‘bridges’ between different modules of the molecule. These bridges were made of anthracene, a type of hydrocarbon. For their ‘mix-and-match’ molecules, the researchers attached a bright light-emitting radical to an anthracene molecule. After a photon of light is absorbed by the radical, the excitation spreads out onto the neighbouring anthracene, causing three electrons to start spinning in the same way. When a further radical group is attached to the other side of the anthracene molecules, its electron is also coupled, bringing four electrons to spin in the same direction

“In this example, we can switch on interaction between two electrons on opposite ends of the molecule by aligning electron spins on the bridge absorbing a photon of light,” said Gorgon. “After relaxing back, the distant electrons remember they were together even after the bridge is gone.

“In these materials we’ve designed, absorbing a photon is like turning a switch on. The fact that we can start to control these quantum objects by reliably coupling spins at room temperature could open up far more flexibility in the world of quantum technologies. There’s a huge potential here to go in lots of new directions.”

Ultra-short pulse laser acceleration of protons to 80 MeV from cryogenic hydrogen jets tailored to near-critical density

by Martin Rehwald, Stefan Assenbaum, Constantin Bernert, et ail in Nature Communications

Bringing protons up to speed with strong laser pulses — this still young concept promises many advantages over conventional accelerators. For instance, it seems possible to build much more compact facilities. Prototypes to date, however, in which laser pulses are fired at ultra-thin metal foils, show weaknesses — especially in the frequency with which they can accelerate protons. At the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), an international working group has tested a new technique: In this approach, frozen hydrogen acts as a “target” for the laser pulses. In the future, the method could serve as a basis for advanced tumor therapy concepts, as the team describes.

Conventional proton accelerators such as the Large Hadron Collider at CERN in Geneva are based on the particle acceleration via strong radio frequency waves. In laser acceleration, on the other hand, ultra-bright light pulses give the particles a boost: Extremely short and powerful laser pulses are fired at wafer-thin metal foils. The light heats the material to such an extent that electrons are ejected in large numbers, while the heavy atomic nuclei remain in place. Since the electrons are negatively charged and the atomic nuclei are positively charged, a strong electric field forms between them.

This field can then launch a pulse of protons with enormous force over a distance of only a few micrometers, thus bringing them to energies for which much longer systems would be needed with conventional accelerator technology. Another advantage: “With laser acceleration, we can pack a huge number of particles into one proton bunch,” explains HZDR physicist Dr. Karl Zeil. “This could be interesting for radiation therapy of tumors.”

However, the previous method of firing laser pulses at metal foils has drawbacks. Firstly, it is difficult to generate several proton pulses per second — the foil is already destroyed by a single laser shot and therefore has to be replaced again and again. Secondly, the acceleration process is quite complex and relatively difficult to control. The reason: The protons to be accelerated come from hydrocarbons that have accumulated on the metal foils as a layer of contaminants — not exactly ideal for perfect control of the experiment.

Target expansion study and hydrodynamic simulation.

Therefore, the German-American research team around Karl Zeil came up with an alternative: “Instead of a metal foil, we use a fine, strongly cooled hydrogen jet,” the researcher describes. “This jet serves as a target for our high-intensity laser pulses.” Specifically, the experts cool hydrogen gas in a copper block to such an extent that it becomes liquid. The liquid hydrogen then flows through a nozzle into a vacuum chamber. It thus cools further and solidifies into a micrometer-thin filament: the target for the laser pulses. And since the hydrogen filament renews itself, the laser has a new, intact target in its sight for every shot.

Another benefit is that the setup allows for a more favorable acceleration mechanism: Instead of just heating the material, the laser pulses use radiation pressure to push the electrons out of the hydrogen and create the extreme electric fields needed to accelerate the protons. The team was able to optimize the process by sending a short, weaker light pulse in front of the main laser pulse. This preheated the frozen hydrogen filament, causing it to expand and its cross-section to grow from five micrometers to several times that size. This made it possible to increase the acceleration distance and optimize the process.

The result: “We were able to bring protons up to an energy of 80 MeV,” reports Karl Zeil. “This is close to the previous record for laser proton acceleration. But unlike previous facilities, our technique has the potential to generate multiple proton bunches per second.” Furthermore, the acceleration process is comparatively easy to simulate for hydrogen targets using high-performance computing — a task that also involved the Center for Advanced Systems Understanding (CASUS) at HZDR. “This allows us to better understand and optimize the interaction between laser and matter,” Zeil said. Now the experts want to use AI algorithms to increase the “hit rate” between the laser pulses and the frozen hydrogen jet.

The technology could be interesting for a future type of radiation therapy. Already today, some tumors are successfully irradiated with protons. Laser acceleration could increase the dose and thus shorten the irradiation time. And — as an HZDR study suggests — this could better protect the healthy tissue surrounding the tumor.

Critical slowing down near a magnetic quantum phase transition with fermionic breakdown

by Yang, CJ., Kliemt, K., Krellner, C. et al in Nature Physics

Many substances change their properties when they are cooled below a certain critical temperature. Such a phase transition occurs, for example, when water freezes. However, in certain metals there are phase transitions that do not exist in the macrocosm. They arise because of the special laws of quantum mechanics that apply in the realm of nature’s smallest building blocks. It is thought that the concept of electrons as carriers of quantized electric charge no longer applies near these exotic phase transitions. Researchers at the University of Bonn and ETH Zurich have now found a way to prove this directly. Their findings allow new insights into the exotic world of quantum physics.

If you cool water below zero degrees Celsius, it solidifies into ice. In the process, it abruptly changes its properties. As ice, for example, it has a much lower density than in a liquid state — which is why icebergs float. In physics, this is referred to as a phase transition. But there are also phase transitions in which characteristic features of a substance change gradually. If, for example, an iron magnet is heated up to 760 degrees Celsius, it loses its attraction to other pieces of metal — it is then no longer ferromagnetic, but paramagnetic. However, this does not happen abruptly, but continuously: The iron atoms behave like tiny magnets. At low temperatures, they are oriented parallel to each other. When heated, they fluctuate more and more around this rest position until they are completely randomly aligned, and the material loses its magnetism completely. So while the metal is being heated, it can be both somewhat ferromagnetic and somewhat paramagnetic.

The phase transition thus takes place gradually, so to speak, until finally all the iron is paramagnetic. Along the way, the transition slows down more and more. This behavior is characteristic of all continuous phase transitions. “We call it ‘critical slowing down,’” explains Prof. Dr. Hans Kroha of the Bethe Center for Theoretical Physics at the University of Bonn. “The reason is that with continuous transitions, the two phases get energetically closer and closer together.” It is similar to placing a ball on a ramp: It then rolls downhill, but the smaller the difference in altitude, the more slowly it rolls. When iron is heated, the energy difference between the phases decreases more and more, in part because the magnetization disappears progressively during the transition.

Such a “slowing down” is typical for phase transitions based on the excitation of bosons. Bosons are particles that “generate” interactions (on which, for example, magnetism is based). Matter, on the other hand, is not made up of bosons but of fermions. Electrons, for example, belong to the fermions. Phase transitions are based on the fact that particles (or also the phenomena triggered by them) disappear. This means that the magnetism in iron becomes smaller and smaller as fewer atoms are aligned in parallel.

“Fermions, however, cannot be destroyed due to fundamental laws of nature and therefore cannot disappear,” Kroha explains. “That’s why normally they are never involved in phase transitions.”

Exploring fermionic quantum criticality by time-resolved THz reflectivity.

Electrons can be bound in atoms; they then have a fixed place which they cannot leave. Some electrons in metals, on the other hand, are freely mobile — which is why these metals can also conduct electricity. In certain exotic quantum materials, both varieties of electrons can form a superposition state. This produces what are known as quasiparticles. They are, in a sense, immobile and mobile at the same timetime — a feature that is only possible in the quantum world. These quasiparticles — unlike “normal” electrons — can be destroyed during a phase transition. This means that the properties of a continuous phase transition can also be observed there, in particular, critical slowing down.

So far, this effect could be observed only indirectly in experiments. Researchers led by theoretical physicist Hans Kroha and Manfred Fiebig’s experimental group at ETH Zurich have now developed a new method, which allows direct identification of the collapse of quasiparticles at a phase transition, in particular the associated critical slowing down.

“This has enabled us to show for the first time directly that such a slowdown can also occur in fermions,” says Kroha, who is also a member of the Transdisciplinary Research Area “Matter” at the University of Bonn and the Cluster of Excellence “Matter and Light for Quantum Computing” of the German Research Foundation. The result contributes to a better understanding of phase transitions in the quantum world. On the long term, the findings might also be useful for applications in quantum information technology.

Test of charged baryon interaction with high-resolution vibrational spectroscopy of molecular hydrogen ions

by S. Alighanbari, I. V. Kortunov, G. S. Giri, S. Schiller in Nature Physics

Using ultra-high-precision laser spectroscopy on a simple molecule, a group of physicists led by Professor Stephan Schiller Ph.D. from Heinrich Heine University Düsseldorf (HHU) has measured the wave-like vibration of atomic nuclei with an unprecedented level of precision. The physicists report that they can thus confirm the wave-like movement of nuclear material more precisely that ever before and that they have found no evidence of any deviation from the established force between atomic nuclei.

Simple atoms have been the subjects of precision experimental and theoretical investigations for nearly 100 years, with pioneering work carried out on the description and measurement of the hydrogen atom, the simplest atom with just one electron. Currently, hydrogen atom energies — and thus their electromagnetic spectrum — are the most precisely computed energies of a bound quantum system. As extremely precise measurements of the spectrum can also be made, the comparison of theoretical predictions and measurements enables testing of the theory on which the prediction is based. Such tests are very important. Researchers around the world are seeking — albeit unsuccessfully to date — evidence of new physical effects that could occur as a result of the existence of Dark Matter. These effects would lead to a discrepancy between measurement and prediction.

By contrast with the hydrogen atom, the simplest molecule was not a subject for precision measurements for a long time. However, the research group headed by Professor Stephan Schiller Ph.D. from the Chair of Experimental Physics at HHU has dedicated itself to this topic. In Düsseldorf, the group has conducted pioneering work and developed experimental techniques that are among the most accurate in the world.

The simplest molecule is the molecular hydrogen ion (MHI): a hydrogen molecule, which is missing an electron and comprises three particles. One variant, H2+, comprises two protons and an electron, while HD+ comprises a proton, a deuteron — a heavier hydrogen isotope — and an electron. Protons and deuterons are charged “baryons,” i.e. particles which are subject to the so-called strong force.

Within the molecules, the components can behave in various ways: The electrons move around the atomic nuclei, while the atomic nuclei vibrate against or rotate around each other, with the particles acting like waves. These wave motions are described in detail by quantum theory. The different modes of motion determine the spectra of the molecules, which are reflected in different spectral lines. The spectra arise in a similar way to atom spectra, but are significantly more complex.

Spectral characteristics of the 1.15 μm spectroscopy laser.

The art of current physics research now involves measuring the wavelengths of the spectral lines extremely precisely and — with the help of quantum theory — also calculating these wavelengths extremely precisely. A match between the two results is interpreted as proof of the accuracy of the predictions, while a mismatch could be a hint for “new Physics.” Over the years, the team of physicists at HHU has refined the laser spectroscopy of the MHI, developing techniques that have improved the experimental resolution of the spectra by multiple orders of magnitude. Their objective: the more precisely the spectra can be measured, the better the theoretical predictions can be tested. This enables the identification of any potential deviations from the theory and thus also starting points for how the theory might need to be modified.

Professor Schiller’s team has improved experimental precision to a level better than theory. To achieve this, the physicists in Düsseldorf confine a moderate number of around 100 MHI in an ion trap in an ultra-high vacuum container, using laser cooling techniques to cool the ions down to a temperature of 1 milli kelvin. This enables extremely precise measurement of the molecular spectra of rotational and vibrational transitions. Following earlier investigations of spectral lines with wavelengths of 230 ?m and 5.1 ?m, the authors now present measurements for a spectral line with the significantly shorter wavelength of 1.1 ?m in Nature Physics.

Professor Schiller: “The experimentally determined transition frequency and the theoretical prediction agree. In combination with previous results, we have established the most precise test of the quantum motion of charged baryons: Any deviation from the established quantum laws must be smaller than 1 part in 100 billion, if it exists at all.”

The result can also be interpreted in an alternative way: Hypothetically, a further fundamental force could exist between the proton and deuteron in addition to the well-known Coulomb force (the force between electrically charged particles).

Lead author Dr Soroosh Alighanbari: “Such a hypothetical force may exist in connection with the phenomenon of Dark Matter. We have not found any evidence for such a force in the course of our measurements, but we will continue our search.”

Coupled topological flat and wide bands: Quasiparticle formation and destruction

by Haoyu Hu, Qimiao S in Science Advances

Rice University physicists have shown that immutable topological states, which are highly sought for quantum computing, can be entangled with other, manipulable quantum states in some materials.

“The surprising thing we found is that in a particular kind of crystal lattice, where electrons become stuck, the strongly coupled behavior of electrons in d atomic orbitals actually act like the f orbital systems of some heavy fermions,” said Qimiao Si, co-author of a study about the research.

The unexpected find provides a bridge between subfields of condensed matter physics that have focused on dissimilar emergent properties of quantum materials. In topological materials, for example, patterns of quantum entanglement produce “protected,” immutable states that could be used for quantum computing and spintronics. In strongly correlated materials, the entanglement of billions upon billions of electrons gives rise to behaviors like unconventional superconductivity and the continual magnetic fluctuations in quantum spin liquids.

In the study, Si and co-author Haoyu Hu, a former graduate student in his research group, built and tested a quantum model to explore electron coupling in a “frustrated” lattice arrangement like those found in metals and semimetals that feature “flat bands,” states where electrons become stuck and strongly correlated effects are amplified. The research is part of an ongoing effort by Si, who won a Vannevar Bush Faculty Fellowship from the Defense Department in July to pursue the validation of a theoretical framework for controlling topological states of matter.

In the study, Si and Hu showed that electrons from d atomic orbitals could become part of larger, molecular orbitals that are shared by several atoms in the lattice. The research also showed that electrons in molecular orbitals could become entangled with other frustrated electrons, producing strongly correlated effects that were very familiar to Si, who has spent years studying heavy fermion materials.

“These are completely d-electron systems,” Si said. “In the d-electron world, it’s like you have a highway with multiple lanes. In the f-electron world, you can think of electrons moving in two tiers. One is like the d-electron highway, and the other is like a dirt road, where movement is very slow.”

Si said f-electron systems host very clean examples of strongly correlated physics, but they aren’t practical for everyday use.

“This dirt road lies so far from the highway,” he said. “The influence from the highway is very small, which translates to a minute energy scale and very low-temperature physics. Meaning you need to go to temperatures around 10 Kelvin or so to even see the effects of coupling.

“That is not the case in the d-electron world. Things couple to one another quite efficiently on the multilane highway there.”

And that coupling efficiency persists, even when there is a flat band. Si likened it to one of the highway’s lanes becoming as inefficient and slow as the f-electron dirt road.

“Even when it has faded into a dirt road, it still shares status with the other lanes, because they all came from the d orbital,” Si said. “It is effectively a dirt road, but it is much more strongly coupled, and that translates to physics at much higher temperatures.

“That means I can have all of the exquisite, f-electron-based physics, for which I have well-defined models and a lot of intuition from years of study, but instead of having to go to 10 Kelvin, I can potentially work at, say, 200 Kelvin, or possibly even 300 Kelvin, or room temperature. So, from a functionality perspective, it is extremely promising.”

Correlated insulator collapse due to quantum avalanche via in-gap ladder states

by Jong E. Han, Camille Aron, Xi Chen, Ishiaka Mansaray, Jae-Ho Han, Ki-Seok Kim, Michael Randle, Jonathan P. Bird in Nature Communications

Looking only at their subatomic particles, most materials can be placed into one of two categories.

Metals — like copper and iron — have free-flowing electrons that allow them to conduct electricity, while insulators — like glass and rubbe r — keep their electrons tightly bound and therefore do not conduct electricity. Insulators can turn into metals when hit with an intense electric field, offering tantalizing possibilities for microelectronics and supercomputing, but the physics behind this phenomenon called resistive switching is not well understood. Questions, like how large an electric field is needed, are fiercely debated by scientists, like University at Buffalo condensed matter theorist Jong Han.

“I have been obsessed by that,” he says.

Han, PhD, professor of physics in the College of Arts and Sciences, is the lead author on a study that takes a new approach to answer a long-standing mystery about insulator-to-metal transitions.

The difference between metals and insulators lies in quantum mechanical principles, which dictate that electrons are quantum particles and their energy levels come in bands that have forbidden gaps, Han says. Since the 1930s, the Landau-Zener formula has served as a blueprint for determining the size of electric field needed to push an insulator’s electrons from its lower bands to its upper bands. But experiments in the decades since have shown materials require a much smaller electric field — approximately 1,000 times smaller — than the Landau-Zener formula estimated.

CDW-like two-stage IMT.

“So, there is a huge discrepancy, and we need to have a better theory,” Han says.

To solve this, Han decided to consider a different question: What happens when electrons already in the upper band of an insulator are pushed? Han ran a computer simulation of resistive switching that accounted for the presence of electrons in the upper band. It showed that a relatively small electric field could trigger a collapse of the gap between the lower and upper bands, creating a quantum path for the electrons to go up and down between the bands.

To make an analogy, Han says, “Imagine some electrons are moving on a second floor. When the floor is tilted by an electric field, electrons not only begin to move but previously forbidden quantum transitions open up and the very stability of the floor abruptly falls apart, making the electrons on different floors flow up and down. “Then, the question is no longer how the electrons on the bottom floor jump up, but the stability of higher floors under an electric field.”

This idea helps solve some of the discrepancies in the Landau-Zener formula, Han says. It also provides some clarity to the debate over insulator-to-metal transitions caused by electrons themselves or those caused by extreme heat. Han’s simulation suggests the quantum avalanche is not triggered by heat. However, the full insulator-to-metal transition doesn’t happen until the separate temperatures of the electrons and phonons — quantum vibrations of the crystal’s atoms — equilibrate. This shows that the mechanisms for electronic and thermal switching are not exclusive of each other, Han says, but can instead arise simultaneously.

“So, we have found a way to understand some corner of this whole resistive switching phenomenon,” Han says. “But I think it’s a good starting point.”

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