TL;DR

• Absolute zero cannot be reached — unless you have an infinite amount of energy or an infinite amount of time. Scientists in Vienna (Austria) studying the connection between thermodynamics and quantum physics have now found out that there is a third option: Infinite complexity. It turns out that reaching absolute zero is in a way equivalent to perfectly erasing information in a quantum computer, for which an infinetly complex quantum computer would be required.
• Scientists analyzed each element of the neutrino mass matrix belonging to leptons and showed theoretically that the intergenerational mixing of lepton flavors is large. By using the mathematics of random matrix theory, the research team was able to demonstrate why the calculation of the squared difference of the neutrino masses are in close agreement with the experimental results in the case of the seesaw model with the random Dirac and Majorana matrices. The results of this research are expected to contribute to the further development of particle theory research, which largely remains a mystery.
• Nuclear physicists may have finally pinpointed where in the proton a large fraction of its mass resides. A recent experiment has revealed the radius of the proton’s mass that is generated by the strong force as it glues together the proton’s building block quarks.
• An international group of researchers has created a mixed magnon state in an organic hybrid perovskite material by utilizing the Dzyaloshinskii — Moriya-Interaction (DMI). The resulting material has potential for processing and storing quantum computing information.
• Physicists predict that layered electronic 2D semiconductors can host a curious quantum phase of matter called the supersolid. This counterintuitive quantum material simultaneously forms a rigid crystal, and yet at the same time allows particles to flow without friction, with all the particles belong to the same single quantum state.
• Shooting ions is very different from shooting a gun: By firing highly charged ions onto tiny gold structures, these structures can be modified in technologically interesting ways. Surprisingly, the key is not the force of impact, but the electric charge of the projectiles.
• Stacked layers of ultrathin semiconductor materials feature phenomena that can be exploited for novel applications. Physicists have studied effects that emerge by giving two layers a slight twist.
• In a major breakthrough in the fields of nanophotonics and ultrafast optics, a research team has demonstrated the ability to dynamically steer light pulses from conventional, so-called incoherent light sources.
• By adapting technology used for gamma-ray astronomy, researchers has found X-ray transitions previously thought to have been unpolarized according to atomic physics, are in fact highly polarized.
• Trapped electrons traveling in circular loops at extreme speeds inside graphene quantum dots are highly sensitive to external magnetic fields and could be used as novel magnetic field sensors with unique capabilities, according to a new study.
• 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

Landauer Versus Nernst: What is the True Cost of Cooling a Quantum System?

by Philip Taranto, Faraj Bakhshinezhad, Andreas Bluhm, et al in PRX Quantum

The absolute lowest temperature possible is -273.15 degrees Celsius. It is never possible to cool any object exactly to this temperature — one can only approach absolute zero. This is the third law of thermodynamics.

A research team at TU Wien (Vienna) has now investigated the question: How can this law be reconciled with the rules of quantum physics? They succeeded in developing a “quantum version” of the third law of thermodynamics: Theoretically, absolute zero is attainable. But for any conceivable recipe for it, you need three ingredients: Energy, time and complexity. And only if you have an infinite amount of one of these ingredients can you reach absolute zero.

Framework. The task of cooling a quantum system in two extremal control scenarios, with each step of both paradigms comprising two primitives.

When quantum particles reach absolute zero, their state is precisely known: They are guaranteed to be in the state with the lowest energy. The particles then no longer contain any information about what state they were in before. Everything that may have happened to the particle before is perfectly erased. From a quantum physics point of view, cooling and deleting information are thus closely related.

At this point, two important physical theories meet: Information theory and thermodynamics. But the two seem to contradict each other: “From information theory, we know the so-called Landauer principle. It says that a very specific minimum amount of energy is required to delete one bit of information,” explains Prof. Marcus Huber from the Atomic Institute of TU Wien. Thermodynamics, however, says that you need an infinite amount of energy to cool anything down exactly to absolute zero. But if deleting information and cooling to absolute zero are the same thing — how does that fit together?

Complexity. Structural (left) and control complexity (right). Structural complexity concerns properties of the machine Hamiltonian. For perfect cooling it is necessary that the largest energy gap diverges.

The roots of the problem lie in the fact that thermodynamics was formulated in the 19th century for classical objects — for steam engines, refrigerators or glowing pieces of coal. At that time, people had no idea about quantum theory. If we want to understand the thermodynamics of individual particles, we first have to analyse how thermodynamics and quantum physics interact — and that is exactly what Marcus Huber and his team did.

“We quickly realised that you don’t necessarily have to use infinite energy to reach absolute zero,” says Marcus Huber. “It is also possible with finite energy — but then you need an infinitely long time to do it.”

Up to this point, the considerations are still compatible with classical thermodynamics as we know it from textbooks. But then the team came across an additional detail of crucial importance:

“We found that quantum systems can be defined that allow the absolute ground state to be reached even at finite energy and in finite time — none of us had expected that,” says Marcus Huber. “But these special quantum systems have another important property: they are infinitely complex.”

So you would need infinitely precise control over infinitely many details of the quantum system — then you could cool a quantum object to absolute zero in finite time with finite energy. In practice, of course, this is just as unattainable as infinitely high energy or infinitely long time.

“So if you want to perfectly erase quantum information in a quantum computer, and in the process transfer a qubit to a perfectly pure ground state, then theoretically you would need an infinitely complex quantum computer that can perfectly control an infinite number of particles,” says Marcus Huber. In practice, however, perfection is not necessary — no machine is ever perfect. It is enough for a quantum computer to do its job fairly well. So the new results are not an obstacle in principle to the development of quantum computers. In practical applications of quantum technologies, temperature plays a key role today — the higher the temperature, the easier it is for quantum states to break and become unusable for any technical use.

“This is precisely why it is so important to better understand the connection between quantum theory and thermodynamics,” says Marcus Huber. “There is a lot of interesting progress in this area at the moment. It is slowly becoming possible to see how these two important parts of physics intertwine.”

Neutrino mass square ratio and neutrinoless double-beta decay in random neutrino mass matrices

by Naoyuki Haba, Yasuhiro Shimizu, Toshifumi Yamada in Progress of Theoretical and Experimental Physics

When any matter is divided into smaller and smaller pieces, eventually all you are left with — when it cannot be divided any further — is a particle. Currently, there are 12 different known elementary particles, which in turn are made up of quarks and leptons each of which come in six different flavors. These flavors are grouped into three generations — each with one charged and one neutral lepton — to form different particles, including the electron, muon, and tau neutrinos. In the Standard Model, the masses of the three generations of neutrinos are represented by a three-by-three matrix.

A research team led by Professor Naoyuki Haba from the Osaka Metropolitan University Graduate School of Science, analyzed the collection of leptons that make up the neutrino mass matrix. Neutrinos are known to have less difference in mass between generations than other elementary particles, so the research team considered that neutrinos are roughly equal in mass between generations. They analyzed the neutrino mass matrix by randomly assigning each element of the matrix. They showed theoretically, using the random mass matrix model that the lepton flavor mixings are large.

“Clarifying the properties of elementary particles leads to the exploration of the universe and ultimately to the grand theme of where we came from!” Professor Haba explained. “Beyond the remaining mysteries of the Standard Model, there is a whole new world of physics.”

Histograms of the ratios R1 (left) and R2 (right) in each model. The red and blue lines correspond to the central value and the 3σ range of the current experimental value, respectively.

After studying the neutrino mass anarchy in the Dirac neutrino, seesaw, double seesaw models, the researchers found that the anarchy approach requires that the measure of the matrix should obey the Gaussian distribution. Having considered several models of light neutrino mass where the matrix is composed of the product of several random matrices, the research team was able to prove, as best they could at this stage, why the calculation of the squared difference of the neutrino masses are closest with the experimental results in the case of the seesaw model with the random Dirac and Majorana matrices.

“In this study, we showed that the neutrino mass hierarchy can be mathematically explained using random matrix theory. However, this proof is not mathematically complete and is expected to be rigorously proven as random matrix theory continues to develop,” said Professor Haba. “In the future, we will continue with our challenge of elucidating the three-generation copy structure of elementary particles, the essential nature of which is still completely unknown both theoretically and experimentally.”

Determining the gluonic gravitational form factors of the proton

by B. Duran, Z.-E. Meziani, S. Joosten, M. K. Jones, S. Prasad, et al in Nature

Nuclear physicists may have finally pinpointed where in the proton a large fraction of its mass resides. A recent experiment carried out at the U.S. Department of Energy’s Thomas Jefferson National Accelerator Facility has revealed the radius of the proton’s mass that is generated by the strong force as it glues together the proton’s building block quarks.

One of the biggest mysteries of the proton is the origin of its mass. It turns out that the proton’s measured mass doesn’t just come from its physical building blocks, its three so-called valence quarks.

“If you add up the Standard Model masses of the quarks in a proton, you only get a small fraction of the proton’s mass,” explained experiment co-spokesperson Sylvester Joosten, an experimental physicist at DOE’s Argonne National Laboratory.

Over the last few decades, nuclear physicists have tentatively pieced together that the proton’s mass comes from several sources. First, it gets some mass from the masses of its quarks, and some more from their movements. Next, it gets mass from the strong force energy that glues those quarks together, with this force manifesting as ‘gluons.’ Lastly, it gets mass from the dynamic interactions of the proton’s quarks and gluons.

This new measurement may have finally shed some light on the mass that is generated by the proton’s gluons by pinpointing the location of the matter generated by these gluons. The radius of this core of matter was found to reside at the center of the proton. The result also seems to indicate that this core has a different size than the proton’s well-measured charge radius, a quantity that is often used as a proxy for the proton’s size.

“The radius of this mass structure is smaller than the charge radius, and so it kind of gives us a sense of the hierarchy of the mass versus the charge structure of the nucleon,” said experiment co-spokesperson Mark Jones, Jefferson Lab’s Halls A&C leader.

Mass radius and trace anomaly.

According to experiment co-spokesperson Zein-Eddine Meziani, a staff scientist at DOE’s Argonne National Laboratory, this result actually came as somewhat of a surprise.

“What we have found is something that we really weren’t expecting to come out this way. The original goal of this experiment was a search for a pentaquark that has been reported by researchers at CERN,” Meziani said

The experiment was performed in Experimental Hall C in Jefferson Lab’s Continuous Electron Beam Accelerator Facility, a DOE Office of Science user facility. In the experiment, energetic 10.6 GeV (billion electron-volt) electrons from the CEBAF accelerator were sent into a small block of copper. The electrons were slowed down or deflected by the block, causing them to emit bremsstrahlung radiation as photons. This beam of photons then struck the protons inside a liquid hydrogen target. Detectors measured the remnants of these interactions as electrons and positrons.

The experimenters were interested in those interactions that produced J/ particles amongst the hydrogen’s proton nuclei. The J/ is a short-lived meson that is made of charm/anti-charm quarks. Once formed, it quickly decays into an electron/positron pair. Of the billions of interactions, the experimenters found about 2,000 J/ particles in their cross section measurements of these interactions by confirming the coincident electron/positron pairs.

“It’s similar to what we’ve been doing all along. By doing elastic scattering of the electron on the proton, we’ve been getting the proton’s charge distribution,” said Jones. “In this case, we did exclusive photo-production of the J/ from the proton, and we’re getting the gluon distribution instead of the charge distribution.”

The collaborators were then able to insert these cross section measurements into theoretical models that describe the gluonic gravitational form factors of the proton. The gluonic form factors detail the mechanical characteristics of the proton, such as its mass and pressure.

“There were two quantities, known as gravitational form factors, that we were able to pull out, because we had access to these two models: the generalized parton distributions model and the holographic quantum chromodynamics (QCD) model. And we compared the results from each of these models with lattice QCD calculations,” Meziani added.

From two different combinations of these quantities, the experimenters determined the aforementioned gluonic mass radius dominated by graviton-like gluons, as well as a larger radius of attractive scalar gluons that extend beyond the moving quarks and confine them.

“One of the more puzzling findings from our experiment is that in one of the theoretical model approaches, our data hint at a scalar gluon distribution that extends well beyond the electromagnetic proton radius,” Joosten said. “To fully understand these new observations and their implications on our understanding of confinement, we will need a new generation of high-precision J/ experiments.”

One possibility for further exploration of this tantalizing new result is the Solenoidal Large Intensity Device experiment program, called SoLID. The SoLID program is still in the proposal stage. If approved to move forward, experiments conducted with the SoLID apparatus would provide new insight into J/ physics.

“The big next step is to measure J/ production with the SoLID detector. It will really be able to make high-precision measurements in this region. One of the major pillars of that program is J/ production, along with transverse momentum distribution measurements and parity-violating deep inelastic scattering measurements,” Jones said.

Jones, Joosten and Meziani represent an experimental collaboration that includes more than 50 nuclear physicists from 10 institutions. The spokespeople also want to highlight Burcu Duran, the lead author and a postdoctoral research associate at the University of Tennessee, Knoxville.

Hybrid magnonics in hybrid perovskite antiferromagnets

by Andrew H. Comstock, Chung-Tao Chou, Zhiyu Wang, Tonghui Wang, Ruyi Song, Joseph Sklenar, Aram Amassian, Wei Zhang, Haipeng Lu, Luqiao Liu, Matthew C. Beard, Dali Sun in Nature Communications

An international group of researchers has created a mixed magnon state in an organic hybrid perovskite material by utilizing the Dzyaloshinskii-Moriya-Interaction (DMI). The resulting material has potential for processing and storing quantum computing information. The work also expands the number of potential materials that can be used to create hybrid magnonic systems.

In magnetic materials, quasi-particles called magnons direct the electron spin within the material. There are two types of magnons — optical and acoustic — which refer to the direction of their spin.

“Both optical and acoustic magnons propagate spin waves in antiferromagnets,” says Dali Sun, associate professor of physics and member of the Organic and Carbon Electronics Lab (ORaCEL) at North Carolina State University. “But in order to use spin waves to process quantum information, you need a mixed spin wave state.”

“Normally two magnon modes cannot generate a mixed spin state due to their different symmetries,” Sun says. “But by harnessing the DMI we discovered a hybrid perovskite with a mixed magnon state.” Sun is also a corresponding author of the research.

Magnetic properties of a hybrid perovskite antiferromagnet.

The researchers accomplished this by adding an organic cation to the material, which created a particular interaction called the DMI. In short, the DMI breaks the symmetry of the material, allowing the spins to mix.

The team utilized a copper based magnetic hybrid organic-inorganic perovskite, which has a unique octahedral structure. These octahedrons can tilt and deform in different ways. Adding an organic cation to the material breaks the symmetry, creating angles within the material that allow the different magnon modes to couple and the spins to mix.

“Beyond the quantum implications, this is the first time we’ve observed broken symmetry in a hybrid organic-inorganic perovskite,” says Andrew Comstock, NC State graduate research assistant and first author of the research.

“We found that the DMI allows magnon coupling in copper-based hybrid perovskite materials with the correct symmetry requirements,” Comstock says. “Adding different cations creates different effects. This work really opens up ways to create magnon coupling from a lot of different materials — and studying the dynamic effects of this material can teach us new physics as well.”

Chester Supersolid of Spatially Indirect Excitons in Double-Layer Semiconductor Heterostructures

by Sara Conti, Andrea Perali, Alexander R. Hamilton, Milorad V. Milošević, François M. Peeters, David Neilson in Physical Review Letters

A collaboration of Australian and European physicists predict that layered electronic 2D semiconductors can host a curious quantum phase of matter called the supersolid.

The supersolid is a very counterintuitive phase indeed. It is made up of particles that simultaneously form a rigid crystal and yet at the same time flow without friction since all the particles belong to the same single quantum state. A solid becomes ‘super’ when its quantum properties match the well-known quantum properties of superconductors. A supersolid simultaneously has two orders, solid and super:

• solid because of the spatially repeating pattern of particles,
• super because the particles can flow without resistance.

“Although a supersolid is rigid, it can flow like a liquid without resistance,” explains Lead author Dr Sara Conti (University of Antwerp).

Energies of the superfluid (sf), exciton supersolid (ss), and exciton normal solid (ns) phases, as labeled, for different layer separations d.

Geoffrey Chester, a Professor at Cornell University, predicted in 1970 that solid helium-4 under pressure should at low temperatures display:

• Crystalline solid order, with each helium atom at a specific point in a regularly ordered lattice and, at the same time,
• Bose-Einstein condensation of the atoms, with every atom in the same single quantum state, so they flow without resistance.

However in the following five decades the Chester supersolid has not been unambiguously detected. Alternative approaches to forming a supersolid-like state have reported supersolid-like phases in cold-atom systems in optical lattices. These are either clusters of condensates or condensates with varying density determined by the trapping geometries. These supersolid-like phases should be distinguished from the original Chester supersolid in which each single particle is localised in its place in the crystal lattice purely by the forces acting between the particles.

The new Australia-Europe study predicts that such a state could instead be engineered in two-dimensional (2D) electronic materials in a semiconductor structure, fabricated with two conducting layers separated by an insulating barrier of thickness d. One layer is doped with negatively-charged electrons and the other with positively-charged holes. The particles forming the supersolid are interlayer excitons, bound states of an electron and hole tied together by their strong electrical attraction. The insulating barrier prevents fast self-annihilation of the exciton bound pairs. Voltages applied to top and bottom metal ‘gates’ tune the average separation r0 between excitons.

The research team predicts that excitons in this structure will form a supersolid over a wide range of layer separations and average separations between the excitons. The electrical repulsion between the excitons can constrain them into a fixed crystalline lattice.

“A key novelty is that a supersolid phase with Bose-Einstein quantum coherence appears at layer separations much smaller than the separation predicted for the non-super exciton solid that is driven by the same electrical repulsion between excitons,” says co-corresponding author Prof David Neilson (University of Antwerp). “In this way, the supersolid pre-empts the non-super exciton solid. At still larger separations, the non-super exciton solid eventually wins, and the quantum coherence collapses.”

“This is an extremely robust state, readily achievable in experimental setups,” adds co-corresponding author Prof Alex Hamilton (UNSW). “Ironically, the layer separations are relatively large and are easier to fabricate than the extremely small layer separations in such systems that have been the focus of recent experiments aimed at maximising the interlayer exciton binding energies.”

As for detection, for a superfluid it is well known that this cannot be rotated until it can host a quantum vortex, analogous to a whirlpool. But to form this vortex requires a finite amount of energy, and hence a sufficiently strong rotational force. So up to this point, the measured rotational moment of inertia (the extent to which an object resists rotational acceleration) will remain zero. In the same way, a supersolid can be identified by detecting such an anomaly in its rotational moment of inertia. The research team has reported the complete phase diagram of this system at low temperatures.

“By changing the layer separation relative to the average exciton spacing, the strength of the exciton-exciton interactions can be tuned to stabilise either the superfluid, or the supersolid, or the normal solid,” says Dr Sara Conti.

“The existence of a triple point is also particularly intriguing. At this point, the boundaries of supersolid and normal-solid melting, and the supersolid to normal-solid transition, all cross. There should be exciting physics coming from the exotic interfaces separating these domains, for example, Josephson tunnelling between supersolid puddles embedded in a normal-background.”

Charge‐State‐Enhanced Ion Sputtering of Metallic Gold Nanoislands

by Gabriel L. Szabo, Benedykt R. Jany, Helmut Muckenhuber, Anna Niggas, Markus Lehner, Arkadiusz Janas, Paul S. Szabo, Ziyang Gan, Antony George, Andrey Turchanin, Franciszek Krok, Richard A. Wilhelm in Small

Normally, we have to make a choice in physics: Either we deal with big things — such as a metal plate and its material properties, or with tiny things — such as individual atoms. But there is also a world in between: The world of small but not yet tiny things, in which both effects of the macroscopic world and effects of the microscopic world play a role.

The experiments conducted at TU Wien are located in this complicated in-between world: Extremely small pieces of gold, consisting of a few thousand atoms and with a diameter in the order of ten nanometres, are bombarded with highly charged ions. This makes it possible to change the shape and size of these gold pieces in a targeted manner. The results show: What happens in the process cannot simply be pictured like the impact of a golf ball in a sand bunker — the interaction of ion and gold piece is much more subtle.

“We work with multiply-ionized xenon atoms. Up to 40 electrons are removed from these atoms, so they are highly electrically charged,” says Prof. Richard Wilhelm from the Institute of Applied Physics at TU Wien. These highly charged ions then hit small gold islands placed on an insulating substrate — and then different things can happen: The gold islands may become flatter, they can melt, they can even evaporate. “Depending on how highly our ions are electrically charged, we can trigger different effects,” says Gabriel Szabo, first author of the current study, who is currently working on his dissertation in Richard Wilhelm’s team.

This graph shows the height change of the gold nanoislands after irradiation with Xeq+ (q = 1, 18, 25, 32, 40), with the histograms surrounding the graph showing the height distributions for each charge state before (blue) and after (orange) irradiation. In the graph a clear charge-state dependence of the height loss is visible.

The highly charged ions hit the tiny gold nuggets at elevated speed — at around 500 kilometres per second. Nevertheless, it is remarkably not the force of the impact that changes the gold islands. The process is completely different from the impact of a golf ball in a pile of sand, or the accidental impact of a tennis ball in a nicely decorated birthday cake.

“If you shoot uncharged xenon atoms at the gold islands with the same kinetic energy, the gold islands remain practically unchanged,” says Gabriel Szabo. “So the decisive factor is not the kinetic energy, but the electrical charge of the ions. This charge also carries energy, and it is deposited exactly at the point of impact.”

As soon as the extremely strongly positively charged ions hit the nano gold piece, they snatch electrons away from the gold. In a large piece of gold, this would have no significant effect: Gold is an excellent conductor, the electrons can move freely, and more electrons would be supplied from other areas of the gold nugget. But the nano-gold structures are so small that they can no longer be regarded as an inexhaustible reservoir of electrons. It is precisely here that one enters the intermediate world between macroscopic metal and tiny atomic clusters and their nanoscale properties.

“The charge energy of the impacting ion is transferred to the gold, thus the electronic structure of the entire nano-gold object is thrown completely out of balance, the atoms start to move and the crystal structure of the gold is destroyed,” explains Richard Wilhelm. “Depending on how much energy you deposit, it may even happen that the entire nano-gold piece melts or is vaporised.”

The effects of the ion bombardment can then be studied in an atomic force microscope: Depending on the charge of the ions, the height of the gold pieces is reduced to a lesser or greater extent, Gabriel Szabo reports: “Just as our models had also predicted, we can control the impact of the ions on the gold — and not by the speed we give our projectiles, but rather by their charge.”

Improved control and deeper understanding of such processes is important for making a wide variety of nanostructures. “It’s a technique that allows you to selectively edit the geometry of particularly small structures. That’s just as interesting for the creation of microelectronic components as it is for so-called quantum dots — tiny structures that allow very specific tailor-made electronic or optical effects due to their quantum physical properties,” says Richard Wilhelm.

And it is another insight into the world of small but not yet tiny things — into the multifaceted intermediate world between quantum physics and solid-state physics, which can only be understood by keeping quantum and many-particle phenomena in mind at the same time.

Excitons in mesoscopically reconstructed moiré heterostructures

by Shen Zhao, Zhijie Li, Xin Huang, Anna Rupp, Jonas Göser, Ilia A. Vovk, Stanislav Yu. Kruchinin, Kenji Watanabe, Takashi Taniguchi, Ismail Bilgin, Anvar S. Baimuratov, Alexander Högele in Nature Nanotechnology

Stacked layers of ultrathin semiconductor materials feature phenomena that can be exploited for novel applications. A team led by LMU physicist Alexander Högele has studied effects that emerge by giving two layers a slight twist.

Novel, ultrathin nanomaterials exhibit remarkable properties. If you stack individual atomically thin layers of crystals in a vertical assembly, for example, fascinating physical effects can occur. For instance, bilayers of the wonder material graphene twisted by the magic angle of 1.1 degrees may exhibit superconductivity. And researchers are also focusing their attention on bilayer semiconducting heterostructures made of so-called transition metal dichalcogenides, which are held together weakly by van der Waals forces.

The research group led by Alexander Högele investigates such novel heterostructures, which do not occur in nature. “The combination of materials, the number of layers, and their relative orientation give rise to a wide variety of novel phenomena,” says the LMU physicist. “In the lab, we can tailor physical phenomena for various applications in electronics, photonics, or quantum technology with properties that are unknown in naturally occurring crystals.” Experimentally observed phenomena are not always easy to interpret, however, as a new paper.

Characteristics of MoSe2–WSe2 HBLs in H- and R-type stacking.

Högele’s team investigated a heterobilayer system held together by van der Waals forces and fabricated from semiconductor monolayers of molybdenum diselenide (MoSe2) and tungsten diselenide (WSe2). Depending on the orientation of the individual layers, moiré effects can emerge. These effects, which we are familiar with from everyday life, also arise in the nano-world when two different atomic lattices are stacked upon each other, or two identical lattices are twisted with respect to each other. The difference in the nano case is that it is not an optical effect. In the quantum mechanical world of atomically thin crystal heterostructures, moiré interference dramatically affects the properties of the composite system, also impacting electrons and strongly bound electron-hole pairs, or excitons, explains Högele.

“Our work shows that the naïve notion of a perfect moiré pattern in heterobilayer MoSe2-WSe2 does not necessarily hold true, particularly for small angles of rotation. Therefore, the interpretation of the phenomenology observed to date will have to be partially revised,” says Högele.

Instead of periodic moiré patterns, there are laterally extended areas that are free from moiré interferences. Moreover, there are zones with interesting quantum mechanical effects such as one-dimensional quantum wires or quasi zero-dimensional quantum dots that are potentially viable for applications in quantum communication based on spatially localized excitons with single-photon emission characteristics. In the latter case, ideal moiré patterns presumably transform into periodic patterns with triangular or hexagonal tiling.

The reason seems to lie in an elastic deformation of the lattice structure that depends on the orientation of the layers. The atoms are displaced out of their equilibrium positions, which comes at the expense of increased strain in individual layers but promotes better adhesion among the layers. The result is an energy landscape in the heterobilayer system that can be engineered and potentially exploited by means of rational design.

“We also observe collective phenomena in synthetic crystals, where periodic moiré patterns have a dramatic effect on the motion of electrons as well as their mutual interactions,” says Högele.

Of decisive importance is the understanding of excitons -electron-hole pairs — that are characteristic for the distinct types of atomic registries in bilayer crystal heterostructures and which could potentially be utilized in future opto-electronic applications. These excitons are generated in semiconducting transition metal dichalcogenides by means of light absorption, and convert back into light again. “Excitons thus act as mediators of light-matter interaction in semiconductor crystals,” says Högele. As the current paper shows, different types of excitons arise depending on the actual structure of the heterobilayer systems in parallel or antiparallel alignment. “We want to learn how to manufacture van der Waals heterostructures with customized properties in a deterministic approach to control the rich emergent phenomenology of correlated effects such as magnetism or superconductivity.”

Sub-picosecond steering of ultrafast incoherent emission from semiconductor metasurfaces

by Iyer, P.P., Karl, N., Addamane, S. et al. Nature Photonics

In a major breakthrough in the fields of nanophotonics and ultrafast optics, a Sandia National Laboratories research team has demonstrated the ability to dynamically steer light pulses from conventional, so-called incoherent light sources.

This ability to control light using a semiconductor device could allow low-power, relatively inexpensive sources like LEDs or flashlight bulbs to replace more powerful laser beams in new technologies such as holograms, remote sensing, self-driving cars and high-speed communication.

“What we’ve done is show that steering a beam of incoherent light can be done,” said Prasad Iyer, Sandia scientist and lead author of the research.

Incoherent light is emitted by many common sources, such as an old-fashioned incandescent light bulb or an LED bulb. This light is called incoherent since the photons are emitted with different wavelengths and in a random fashion. A beam of light from a laser, however, does not spread and diffuse because the photons have the same frequency and phase and is thus called coherent light.

In the team’s research, they manipulated incoherent light by using artificially structured materials called metasurfaces, made from tiny building blocks of semiconductors called meta-atoms that can be designed to reflect light very efficiently. Although metasurfaces had previously shown promise for creating devices that could steer light rays to arbitrary angles, they also presented a challenge because they had only been designed for coherent light sources. Ideally, one would want a semiconductor device that can emit light like an LED, steer the light emission to a set angle by applying a control voltage and shift the steering angle at the fastest speed possible.

The researchers started with a semiconductor metasurface that had embedded tiny light sources called quantum dots. By using a control optical pulse, they were able to change, or reconfigure, the way the surface reflected light and steer the light waves emitted from the quantum dots in different directions over a 70-degree range for less than a trillionth-of-a-second, marking a significant success. Similar to laser-based steering, the steered beam restrained the tendency of incoherent light to spread over a wider viewing angle and instead produced bright light at a distance.

A metasurface sample is used for the beam steering with each reflective patch containing thousands of meta-atoms designed to dynamically steer incoherent light. (Photo by Craig Fritz)

A feat previously considered impossible, the team’s proof-of-principle work paves the way for developments in the fields of nanophotonics and ultrafast optics. The ability to dynamically control incoherent light sources and manipulate their properties offers a wide range of applications.

One low-power use would be to brighten military helmet screens used to overlay maps or blueprints over ordinary vision.

“In applications where space is valuable,” Iyer said, “steering light emission with low-size-and-weight metasurface-LED displays could be made possible in the future with this technology. We can use the light emitted in a better way rather than just turning them off and on.”

The technique could also provide a new kind of small display that can project holographic images onto eyeballs using low-power LEDs, a capability of particular interest for augmented and virtual reality devices. Other uses could be in self-driving cars where LIDAR is used to sense objects in the path of the car.

In terms of expressions of interest, the team has had several inquiries from commercial sources, said Sandia researcher Igal Brener, a paper author and lead scientist on the project.

“A commercial product could be 5–10 years out, especially if we want to have all the functionality on-chip,” Brener said. “You wouldn’t use a control optical pulse to impart the changes in the metasurface needed to steer the light, but rather you would do this control electrically. We have ideas and plans, but it’s still early. Imagine an LED light bulb that can emit light to follow you. Then you wouldn’t waste all that illumination where there’s nobody. This is one of the many applications that we dreamed about with DOE years ago for energy efficiency for office lighting, for example.”

Similarly, tamed light may one day offer benefits in scenarios where focused illumination is only needed in a specific area of interest, such as surgery or in autonomous vehicles.

Strong Polarization of a J=1/2 to 1/2 Transition Arising from Unexpectedly Large Quantum Interference

by Nobuyuki Nakamura, Naoki Numadate, Simpei Oishi, Xiao-Min Tong, Xiang Gao, Daiji Kato, Hirokazu Odaka, Tadayuki Takahashi, Yutaka Tsuzuki, Yuusuke Uchida, Hirofumi Watanabe, Shin Watanabe, Hiroki Yoneda in Physical Review Letters

By adapting technology used for gamma-ray astronomy, a group of experimental researchers has found that X-ray transitions previously thought to have been unpolarized according to atomic physics, are in fact highly polarized, reports a new study.

When electrons recombine with highly charged ions, X-ray polarization becomes important for testing fundamental atomic physics involving relativistic and quantum electrodynamics effects. But to date, experimental researchers have been challenged by the technical difficulties these experiments require.

A team of researchers led by the University of Electro-Communications Insitute for Laser Science Professor Nobuyuki Nakamura, and including Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU) Professor Tadayuki Takahashi and graduate student Yutaka Tsuzuki, and Institute of Space and Astronautical Science (ISAS/JAXA) Associate Professor Shin Watanabe, successfully combined two state-of-the-art instruments and technologies to measure the polarization of high-energy X-rays emitted when highly charged ions capture high-energy electrons.

Two-dimensional plot of x-ray spectra from highly charged Pb ions as a function of the electron beam energy (bottom panel).

The first is the electron beam ion trap the Tokyo-EBIT, which is one of the world’s leading highly charged ion generators and experimental instruments owned by the University of Electro-Communications, and the second is the Si/CdTe Compton Camera for high-energy X-rays, which was developed for astronomical observations mainly at ISAS/JAXA and improved for this research.

The technology behind the Si/CdTe Compton Camera was originally developed by a team led by Takahashi to study X-rays and gamma rays in the universe released by highly energized black holes, supernovae and galaxy clusters, and was built into the Japan Aerospace Exploration Agency (JAXA) ASTRO-H satellite, launched in 2016.

Takahashi had been looking for a way to adapt the technology to other fields. After a meeting with Nakamura, Takahashi began to work on designing the X-ray polarization experiment and implementing the Si/CdTe Compton Camera into the method. Tsuzuki carried out a large part of the calibration and simulation of the Compton camera.

Giant orbital magnetic moments and paramagnetic shift in artificial relativistic atoms and molecules

by Zhehao Ge, Sergey Slizovskiy, Peter Polizogopoulos, Toyanath Joshi, Takashi Taniguchi, Kenji Watanabe, David Lederman, Vladimir I. Fal’ko, Jairo Velasco in Nature Nanotechnology

Trapped electrons traveling in circular loops at extreme speeds inside graphene quantum dots are highly sensitive to external magnetic fields and could be used as novel magnetic field sensors with unique capabilities, according to a new study.

Electrons in graphene (an atomically thin form of carbon) behave as if they were massless, like photons, which are massless particles of light. Although graphene electrons do not move at the speed of light, they exhibit the same energy-momentum relationship as photons and can be described as “ultra-relativistic.” When these electrons are confined in a quantum dot, they travel at high velocity in circular loops around the edge of the dot.

“These current loops create magnetic moments that are very sensitive to external magnetic fields,” explained Jairo Velasco Jr., associate professor of physics at UC Santa Cruz. “The sensitivity of these current loops stems from the fact that graphene electrons are ultra-relativistic and travel at high velocity.”

Velasco is a corresponding author of a paper on the new findings. His group at UC Santa Cruz used a scanning tunneling microscope (STM) to create the quantum dots in graphene and study their properties. His collaborators on the project include scientists at the University of Manchester, U.K., and the National Institute for Materials Science in Japan.

“This was highly collaborative work,” Velasco said. “We did the measurements in my lab at UCSC, and then we worked very closely with theoretical physicists at the University of Manchester to understand and interpret our data.”

Deviation between experimental potential well and parabolic potential well.

The unique optical and electrical properties of quantum dots — which are often made of semiconductor nanocrystals — are due to electrons being confined within a nanoscale structure such that their behavior is governed by quantum mechanics. Because the resulting electronic structure is like that of atoms, quantum dots are often called “artificial atoms.” Velasco’s approach creates quantum dots in different forms of graphene using an electrostatic “corral” to confine graphene’s speeding electrons.

“Part of what makes this interesting is the fundamental physics of this system and the opportunity to study atomic physics in the ultra-relativistic regime,” he said. “At the same time, there are interesting potential applications for this as a new type of quantum sensor that can detect magnetic fields at the nano scale with high spatial resolution.”

Additional applications are also possible, according to co-first author Zhehao Ge, a UCSC graduate student in physics.

“The findings in our work also indicate that graphene quantum dots can potentially host a giant persistent current (a perpetual electric current without the need of an external power source) in a small magnetic field,” Ge said. “Such current can potentially be used for quantum simulation and quantum computation.”

The study looked at quantum dots in both monolayer graphene and twisted bilayer graphene. The graphene rests on an insulating layer of hexagonal boron nitride, and a voltage applied with the STM tip creates charges in the boron nitride that serve to electrostatically confine electrons in the graphene.

Although Velasco’s lab uses STM to create and study graphene quantum dots, a simpler system using metal electrodes in a cross-bar array could be used in a magnetic sensor device. Because graphene is highly flexible, the sensor could be integrated with flexible substrates to enable magnetic field sensing of curved objects.

“You could have many quantum dots in an array, and this could be used to measure magnetic fields in living organisms, or to understand how the magnetic field is distributed in a material or a device,” Velasco said.

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