Synchronising ultrashort X-ray pulses

At the ATHOS beamline of SwissFEL, PSI researchers demonstrated a technique known as mode-locking, which allows fully coherent, ultrashort X-ray pulses to be produced. In the photo, several undulator modules are visible (blue); between each pair are magnetic chicanes used to delay the electrons.
Photo Credit: © Paul Scherrer Institute PSI/Markus Fischer

Scientists at the Paul Scherrer Institute PSI have, for the first time, demonstrated a technique that synchronises ultrashort X-ray pulses at the X-ray free-electron laser SwissFEL. This achievement opens new possibilities for observing ultrafast atomic and molecular processes with attosecond precision.

Scrutinising fast atomic and molecular processes in action requires bright and short X-ray pulses – a task in which free-electron lasers such as SwissFEL excel. However, within these X-ray pulses the light is internally disordered: its temporal structure is randomly distributed and varies from shot to shot. This limits the accuracy of certain experiments.

To tame this inherent randomness, a team of PSI researchers has succeeded in implementing a technique known as mode-locking to generate trains of pulses that are coherent in time. “We can now obtain fully ordered pulses in time and frequency in a very controlled manner,” says accelerator physicist Eduard Prat, who led the study, published in Physical Review Letters. Selected by the journal as Editor’s Suggestion, the study, funded by the EU/ERC project “HERO”, represents a significant step towards the generation of tailored attosecond X-ray pulses and a range of new experiments that are only possible with precisely timed, synchronized light pulses.

A Clear Signal Emerging from Quantum Noise

Surprising signals can arise from the coupling of light particles.
Image Credit: © Oliver Diekmann

Researchers at TU Wien and the Okinawa Institute of Science and Technology (OIST) have demonstrated an unexpected effect: in a quantum system that is highly disordered, coherent microwave radiation can suddenly emerge. 

Two candles emit twice as much light as one. And ten candles have ten times the intensity. This rule seems completely trivial—but in the quantum world it can be broken. When quantum particles are excited to a higher-energy state, they can emit light as they relax back to a lower-energy state. However, when many such quantum particles are coupled together, they can collectively generate a light pulse that is far stronger than the sum of individual contributions. The pulse intensity scales with the square of the number of particles—this phenomenon is known as superradiance. It is a form of collective emission in which all quantum particles in the system release energy almost instantaneously and, so to speak, “in lockstep.” 

TU Wien and the Okinawa Institute of Science and Technology (Japan) have now discovered a different, completely unexpected manifestation of this phenomenon. They observed superradiance in irregular diamonds and found that after the initial superradiant pulse, a series of additional pulses follows, emitting further radiation in a coherent and perfectly regular manner. This is about as surprising as if the uncoordinated chirping of many crickets were suddenly to merge into a single, synchronized bang. 

MicroBooNE finds no evidence for a sterile neutrino

Members of the MicroBooNE collaboration pose in front of Wilson Hall with a 3D-printed model of the MicroBooNE detector. The collaboration consists of 193 scientists from 40 institutions.
Photo Credit: Dan Svoboda, Fermilab

Scientists on the MicroBooNE experiment further ruled out the possibility of one sterile neutrino as an explanation for results from previous experiments. In the latest MicroBooNE result, the collaboration used one detector and two beams to study neutrino behavior, ruling out the single sterile neutrino model with 95% certainty.

Scientists are closing the door on one explanation for a neutrino mystery that has plagued them for decades.

An international collaboration of scientists working on the MicroBooNE experiment at the U.S. Department of Energy’s Fermi National Accelerator Laboratory announced that they have found no evidence for a fourth type of neutrino. The paper was published today in Nature.

Physics: In-Depth Description

Image Credit: Scientific Frontline / AI generated

Physics is the fundamental natural science dealing with the study of matter, energy, space, and time, and the interactions between them. Its primary goal is to understand how the universe behaves at every scale, from the subatomic particles that constitute matter to the vast structure of the cosmos.

Anything-goes “anyons” may be at the root of surprising quantum experiments

MIT physicists propose that under certain conditions, a magnetic material’s electrons could splinter into fractions of themselves to form quasiparticles known as “anyons.”

In the past year, two separate experiments in two different materials captured the same confounding scenario: the coexistence of superconductivity and magnetism. Scientists had assumed that these two quantum states are mutually exclusive; the presence of one should inherently destroy the other.

Now, theoretical physicists at MIT have an explanation for how this Jekyll-and-Hyde duality could emerge. In a paper appearing today in the Proceedings of the National Academy of Sciences, the team proposes that under certain conditions, a magnetic material’s electrons could splinter into fractions of themselves to form quasiparticles known as “anyons.” In certain fractions, the quasiparticles should flow together without friction, similar to how regular electrons can pair up to flow in conventional superconductors.

Surfing on the waves of the microcosm

A particle (red sphere) is guided from left to its destination (right) using a laser trap (double-cone) by means of a protocol developed in the study, which is described by the parameter λ. A known time-dependent external force field F (t) acts on this environment. The optimised protocol exploits this force field in a way that extracts the maximum amount of work. This can be applied to various external fields, to active particles and to micro-robot transport problems. 
Image Credit: HHU/Kristian S. Olsen

Conditions can get rough in the micro- and nanoworld. To ensure that e.g. nutrients can still be optimally transported within cells, the minuscule transporters involved need to respond to the fluctuating environment. Physicists at Heinrich Heine University Düsseldorf (HHU) and Tel Aviv University in Israel have used model calculations to examine how this can succeed. They have now published their results – which could also be relevant for future microscopic machines – in the scientific journal Nature Communications. 

When planning an ocean crossing, sailors seek a course, which makes optimum use of favorable wind and ocean currents, and maneuver to save time and energy. They also react to random fluctuations in wind and currents and take advantage of fair winds and waves. Such considerations regarding energy costs are also important for transport processes at the micro- and nanoscale. For example, molecular motors should use as little energy as possible when transporting nutrients from A to B between and within biological cells.  

Scientists create stable, switchable vortex knots inside liquid crystals

Vortex knots inside a chiral nematic liquid crystal
Image Credit: Ivan Smalyukh

The knots in your shoelaces are familiar, but can you imagine knots made from light, water, or from the structured fluids that make LCD screens shine? 

They exist, and in a new Nature Physics study, researchers created particle-like so-called “vortex knots” inside chiral nematic liquid crystals, a twisted fluid like those used in LCD screens. For the first time, these knots are stable and could be reversibly switched between different knotted forms, using electric pulses to fuse and split them. 

“These particle-like topological objects in liquid crystals share the same kind of topology found in theoretical models of glueballs, experimentally-elusive theoretical subatomic particles in high-energy physics, in hopfions and heliknotons studied in light, magnetic materials, and in vortex knots found across many other systems,” explains Ivan Smalyukh, director of the Hiroshima University WPI-SKCM² Satellite at the University of Colorado Boulder and a professor in CU Boulder’s Department of Physics. 

Rice researchers uncover the hidden physics of knot formation in fluids

From left to right, top to bottom: Sibani Lisa Biswal, Fred MacKintosh, Lucas H.P. Cunha and Luca Tubiana.
Photo Credit: Courtesy of Rice University

Knots are everywhere — from tangled headphones to DNA strands packed inside viruses — but how an isolated filament can knot itself without collisions or external agitation has remained a longstanding puzzle in soft-matter physics.

Now, a team of researchers at Rice University, Georgetown University and the University of Trento in Italy has uncovered a surprising physical mechanism that explains how a single filament, even one too short or too stiff to easily wrap around itself, can form a knot while sinking through a fluid under strong gravitational forces. The discovery, published in Physical Review Letters, provides new insight into the physics of polymer dynamics, with implications ranging from understanding how DNA behaves under confinement to designing next-generation soft materials and nanostructures.

“It is inherently difficult for a single, isolated filament to knot on its own,” said Sibani Lisa Biswal, corresponding author, chair of Rice’s Department of Chemical and Biomolecular Engineering and the William M. McCardell Professor in Chemical Engineering. “What’s remarkable about this study is that it shows a surprisingly simple and elegant mechanism that allows a filament to form a knot purely because of stochastic forces as it sediments through a fluid under strong gravitational forces.”

Icy Hot Plasmas: Fluffy, Electrically Charged Ice Grains Reveal New Plasma Dynamics

Ice grains, illuminated by a green sheet of laser light, are suspended in the plasma discharge (purple). Insets show individual ice grains imaged with 20x magnification.
Image Credit: Bellan Plasma Group/Caltech

When a gas is highly energized, its electrons get torn from the parent atoms, resulting in a plasma—the oft-forgotten fourth state of matter (along with solid, liquid, and gas). When we think of plasmas, we normally think of extremely hot phenomena such as the Sun, lightning, or maybe arc welding, but there are situations in which icy cold particles are associated with plasmas. Images of distant molecular clouds from the James Webb Space Telescope feature such hot–cold interactions, with frozen dust illuminated by pockets of shocked gas and newborn stars.

Now a team of Caltech researchers has managed to recreate such an icy plasma system in the lab. They created a plasma in which electrons and positively charged ions exist between ultracold electrodes within a mostly neutral gas environment, injected water vapor, and then watched as tiny ice grains spontaneously formed. They studied the behavior of the grains using a camera with a long-distance microscope lens. The team was surprised to find that extremely "fluffy" grains developed under these conditions and grew into fractal shapes—branching, irregular structures that are self-similar at various scales. And that structure leads to some unexpected physics.

A new approach links quantum physics and gravitation

Quantum-Geodesics 
Large masses – such as a galaxy – curve space-time. Objects move along a geodesic. If we take into account that space-time itself has quantum properties, deviations arise (dashed line vs. solid line).
Image Credit: © TU Wien  

A team at TU Wien combines quantum physics and general relativity theory – and discovers striking deviations from previous results. 

It is something like the “Holy Grail” of physics: unifying particle physics and gravitation. The world of tiny particles is described extremely well by quantum theory, while the world of gravitation is captured by Einstein’s general theory of relativity. But combining the two has not yet worked – the two leading theories of theoretical physics still do not quite fit together. 

There are many ideas for such a unification – with names like string theory, loop quantum gravity, canonical quantum gravity or asymptotically safe gravity. Each of them has its strengths and weaknesses. What has been missing so far, however, are observable predictions for measurable quantities and experimental data that could reveal which of these theories describes nature best. A new study from TU Wien may now have brought us a small step closer to this ambitious goal. 

Untangling magnetism

Spin-wave spectrum of CoFe₂O₄ measured on the MAPS spectrometer (left) and the corresponding spin-wave calculation (right). The large ~60 meV splitting between the two magnon branches originates from the strong imbalance of molecular fields on the A and B cation sites, as illustrated in the inset crystal structure.
Image Credit: KyotoU / Yusuke Nambu

Magnetostriction and spin dynamics are fundamental properties of magnetic materials.  Despite having been studied for decades, finding a decisive link between the two in bulk single crystals had remained elusive. That is until a research team from several institutions, including Kyoto University, sought to examine these properties in the compound CoFe2O4, a spinel oxide (chemical formula AB2O4) widely used in numerous medical and industrial applications.

Spin dynamics describe how the tiny magnetic moments of atoms in a magnetic material interact and change orientation with time, while magnetostriction describes how a material changes shape or dimensions in response to a change in magnetization. These properties are central to the operation of sensors and actuators that employ magnetoelastic materials that change their magnetization under mechanical stress.

When Quantum Gases Refuse to Follow the Rules

The team  Frederik Møller, Philipp Schüttelkopf and Jörg Schmiedmayer
Photo Credit: © Technische Universität Wien

At TU Wien, researchers have created a one-dimensional “quantum wire” made from a gas of ultracold atoms, where mass and energy flow without friction or loss. 

In physical systems, transport takes many forms, such as electric current through a wire, heat through metal, or even water through a pipe. Each of these flows can be described by how easily the underlying quantity—charge, energy, or mass—moves through a material. Normally, collisions and friction lead to resistance causing these flows to slow down or fade away. But in a new experiment at TU Wien, scientists have observed a system where that doesn’t happen at all. 

By confining thousands of rubidium atoms to move along a single line using magnetic and optical fields, they created an ultracold quantum gas in which energy and mass move with perfect efficiency. The results, now published in the journal Science, show that even after countless collisions, the flow remains stable and undiminished, thus revealing a kind of transport that defies the rules of ordinary matter. 

Consciousness as the foundation – new theory of the nature of reality

Maria Strømme, Professor of Materials Science.
Inset Photo Credit: Courtesy of Uppsala University

Consciousness is fundamental; only thereafter do time, space and matter arise. This is the starting point for a new theoretical model of the nature of reality, presented by Maria Strømme, Professor of Materials Science at Uppsala University, in the scientific journal AIP Advances. The article has been selected as the best paper of the issue and featured on the cover. 

Strømme, who normally conducts research in nanotechnology, here takes a major leap from the smallest scales to the very largest – and proposes an entirely new theory of the origin of the universe. The article presents a framework in which consciousness is not viewed as a byproduct of brain activity, but as a fundamental field underlying everything we experience – matter, space, time, and life itself. 

Particle accelerator waste could help produce cancer-fighting materials

Photo Credit: Courtesy of University of York

Energy that would normally go to waste inside powerful particle accelerators could be used to create valuable medical isotopes, scientists have found. 

The next step is to explore how the method could be scaled up to deliver clinically use 

Researchers at the University of York have shown that intense radiation captured in particle accelerator “beam dumps” could be repurposed to produce materials used in cancer therapy.  

Scientists have now found a way to make those leftover photons do a second job, without affecting the main physics experiments. 

A beam of photons designed to investigate things like the matter that makes up our universe, could at the same time, be used to create useful medical isotopes in the diagnosis and treatment of cancer. 

Light causes atomic layers to do the twist

Fang Liu, assistant professor of chemistry in Stanford’s School of Humanities and Sciences
Photo Credit: Fawn Hallenbeck/Stanford University

A study led by Stanford and Cornell researchers shows how light could be used to control the behavior of moiré materials, atomically thin layers that gain unusual properties when stacked and offset. The research has implications for developing superconductivity, magnetism, and quantum electronics.

A pulse of light sets the tempo in the material. Atoms in a crystalline sheet just a few atoms thick begin to move—not randomly, but in a coordinated rhythm, twisting and untwisting in sync like dancers following a beat.

Until now, researchers hadn’t been able to directly observe how those layers physically respond to a burst of light. In a recent study, a team led by Stanford and Cornell University researchers showed that the atomic layers can briefly twist more tightly together, then spring back, like a coiled ribbon releasing its energy.

Rare Particle Pairs Point to Primordial Soup's Temperature at Different Stages

The STAR detector, which is as large as a house, specializes in tracking the thousands of particles produced by each ion collision at the Relativistic Heavy Ion Collider.
Photo Credit: Kevin Coughlin/Brookhaven National Laboratory

At the Relativistic Heavy Ion Collider (RHIC), a U.S. Department of Energy (DOE) Office of Science user facility for nuclear physics research at DOE’s Brookhaven National Laboratory, scientists recreate the ultra-hot conditions of the early universe by smashing particles together at nearly the speed of light. RHIC's collisions delve into mysteries about the properties of matter by melting the colliding particles into a quark-gluon plasma (QGP) — a soup of fundamental particles that are the building blocks of protons and neutrons.

A new analysis of data captured by the STAR detector at RHIC revealed the QGP’s temperature at different stages of its evolution following collisions of gold ions — the nuclei of gold atoms stripped of their electrons. These measurements are key to mapping out how nuclear matter changes as quarks and gluons in the hot soup cool and coalesce to form more ordinary nuclear particles. Studying this phase transition at RHIC is helping physicists understand what happened in the briefest moments at the beginning of the universe, the last time the QGP existed in nature.

Physicists observe key evidence of unconventional superconductivity in magic-angle graphene

MIT researchers observed clear signatures of unconventional superconductivity in magic-angle twisted trilayer graphene (MATTG). The image illustrates pairs of superconducting electrons (yellow spheres) traveling through MATTG, as the team’s new method (represented by magnifying glass) probes the material’s unconventional superconducting gap (represented by the V-shaped beam).
Image Credit: Sampson Wilcox and Emily Theobald, MIT RLE

Superconductors are like the express trains in a metro system. Any electricity that “boards” a superconducting material can zip through it without stopping and losing energy along the way. As such, superconductors are extremely energy efficient, and are used today to power a variety of applications, from MRI machines to particle accelerators.

But these “conventional” superconductors are somewhat limited in terms of uses because they must be brought down to ultra-low temperatures using elaborate cooling systems to keep them in their superconducting state. If superconductors could work at higher, room-like temperatures, they would enable a new world of technologies, from zero-energy-loss power cables and electricity grids to practical quantum computing systems. And so scientists at MIT and elsewhere are studying “unconventional” superconductors — materials that exhibit superconductivity in ways that are different from, and potentially more promising than, today’s superconductors.

In a promising breakthrough, MIT physicists have today reported their observation of new key evidence of unconventional superconductivity in “magic-angle” twisted tri-layer graphene (MATTG) — a material that is made by stacking three atomically-thin sheets of graphene at a specific angle, or twist, that then allows exotic properties to emerge.

Physicists discover new state of matter in electrons, platform to study quantum phenomena

From left, researchers Cyprian Lewandowski, Aman Kumar and Hitesh Changlani.
Photo Credit: Devin Bittner/FSU College of Arts and Sciences

Electricity powers our lives, including our cars, phones, computers and more, through the movement of electrons within a circuit. While we can’t see these electrons, electric currents moving through a conductor flow like water through a pipe to produce electricity.

Certain materials, however, allow that electron flow to “freeze” into crystallized shapes, triggering a transition in the state of matter that the electrons collectively form. This turns the material from a conductor to an insulator, stopping the flow of electrons and providing a unique window into their complex behavior. This phenomenon makes possible new technologies in quantum computing, advanced superconductivity for energy and medical imaging, lighting, and highly precise atomic clocks. 

A team of Florida State University-based physicists, including National High Magnetic Field Laboratory Dirac Postdoctoral Fellow Aman Kumar, Associate Professor Hitesh Changlani and Assistant Professor Cyprian Lewandowski, have shown the conditions necessary to stabilize a phase of matter in which electrons exist in a solid crystalline lattice but can “melt” into a liquid state, known as a generalized Wigner crystal. Their work was published in npj Quantum Materials, a Nature publication. 

The Saltwater Formula

Mannum Waterfalls in South Australia
Photo Credit: © denisbin Creative Commons 2.0  

A solution to a tricky groundwater riddle from Australia: Researchers at TU Wien have developed numerical models to simulate the movement of fluids in porous materials.

Things are complicated along the Murray–Darling River in southern Australia. Agricultural irrigation washes salt out of the upper soil layers, and this salt eventually ends up in the river. To prevent the river’s salt concentration from rising too much, part of the salty water is diverted into special basins. Some of these basins are designed to let the salty water evaporate, others to slowly release it in a controlled manner in the underground. That keeps salt temporarily out of the river and allows a better management of the river’s water—but increases the salinity in the ground. How can we calculate how this saltwater spreads underground and what its long-term effects will be?

Such questions are extremely difficult to answer, as several physical effects interact in complex ways. At TU Wien, researchers have now developed an efficient computer model that can run on supercomputers to calculate the spreading of fluids in porous materials—allowing the movement of saltwater in the soils, like in the case of the Murray–Darling River, to be predicted much more accurately. The same approach can also be applied to other problems, such as the dispersion of pollutants in groundwater.

Birch leaves and peanuts turned into advanced laser technology

Upper: The biomaterial-based random laser when activated. Lower: The same laser seen in daylight.
 Photo Credit: Zhihao Huang

Physicists at Umeå University, in collaboration with researchers in China, have developed a laser made entirely from biomaterials – birch leaves and peanut kernels. The environmentally friendly laser could become an inexpensive and accessible tool for medical diagnostics and imaging.

The results have been published in the scientific journal Nanophotonics and show how a so-called random laser can be made entirely from biological materials.

“Our study shows that it is possible to create advanced optical technology in a simple way using only local, renewable materials,” says Jia Wang, Associate Professor at the Department of Physics, Umeå University, and one of the authors of the study.

A random laser is a type of laser in which light scatters many times inside a disordered material before emerging as a focused beam. It holds great promise for applications such as medical imaging and early disease detection, and has therefore attracted significant research attention. However, conventional random laser materials are often toxic or expensive and complex to produce.