Category: Physics

Gigahertz femtosecond lasers are suited to enhance and regulate laser machining quality to engineer the physicochemical properties of materials. Materials scientists seek to understand the laser-material interactions by gigahertz femtosecond lasers, although the method is complex due to the associated ablation dynamics.

In a new report now published in Science Advances, Minok Park and a team of scientists in laser technologies and mechanical engineering at the University of California, Berkeley, studied the ablation dynamics of copper using gigahertz femtosecond bursts via time-resolved scattering imaging, emission imaging and emission spectroscopy.

Researchers have combined several methods to reveal the process of Gigahertz femtosecond bursts, which rapidly removed molten copper from an irradiated spot, for material ejection. The process of material ejection stopped after burst irradiation due to the limited amounts of remnant matter to provide insights into the mechanisms of complex ablation triggered via Gigahertz femtosecond bursts that are employed to select optimal laser conditions in cross-cutting processes, nano-/micro-fabrication and spectroscopy.

Gigahertz and femtosecond laser ablation

Laser ablation is a process of removing material from surfaces via the interaction of high-power lasers with significant impact across energy harvesting and storage, biomedicine, optoelectronics and spectroscopy. Materials scientists have achieved significant capacities to offer a direct, one-step, chemical-free pathway for material machining and ablation sampling by using ultrafast, femtosecond laser ablation. The process is suited to precisely regulate the ablation features.

In this study, Park and colleagues developed a variety of methods to examine real-time laser ablation dynamics. They studied the ablation of copper with a gigahertz femtosecond laser pulse and compared the outcomes to femtosecond pulse ablation. The combined methods resulted in the fast removal of molten liquid material, while halting material removal after burst irradiation. The researchers obtained direct insights into the dynamics and dominant mechanism of gigahertz ablation with femtosecond pulses.

The ultrafast laser experiments

During the experiments, the team used an optical system to investigate the ablation mechanisms of copper with a single femtosecond laser pulse and gigahertz femtosecond bursts under atmospheric pressure. Using time-resolved scattering and emission images, the researchers visualized light emitting and non-emitting species. They characterized the crater morphology with white light interferometry and scanning electron microscopy to ablate a pristine copper surface to a depth of 500 nm. The scientists noted the appearance of irregular, resolidified structures on the irradiated spot. The ablation efficiency of the gigahertz bursts improved by manifolds compared to single-pulse irradiation.

• GHz fs burst ablation with 200 pulses. Time-resolved (A) emission imaging, (B) optical emission spectroscopy, and (C) scattering imaging to investigate ablation dynamics by GHz fs laser with 200 pulses at a fluence of 18.7 J/cm2 (0.09 J/cm2 per pulse, 155 ns total irradiation time), across different time scales. Scattering images were captured for 100 ns, 200 ns, 500 ns, and 1 μs, respectively. The blue lines represent the target Cu surface. White scale bars, 50 μm; blue scale bars, 10 μm. Credit: Science Advances (2023). DOI: 10.1126/sciadv.adf6397
• Single-pulse fs laser irradiation. Time-resolved (A) emission imaging, (B) optical emission spectroscopy, and (C) scattering imaging showing the ablation dynamics at a fluence of 18.7 J/cm2, across different time scales. a.u., arbitrary units. Scattering images were acquired for varying ICCD gate widths of 100 ns, 200 ns, 500 ns, and 1 μs, respectively. The blue lines in these images represent the Cu target surface, and images below the lines are mirror reflections from the polished Cu surface. White scale bars, 50 μm; blue scale bars, 10 μm. Credit: Science Advances (2023). DOI: 10.1126/sciadv.adf6397
• GHz fs burst ablation with 50 pulses. Time-resolved (A) emission imaging, (B) optical emission spectroscopy, and (C) scattering imaging showing ablation dynamics and mechanisms at a fluence of 18.7 J/cm2 (0.37 J/cm2 per pulse, 38-ns dwell time). Scattering images were acquired for 100 ns, 200 ns, 500 ns, and 1 μs, respectively. The blue lines show the target Cu surface. White scale bars, 50 μm. Credit: Science Advances (2023). DOI: 10.1126/sciadv.adf6397
• GHz fs burst ablation with 200 pulses. Time-resolved (A) emission imaging, (B) optical emission spectroscopy, and (C) scattering imaging to investigate ablation dynamics by GHz fs laser with 200 pulses at a fluence of 18.7 J/cm2 (0.09 J/cm2 per pulse, 155 ns total irradiation time), across different time scales. Scattering images were captured for 100 ns, 200 ns, 500 ns, and 1 μs, respectively. The blue lines represent the target Cu surface. White scale bars, 50 μm; blue scale bars, 10 μm. Credit: Science Advances (2023). DOI: 10.1126/sciadv.adf6397
• Single-pulse fs laser irradiation. Time-resolved (A) emission imaging, (B) optical emission spectroscopy, and (C) scattering imaging showing the ablation dynamics at a fluence of 18.7 J/cm2, across different time scales. a.u., arbitrary units. Scattering images were acquired for varying ICCD gate widths of 100 ns, 200 ns, 500 ns, and 1 μs, respectively. The blue lines in these images represent the Cu target surface, and images below the lines are mirror reflections from the polished Cu surface. White scale bars, 50 μm; blue scale bars, 10 μm. Credit: Science Advances (2023). DOI: 10.1126/sciadv.adf6397

Visualizing the outcome

The research team observed time-resolved images, emission spectra and scattering images to investigate the ablation dynamics of a single-pulse femtosecond laser on a copper surface. The images revealed the ejection of two different particle types from the substrate including those released after different timescales: (1) after a 0–200 nanoseconds delay, and (2) those ejected between 300 nanoseconds to 4 microseconds.

The researchers explored time-resolved emission imaging and spectroscopy alongside images of ablated plumes induced via gigahertz bursts composed of 50 pulses. They noted spherically shaped copper plasmas for a period of 30 nanoseconds during the experiments.

Laser-ablation dynamics

After a time period of 200 nanoseconds, the team did not observe ejecta at the center of the laser-matter interaction zone; indicating that the target was not ablated further. This behavior distinctly differed from the dynamics of single-pulse ablation.

The team devised two contributing mechanisms to the underlying process of material ejection, including (1) the vaporization of materials at the center, and (2) the ejection of liquid form the molten pool edge via fast, radially outward fluid motion, to recoil pressure exerted by vaporization. While the copper nanoparticles were expelled from the edge of the molten pool, a limited amount of liquid remained freezing on the crater surface, which they verified using scanning electron microscopy.

Comparative laser-ablation dynamics

The scientists used time-resolved emission imaging, emission spectroscopy and scattering images of ablation, driven by gigahertz femtosecond laser bursts. When they released the scattering images at a timescale later than 300s, the ejecta showed how the irradiation spot cooled down to inhibit materials removal.

The researchers compared the two experimental conditions and further studied the early ablation dynamics of copper driven by gigahertz bursts to note distinctly different ablation dynamics of a gigahertz burst driven with 200 pulses, compared to the gigahertz burst with 50 pulses. The outcomes provided direct confirmation of the different mechanisms of gigahertz-directed laser-induced ablation when compared to single-pulse irradiation.

Outlook

In this way, Minok Park and colleagues observed the ablation dynamics of copper by using single femtosecond laser pulses and gigahertz bursts with 50–200 pulses via multimodal probing methods. The single-pulse femtosecond laser irradiation produced two types of particles with different ejection speeds at different timescales.

The outcomes provide insights to comprehensively understand ablation mechanisms underlying gigahertz femtosecond bursts that are critical to explore a variety of applications across laser processing, machining, printing and spectroscopic diagnostics.

More information: Minok Park et al, Mechanisms of ultrafast GHz burst fs laser ablation, Science Advances (2023). DOI: 10.1126/sciadv.adf6397

Jan Kleinert et al, Ultrafast laser ablation of copper with ~GHz bursts, Laser Applications in Microelectronic and Optoelectronic Manufacturing (LAMOM) XXIII (2018). DOI: 10.1117/12.2294041

Journal information: Science Advances 

© 2023 Science X Network

Having more tools helps; having the right tools is better. Utilizing multiple dimensions may simplify difficult problems—not only in science fiction but also in physics—and tie together conflicting theories.

For example, Einstein’s theory of general relativity—which resides in the fabric of space-time warped by planetary or other massive objects—explains how gravity works in most cases. However, the theory breaks down under extreme conditions such as those existing in black holes and cosmic primordial soups.

An approach known as superstring theory could use another dimension to help bridge Einstein’s theory with quantum mechanics, solving many of these problems. But the necessary evidence to support this proposal has been lacking.

Now, a team of researchers led by Kyoto University is exploring ‘de Sitter space’ to invoke a higher dimension to explain gravity in the expanding early universe. They have developed a concrete method to compute correlation functions among fluctuations on expanding universe by making use of holography.

“We came to realize that our method can be applied more generically than we expected while dealing with quantum gravity,” says Yasuaki Hikida, from the Yukawa Institute for Theoretical Physics.

Dutch astronomer Willem de Sitter’s theoretical models describe space in a way that fits with Einstein’s general theory of relativity, in that the positive cosmological constant accounts for the expansion of the universe.

Starting with existing methods for handling gravity in anti-de Sitter space, Hikida’s team reshaped them to work in expanding de Sitter space to more precisely account for what is already known about the universe.

“We are now extending our analysis to investigate cosmological entropy and quantum gravity effects,” adds Hikida.

Although the team’s calculations only considered a three-dimensional universe as a test case, the analysis may easily be extended to a four-dimensional universe, allowing for the extraction of information from our real world.

“Our approach possibly contributes to validating superstring theory and allows for practical calculations about the subtle changes that rippled across the fabric of our early universe.”

The study is published in the journal Physical Review Letters.

More information: Heng-Yu Chen et al, Three-Dimensional de Sitter Holography and Bulk Correlators at Late Time, Physical Review Letters (2022). DOI: 10.1103/PhysRevLett.129.061601

Journal information: Physical Review Letters 

Provided by Kyoto University 

Researchers at University of Tokyo, JTS PRESTO, Ludwig Maximilians Universität and Kindai University recently demonstrated the modulation of an electron source by applying laser light to a single fullerene molecule. Their study, featured in Physical Review Letters, could pave the way for the development of better performing computers and microscopic imaging devices.

“By irradiating a sharp metallic needle with femtosecond pulses, we had previously demonstrated optical control of electron emission sites on a scale of approximately 10 nm,” Hirofumi Yanagisawa, one of the researchers who carried out the study, told Phys.org. “The optical control was achieved using plasmonic effects, but it was technically difficult to miniaturize such an electron source using the same principle. We were seeking a way to miniaturize the electron source and we hit upon the idea of using a single molecule and its molecular orbitals.”

Yanagisawa and his colleagues set out to realize their idea experimentally using electrons emitted from molecules on a sharp metallic needle. However, they were well-aware of the difficulties they would encounter, due to unresolved difficulties associated with the use of electron emissions from molecule-covered needles.

“For one, it was not clear whether electron emissions originate from single molecules or not, and beyond that, the interpretation of the emission patterns was not clear,” Yanagisawa explained. “Although there were mysteries that we had to clarify, we thought that light-induced electron emissions from molecule-covered needles would be a new phenomenon anyway, if we could observe this, and that the phenomena would give us answers to those intractable questions.”

One year after they started conducting their experiments, the researchers successfully observed light-induced changes in electron emission patterns. Understanding the physics underpinning this observed phenomenon, required an additional four years of research.

To miniaturize a site-selective electron source via the so-called plasmonic effect, researchers first need to change the shape of an electron emitter at an atomic scale, which is a highly technical and challenging task. Instead of changing the shape of the emitter, therefore, Yanagisawa and his colleagues tried to change the electronic structure (i.e., molecular orbital) of electrons passing through their single-molecule emitter.

“In this case, the electronic structure in a single molecule defines a sort of aperture for incoming electron waves, where the shape of the outgoing electron waves will become the shape of the aperture,” Yanagisawa said. “For example, if the aperture is a ring shape, the outgoing electron waves also become a ring shape. The important thing is that the aperture’s shape varies with the energy of the incoming electrons in quantum mechanics.”

Essentially, the researchers were able to change the shape of the aperture on their emitter by exciting electrons with laser pulses and changing their energies. This in turn changed the shape of the emitted electron waves.

“We observed subnanometric modulation in electron emission sites by light,” Yanagisawa said. “Optically selecting emission sites can lead to an integration of ultrafast switches that can be from three to six orders of magnitude faster than switches in a computer.”

The technique introduced by the researchers could theoretically enable the integration of ultrafast switches into a single fullerene molecule. Yanagisawa and his colleagues also propose an integration scheme that would allow the integration of as many switches as desired without the need to increase the size of devices, which would generally be necessary.

In their next studies, they would like to further improve their ability to control electron emission using their technique, as this could facilitate the future integration of ultrafast switches into single molecules. In addition, they plan to explore the possibility of applying their method to electron microscopy technology.

In addition to informing the creation of vacuum nanoelectronics, in fact, their proposed method could be applied in the field of electron microscopy. The irradiation of solids using light can excite electrons within them, and some of these electrons can then be emitted into a vacuum; a process known as photoelectron emission.

“A photoelectron emission microscope (PEEM) can be used to observe femtosecond to attosecond electron dynamics on a nanoscale area,” Yanagisawa said. “Ultrafast electron dynamics play an important role, even at a single-molecule scale. However, the spatial resolution of a PEEM is around sub-10 nm or so, and hence, it was not possible to resolve electron dynamics in a single molecule.”

The modulation of light-induced electron emissions from a single molecule demonstrated by this team of researchers can be combined with PEEM technology. Yanagisawa and his colleagues showed that a PEEM based on their approach attains a spatial resolution of approximately 0.3 nm, while also resolving single-molecule molecular orbitals.

“In the future, we will be using our microscope to investigate ultrafast electron dynamics in a single molecule,” Yanagisawa added. “Because our PEEM uses low-energy electrons, we expect less damage to biomolecules so that we can observe a specific biomolecule without destroying it. Femtosecond electron dynamics play a crucial role even in photosynthesis, thus we would soon like to investigate the photosynthetic process at a single-molecule scale using our PEEM.”

More information: Hirofumi Yanagisawa et al, Light-Induced Subnanometric Modulation of a Single-Molecule Electron Source, Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.130.106204

Hirofumi Yanagisawa et al, Field emission microscope for a single fullerene molecule, Scientific Reports (2022). DOI: 10.1038/s41598-022-06670-1

Journal information: Physical Review Letters  , Scientific Reports 

© 2023 Science X Network

Researchers at the Université de Lorraine in France and Tohoku University in Japan have demonstrated a sub-picosecond magnetization reversal in rare-earth-free archetypical spin valves. Their discovery was published in the journal Nature Materials on March 9, 2023.

Manipulating magnetic materials without using magnetic fields is of paramount importance for many applications, such as non-volatile memory. Two decades ago, it was discovered that a magnetization reversal could be induced by a charge current. A decade later, a much faster, sub-picosecond control of the magnetization was achieved by shining femtosecond light pulses. This process became known as all-optical magnetization switching. However, only a few specific rare-earth-based material systems containing antiparallel alignment in magnetic sub-lattices experience such ultrafast phenomena.

In their work, the research group demonstrated sub-picosecond optical control of magnetization in rare-earth-free archetypical spintronic structures, consisting of [Pt/Co]/Cu/[Co/Pt], at ultrafast timescales.

Furthermore, they observed magnetization reversal from parallel alignment, which was previously unseen and unexpected in ultrafast magnetism. Like the discovery of magnetization reversal by a charge current two decades ago, this breakthrough has the potential to drastically extend the bandwidth of common spintronic devices. This can be done by exploiting common spintronics phenomena in a strongly out-of-equilibrium context.

“Our findings provide a route for ultrafast magnetization control by bridging concepts from spintronics and ultrafast magnetism,” says Dr. Junta Igarashi of the Université de Lorraine (JSPS Overseas Research Fellowships, an alumnus of Tohoku University) and first author of the paper.

Professor Stéphane Mangin of the Université de Lorraine, also serving as a visiting professor at the Center for Science and Innovation in Spintronics (CSIS), Tohoku University, added, “Our findings are a milestone in the development of ultra-fast spintronics and could open the way to new applications for ultra-fast and energy efficient memories.”

More information: Junta Igarashi et al, Optically induced ultrafast magnetization switching in ferromagnetic spin valves, Nature Materials (2023). DOI: 10.1038/s41563-023-01499-z

Journal information: Nature Materials 

Provided by Tohoku University 

In work published in Science, the team led by Prof. Dr. Ulf Peschel reports on measurements on a sequence of pulses that travel thousands of kilometers through glass fibers that are only a few microns thin. The researchers were surprised by the results.

“We have found that the light pulses organize themselves after about a hundred kilometers and then behave more like molecules of a conventional gas, such as air, for example,” reports Prof. Ulf Peschel, the head of the group in Jena.

In a gas the particles move back and forth at different speeds, but still they have a mean velocity defined by their temperature. Although light pulses propagate through the glass fiber at an average speed of about 200,000 kilometers per second , they are not all equally fast. “The statistical distribution of their velocities equals exactly that of a conventional gas with a fixed temperature,” says Peschel.

As the researchers have now demonstrated for the first time in their recent publication, this photon gas can be cooled, for example, by a process known as adiabatic expansion. As in a real gas, the velocity differences of the particles decrease during cooling and the order in the signal sequence automatically increases. When the absolute temperature zero of 0 Kelvin is reached, all pulses propagate at exactly the same velocity.

The reverse process is also possible. “When the optical gas is heated, velocity differences increase,” explains Peschel. If all pulse velocities occur equally often, the disorder is at a maximum and the temperature is infinite—a state which cannot be reached in a real gas as it would require an infinite amount of energy.

“In contrast, a periodic modulation of the refractive index can limit the range of allowed pulse velocities in the glass fiber. In this way, all available velocity states can be equally excited, creating a photon gas of infinite temperature. If even more energy is added, states of extreme velocities are preferentially populated—the photon gas becomes hotter than infinitely hot.”

“For this state, which has so far only been described theoretically for light, a temperature below absolute zero is mathematically assumed,” says Peschel. He and his colleagues have now been able to create such a photon gas with negative temperature and show for the first time that it obeys conventional laws of thermodynamics.

“Our results will contribute to a better understanding of the collective behavior of large ensembles of optical signals. If we take the laws of thermodynamics into account, we can make optical data transmission more robust and reliable, for example by structuring pulse distributions to better match thermal distributions.”

More information: A. L. Marques Muniz et al, Observation of photon-photon thermodynamic processes under negative optical temperature conditions, Science (2023). DOI: 10.1126/science.ade6523. www.science.org/doi/10.1126/science.ade6523

Journal information: Science 

Provided by Friedrich Schiller University of Jena

Physicists are learning more about the bizarre behavior of “strange metals,” which operate outside the normal rules of electricity.

Theoretical physicist Yashar Komijani, an assistant professor at the University of Cincinnati, contributed to an international experiment using a strange metal made from an alloy of ytterbium, a rare earth metal. Physicists in a lab in Hyogo, Japan, fired radioactive gamma rays at the strange metal to observe its unusual electrical behavior.

Led by Hisao Kobayashi with the University of Hyogo and RIKEN, the study was published in the journal Science. The experiment revealed unusual fluctuations in the strange metal’s electrical charge.

“The idea is that in a metal, you have a sea of electrons moving in the background on a lattice of ions,” Komijani said. “But a marvelous thing happens with quantum mechanics. You can forget about the complications of the lattice of ions. Instead, they behave as if they are in a vacuum.”

Komijani for years has been exploring the mysteries of strange metals in relation to quantum mechanics.

“You can put something in a black box and I can tell you a lot about what’s inside it without even looking at it just by measuring things like resistivity, heat capacity and conductivity,” he said.

“But when it comes to strange metals, I have no idea why they are showing the behavior they do. The mystery is what is happening inside this strange system. That is the question.”

Strange metals are of interest to a wide range of physicists studying everything from particle physics to quantum mechanics. One reason is because of their oddly high conductivity, at least under extremely cold temperatures, which gives them potential as superconductors for quantum computing.

“The thing that is really exciting about these new results is that they provide a new insight into the inner machinery of the strange metal,” said study co-author Piers Coleman, a distinguished professor at Rutgers University.

“These metals provide the canvas for new forms of electronic matter—especially exotic and high temperature superconductivity,” he said.

Coleman said it’s too soon to speculate about what new technologies strange metals might inspire.

“It is said that after Michael Faraday discovered electromagnetism, the British Chancellor William Gladstone asked what it would be good for,” Coleman said. “Faraday answered that while he didn’t know, he was sure that one day the government would tax it.”

Faraday’s discoveries opened a world of innovation.

“We feel a bit the same about the strange metal,” Coleman said. “Metals play such a central role today—copper, the archetypal conventional metal, is in all devices, all power lines, all around us.”

Coleman said strange metals one day could be just as ubiquitous in our technology.

The Japan experiment was groundbreaking in part because of the way that researchers created the gamma particles using a particle accelerator called a synchrotron.

“In Japan, they use a synchrotron like they have at CERN [the European Organization for Nuclear Research] that accelerates a proton and smashes it into a wall and it emits a gamma ray,” Komijani said. “So they have an on-demand source of gamma rays without using radioactive material.”

Researchers used spectroscopy to study the effects of gamma rays on the strange metal.

Researchers also examined the speed of the metal‘s electrical charge fluctuations, which take just a nanosecond—a billionth of a second. That might seem incredibly fast, Komijani said.

“However, in the quantum world, a nanosecond is an eternity,” he said. “For a long time we have been wondering why these fluctuations are actually so slow. We came up with a theory with collaborators that there might be vibrations of the lattice and indeed that was the case.”

More information: Hisao Kobayashi et al, Observation of a critical charge mode in a strange metal, Science (2023). DOI: 10.1126/science.abc4787

Journal information: Science 

Provided by University of Cincinnati 

The peculiar topological properties of some forms of matter have been researched for decades. Now, researchers at the Institute of Science and Technology Austria (ISTA) have discovered topological properties of simple diatomic molecules driven to rotation by laser pulses.

The scientists apply similar mathematics to describe them as for solid matter systems, thus bridging two different fields of physics. Their findings promise possible applications in chemistry.

Sometimes, unforeseen connections between disparate research fields in physics can emerge. This is the case for the topological properties of quantum states in rotating molecules.

In a new study, Ph.D. student Volker Karle, Postdoc Areg Ghazaryan, and Professor Mikhail Lemeshko from the Institute of Science and Technology Austria (ISTA), have now revealed that a simple rotating molecule made from just two atoms can feature quantum states with topological properties, similar to what happens in graphene and other solid-state topological materials.

“The interesting thing is that these two systems—a single rotating molecule and a solid sheet of graphene made from millions of carbon atoms—are very different and yet, some of their properties can be described by similar mathematics,” Karle explains. “We are building a bridge between the fields of physical chemistry and solid-state physics.”

The three researchers published their new findings in the journal Physical Review Letters.

A doughnut stays a doughnut

“Topology is the study of the geometrical properties of an object which are unaffected by the continuous change of its shape and size. Realizing that one can classify quantum states not only by their energy and symmetry but also by their topology led to a real breakthrough in our understanding of solid-state physics in the last decades,” Lemeshko explains.

“A simple example of a topological property would be a doughnut. From a mathematical perspective, a doughnut is just a ring with one hole,” Karle adds. “No matter how you stretch or squeeze it, it remains a doughnut as long as you do not do anything as drastic as adding or removing a hole. The property of being a doughnut is therefore topologically protected from ‘small’ disturbances like changing its shape or size.”

In systems like topological insulators, these topological effects emerge from the effects of millions of atoms interacting with one another. However, Karle, Ghazaryan, and Lemeshko have shown that this kind of phenomenon can also be found in much simpler systems like a single molecule.

Pushing a molecule with laser light

“The system we are studying is a single molecule formed by two atoms bonded together,” Karle says. The researchers created a model that describes what happens in such a molecule being pushed by short laser pulses to make it rotate around the midpoint between the two atoms. “At just the right wavelength and timing of the laser pulses, we can create topologically nontrivial quantum states in the molecule that behave like to ones found in solid-state systems.”

For decades now, scientists have studied the topological properties of many different materials and systems—even leading to a Nobel Prize in 2016. However, finding them in a system like a simple molecule allows for new kinds of experiments and applications.

“We are envisioning an experiment where a stream of such molecules is being shot out of a source and then hit with laser pulses,” Karle says. “They then fly into a detector where we can study their quantum states in much greater detail than what’s possible with solid-state systems.” The researchers hope to gain many more insights from future experiments perhaps laying the foundations for new applications in chemistry.

Controlling reactivity

Non-trivial topological properties, like the ones described in this new publication, could lead to topologically protected quantum states. These are especially interesting for any application that needs to be resilient against outside disturbances like heat, magnetic fields, or material impurities. A well-known example that has garnered lots of research interest during the last few years are quantum computers based on topological quantum bits.

However, the molecules that Karle and his colleagues are studying would find different applications. “We hope that this research will allow us to better understand many chemical reactions and may one-day lead to new ways of controlling them,” Lemeshko says. “We could use lasers to create topologically protected quantum states in molecules that increase or decrease their reactivity with other chemicals just as we need it. The topological protection would stabilize the quantum state of the molecule which would otherwise quickly vanish.”

More information: Volker Karle et al, Topological Charges of Periodically Kicked Molecules, Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.130.103202

Journal information: Physical Review Letters 

Provided by Institute of Science and Technology Austria 

Super-resolution microscopy methods are essential for uncovering the structures of cells and the dynamics of molecules. Since researchers overcame the resolution limit of around 250 nanometers (while winning the 2014 Nobel Prize in Chemistry for their efforts), which had long been considered absolute, the methods of microscopy have progressed rapidly.

Now a team led by LMU chemist Prof. Philip Tinnefeld has made a further advance through the combination of various methods, achieving the highest resolution in three-dimensional space and paving the way for a fundamentally new approach for faster imaging of dense molecular structures. The new method permits axial resolution of under 0.3 nanometers.

The researchers combined the so-called pMINFLUX method developed by Tinnefeld’s team with an approach that utilizes special properties of graphene as an energy acceptor. pMINFLUX is based on the measurement of the fluorescence intensity of molecules excited by laser pulses. The method makes it possible to distinguish their lateral distances with a resolution of just 1 nanometer.

Graphene absorbs the energy of a fluorescent molecule that is no more than 40 nanometers distant from its surface. The fluorescence intensity of the molecule therefore depends on its distance from graphene and can be used for axial distance measurement.

DNA-PAINT increases the speed

Consequently, the combination of pMINFLUX with this so-called graphene energy transfer (GET) furnishes information about molecular distances in all three dimensions—and does this in the highest resolution achievable to date of under 0.3 nanometers. “The high precision of GET-pMINFLUX opens the door to new approaches for improving super-resolution microscopy,” says Jonas Zähringer, lead author of the paper.

The researchers also used this to further increase the speed of super-resolution microscopy. To this end, they drew on DNA nanotechnology to develop the so-called L-PAINT approach. In contrast to DNA-PAINT, a technique that enables super-resolution through the binding and unbinding of a DNA strand labeled with a fluorescent dye, the DNA strand in L-PAINT has two binding sequences.

In addition, the researchers designed a binding hierarchy, such that the L-PAINT DNA strand binds longer on one side. This allows the other end of the strand to locally scan the molecule positions at a rapid rate.

“As well as increasing the speed, this permits the scanning of dense clusters faster than the distortions arising from thermal drift,” says Tinnefeld. “Our combination of GET-pMINFLUX and L-PAINT enables us to investigate structures and dynamics at the molecular level that are fundamental to our understanding of biomolecular reactions in cells.”

The findings are published in the journal Light: Science & Applications.

More information: Jonas Zähringer et al, Combining pMINFLUX, graphene energy transfer and DNA-PAINT for nanometer precise 3D super-resolution microscopy, Light: Science & Applications (2023). DOI: 10.1038/s41377-023-01111-8

Journal information: Light: Science & Applications 

Provided by Ludwig Maximilian University of Munich 

Self-testing is a promising method to infer the physics underlying specific quantum experiments using only collected measurements. While this method can be used to examine bipartite pure entangled states, so far it could only be applied to limited kinds of quantum states involving an arbitrary number of systems.

Researchers at Sorbonne University, ICFO-Institute of Photonic Sciences and Quantinuum recently introduced a framework for the quantum network-assisted self-testing of all pure entangled states of an arbitrary number of systems. Their paper, published in Nature Physics, could inform future research efforts aimed at certifying quantum phenomena.

“I was a postdoctoral researcher in Barcelona in 2014 in the group of Antonio Acín when the first author, Ivan Šupić and I began working on self-testing quantum states together,” Matty Hoban, one of the researchers who carried out the study, told Phys.org. “That is, certifying that you have systems in particular quantum states without trusting the devices and treating them as black boxes (called the device-independent setting). Part of this work involved exploring different kinds of scenarios of trust.”

During their initial collaboration, Hoban and Šupić investigated scenarios in which quantum physicists trust some of their experimental components and distrust others. Their goal was to then identify strategies that could simplify the certification of quantum states in these different scenarios.

“I had already moved back to the U.K. and was at the University of Oxford when Ivan visited me and we started exploring a setting where you could prepare particular, simple quantum states and trust this preparation, and then use these states to probe larger systems with more complex quantum states,” Hoban said. “This is a bit like using a small magnet (e.g., a compass) to characterize the magnetic field of the Earth. With the other authors Antonio Acín and Laia Domingo Colomer, we showed how you could self-test arbitrary quantum states in a setting called the Measurement-Device-Independent setting. Meanwhile Ivan was working with Joseph Bowles and Antonio Acín and Daniel Cavalcanti on the detection of entanglement in this completely black-box setting.”

In their new studies, Hoban and his colleagues found that the self-testing of simple quantum states could be a building block for the detection of entanglement. Specifically, this could be achieved by self-testing the maximally entangled state and transferring it in a networked scenario with more systems.

Combining their research efforts, the researchers were able to remove the assumption of characterized quantum state preparation in the measurement-device-independent setting outlined in one of their previous works. They then also teamed up with Marc-Olivier Renou, who was experienced in the study of device-independent quantum systems in networked scenarios.

“In traditional self-testing, if you want to certify that N parties have a particular N-partite quantum state, you would just ask questions of N devices,” Hoban explained. “But now imagine you had a large network of M devices and they can share information, and M could be larger than N. Network-assisted self-testing allows you to ask questions of this larger network to determine the behavior of a smaller number of devices. In the classical world adding additional devices might not seem to add anything: if I ask one person what time it is, their watch shouldn’t depend on whether they had a friend with them or not. But adding quantum systems can add something more.”

A significant difference between quantum systems and classical systems lays in the connections between different systems, particularly in the concept of quantum entanglement. Quantum entanglement underpins many quantum information tasks, such as quantum teleportation.

“If we have two parties, Alice can send an arbitrary unknown quantum state to Bob if they initially share a maximally entangled state,” Hoban said. “So not only entanglement, but maximal entanglement, allows us to move quantum information around from party to party. Instead of just Alice and Bob we can have multiple parties moving this information around, in a network.”

The network-assisted self-testing strategy introduced by Hoban and his colleagues exploits the fact that devices can be entangled with other devices to implement features of quantum theory, including teleportation. As part of their study, the researchers showed that their strategy successfully enables the self-testing of arbitrary pure quantum states.

“On a more foundational level, our results show that you can treat a system as a complete black box, yet from the statistics from interacting with it, you can conclude what the properties of the system are,” Hoban said. “It’s a bit like when you ask a witness to a crime to reconstruct what the alleged criminal looks like; the resulting image can look hilariously wrong or be completely generic. Furthermore, the witness could be lying and you would not know. In our work, you can perform a perfect reconstruction of the quantum description of a system just through asking questions to a black box and you could catch the system out if it tries to lie about what’s inside.”

The recent work by this team of researchers could soon open new opportunities for the certifying quantum devices and entangled quantum states. Notably, their proposed technique is generic, so it could be used to self-test a wide range of quantum states without requiring particular adaptations. Hoban and his colleagues are now working on making their strategy increasingly applicable to real-world problems.

“Our results are more proof-of-principle and require that you achieve some task perfectly; we need to allow for the possibility of some small error,” Hoban added. “This is called robust self-testing in the literature. Also, the methods we used are generic, and we would like to adapt them to particular settings to reduce the resource requirements. I would also like to find applications in delegating quantum computation and quantum cryptography.”

More information: Ivan Šupić et al, Quantum networks self-test all entangled states, Nature Physics (2023). DOI: 10.1038/s41567-023-01945-4

Journal information: Nature Physics 

Bubbles are fun for everyone. But, it turns out, they can also be little menaces.

When a bubble pops, it can concentrate and aerosolize any particles stuck on it. Not a big deal when it’s a store-bought soapy bubble bursting in the yard or on your hand.

But it’s a major concern when the particles it carries are potentially hazardous: bubbles caught in a crashing wave can send vaporized microplastics into the air where they might mess with the Earth’s atmosphere; bubbles burst by a flushing toilet can fling bacteria meters and onto nearby surfaces; a frothing cruise ship hot tub was once shown to be a Legionnaires’ disease super-spreader.

Now, a new study by Boston University engineers illustrates why bubbles fire some contaminants into the air, while allowing others to sink harmlessly. After taking a close-up look at what happens when bubbles pop, the researchers found a new way to predict which particles are flung high—and which ones fall—overhauling a 40-year-old theory of fluid dynamics. Their results, which were published in Physical Review Letters, could help scientists track marine pollution or more accurately predict a virus’ transmissibility.

“With this new theory, we can better model potential ocean sources of pollutants or how other particles in the ocean can get into the atmosphere and act as cloud condensation nuclei, altering the climate,” says Lena Dubitsky, a doctoral student in the BU Fluid Lab and joint lead author on the paper. “In terms of public health, this model helps predict what drop size might contain the most pathogens.”

And that can be crucial in determining how easily a disease might spread or whether a small drop can sneak a virus through the defenses protecting our lower respiratory tract.

At their simplest, bubbles are a thin layer of liquid surrounding a gas. The bubbles kids love blowing, for example, are a layer of water trapped between two layers of soap molecules, with air in the middle.Play

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If you poke the bubble, it creates a hole, which quickly widens—the whole bubble pops in less than one-tenth of a second—forcing the outer soapy layer to collapse, packing its molecules together in a denser space. All of that movement and change in density—as well as the air inside flying up and out—propels drops of water and soap into the sky in a quick pop.

The retreat of that outer layer and the ejection of those drops—particularly the first, or top, drop to exit—is central to the BU Fluid Lab’s new theory. “We focus on jet drops in this study, which are created when the bubble cavity collapses and shoots up into a liquid jet, which pinches off into drops,” says Dubitsky. “In particular, we study the first jet drop since it tends to be the smallest and fastest, making it more likely to stay suspended in the atmosphere, to be transported the furthest, or be inhaled deeply into the respiratory tracts.”

Any particles trapped in that first explosive drop are also more likely to become highly concentrated.

For the past four decades, researchers studying bubbles thought the all-important top drop was drawn from a uniform fluid layer surrounding the entire bubble—only particles small enough to sit in that layer would get pulled into it, meaning bigger particles would get left behind.

“We decided to use really big particles to stress test this old theory and found it didn’t apply at all,” says Dubitsky.

Instead, they discovered that the fluid forming the top drop doesn’t always surround the whole bubble, and that a bubble’s size and where a particle sits on it also determine what gets ejected and when. If that all seems a bit esoteric and technical, just think about SARS-CoV-2. For the past couple of years, our health has been inextricably tied to droplets—how they spread, what they carry with them, how long they linger in the air.

“In order to predict the infectivity of a particular pathogen, one needs to know the infectious dose, so when these droplets become ultraconcentrated, it really matters what size is becoming ultraconcentrated,” says Oliver McRae, a joint lead author on the paper and BU College of Engineering postdoctoral associate.Play

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“If you have a 50-micron droplet [one micron is one one-thousandth of a millimeter], we don’t really care about that much. If you’re able to get larger particles and transport them much further than previously thought, that is a key takeaway.”

To capture the bubbles in action, the research team set up a container filled with fluid and small microplastic particles—little pieces of polystyrene. They then blew bubbles of different sizes into the liquid, using high-speed cameras to watch them rise to the surface and burst. The top drop would splat onto a glass slide above the fluid’s surface, allowing researchers to analyze the particles left behind. McRae then created computer simulations of the bursting bubbles so they could dissect their speedy demise.

“What we saw is that as the bubble is collapsing and the fluid is being swept down toward the base, eventually becoming a jet, the fluid layer is getting thicker,” says McRae, “and so that compression allows for larger particles to get in.”

With larger bubbles, the outer layer was pretty uniform to start with, completely surrounding the bubble; on smaller bubbles though, it barely covered the bottom half.

“That means if you’re a bacteria or a virus and you’re stuck on the upper half of the bubble, you’ll never get in the top drop in a small bubble,” says McRae. “That wasn’t taken into account or wasn’t predicted in previous theories.”

According to James Bird, an ENG associate professor of mechanical engineering and the Fluid Lab’s principal investigator, the research is exciting because it “opens up the possibility that there’s so much more going on than we had appreciated, than we even had the framework to appreciate.”

As an example, he says, the Legionella bacteria, which causes Legionnaires’ disease and is transported by bursting bubbles, has an elongated rather than circular shape. What could his team’s latest findings mean for how it gets swept up in a bubble’s pop? And what could that mean for stemming outbreaks?

“Maybe in a toilet or swimming pool there are strategies one can take to make sure some of these places aren’t going to be as pathogenic,” says Bird. “Or maybe when you have something new, a novel virus or bacteria, there’s ways to predict, just based on the chemistry and the shape, how likely it is to be aerosolized. This work is a stepping stone.”

More information: Lena Dubitsky et al, Enrichment of Scavenged Particles in Jet Drops Determined by Bubble Size and Particle Position, Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.130.054001

Journal information: Physical Review Letters 

Provided by Boston University