Category: Physics

Physicists at the University of Bonn have experimentally proven that an important theorem of statistical physics applies to so-called “Bose-Einstein condensates.” Their results now make it possible to measure certain properties of the quantum “superparticles” and deduce system characteristics that would otherwise be difficult to observe. The study has now been published in Physical Review Letters.

Suppose in front of you there is a container filled with an unknown liquid. Your goal is to find out by how much the particles in it (atoms or molecules) move back and forth randomly due to their thermal energy. However, you do not have a microscope with which you could visualize these position fluctuations known as “Brownian motion”.

It turns out you do not need that at all: You can also simply tie an object to a string and pull it through the liquid. The more force you have to apply, the more viscous your liquid. And the more viscous it is, the lesser the particles in the liquid change their position on average. The viscosity at a given temperature can therefore be used to predict the extent of the fluctuations.

The physical law that describes this fundamental relationship is the fluctuation-dissipation theorem. In simple words, it states: The greater the force you need to apply to perturb a system from the outside, the less it will also fluctuate randomly (i.e., statistically) on its own if you leave it alone.

“We have now confirmed the validity of the theorem for a special group of quantum systems for the first time: the Bose-Einstein condensates,” explains Dr. Julian Schmitt from the Institute of Applied Physics at the University of Bonn.

‘Super photons’ made of thousands of light particles

Bose-Einstein condensates are exotic forms of matter that can arise due to a quantum mechanical effect: Under certain conditions, particles, be they atoms, molecules, or even photons (particles that constitute light), become indistinguishable. Many hundreds or thousands of them merge into a single “super particle”—the Bose-Einstein condensate (BEC).

In a liquid at finite temperature, molecules move back and forth at random. The warmer the liquid, the more pronounced are these thermal fluctuations. Bose-Einstein condensates can also fluctuate: The number of condensed particles varies. And this fluctuation also increases with rising temperature.

“If the fluctuation-dissipation theorem applies to BECs, the greater the fluctuation in their particle number, the more sensitively they should respond to an external perturbation,” Schmitt says. “Unfortunately, the number [of] fluctuations in the usually studied BECs in ultracold atomic gases is too small to test this relationship.”

However, the research group of Prof. Dr. Martin Weitz, within which Schmitt is a junior research group leader, works with Bose-Einstein condensates made of photons. And for this system, the limitation does not apply. “We make the photons in our BECs interact with dye molecules,” explains the physicist. When photons interact with dye molecules, it frequently happens that a molecule “swallows” a photon. The dye thereby becomes energetically excited. It can later release this excitation energy by “spitting out” a photon.

Low-energy photons are swallowed less often

“Due to the contact to the dye molecules, the number of photons in our BECs shows large statistical fluctuations,” says the physicist. In addition, the researchers can precisely control the strength of this variation: In the experiment, the photons are trapped between two mirrors, where they are reflected back and forth in a ping-pong game manner.

The distance between the mirrors can be varied. The larger it becomes, the lower the energy of the photons. Since low-energy photons are less likely to excite a dye molecule (so they are swallowed less often), the number of condensed light particles now fluctuates much less.

The Bonn physicists now investigated how the extent of the fluctuation is related to the “response” of the BEC. If the fluctuation-dissipation theorem holds, this sensitivity should decrease as fluctuation decreases.

“In fact, we were able to confirm this effect in our experiments,” emphasizes Schmitt, who is also a member of the Transdisciplinary Research Area (TRA) “Matter” at the University of Bonn and the Cluster of Excellence “ML4Q—Matter and Light for Quantum Computing.”

As with liquids, it is now possible to infer the microscopic properties of Bose-Einstein condensates from macroscopic response parameters that can be more easily measured. “This opens a way to new applications, such as the precise temperature determination in complex photonic systems,” says Schmitt.

More information: Fahri Emre Öztürk et al, Fluctuation-Dissipation Relation for a Bose-Einstein Condensate of Photons, Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.130.033602

Journal information: Physical Review Letters 

Provided by Rheinische Friedrich-Wilhelms-Universität Bonn

Scientists have advanced in discovering how to use ripples in space-time known as gravitational waves to peer back to the beginning of everything we know. The researchers say they can better understand the state of the cosmos shortly after the Big Bang by learning how these ripples in the fabric of the universe flow through planets and the gas between the galaxies.

“We can’t see the early universe directly, but maybe we can see it indirectly if we look at how gravitational waves from that time have affected matter and radiation that we can observe today,” said Deepen Garg, lead author of a paper reporting the results in the Journal of Cosmology and Astroparticle Physics. Garg is a graduate student in the Princeton Program in Plasma Physics, which is based at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL).

Garg and his advisor Ilya Dodin, who is affiliated with both Princeton University and PPPL, adapted this technique from their research into fusion energy, the process powering the sun and stars that scientists are developing to create electricity on Earth without emitting greenhouse gases or producing long-lived radioactive waste. Fusion scientists calculate how electromagnetic waves move through plasma, the soup of electrons and atomic nuclei that fuels fusion facilities known as tokamaks and stellarators.

It turns out that this process resembles the movement of gravitational waves through matter. “We basically put plasma wave machinery to work on a gravitational wave problem,” Garg said.

Gravitational waves, first predicted by Albert Einstein in 1916 as a consequence of his theory of relativity, are disturbances in space-time caused by the movement of very dense objects. They travel at the speed of light and were first detected in 2015 by the Laser Interferometer Gravitational Wave Observatory (LIGO) through detectors in Washington State and Louisiana.

Garg and Dodin created formulas that could theoretically lead gravitational waves to reveal hidden properties about celestial bodies, like stars that are many light years away. As the waves flow through matter, they create light whose characteristics depend on the matter’s density.

A physicist could analyze that light and discover properties about a star millions of light years away. This technique could also lead to discoveries about the smashing together of neutron stars and black holes, ultra-dense remnants of star deaths. They could even potentially reveal information about what was happening during the Big Bang and the early moments of our universe.

The research began without any sense of how important it might become. “I thought this would be a small, six-month project for a graduate student that would involve solving something simple,” Dodin said. “But once we started digging deeper into the topic, we realized that very little was understood about the problem and we could do some very basic theory work here.”

The scientists now plan to use the technique to analyze data in the near future. “We have some formulas now, but getting meaningful results will take more work,” Garg said.

More information: Deepen Garg et al, Gravitational wave modes in matter, Journal of Cosmology and Astroparticle Physics (2022). DOI: 10.1088/1475-7516/2022/08/017

Provided by Princeton Plasma Physics Laboratory

Quantum materials are materials with unique electronic, magnetic or optical properties, which are underpinned by the behavior of electrons at a quantum mechanical level. Studies have showed that interactions between these materials and strong laser fields can elicit exotic electronic states.

In recent years, many physicists have been trying to elicit and better understand these exotic states, using different material platforms. A class of materials that was found to be particularly promising for studying some of these states are monolayer transition metal dichalcogenides.

Monolayer transition metal dichalcogenides are 2D materials that consist in single layers of atoms from a transition metal (e.g., tungsten or molybdenum) and a chalcogen (e.g., sulfur or selenium), which are arranged into a crystal lattice. These materials have been found to offer exciting opportunities for Floquet engineering (a technique to manipulate the properties of materials using lasers) of excitons (quasiparticle electron-hole correlated states).

Researchers at the SLAC National Accelerator Laboratory, Stanford University and University of Rochester have recently demonstrated the Floquet engineering of excitons driven by strong fields in a monolayer transition metal dichalcogenide. Their findings, presented in a paper in Nature Physics, could open new possibilities for the study of excitonic phenomena.

“Our group has been studying strong-field driven processes such as high-harmonic generation (HHG) in 2D-crystals subjected to intense mid-infrared laser fields,” Shambhu Ghimire, one of the researchers who carried out the study, told Phys.org.

“We are very interested to understand the detailed mechanism of the HHG process, and 2D-crystals seem to be a fascinating platform for this, as they are something in between isolated atoms in the gas phase and the bulk crystals. In the gas phase, the process is understood by considering the dynamics of the laser field ionized electron and its recombination to the parent ion.”

When exposed to strong laser fields, 2D crystals can host strongly-driven excitons. In their previous research, Ghimire and his colleagues explored whether driving these quasiparticles with strong laser fields and measuring high harmonics would allow them to better understand the solid-state HHG process.

“While this previous work was the inspiration for our study, we also started measuring the change in absorption on these driven systems and learned more about the non-equilibrium state of the material itself,” Ghimire explained. “Indeed, we find that there are previously not observed absorption features that can be linked to what’s known in the literature as the Floquet states of the materials subjected to strong periodic drives.”

In their experiments, the researchers used high-power ultrafast laser pulses in the mid-infrared wavelength range to monolayer tungsten disulfide (TMDs). The use of these ultrafast pulses allowed them to avoid the sample damage that typically results from strong light-matter interactions.

More specifically, the photon energy of mid-infrared laser pulses is around 0.31 eV, which is significantly below the optical bandgap of monolayer TMDs (~2 eV). Therefore, the team did not expect to observe a particularly sizable generation of charge carriers.

“At the same time, the photon energy in our set up is tunable and can be resonant to exciton energies of the monolayer,” Ghimire said. “To fabricate our material samples, we collaborated with the team of Prof. Fang Liu at Stanford Chemistry. This group has pioneered a new approach to fabricate millimeter scale monolayer samples, which was also a key to the success of these experiments.”

Yuki Kobayashi, a postdoctoral scholar, who is the lead author of the paper said that they unveiled two new mechanisms for creating quantum virtual states in monolayer TMDs. The first of these involves Floquet states, which are attained by mixing the quantum states of materials with external photons, while the second involves the so-called Franz-Keldvsh effect.

“We found that an originally dark exciton state can be optically bright by mixing with single photon, being manifested as a separate absorption signal below the optical bandgap,” Kobayahsi said. “The second mechanism we unveiled is the dynamic Franz-Keldysh effect. This is caused by the external laser field triggering momentum kick to the excitons, leading to universal blue shift of the spectral features. This effect was observed because we applied a high-field laser pulse (~0.3 V/nm) that is strong enough to break apart the electron-hole pair.”

Combining the two mechanisms they unveiled, Kobayashi and his colleagues were able to achieve an energy tuning over 100 meV in their sample of monolayer TMDs. These notable results highlight the huge potential of this monolayer transition metal dichalcogenide as a platform to realize strong-field excitonic phenomena.

“One of the unanswered questions in our work is the real-time response of strong-field excitonic phenomena: how fast can we turn on and off the virtual quantum states?” Ghimire added. “We expect that, by going beyond the perturbative domain, it will be possible to imprint the oscillation patterns of laser carrier waves in the virtual quantum states, approaching the sub-petahertz regime of optical property control.”

More information: Yuki Kobayashi et al, Floquet engineering of strongly driven excitons in monolayer tungsten disulfide, Nature Physics (2023). DOI: 10.1038/s41567-022-01849-9

Hanzhe Liu et al, High-harmonic generation from an atomically thin semiconductor, Nature Physics (2016). DOI: 10.1038/nphys3946

P. B. Corkum, Plasma perspective on strong field multiphoton ionization, Physical Review Letters (2002). DOI: 10.1103/PhysRevLett.71.1994

Shambhu Ghimire et al, High-harmonic generation from solids, Nature Physics (2018). DOI: 10.1038/s41567-018-0315-5

Fang Liu, Mechanical exfoliation of large area 2D materials from vdW crystals, Progress in Surface Science (2021). DOI: 10.1016/j.progsurf.2021.100626

Journal information: Physical Review Letters  , Nature Physics 

© 2023 Science X Network

A birefringence based light modulator that works in the wavelength region of < 350 nm plays a vital role in DUV beam shaping, high density data storage, semiconductor micro-nano processing and photolithography. Actually, a series of DUV birefringent materials, including single crystals of α-BBO, MgF₂, Ca(BO₂)₂, and α-SnF₂, have thus been made and commercially used. However, these birefringent elements have fixed birefringence, limiting their capability of continuous light modulation.

Liquid crystals (LCs) are another kind of birefringent materials, of which birefringence is tunable via the molecular alignment by external electrical or magnetic stimuli. Up to now, the commonly used LCs are mainly based on organic molecules or polymers, which are not stable under DUV light due to photochemical degradation effects. Meanwhile, DUV can also induce free radicals in some organic groups, and initiate their polymerization, which disorders the alignment and the resultant birefringence of LC. Therefore, organic LC cannot modulate DUV light.

In a new paper published in Light Science & Application, three teams of scientists, led by Professor Hui-Ming Cheng and Associate Professor Baofu Ding from Shenzhen Institute of Advanced Technology, CAS, Professor Wei Cai from Xi’an Research Institute of High Technology, Professor Bilu Liu from Tsinghua University, China, cooperatively synthesized two-dimensional (2D) inorganic cobalt-doped titanate (CTO) LC by using a wet chemical method. The 2D LC has large magnetic & optical anisotropy as well as high transmittance of > 70% in the wavelength of 300 ~ 350 nm, which enables the transmitted DUV modulation in a magnetic and portable way (Fig. 1).

Fig. 2 summarizes the desired performance of the 2D CTO LC based DUV modulator, as evidenced by its good reversibility, fast response at the millisecond level, excellent durability and DUV stability.

The magneto-birefringence effect of 2D CTO LC make it applicable in the preparation of flexible DUV birefringent optical hydrogel. By adding a small amount of monomer and photo-initiator into 2D CTO suspension, a DUV birefringent hydrogel was prepared via UV curing during exertion of magnetic field.

Once the hydrogelation is completed, the magnetically aligned 2D CTO nanosheets can be fixed inside the hydrogel and all their long axes parallel each other, even after removal of the magnetic field. The CTO hydrogel can serve as a transparent mechano-optical crystal, through which the DUV light can be in-situ modulated without direction alteration in a mechanical way (Fig. 3). The 2D CTO based hydrogel is the first birefringence-tunable element that can continuously tune the DUV light in a mechanical way.

This work may extend birefringence-tunable optics that are currently used in visible and infrared regions to the DUV region, which is important for high density data storage, semiconductor micro-nano processing and photolithography.

More information: Youan Xu et al, Deep ultraviolet hydrogel based on 2D cobalt-doped titanate, Light: Science & Applications (2023). DOI: 10.1038/s41377-022-00991-6

Journal information: Light: Science & Applications 

Provided by Light Publishing Center, Changchun Institute of Optics, Fine Mechanics And Physics, CAS

In northern communities, seasonal snow plays a central role in day-to-day activities.

For some, it means a day off from school. For others, it’s a signal that skiing season is starting. Or maybe it’s a harbinger of an extra long commute to work. It’s remarkable how many memories and emotions can be evoked by a few billion tiny ice crystals.

We may see snow as a blanket or drifts across the landscape or our driveway. But when was the last time you took a closer look at snow, and I mean a really close look?

Many a writer has mused about snowflakes as a natural work of art. Here’s a scientific look at the amazing nature of snowflakes and snow.

How do snowflakes form?

While different catalogs will say that there are seven types of snowflakes, or eight or 35, we are probably most familiar with the classic six-sided dendrite forms, characterized by elaborate and nearly symmetrical branches. You know, the type that you would cut out of a piece of paper.

The dendrite form is a study in water chemistry. When ice forms at the molecular level, the angle between the hydrogen and oxygen atoms will always be 120 degrees; put three of these together to get a full ring of molecules with a six-sided structure. In fact, every time a water molecule attaches itself to this ring, it will do so at the same angle.

As the snowflake grows, the attachment of water molecules is determined by the temperature and humidity of the air. Since these characteristics don’t change too much at the size of a growing snowflake, those attachments tend to occur evenly across the six points of the hexagonal flake.

Molecule by molecule, the snowflake grows and eventually begins to fall. This takes the snowflake to a new part of the atmosphere, where temperature and humidity are different, resulting in new ice structures forming, but still with the same set of angles.

Is each snowflake really unique?

A typical dendrite is made up of about a quintillion (that’s a one with 18 zeroes after it) individual water molecules. Given slight changes in temperature and humidity and the huge number of molecules and bonding opportunities involved, the ice structures created can be incredibly diverse and complicated.

For this reason, it is entirely likely that no two snowflakes form in exactly the same way, and consequently no two snowflakes are alike.

Twin snowflakes have been grown in a lab, where temperature and humidity are closely controlled, but that’s a bit of a cheat.

Why is some snow light and fluffy and some is heavy?

The story of snow crystal growth doesn’t end high above in the clouds. Once the snowflakes reach the ground and accumulate as a blanket of snow, they begin to change.

Freshly fallen snow tends to be light and fluffy because the flakes take up a lot of space and there is a lot of air between and within them. But over time, they break apart, pack tighter together and the density increases.

This process is known as sintering and is useful for building snow shelters like igloos and quinzees. But some of the most remarkable changes happen at the bottom of the snowpack, where warmth from the ground below and cold from the air above interact.

Through a process of sublimation—water molecules change from ice directly to vapor, skipping the liquid phase—and refreezing, cup-shaped crystals a few centimeters across known as depth hoar can form. Though beautiful to look at, depth hoar has a low density and when it forms on a steep slope there is a chance for the snowpack to slide as an avalanche.

So next time you’re out in the snow, even if you’re grumbling about having to shovel the driveway for the umpteenth time this winter, take a moment to catch a snowflake on your mitten and have a look at it. You’re looking at a formation no one has ever seen before.

Check out physics professor Kenneth Libbrecht’s website for a full description of snowflake forms.

Since the introduction of the first ruby laser—a solid-state laser that uses the synthetic ruby crystal as its laser medium—in 1960, the use of lasers has expanded significantly in scientific, medical and industrial fields.

With the advancement of science and technology, lasers with extremely narrow linewidths have become the key to research in frontier scientific fields. The much-anticipated gravitational wave detection program Laser Interferometer Gravitational-Wave Observatory (LIGO) in the US, for instance, has extremely strict requirements for the coherence of lasers. While Brillouin lasers have great potential for such applications because of their linewidth narrowing effect, the lasing threshold of on-chip Brillouin lasers is high due to the intrinsic loss of the Brillouin waveguide and the large mode volume.

To circumvent this issue, a team from the Nano-OptoElectronics Lab led by Professor Yidong Huang at Tsinghua University proposes a similar scattering photon lasing phenomenon occurring in an optomechanical microcavity, which can help realize a new on-chip narrow linewidth laser with a lower lasing threshold.

“The new laser can be achieved from a one-dimensional optomechanical crystal with both photon and phonon excitation within a chip size of just tens of microns by a small pump threshold of only 500 microwatts,” shares Associate Professor Kaiyu Cui, a researcher involved in the study.

The team observed that the linewidth of the new laser was narrowed by four orders of magnitude to 5.4 kHz after phonon lasing at 6.2 GHz. This highly coherent phonon laser has important applications in fields such as high-precision mass sensing, spectral sensing and signal processing. At the same time, the excited photon also exhibits a significant threshold effect, which can be applied in coherent wavelength conversion.

Notably, achieving simultaneous photon and phonon lasing in one-dimensional optomechanical crystals is no easy feat. Periodically aligned nanostructures are needed to confine both light and mechanical waves in a very small volume by a physical mechanism known as defect modes. Only then could the localized photons and phonons within the microcavity undergo strong energy coupling, and in turn enabling coherent lasing at very low pump power.

Nonetheless, the team has successfully fabricated one-dimensional optomechanical crystals on a silicon chip using electron-beam lithography. When the incident pump power exceeded the threshold, significant lasing was observed on the spectrometer. Indeed, the experimental results matched those of theoretical expectations.

The researchers published their latest findings, which could pave the way for silicon-based photonic and phononic lasers to fulfill the urgent need for new laser technologies, in the journal Fundamental Research.

“In optomechanical crystals, nonlinear equations can be used to describe the behavior of photons and phonons. Since nonlinear systems cannot be solved analytically in general, most previous studies have been conducted based on linearized equations,” explains Prof Huang.

“Based on our findings, we propose that the nonlinear equations can be analyzed directly by means of limit-cycle theory, which gives the first analytical formulation of the laser linewidth under the effect of phase noise.”

More information: Jian Xiong et al, Phonon and photon lasing dynamics in optomechanical cavities, Fundamental Research (2022). DOI: 10.1016/j.fmre.2022.10.008

Provided by KeAi Communications Co., Ltd.

New research led by a team of scientists at The Australian National University (ANU) has outlined a way to achieve more accurate measurements of microscopic objects using quantum computers—a step that could prove useful in a huge range of next-generation technologies, including biomedical sensing.

Examining the various individual properties of a large everyday object like a car is fairly simple: a car has a well-defined position, color and speed. However, this becomes much trickier when trying to examine microscopic quantum objects like photons—tiny little particles of light.

That’s because certain properties of quantum objects are connected, and measuring one property can disturb another property. For example, measuring the position of an electron will affect its speed and vice versa.

Such properties are called conjugate properties. This is a direct manifestation of Heisenberg’s famous uncertainty principle—it is not possible to simultaneously measure two conjugate properties of a quantum object with arbitrary accuracy.

According to lead author and ANU Ph.D. researcher Lorcán Conlon, this is one of the defining challenges of quantum mechanics.

“We were able to design a measurement to determine conjugate properties of quantum objects more accurately. Remarkably, our collaborators were able to implement this measurement in various labs around the world,” Conlon said.

“More accurate measurements are crucial, and can in turn open up new possibilities for all sorts of technologies, including biomedical sensing, laser ranging, and quantum communications.”

The new technique revolves around a strange quirk of quantum systems, known as entanglement. According to the researchers, by entangling two identical quantum objects and measuring them together, scientists can determine their properties more precisely than if they were measured individually.

“By entangling two identical quantum systems, we can acquire more information,” co-author Dr. Syed Assad said. “There is some unavoidable noise associated with measuring any property of a quantum system. By entangling the two, we’re able to reduce this noise and get a more accurate measurement.”

In theory, it is possible to entangle and measure three or more quantum systems to achieve even better precision, but in this case the experiments failed to agree with the theory. Nevertheless, the authors are confident that future quantum computers will be able to overcome these limitations.

“Quantum computers with error-corrected qubits will be able to gainfully measure with more and more copies in the future,” Conlon said.

According to Professor Ping Koy Lam, A*STAR chief quantum scientist at Institute of Materials Research and Engineering (IMRE), one of the key strengths of this work is that a quantum-enhancement can still be observed in noisy scenarios.

“For practical applications, such as in biomedical measurements, it is important that we can see an advantage even when the signal is inevitably embedded in a noisy real-world environment,” he said.

The study was conducted by experts at the ARC Centre of Excellence for Quantum Computation and Communication Technology (CQC2T), in collaboration with researchers from A*STAR’s Institute of Materials Research and Engineering (IMRE), the University of Jena, the University of Innsbruck, and Macquarie University. Amazon Web Services collaborated by providing research and architectural support, and by making available the Rigetti Aspen-9 device using Amazon Braket.

The researchers tested their theory on 19 different quantum computers, across three different platforms: superconducting, trapped ion and photonic quantum computers. These world leading devices are located across Europe and America and are cloud-accessible, allowing researchers from across the globe to connect and carry out important research.

The research has been published in Nature Physics.

More information: Lorcán O. Conlon et al, Approaching optimal entangling collective measurements on quantum computing platforms, Nature Physics (2023). DOI: 10.1038/s41567-022-01875-7

Journal information: Nature Physics 

Provided by Australian National University 

Seemingly spontaneously coordinated swarm behavior exhibited by large groups of animals is a fascinating and striking collective phenomenon. Experiments conducted by researchers at Leipzig University on laser-controlled synthetic microswimmers now show that supposed swarm intelligence can sometimes also be the result of simple and generic physical mechanisms.

A team of physicists led by Professor Frank Cichos and Professor Klaus Kroy found that swarms of synthetically produced Brownian microswimmers appear to spontaneously decide to orbit their target point instead of heading for it directly. They have just published their findings in the renowned journal Nature Communications.

“Scientific research on herd and flock behavior is usually based on field observations. In such cases, it is usually difficult to reliably record the internal states of the herd animals,” Kroy said. As a result, the interpretation of observations frequently relies on plausible assumptions as to which individual behavioral rules are necessary for the complex collective groups under observation.

Researchers at Leipzig University therefore developed an experimental model system of microswimmers that elicits properties of natural swarm intelligence and provides complete control over the individuals’ internal states, strategies, and transformation of signal perception into a navigational reaction.

Thanks to a sophisticated laser heating system (see image), the colloidal swimmers, which are visible only under the microscope, can actively self-propel in a water container by a kind of “thermophoretic self-propulsion” while their travel is permanently disturbed in a random manner by Brownian motion.

“Apart from Brownian random motion, which is ubiquitous in microphysics, the experimental set-up provides complete control over the physical parameters and navigation rules of the individual colloidal swimmers and allows long-term observations of swarms of variable sizes,” Cichos said.

According to Cichos, when just a very simple and generic navigation rule is followed identically by all of the swimmers, a surprisingly complex swarm behavior results. For example, if the swimmers are aiming at the same fixed point, instead of them gathering at the same place a kind of carousel can form. Similar to satellites or atomic electrons, the swimmers then orbit their attractive center on circular paths of varying heights.

The only “intelligent” behavioral rule required for this is that the self-propulsion responds to environmental perception with a certain time delay, which usually occurs in natural swarm phenomena from mosquito dances to road traffic anyway. It turns out that such a “delayed” effect alone is sufficient to form complex dynamic patterns such as the carousel described above.

“Physically speaking, each individual swimmer can spontaneously break the radial symmetry of the system and go into circular motion if the product of the delayed time and swimming speed is large enough,” Kroy said. In contrast, the orbits of larger swarms and their synchronization and stabilization depend on additional details such as the steric, phoretic and hydrodynamic interactions between the individual swimmers.

Since all signal-response interactions in the living world occur in a time-delayed manner, these findings should also further the understanding of dynamic pattern formation in natural swarm ensembles. The researchers deliberately chose primitive and uniform navigation rules for their experiment. This allowed them to develop a stringent mathematical description of the observed phenomena.

In the analysis of the delayed stochastic differential equations used for this purpose, the delay-induced effective synchronization of the swimmers with their own past turned out to be the key mechanism for the spontaneous circular motion. To a large extent, the theory allows us to mathematically predict the experimental observations.

“All in all, we have succeeded in creating a laboratory for swarms of Brownian microswimmers. This can serve as a building block for future systematic studies of increasingly complex and possibly still unknown swarm behavior, and it may also explain why puppies often circle their food bowl when they are being fed,” Cichos said.

More information: Xiangzun Wang et al, Spontaneous vortex formation by microswimmers with retarded attractions, Nature Communications (2023). DOI: 10.1038/s41467-022-35427-7

Journal information: Nature Communications 

Provided by Leipzig University 

It was an anomaly detected in the storm of a nuclear reactor so puzzling that physicists hoped it would shine a light on dark matter, one of the universe’s greatest mysteries.

However new research has definitively ruled out that this strange measurement signaled the existence of a “sterile neutrino“, a hypothetical particle that has long eluded scientists.

Neutrinos are sometimes called “ghost particles” because they barely interact with other matter—around 100 trillion are estimated to pass through our bodies every second.

Since neutrinos were first theorized in 1930, scientists have been trying to nail down the properties of these shape-shifters, which are one of the most common particles in the universe.

They appear “when the nature of the nucleus of an atom has been changed”, physicist David Lhuillier of France’s Atomic Energy Commission told AFP.

That could happen when they come together in the furious fusion in the heart of stars like our Sun, or are broken apart in nuclear reactors, he said.

There are three confirmed flavors of neutrino: electron, muon and tau.

However physicists suspect there could be a fourth neutrino, dubbed “sterile” because it does not interact with ordinary matter at all.

In theory, it would only answer to gravity and not the fundamental force of weak interactions, which still hold sway over the other neutrinos.

The sterile neutrino has a place ready for it in theoretical physics, “but there has not yet been a clear demonstration that is exists,” he added.

Dark matter candidate

So Lhuillier and the rest of the STEREO collaboration, which brings together French and German scientists, set out to find it.

Previous nuclear reactor measurements had found fewer neutrinos than the amount expected by theoretical models, a phenomenon dubbed the “reactor antineutrino anomaly”.

It was suggested that the missing neutrinos had changed into the sterile kind, offering a rare chance to prove their existence.

To find out, the STEREO collaboration installed a dedicated detector a few meters away from a nuclear reactor used for research at the Laue–Langevin institute in Grenoble, France.

After four years of observing more than 100,000 neutrinos and two years analyzing the data, the verdict was published in the journal Nature on Wednesday.

The anomaly “cannot be explained by sterile neutrinos,” Lhuillier said.

But that “does not mean there are none in the universe”, he added.

The experiment found that previous predictions of the amount of neutrinos being produced were incorrect.

But it was not a total loss, offering a much clearer picture of neutrinos emitted by nuclear reactors.

This could help not just with future research, but also for monitoring nuclear reactors.

Meanwhile, the search for the sterile neutrino continues. Particle accelerators, which smash atoms, could offer up new leads.

Despite the setback, interest could remain high because sterile neutrinos have been considered a suspect for dark matter, which makes up more than quarter of the universe but remains shrouded in mystery.

Like dark matter, the sterile neutrino does not interact with ordinary matter, making it incredibly difficult to observe.

“It would be a candidate which would explain why we see the effects of dark matter—and why we cannot see dark matter,” Lhuillier said.

More information: David Lhuillier, STEREO neutrino spectrum of 235U fission rejects sterile neutrino hypothesis, Nature (2023). DOI: 10.1038/s41586-022-05568-2. www.nature.com/articles/s41586-022-05568-2

Journal information: Nature 

© 2023 AFP

A team from Nagoya University in Japan has observed, for the first time, the energy transferring from resonant electrons to whistler-mode waves in space. Their findings offer direct evidence of previously theorized efficient growth, as predicted by the non-linear growth theory of waves. This should improve our understanding of not only space plasma physics but also space weather, a phenomenon that affects satellites.

When people imagine outer space, they often envision it as a perfect vacuum. In fact, this impression is wrong because the vacuum is filled with charged particles. In the depths of space, the density of charged particles becomes so low that they rarely collide with each other.

Instead of collisions, the forces related to the electric and magnetic fields filling space, control the motion of charged particles. This lack of collisions occurs throughout space, except for very near to celestial objects, such as stars, moons, or planets. In these cases, the charged particles are no longer traveling through the vacuum of space but instead through a medium where they can strike other particles.

Around the Earth, these charged-particle interactions generate waves, including electromagnetic whistler-mode waves, which scatter and accelerate some of the charged particles. When diffuse auroras appear around the poles of planets, observers are seeing the results of an interaction between waves and electrons. Since electromagnetic fields are so important in space weather, studying these interactions should help scientists predict variations in the intensity of highly energetic particles. This might help protect astronauts and satellites from the most severe effects of space weather.

A team comprising Designated Assistant Professor Naritoshi Kitamura and Professor Yoshizumi Miyoshi of the Institute for Space and Earth Science (ISEE) at Nagoya University, together with researchers from the University of Tokyo, Kyoto University, Tohoku University, Osaka University, and Japan Aerospace Exploration Agency (JAXA), and several international collaborators, mainly used data obtained using low-energy electron spectrometers, called Fast Plasma Investigation-Dual Electron Spectrometers, on board NASA’s Magnetospheric Multiscale spacecraft.

They analyzed interactions between electrons and whistler-mode waves, which were also measured by the spacecraft. By applying a method of using a wave particle interaction analyzer, they succeeded in directly detecting the ongoing energy transfer from resonant electrons to whistler-mode waves at the location of the spacecraft in space. From this, they derived the growth rate of the wave. The researchers published their results in Nature Communications.

The most important finding was that the observed results were consistent with the hypothesis that non-linear growth occurs in this interaction. “This is the first time anybody has directly observed the efficient growth of waves in space for the wave-particle interaction between electrons and whistler-mode waves,” explains Kitamura.

“We expect that the results will contribute to research on various wave-particle interactions and to also improve our understanding of the progress of plasma physics research. As more specific phenomena, the results will contribute to our understanding of the acceleration of electrons to high energies in the radiation belt, which are sometimes called ‘killer electrons’ because they inflict damage on satellites, as well as the loss of high-energy electrons in the atmosphere, which form diffuse auroras.”

More information: N. Kitamura et al, Direct observations of energy transfer from resonant electrons to whistler-mode waves in magnetosheath of Earth, Nature Communications (2022). DOI: 10.1038/s41467-022-33604-2

Journal information: Nature Communications 

Provided by Nagoya University