New possibilities for working with quantum information

Small particles can have an angular momentum that points in a certain direction — the spin. This spin can be manipulated by a magnetic field. This principle, for example, is the basic idea behind magnetic resonance imaging as used in hospitals. An international research team has now discovered a surprising effect in a system that is particularly well suited for processing quantum information: the spins of phosphorus atoms in a piece of silicon, coupled to a microwave resonator. If these spins are cleverly excited with microwave pulses, a so-called spin echo signal can be detected after a certain time — the injected pulse signal is re-emitted as a quantum echo. Surprisingly, this spin echo does not occur only once, but a whole series of echoes can be detected. This opens up new possibilities of how information can be processed with quantum systems.

The experiments were carried out at the Walther-Meissner-Institute in Garching by researchers from the Bavarian Academy of Sciences and Humanities and the Technical University of Munich, the theoretical explanation was developed at TU Wien (Vienna). Now the joint work has been published in the journal Physical Review Letters.

The echo of quantum spins

“Spin echoes have been known for a long time, this is nothing unusual,” says Prof. Stefan Rotter from TU Wien (Vienna). First, a magnetic field is used to make sure that the spins of many atoms point in the same magnetic direction. Then the atoms are irradiated with an electromagnetic pulse, and suddenly their spins begin to change direction.

However, the atoms are embedded in slightly different environments. It is therefore possible that slightly different forces act on their spins. “As a result, the spin does not change at the same speed for all atoms,” explains Dr. Hans Hübl from the Bavarian Academy of Sciences and Humanities. “Some particles change their spin direction faster than others, and soon you have a wild jumble of spins with completely different orientations.”

But it is possible to rewind this apparent chaos — with the help of another electromagnetic pulse. A suitable pulse can reverse the previous spin rotation so that the spins all come together again. “You can imagine it’s a bit like running a marathon,” says Stefan Rotter. “At the start signal, all the runners are still together. As some runners are faster than others, the field of runners is pulled further and further apart over time. However, if all runners were now given the signal to return to the start, all runners would return to the start at about the same time, although faster runners have to cover a longer distance back than slower ones.”

In the case of spins, this means that at a certain point in time all particles have exactly the same spin direction again — and this is called the “spin echo.” “Based on our experience in this field, we had already expected to be able to measure a spin echo in our experiments,” says Hans Hübl. “The remarkable thing is that we were not only able to measure a single echo, but a series of several echoes.”

The spin that influences itself

At first, it was unclear how this novel effect comes about. But a detailed theoretical analysis now made it possible to understand the phenomenon: It is due to the strong coupling between the two components of the experiment — the spins and the photons in a microwave resonator, an electrical circuit in which microwaves can only exist at certain wavelengths. “This coupling is the essence of our experiment: You can store information in the spins, and with the help of the microwave photons in the resonator you can modify it or read it out,” says Hans Hübl.

The strong coupling between the atomic spins and the microwave resonator is also responsible for the multiple echoes: If the spins of the atoms all point in the same direction in the first echo, this produces an electromagnetic signal. “Thanks to the coupling to the microwave resonator, this signal acts back on the spins, and this leads to another echo — and on and on,” explains Stefan Rotter. “The spins themselves cause the electromagnetic pulse, which is responsible for the next echo.”

The physics of the spin echo has great significance for technical applications — it is an important basic principle behind magnetic resonance imaging. The new possibilities offered by the multiple echo, such as the processing of quantum information, will now be examined in more detail. “For sure, multiple echos in spin ensembles coupled strongly to the photons of a resonator are an exciting new tool. It will not only find useful applications in quantum information technology, but also in spin-based spectroscopy methods,” says Rudolf Gross, co-author and director of the Walther-Meissner-Institute.

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Materials provided by Vienna University of Technology. Original written by Florian Aigner. Note: Content may be edited for style and length.

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Physicists develop basic principles for mini-labs on chips

Colloidal particles have become increasingly important for research as vehicles of biochemical agents. In future, it will be possible to study their behaviour much more efficiently than before by placing them on a magnetised chip. A research team from the University of Bayreuth reports on these new findings in the journal Nature Communications. The scientists have discovered that colloidal rods can be moved on a chip quickly, precisely, and in different directions, almost like chess pieces. A pre-programmed magnetic field even enables these controlled movements to occur simultaneously.

For the recently published study, the research team, led by Prof. Dr. Thomas Fischer, Professor of Experimental Physics at the University of Bayreuth, worked closely with partners at the University of Poznán and the University of Kassel. To begin with, individual spherical colloidal particles constituted the building blocks for rods of different lengths. These particles were assembled in such a way as to allow the rods to move in different directions on a magnetised chip like upright chess figures — as if by magic, but in fact determined by the characteristics of the magnetic field.

In a further step, the scientists succeeded in eliciting individual movements in various directions simultaneously. The critical factor here was the “programming” of the magnetic field with the aid of a mathematical code, which in encrypted form, outlines all the movements to be performed by the figures. When these movements are carried out simultaneously, they take up to one tenth of the time needed if they are carried out one after the other like the moves on a chessboard.

“The simultaneity of differently directed movements makes research into colloidal particles and their dynamics much more efficient,” says Adrian Ernst, doctoral student in the Bayreuth research team and co-author of the publication. “Miniaturised laboratories on small chips measuring just a few centimetres in size are being used more and more in basic physics research to gain insights into the properties and dynamics of materials. Our new research results reinforce this trend. Because colloidal particles are in many cases very well suited as vehicles for active substances, our research results could be of particular benefit to biomedicine and biotechnology,” says Mahla Mirzaee-Kakhki, first author and Bayreuth doctoral student.

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Scientists probe the chemistry of a single battery electrode particle both inside and out

The particles that make up lithium-ion battery electrodes are microscopic but mighty: They determine how much charge the battery can store, how fast it charges and discharges and how it holds up over time — all crucial for high performance in an electric vehicle or electronic device.

Cracks and chemical reactions on a particle’s surface can degrade performance, and the whole particle’s ability to absorb and release lithium ions also changes over time. Scientists have studied both, but until now they had never looked at both the surface and the interior of an individual particle to see how what happens in one affects the other.

In a new study, a research team led by Yijin Liu at the Department of Energy’s SLAC National Accelerator Laboratory did that. They stuck a single battery cathode particle, about the size of a red blood cell, on a needle tip and probed its surface and interior in 3D with two X-ray instruments. They discovered that cracking and chemical changes on the particle’s surface varied a lot from place to place and corresponded with areas of microscopic cracking deep inside the particle that sapped its capacity for storing energy.

“Our results show that the surface and the interior of a particle talk to each other, basically,” said SLAC lead scientist Yijin Liu, who led the study at the lab’s Stanford Synchrotron Radiation Lightsource (SSRL). “Understanding this chemical conversation will help us engineer the whole particle so the battery can cycle faster, for instance.”

The scientists describe their findings in Nature Communications today.

Damage both inside and out

A lithium-ion battery stores and releases energy by moving lithium ions through an electrolyte back and forth between two electrodes, the anode and the cathode. When you charge the battery, lithium ions rush into the anode for storage. When you use the battery, the ions leave the anode and flow into the cathode, where they generate a flow of electrical current.

Each electrode consists of many microscopic particles, and each particle contains even smaller grains. Their structure and chemistry are key to the battery’s performance. As the battery charges and discharges, lithium ions seep in and out of the spaces between the particles’ atoms, causing them to swell and shrink. Over time this can crack and break particles, reducing their ability to absorb and release ions. Particles also react with the surrounding electrolyte to form a surface layer that gets in the way of ions entering and leaving. As cracks develop, the electrolyte penetrates deeper to damage the interior.

This study focused on particles made from a nickel-rich layered oxide, which can theoretically store more charge than today’s battery materials. It also contains less cobalt, making it cheaper and less ethically problematic, since some cobalt mining involves inhumane conditions, Liu said.

There’s just one problem: The particles’ capacity for storing charge quickly fades during multiple rounds of high-voltage charging – the type used to fast-charge electric vehicles.

“You have millions of particles in an electrode. Each one is like a rice ball with many grains,” Liu said. “They’re the building blocks of the battery, and each one is unique, just like every person has different characteristics.”

Taming a next-gen material

Liu said scientists have been working on two basic approaches for minimizing damage and increasing the performance of particles: Putting a protective coating on the surface and packing the grains together in different ways to change the internal structure. “Either approach could be effective,” Liu said, “but combining them would be even more effective, and that’s why we have to address the bigger picture.”

Shaofeng Li, a visiting graduate student at SSRL who will be joining SLAC as a postdoctoral researcher, led X-ray experiments that examined a single needle-mounted cathode particle from a charged battery with two instruments — one scanning the surface, the other probing the interior. Based on the results, theorists led by Kejie Zhao, an associate professor at Purdue University, developed a computer model showing how charging would have damaged the particle over a period of 12 minutes and how that damage pattern reflects interactions between the surface and interior.

“The picture we are getting is that there are variations everywhere in the particle,” Liu said. “For instance, certain areas on the surface degrade more than others, and this affects how the interior responds, which in turn makes the surface degrade in a different manner.”

Now, he said, the team plans to apply this technique to other electrode materials they have studied in the past, with particular attention to how charging speed affects damage patterns. “You want to be able to charge your electric car in 10 minutes rather than several hours,” he said, “so this is an important direction for follow-up studies.”

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A multinational study overturns a 130-year old assumption about seawater chemistry

There’s more to seawater than salt. Ocean chemistry is a complex mixture of particles, ions and nutrients. And for over a century, scientists believed that certain ion ratios held relatively constant over space and time.

But now, following a decade of research, a multinational study has refuted this assumption. Debora Iglesias-Rodriguez, professor and vice chair of UC Santa Barbara’s Department of Ecology, Evolution, and Marine Biology, and her colleagues discovered that the seawater ratios of three key elements vary across the ocean, which means scientists will have to re-examine many of their hypotheses and models. The results appear in the Proceedings of the National Academy of Sciences.

Calcium, magnesium and strontium (Ca, Mg and Sr) are important elements in ocean chemistry, involved in a number of biologic and geologic processes. For instance, a host of different animals and microbes use calcium to build their skeletons and shells. These elements enter the ocean via rivers and tectonic features, such as hydrothermal vents. They’re taken up by organisms like coral and plankton, as well as by ocean sediment.

The first approximation of modern seawater composition took place over 130 years ago. The scientists who conducted the study concluded that, despite minor variations from place to place, the ratios between the major ions in the waters of the open ocean are nearly constant.

Researchers have generally accepted this idea from then on, and it made a lot of sense. Based on the slow turnover of these elements in the ocean — on the order of millions of years — scientists long thought the ratios of these ions would remain relatively stable over extended periods of time.

“The main message of this paper is that we have to revisit these ratios,” said Iglesias-Rodriguez. “We cannot just continue to make the assumptions we have made in the past essentially based on the residency time of these elements.”

Back in 2010, Iglesias-Rodriguez was participating in a research expedition over the Porcupine Abyssal Plain, a region of North Atlantic seafloor west of Europe. She had invited a former student of hers, this paper’s lead author Mario Lebrato, who was pursuing his doctorate at the time.

Their study analyzed the chemical composition of water at various depths. Lebrato found that the Ca, Mg and Sr ratios from their samples deviated significantly from what they had expected. The finding was intriguing, but the data was from only one location.

Over the next nine years, Lebrato put together a global survey of these element ratios. Scientists including Iglesias-Rodriguez collected over 1,100 water samples on 79 cruises ranging from the ocean’s surface to 6,000 meters down. The data came from 14 ecosystems across 10 countries. And to maintain consistency, all the samples were processed by a single person in one lab.

The project’s results overturned the field’s 130-year old assumption about seawater chemistry, revealing that the ratio of these ions varies considerably across the ocean.

Scientists have long used these ratios to reconstruct past ocean conditions, like temperature. “The main implication is that the paleo-reconstructions we have been conducting have to be revisited,” Iglesias-Rodriguez explained, “because environmental conditions have a substantial impact on these ratios, which have been overlooked.”

Oceanographers can no longer assume that data they have on past ocean chemistry represent the whole ocean. It has become clear they can extrapolate only regional conditions from this information.

This revelation also has implications for modern marine science. Seawater ratios of Mg to Ca affect the composition of animal shells. For example, a higher magnesium content tends to make shells more vulnerable to dissolution, which is an ongoing issue as increasing carbon dioxide levels gradually make the ocean more acidic. “Biologically speaking, it is important to figure out these ratios with some degree of certainty,” said Iglesias-Rodriguez.

Iglesias-Rodriguez’s latest project focuses on the application of rock dissolution as a method to fight ocean acidification. She’s looking at lowering the acidity of seawater using pulverized stones like olivine and carbonate rock. This intervention will likely change the balance of ions in the water, which is something worth considering. As climate change continues unabated, this intervention could help keep acidity in check in small areas, like coral reefs.

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Machine learning peeks into nano-aquariums

In the nanoworld, tiny particles such as proteins appear to dance as they transform and assemble to perform various tasks while suspended in a liquid. Recently developed methods have made it possible to watch and record these otherwise-elusive tiny motions, and researchers now take a step forward by developing a machine learning workflow to streamline the process.

The new study, led by Qian Chen, a professor of materials science and engineering at the University of Illinois, Urbana-Champaign, builds upon her past work with liquid-phase electron microscopy and is published in the journal ACS Central Science.

Being able to see — and record — the motions of nanoparticles is essential for understanding a variety of engineering challenges. Liquid-phase electron microscopy, which allows researchers to watch nanoparticles interact inside tiny aquariumlike sample containers, is useful for research in medicine, energy and environmental sustainability and in fabrication of metamaterials, to name a few. However, it is difficult to interpret the dataset, the researchers said. The video files produced are large, filled with temporal and spatial information, and are noisy due to background signals — in other words, they require a lot of tedious image processing and analysis.

“Developing a method even to see these particles was a huge challenge,” Chen said. “Figuring out how to efficiently get the useful data pieces from a sea of outliers and noise has become the new challenge.”

To confront this problem, the team developed a machine learning workflow that is based upon an artificial neural network that mimics, in part, the learning potency of the human brain. The program builds off of an existing neural network, known as U-Net, that does not require handcrafted features or predetermined input and has yielded significant breakthroughs in identifying irregular cellular features using other types of microscopy, the study reports.

“Our new program processed information for three types of nanoscale dynamics including motion, chemical reaction and self-assembly of nanoparticles,” said lead author and graduate student Lehan Yao. “These represent the scenarios and challenges we have encountered in the analysis of liquid-phase electron microscopy videos.”

The researchers collected measurements from approximately 300,000 pairs of interacting nanoparticles, the study reports.

As found in past studies by Chen’s group, contrast continues to be a problem while imaging certain types of nanoparticles. In their experimental work, the team used particles made out of gold, which is easy to see with an electron microscope. However, particles with lower elemental or molecular weights like proteins, plastic polymers and other organic nanoparticles show very low contrast when viewed under an electron beam, Chen said.

“Biological applications, like the search for vaccines and drugs, underscore the urgency in our push to have our technique available for imaging biomolecules,” she said. “There are critical nanoscale interactions between viruses and our immune systems, between the drugs and the immune system, and between the drug and the virus itself that must be understood. The fact that our new processing method allows us to extract information from samples as demonstrated here gets us ready for the next step of application and model systems.”

The team has made the source code for the machine learning program used in this study publicly available through the supplemental information section of the new paper. “We feel that making the code available to other researchers can benefit the whole nanomaterials research community,” Chen said.

See the liquid-phase electron microscopy with combined machine learning in action:

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Thinking small: New ideas in the search for dark matter

Since the 1980s, researchers have been running experiments in search of particles that make up dark matter, an invisible substance that permeates our galaxy and universe. Coined dark matter because it gives off no light, this substance, which constitutes more than 80 percent of matter in our universe, has been shown repeatedly to influence ordinary matter through its gravity. Scientists know it is out there but do not know what it is.

So researchers at Caltech, led by Kathryn Zurek, a professor of theoretical physics, have gone back to the drawing board to think of new ideas. They have been looking into the possibility that dark matter is made up of “hidden sector” particles, which are lighter than particles proposed previously, and could, in theory, be found using small, underground table-top devices. In contrast, scientists are searching for heavier dark matter candidates called WIMPs (weakly interacting massive particles) using large-scale experiments such as XENON, which is installed underground in a 70,000-gallon tank of water in Italy.

“Dark matter is always flowing through us, even in this room” says Zurek, who first proposed hidden sector particles over a decade ago. “As we move around the center of the galaxy, this steady wind of dark matter mostly goes unnoticed. But we can still take advantage of that source of dark matter, and design new ways to look for rare interactions between the dark matter wind and the detector.”

In a new paper accepted for publication in the journal Physical Review Letters, the physicists outline how the lighter-weight dark matter particles could be detected via a type of quasiparticle known as a magnon. A quasiparticle is an emergent phenomenon that occurs when a solid behaves as if it contains weakly interacting particles. Magnons are a type of quasiparticle in which electron spins — which act like little magnets — are collectivity excited. In the researchers’ idea for a table-top experiment, a magnetic crystalized material would be used to look for signs of excited magnons generated by dark matter.

“If the dark matter particles are lighter than the proton, it becomes very difficult to detect their signal by conventional means,” says study author Zhengkang (Kevin) Zhang, a postdoctoral scholar at Caltech. “But, according to many well-motivated models, especially those involving hidden sectors, the dark matter particles can couple to the spins of the electrons, such that once they strike the material, they will induce spin excitations, or magnons. If we reduce the background noise by cooling the equipment and moving it underground, we could hope to detect magnons generated solely by dark matter and not ordinary matter.”

Such an experiment is only theoretical at this point but may eventually take place using small devices housed underground, likely in a mine, where outside influences from other particles, such as those in cosmic rays, can be minimized.

One telltale sign of a dark matter detection in the table-top experiments would be changes to the signal that depend on the time of day. This is due to the fact that the magnetic crystals that would be used to detect the dark matter can be anisotropic, meaning that the atoms are naturally arranged in such a way that they tend to interact with the dark matter more strongly when the dark matter comes in from certain directions.

“As Earth moves through the galactic dark matter halo, it feels the dark matter wind blowing from the direction into which the planet is moving. A detector fixed at a certain location on Earth rotates with the planet, so the dark matter wind hits it from different directions at different times of the day, say, sometimes from above, sometimes from the side,” says Zhang.

“During the day, for example, you may have a higher detection rate when the dark matter comes from above than from the side. If you saw that, it would be pretty spectacular and a very strong indication that you were seeing dark matter.”

The researchers have other ideas about how dark matter may reveal itself, in addition to through magnons. They have proposed that the lighter dark matter particles could be detected via photons as well as with another type of quasiparticle called a phonon, which is caused by vibrations in a crystal lattice. Preliminary experiments based on photons and phonons are underway at UC Berkeley, where the team was based prior to Zurek joining the Caltech faculty in 2019. The researchers say that the use of these multiple strategies to look for dark matter is crucial because they complement each other and would help confirm each other’s results.

“We’re looking into new ways to look for dark matter because, given how little we know about dark matter, it’s worth considering all the possibilities,” says Zhang.

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New research deepens understanding of Earth’s interaction with the solar wind

As the Earth orbits the sun, it plows through a stream of fast-moving particles that can interfere with satellites and global positioning systems. Now, a team of scientists at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) and Princeton University has reproduced a process that occurs in space to deepen understanding of what happens when the Earth encounters this solar wind.

The team used computer simulations to model the movement of a jet of plasma, the charged state of matter composed of electrons and atomic nuclei that makes up all the stars in the sky, including our sun. Many cosmic events can produce plasma jets, from relatively small star burps to gigantic stellar explosions known as supernovae. When fast-moving plasma jets pass through the slower plasma that exists in the void of space, it creates what is known as a collision-less shock wave.

These shocks also occur as Earth moves through the solar wind and can influence how the wind swirls into and around Earth’s magnetosphere, the protective magnetic shield that extends into space. Understanding plasma shock waves could help scientists to forecast the space weather that develops when the solar wind swirls into the magnetosphere and enable the researchers to protect satellites that allow people to communicate across the globe.

The simulations revealed several telltale signs indicating when a shock is forming, including the shock’s features, the three stages of the shock’s formation, and phenomena that could be mistaken for a shock. “By being able to distinguish a shock from other phenomena, scientists can feel confident that what they are seeing in an experiment is what they want to study in space,” said Derek Schaeffer, an associate research scholar in the Princeton University Department of Astrophysics who led the PPPL research team. The findings were reported in a paper published in Physics of Plasmas that followed up on previous research reported here and here.

The plasma shocks that occur in space, like those created by Earth traveling against the solar wind, resemble the shock waves created in Earth’s atmosphere by supersonic jet aircraft. In both occurrences, fast-moving material encounters slow or stationary material and must swiftly change its speed, creating an area of swirls and eddies and turbulence.

But in space, the interactions between fast and slow plasma particles occur without the particles touching one another. “Something else must be driving this shock formation, like the plasma particles electrically attracting or repelling each other,” Schaeffer said. “In any case, the mechanism is not fully understood.”

To increase their understanding, physicists conduct plasma experiments in laboratories to monitor conditions closely and measure them precisely. In contrast, measurements taken by spacecraft cannot be easily repeated and sample only a small region of plasma. Computer simulations then help the physicists interpret their laboratory data.

Today, most laboratory plasma shocks are formed using a mechanism known as a plasma piston. To create the piston, scientists shine a laser on a small target. The laser causes small amounts of the target’s surface to heat up, become a plasma, and move outward through a surrounding, slower-moving plasma.

Schaeffer and colleagues produced their simulation by modeling this process. “Think of a boulder in the middle of fast-moving stream,” Schaeffer said. “The water will come right up to the front of the boulder, but not quite reach it. The transition area between quick motion and zero [standing] motion is the shock.”

The simulated results will help physicists distinguish an astrophysical plasma shock wave from other conditions that arise in laboratory experiments. “During laser plasma experiments, you might observe lots of heating and compression and think they are signs of a shock,” Schaeffer said. “But we don’t know enough about the beginning stages of a shock to know from theory alone. For these kinds of laser experiments, we have to figure out how to tell the difference between a shock and just the expansion of the laser-driven plasma.”

In the future, the researchers aim to make the simulations more realistic by adding more detail and making the plasma density and temperature less uniform. They would also like to run experiments to determine whether the phenomena predicted by the simulations can in fact occur in a physical apparatus. “We’d like to put the ideas we talk about in the paper to the test,” says Schaeffer.

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A potential explanation for urban smog

The effect of nitric acid on aerosol particles in the atmosphere may offer an explanation for the smog seen engulfing cities on frosty days. Under laboratory conditions, researchers at CERN in Switzerland observed the formation of atmospheric aerosols and discovered new information on the link between nitrogen oxides, originating in traffic and the energy industry, and the climate and air quality. These findings were published in the Nature and Science Advances journals.

Based on the findings, nitrogen compounds can, depending on the circumstances, either slow down or accelerate the growth of aerosol particles. This means that reducing sulphur dioxide is not on its own enough to prevent the smog problem seen in large cities. Instead, a comprehensive understanding of the atmospheric particle formation process is needed.

Earlier, nitric acid was not thought to have a significant effect on the formation or early growth of aerosol particles, even though nitrate compounds often occur in larger particles. However, the study published in Nature demonstrates that, in cold climates, nitric acid can boost particle growth to a marked degree, and even form particles together with ammonia in temperatures under -15°C. This is significant, as there are up to a thousand times more nitric acid and ammonia than sulphuric acid in the atmosphere.

The discovery could explain why particles are formed even in highly polluted big cities, notwithstanding the established knowledge according to which pollutants should prevent the formation and growth of new particles. The same mechanism may also generate particles higher up in the atmosphere, where the temperature is always cold and nitrogen oxides are produced as a result of lightnings.

At the same time, nitrogen oxides also affect the oxidation characteristics of organic compounds in the atmosphere. In the project headed by University of Helsinki researchers, it was found that nitrogen oxides increase the volatility of the oxidation products of organic compounds. As a result, particle growth slows down and a smaller share of particles survive compared to circumstances where the air is clean. In areas where particle growth is promoted mainly by organic compounds, such as in the boreal forest zone, the phenomenon can reduce the number of aerosols that form clouds, indirectly resulting in warming the climate. This study was published in the Science Advances journal.

Both studies are based on laboratory experiments carried out at CERN, the European Organization for Nuclear Research. The CLOUD chamber at CERN makes it possible to investigate the formation and growth of aerosol particles with unparalleled precision. The Finnish participants in the CLOUD experiments include the University of Helsinki’s Institute for Atmospheric and Earth System Research, the Finnish Meteorological Institute and the University of Eastern Finland.

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Quantum copycat: Researchers find a new way in which bosons behave like fermions

Bosons and fermions, the two classes into which all particles — from the sub-atomic to atoms themselves — can be sorted, behave very differently under most circumstances. While identical bosons like to congregate, identical fermions tend to be antisocial. However, in one dimension — imagine particles that can only move on a line — bosons can become as stand-offish as fermions, so that no two occupy the same position. Now, new research shows that the same thing — bosons acting like fermions — can happen with their velocities. The finding adds to our fundamental understanding of quantum systems and could inform the eventual development of quantum devices.

“All particles in nature come in one of two types, depending on their ‘spin,’ a quantum property with no real analogue in classical physics,” said David Weiss, Distinguished Professor of Physics at Penn State and one of the leaders of the research team. “Bosons, whose spins are whole integers, can share the same quantum state, while fermions, whose spins are half integers, cannot. When the particles are cold or dense enough, bosons behave completely differently from fermions. Bosons form ‘Bose-Einstein condensates,’ congregating in the same quantum state. Fermions, on the other hand, fill available states one by one to form what is called a ‘Fermi sea.'”

Researchers at Penn State have now experimentally demonstrated that, when bosons expand in one dimension — the line of atoms is allowed spread out to become longer — they can form a Fermi sea. A paper describing the research appears March 27, 2020 in the journal Science.

“Identical fermions are antisocial, you can’t have more than one in the same place so when they are very cold they don’t interact,” said Marcos Rigol, professor of physics at Penn State and the other leader of the research team. “Bosons can be in the same place, but this becomes energetically too costly when their interactions are very strong. As a result, when constrained to move in one-dimension, their spatial distribution can look like that of non-interacting fermions. Back in 2004, David’s research group experimentally demonstrated this phenomenon, which was theoretically predicted in the 1960s.”

Even though the spatial properties of strongly interacting bosons and non-interacting fermions are the same in one dimension, bosons can still have the same velocities as each other, while fermions cannot. This is due to the fundamental nature of the particles.

“In 2005, Marcos, then a graduate student, predicted that when strongly interacting bosons expand in one dimension, their velocity distribution will form a Fermi sea,” said Weiss. “I was very excited to collaborate with him in demonstrating this striking phenomenon.”

The research team creates an array of ultracold one-dimensional gases made up of bosonic atoms (‘Bose gases’) using an optical lattice, which uses laser light to trap the atoms. In the light trap, the system is at equilibrium and the strongly interacting Bose gases have spatial distributions like fermions, but still have the velocity distributions of bosons. When the researchers shut off some of the trapping light, the atoms expand in one dimension. During this expansion, the velocity distribution of the bosons smoothly transforms into a one that is identical to fermions. The researchers can follow this transformation as it happens.

“The dynamics of ultracold gases in optical lattices are the source of many novel fascinating phenomena that only recently have started to be explored,” said Rigol. “For example, Dave’s group showed in 2006 that something as universal as temperature is not well defined after Bose gases undergo dynamics in one dimension. My collaborators and I related this finding to a beautiful underlying mathematical property of the theoretical models describing his experiments, known as ‘integrability.’ Integrability plays a central role in our newly observed dynamical fermionization phenomenon.”

Because the system is “integrable,” the researchers can understand it in great detail and by studying the dynamical behavior of these one-dimensional gases, the Penn State team hopes to address broad issues in physics.

“In the last half century many universal properties of equilibrium quantum systems have been elucidated,” said Weiss. “It has been harder to identify universal behavior in dynamical systems. By fully understanding the dynamics of one-dimensional gases, and then by gradually making the gases less integrable, we hope to identify universal principles in dynamical quantum systems.”

Dynamical, interacting quantum systems are an important part of fundamental physics. They are also increasing technologically relevant, as many actual and proposed quantum devices are based on them, including quantum simulators and quantum computers.

“We now have experimental access to things that if you would have asked any theorist working in the field ten years ago ‘will we see this in our lifetime?’ they would have said ‘no way,'” said Rigol.

In addition to Rigol and Weiss, the research team at Penn State includes Joshua M. Wilson, Neel Malvania, Yuan Le, and Yicheng Zhang. The research was funded by the U.S. National Science Foundation and the U.S. Army Research Office. Computations were performed at the Penn State Institute for Computational and Data Sciences.

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Frozen-planet states in exotic helium atoms

Exotic subatomic particles that are like ‘normal’ particles apart from one, opposite, property — such as the positron, which is like an electron but positively rather than negatively charged — are collectively known as antimatter. Direct studies of collisions between particles of matter and those of antimatter using giant facilities such as those at CERN can advance our understanding of the nature of matter. A new study by Tasko Grozdanov from the University of Belgrade in Serbia and Evgeni Solov’ev from the Institute of Nuclear Research near Moscow in Russia has mapped the energy levels of an exotic form of helium produced in this way. This work, which is published in EPJ D, has been described by one commentator as ‘… a new jewel in the treasure of scientific achievements in atomic physics theory.”

An atom of ordinary helium consists of a nucleus with two protons and two neutrons surrounded by two electrons. Experiments at CERN have involved colliding slow antiprotons with these helium atoms to form an exotic form of helium called antiprotonic helium, in which one of the electrons is replaced with an antiproton (a particle like a proton but with the negative charge of an electron). Thus, an atom of antiprotonic helium is uncharged, like ordinary helium, but includes one negatively-charged particle over 1800 times heavier than an electron.

Antiprotonic helium atoms can only survive in configurations in which the antiproton cannot ‘fall’ into the nucleus and annihilate. Until now, the only widely studied configuration involves antiproton making circular orbits around the nucleus, shielded by the remaining electron. Grozdanov and Solov’ev describe a different configuration, named a ‘frozen planet’ state, in which the electron rapidly circulates round the nucleus, generating a potential well that traps the antiproton. The period of time in which the antiproton can remain trapped in this well depends on its energy and the distance from the nucleus. The researchers plan to extend their studies to include similar configurations that rotate, which they suggest may be more amenable to experimental research.

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Journal Reference:

  1. Tasko P. Grozdanov, Evgeni A. Solov’ev. Hidden-crossing explanation of frozen-planet resonances in antiprotonic helium; their positions and widths. The European Physical Journal D, 2020; 74 (3) DOI: 10.1140/epjd/e2020-100565-0

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