| Physics ru Scientists succeed in measuring electron spin qubit without demolishing it Mon, 02 Mar 2020 10:00:02 +0000 nonadult chiefeditor Наука A group of scientists from the RIKEN Center for Emergent Matter Science in Japan have succeeded in taking repeated measurements of the spin of an electron in a silicon quantum dot (QD), without changing the spin in the process. This type of "non-demolition" measurement is important for creating quantum computers that are fault tolerant. Quantum computers promise to make it easier to perform certain classes of calculations such as many-body problems, which are extremely difficult and time-consuming for conventional computers. Essentially, the involve measuring a quantum value which is never in a single state like a conventional transistor, but instead exists as a "superimposed state" - in the same way that Schrodinger's famous cat cannot be said to be alive or dead until it is observed. Using such systems, it is possible to conduct calculations with a qubit that is a superimposition of two values, and then determine statistically what the correct result is. Quantum computers that use single electron spins in silicon QDs are seen as attractive due to their potential scalability and because silicon is already widely used in electronics technology.

The key difficulty, however, with developing quantum computers is that they are very sensitive to external noise, making error correction critical. So far, researchers have succeeded in developing single electron spins in silicon QDs with a long information retention time and high-precision quantum operation, but quantum non-demolition measurement - a key to effective error correction - has proven elusive. The conventional method for reading out single electron spins in silicon is to convert the spins into charges that can be rapidly detected, but unfortunately, the electron spin is affected by the detection process.

Now, in research published in Nature Communications, the RIKEN team has achieved such non-demolition measurement. The key insight that allowed the group to make the advance was to use the Ising type interaction model - a model of ferromagnetism that looks at how the electron spins of neighboring atoms become aligned, leading to the formation of ferromagnetism in the entire lattice. Essentially, they were able to transfer the spin information - up or down - of an electron in a QD to another electron in the neighboring QD using the Ising type interaction in a magnetic field, and then could measure the spin of the neighbor using the conventional method, so that they could leave the original spin unaffected, and could carry out repeated and rapid measurements of the neighbor.

"Through this," explains Group Director Seigo Tarucha, who led the research group, "we were able to achieve a non-demolition fidelity rate of 99%, and by using repeated measurements would get a readout accuracy of 95%. We have also shown that theoretically, this could be increased to out 99.6%, and plan to continue work toward reaching that level."


Watching magnetic nano ‘tornadoes’ in 3D Mon, 24 Feb 2020 16:00:03 +0000 nonadult chiefeditor Наука The team, from the Universities of Cambridge and Glasgow in the UK and ETH Zurich and the Paul Scherrer Institute in Switzerland, used their technique to observe how the magnetisation behaves, the first time this has been done in three dimensions. The technique, called time-resolved magnetic laminography, could be used to understand and control the behaviour of new types of magnets for next-generation data storage and processing. The results are reported in the journal Nature Nanotechnology.











New green technology from UMass Amherst generates electricity ‘out of thin air’ Mon, 17 Feb 2020 16:00:11 +0000 nonadult chiefeditor Наука Scientists at the University of Massachusetts Amherst have developed a device that uses a natural protein to create electricity from moisture in the air, a new technology they say could have significant implications for the future of renewable energy, climate change and in the future of medicine.











Fragile topology: Two new studies explain the strange electron flow in future materials Thu, 13 Feb 2020 19:00:52 +0000 nonadult chiefeditor Наука Electrons race along the surface of certain unusual crystalline materials, except that sometimes they don't. Two new studies from Princeton researchers and their collaborators explain the source of the surprising behavior and chart a course for restoring conductivity in these remarkable crystals, prized for their potential use in future technologies including quantum computers.

The studies were published in the journal Science.

For the past 15 years, a class of materials known as topological insulators has dominated the search for the materials of the future. These crystals have an uncommon property: Their interiors are insulators - where electrons cannot flow - but their surfaces are perfect conductors, where electrons flow without resistance.

That was the picture until the discovery two years ago that some topological materials are actually unable to conduct current on their surface, a phenomenon that earned the name "fragile topology."

"Fragile topology is a strange beast: It is now predicted to exist in hundreds of materials," said B. Andrei Bernevig, a professor of physics at Princeton and co-author on both papers. "It is as if the usual principle that we have been relying on to experimentally determine a topological state breaks down."

To get a handle on how fragile states form, the researchers turned to two resources: mathematical equations and 3D printers. With Luis Elcoro at the University of the Basque Country, Bernevig and Princeton postdoctoral researcher Zhi-Da Song constructed a mathematical theory to explain what is happening inside the materials.

Next, Sebastian Huber and his team at ETH Zurich, in collaboration with researchers at Princeton, Weizmann Institute of Science in Israel, South China University of Technology, and Wuhan University, tested the theory by building a life-sized topological material out of 3D-printed plastics.

Topological materials draw their name from the field of mathematics that explains how shapes such as donuts and coffee cups are related (they both have one hole). The same principles can explain how electrons hop from atom to atom on the surface of the roughly 20,000 or so topological materials identified to date. The theoretical underpinnings of topological materials earned a 2016 Nobel Prize in Physics for F. Duncan Haldane, Princeton's Sherman Fairchild University Professor of Physics.

What makes these crystals so interesting to scientists is their paradoxical electronic properties. The interior of the crystal has no ability to conduct current - it is an insulator. But cut the crystal in half, and the electrons will skim across the newly revealed surfaces without any resistance, protected by their topological nature.

The explanation lies in the connection between the electrons on the surface and those in the interior, or bulk. Electrons can be thought of not as individual particles but as waves that spread out like ripples of water from a pebble tossed in a pond. In this quantum mechanical view, each electron's location is described by a spreading wave that is called a quantum wavefunction. In a topological material, the quantum wavefunction of an electron in the bulk spreads to the edge of the crystal, or surface boundary. This correspondence between the bulk and the boundary leads to a perfectly conducting surface state.

This principle of "bulk-boundary correspondence" to explain topological surface conduction was widely accepted until two years ago, when a handful of scientific papers revealed the existence of fragile topology. Unlike the usual topological states, fragile topological states do not have conducting surface states.

"The usual bulk-boundary correspondence principle breaks down," Bernevig said.

But exactly how remained a puzzle.

In the first of the two Science papers, Bernevig, Song and Elcoro provide a theoretical explanation for a new bulk-boundary correspondence to explain fragile topology. The collaborators show that the electron wavefunction of fragile topology only extends to the surface under specific conditions, which the researchers call a twisted bulk-boundary-correspondence.

The team further found that the twisted bulk-boundary-correspondence can be tuned so that the conducting surface states reappear.

"Based on the wavefunction shapes, we designed a set of mechanisms to introduce interference on the boundary in such a way that the boundary state necessarily becomes perfectly conducting," said Luis Elcoro, a professor at the University of the Basque Country.

Finding new overarching principles is something that always intrigues physicists, but this new kind of bulk-boundary-correspondence might also have some practical value, according to the researchers.

"The twisted bulk-boundary-correspondence of fragile topology provides a potential procedure to control the surface state, which might be useful in mechanical, electronic and optical applications," Song said.

But proving that the theory works was virtually impossible given that one would have to interfere with the boundaries at infinitesimally small atomic scales. So the team turned to collaborators to build a life-sized model with which to explore their ideas.

In the second Science paper, Sebastian Huber and his team at ETH Zurich built a large-scale mock topological crystal out of plastic using 3D printed parts. They used sound waves to represent the electron wavefunctions. They inserted barriers to block the path of the sound waves, which is analogous to cutting the crystal to reveal the conducting surfaces. In this way, the researchers mimicked the twisted boundary condition, and then showed that by manipulating it, they could demonstrate that a freely conducting sound wave travels across the surface.

"This was a very left-field idea and realization," Huber said. "We can now show that virtually all topological states that have been realized in our artificial systems are fragile, and not stable as was thought in the past. This work provides that confirmation, but much more, it introduces a new overarching principle."

Entangled laser beams improve the resolution of gravitational-wave detectors Tue, 11 Feb 2020 10:47:09 +0000 nonadult chiefeditor Наука Quantum noise is a limiting issue in high-precision measurements of gravitational waves. Physicists lead by Prof. Roman Schnabel at the “Quantum Universe” Excellence cluster at the University of Hamburg proved in a test experiment that quantum entanglement of laser light can increase the sensitivity of gravitational-wave observatories. The results of their studies where published in the recent edition of Nature Photonics.

Gravitational-wave observatories measure short-term stretches and compressions of space-time as caused by inspirals of two black holes or neutron stars. The observatories use laser interferometers that send two laser beams in perpendicular directions to mirrors at distances of several kilometers and combine the reflected beams.

Quantum nature of laser light, as manifested in the uncertainty of the photon rate, limits the measurement sensitivity of the observatories in two ways. On the one hand, the strength of the photon radiation pressure on the mirrors is uncertain, which produces a fluctuating motion of the mirrors, masking signal from gravitational waves. On the other hand, the light power in the output of the interferometer fluctuates, producing noise in the photodetectors. It is possible to reduce these noises by using more light power, heavier mirrors, longer arms in the detector or light with squeezed quantum uncertainty.

Since April 2019, all gravitational-wave observatories use laser light with squeezed quantum uncertainty to increase the measurement sensitivity. However, this technique only allows improvements for signals above 100 Hz, where fluctuations in the output laser power dominates. The newly invented technique enables scientists to increase the sensitivity for gravitational-wave signal below 100 Hz, by additionally suppressing quantum radiation-pressure fluctuations. For this purpose, the laser beam of the observatory is entangled with a second, weak laser beam. The quantum uncertainties of entangled light beams are correlated. When overlapping the entangled beams in the output of the observatory the correlated uncertainties cancel each other almost perfectly at all signal frequencies. This increases the resolution for all gravitational-wave signals. With this technique the costs for the operation of a gravitational-wave observatory can potentially be decreased significantly.

“For many years it was unclear how we can use the quantum correlations to simultaneously reduce the uncertain radiation pressure as well as the uncertain light power in the output”, Roman Schnabel explains. “Our calculations from two years ago showed that two entangled laser fields need to have a little shift in their wavelengths, which in turn depends on the distance of the mirrors in the arms of the observatory”.

Dr. Jan Südbeck has received a doctorate on this topic at the University of Hamburg.

He adds, “This we proved in a small-scale setup, which emulates a gravitational-wave detector. Our technique can directly be transferred to actual gravitational-wave observatories.”

Using sound and light to generate ultra-fast data transfer Tue, 11 Feb 2020 10:00:08 +0000 nonadult chiefeditor Наука Researchers have made a breakthrough in the control of terahertz quantum cascade lasers, which could lead to the transmission of data at the rate of 100 gigabits per second - around one thousand times quicker than a fast Ethernet operating at 100 megabits a second.

What distinguishes terahertz quantum cascade lasers from other lasers is the fact that they emit light in the terahertz range of the electromagnetic spectrum. They have applications in the field of spectroscopy where they are used in chemical analysis.

The lasers could also eventually provide ultra-fast, short-hop wireless links where large datasets have to be transferred across hospital campuses or between research facilities on universities - or in satellite communications.

To be able to send data at these increased speeds, the lasers need to be modulated very rapidly: switching on and off or pulsing around 100 billion times every second.

Engineers and scientists have so far failed to develop a way of achieving this.

A research team from the University of Leeds and University of Nottingham believe they have found a way of delivering ultra- fast modulation, by combining the power of acoustic and light waves. They have published their findings today (February 11th) in Nature Communications.

John Cunningham, Professor of Nanoelectronics at Leeds, said: "This is exciting research. At the moment, the system for modulating a quantum cascade laser is electrically driven - but that system has limitations. Ironically, the same electronics that delivers the modulation usually puts a brake on the speed of the modulation. The mechanism we are developing relies instead on acoustic waves."

A quantum cascade laser is very efficient. As an electron passes through the optical component of the laser, it goes through a series of 'quantum wells' where the energy level of the electron drops and a photon or pulse of light energy is emitted.

One electron is capable of emitting multiple photons. It is this process that is controlled during the modulation.

Instead of using external electronics, the teams of researchers at Leeds and Nottingham Universities used acoustic waves to vibrate the quantum wells inside the quantum cascade laser.

The acoustic waves were generated by the impact of a pulse from another laser onto an aluminium film. This caused the film to expand and contract, sending a mechanical wave through the quantum cascade laser.

Tony Kent, Professor of Physics at Nottingham said "Essentially, what we did was use the acoustic wave to shake the intricate electronic states inside the quantum cascade laser. We could then see that its terahertz light output was being altered by the acoustic wave."

Professor Cunningham added: "We did not reach a situation where we could stop and start the flow completely, but we were able to control the light output by a few percent, which is a great start. We believe that with further refinement, we will be able to develop a new mechanism for complete control of the photon emissions from the laser, and perhaps even integrate structures generating sound with the terahertz laser, so that no external sound source is needed."

Professor Kent said: "This result opens a new area for physics and engineering to come together in the exploration of the interaction of terahertz sound and light waves, which could have real technological applications."

New droplet-based electricity generator: A drop of water generates 140V power, lighting up 100 LED bulbs Wed, 05 Feb 2020 18:00:31 +0000 nonadult chiefeditor Наука Generating electricity from raindrops efficiently has gone one step further. A research team led by scientists from the City University of Hong Kong (CityU) has recently developed a droplet-based energy generator (DEG), featured with a field-effect transistor (FET)-like structure that allows for high energy-conversion efficiency and instantaneous power density increased by thousands times compared to its counterparts without FET-like structure. This would help to advance scientific research of water energy generation and tackle the energy crisis.

The research was led together by Professor Wang Zuankai from CityU's Department of Mechanical Engineering, Professor Zeng Xiao Cheng from University of Nebraska-Lincoln, and Professor Wang Zhong Lin, Founding Director and Chief Scientist from Beijing Institute of Nanoenergy and Nanosystems of Chinese Academy of Sciences. Their findings were published in the latest issue of the highly prestigious scientific journal Nature, titled "A droplet-based electricity generator with high instantaneous power density".

Efficiency of electrical energy conversion greatly improved

Hydropower is nothing new. About 70% of the earth's surface is covered by water. Yet low-frequency kinetic energy contained in waves, tides, and even raindrops are not efficiently converted into electrical energy due to limitations in current technology. For example, a conventional droplet energy generator based on the triboelectric effect can generate electricity induced by contact electrification and electrostatic induction when a droplet hits a surface. However, the amount of charges generated on the surface is limited by the interfacial effect, and as a result, the energy conversion efficiency is quite low.

In order to improve the conversion efficiency, the research team has spent two years developing the DEG. Its instantaneous power density can reach up to 50.1 W/m2, thousands times higher than other similar devices without the use of FET-like design. And the energy conversion efficiency is markedly higher.

Professor Wang from CityU pointed out that there are two crucial factors for the invention. First, the team found that the continuous droplets impinging on PTFE, an electret material with a quasi-permanent electric charge, provides a new route for the accumulation and storage of high-density surface charges. They found that when water droplets continuously hit the surface of PTFE, the surface charges generated will accumulate and gradually reach a saturation. This new discovery helped to overcome the bottleneck of low charge density encountered in previous work.

Unique field-effect transistor-like structure

Another key feature of their design is a unique set of structures similar to a FET, which is a Nobel Prize in Physics winning innovation in 1956 and has become the basic building block of modern electronic devices nowadays. The device consists of an aluminium electrode, and an indium tin oxide (ITO) electrode with a film of PTFE deposited on it. The PTFE/ITO electrode is responsible for the charge generation, storage, and induction. When a falling water droplet hits and spreads on the PTFE/ITO surface, it naturally "bridges" the aluminium electrode and the PTFE/ITO electrode, translating the original system into a closed-loop electric circuit.

With this special design, a high density of surface charges can be accumulated on the PTFE through continuous droplet impinging. Meanwhile, when the spreading water connects the two electrodes, all the stored charges on the PTFE can be fully released for the generation of electric current. As a result, both the instantaneous power density and energy conversion efficiency are much higher.

"Our research shows that a drop of 100 microlitres (1 microlitre = one-millionth litre) of water released from a height of 15 cm can generate a voltage of over 140V. And the power generated can light up 100 small LED light bulbs," said Professor Wang.

He added that the increase in instantaneous power density does not result from additional energy, but from the conversion of kinetic energy of water itself.

"The kinetic energy entailed in falling water is due to gravity and can be regarded as free and renewable. It should be better utilized."

Their research also shows that the reduction in relative humidity does not affect the efficiency of power generation. Also, both rainwater and seawater can be used to generate electricity.

Facilitates the sustainability of the world

Professor Wang hoped that the outcome of this research would help to harvest water energy to respond to the global problem of renewable energy shortage.

"Generating power from raindrops instead of oil and nuclear energy can facilitate the sustainable development of the world," he added.

He believed that in the long run, the new design could be applied and installed on different surfaces, where liquid in contact with solid, to fully utilize the low-frequency kinetic energy in water. This can range from the hull surface of ferry, coastline, to the surface of umbrellas or even inside water bottles.

Breakthrough made on the next big step to building the world’s most powerful particle accelerator Wed, 05 Feb 2020 18:00:05 +0000 nonadult chiefeditor Наука For the first time scientists have observed muon ionization cooling – a major step in being able to create the world’s most powerful particle accelerator. This new muon accelerator will give us a better understanding of the fundamental constituents of matter.

Since the 1930s, accelerators have been used to make ever more energetic proton, electron, and ion beams. These beams have been used in practically every scientific field, from colliding particles in the Large Hadron Collider to measuring the chemical structure of drugs, treating cancers and the manufacture of the ubiquitous silicon microchip.

Now, the international Muon Ionization Cooling Experiment (MICE) collaboration, which includes many UK scientists, has made a major step forward in the quest to create an accelerator for an entirely different sort of particle, a muon. A muon accelerator could replace the Large Hadron Collider (LHC), providing at least a ten-fold increase in energy for the creation of new particles.

Until now, the question has been whether you can channel enough muons into a small enough volume to be able to study physics in new, unexplored systems. This new research, published in Nature today, shows that it is possible. The results of the experiment, carried out using the MICE muon beam-line at the Science and Technology Facilities Council (STFC) ISIS Neutron and Muon Beam facility on the Harwell Campus in the UK, clearly show that ionization cooling works and can be used to channel muons into a tiny volume.

“The enthusiasm, dedication, and hard work of the international collaboration and the outstanding support of laboratory personnel at STFC and from institutes across the world have made this game changing breakthrough possible,” commented Professor Ken Long from Imperial College London, spokesperson for the experiment.

Dr Chris Rogers, based at ISIS and the collaboration’s Physics Co-ordinator, explained: “MICE has demonstrated a completely new way of squeezing a particle beam into a smaller volume. This technique is necessary for making a successful muon collider, which could outperform even the LHC.”

Muons have many uses – they can be used to study the atomic structure of materials, they can be used as a catalyst for nuclear fusion and they can be used to see through really dense materials which X-rays can't get through. The research team hopes that this technique can help produce good quality muon beams for these applications as well.

Muons are produced by smashing a beam of protons into a target. The muons can then be separated off from the debris created at the target and directed through a series of magnetic lenses. Because of this rough-and-ready production mechanism, these muons form a diffuse cloud –so when it comes to colliding the muons, the chances of them hitting each other and producing interesting physical phenomena is really low.

To make the cloud less diffuse, a process called beam cooling is used. This involves getting the muons closer together and moving in the same direction. Magnetic lenses can get the muons closer together, or get them moving in the same direction, but not both at the same time.

A major obstacle to cooling a muon beam this is that muons only live for two millionths of a second, and previous methods developed to cool beams take hours to achieve an effect. In the 1970s a new method called 'ionization cooling' had been suggested, and developed into theoretically operable schemes in the in the 1990s. The hurdle of testing this idea in practice remained formidable.

The MICE Collaboration developed the completely new method to tackle this unique challenge, cooling the muons by putting them through specially-designed energy-absorbing materials such as lithium hydride, a compound of lithium metal and hydrogen, or liquid hydrogen cooled to around minus 250 degrees Celsius and encased by incredibly thin aluminium windows. This was done while the beam was very tightly focused by powerful superconducting magnetic lenses. The measurement is so delicate that it requires measuring the beam particle-by-particle using particle physics techniques rather than the usual accelerator diagnostics.

After cooling the beam, the muons can be accelerated by a normal particle accelerator in a precise direction, making it much more likely for the muons to collide. Alternatively, the cold muons can be slowed down so that their decay products can be studied.

Professor Alain Blondel, spokesperson of MICE from 2001 to 2013, and Emeritus Professor at the University of Geneva, commented: “We started MICE studies in 2000 with great enthusiasm and a strong team from all continents. It is a great pride to see the demonstration achieved, just at a time when it becomes evident to many new people that we must include muon machines in the future of particle physics."

“In this era of ever more-expensive particle accelerators, MICE points the way to a new generation of cost-effective muon colliders,” said Professor Dan Kaplan, Director of the IIT Center for Accelerator and Particle Physics in Chicago.

Professor Paul Soler from the University of Glasgow and UK Principal Investigator said: "Ionization cooling is a game-changer for the future of high-energy muon accelerators, such as a muon collider, and we are extremely grateful to all the international funding agencies, including STFC in the UK, for supporting the experiment and to the staff at the ISIS Neutron and Muon Source for hosting the facility that made this result possible.”

Quantum fluctuations stabilize high-temperature superconductors Wed, 05 Feb 2020 18:00:00 +0000 nonadult chiefeditor Наука An international team of researchers from Spain, Italy, France, Germany, and Japan show that the crystal structure of the record superconducting LaH10 compound is stabilized by nuclear quantum fluctuations. Their result suggests that superconductivity approaching room temperature may be possible in hydrogen-rich compounds at much lower pressures than previously expected. The results are published today by Nature.

The physicist Heike Kamerlingh Onnes discovered in 1911 that the electrical resistance of mercury disappears when it is cooled to a few kelvins (-268.95°Celsius). This discovery marked the opening of the superconductivity research field. Although the field has since set milestones, for example in explaining the phenomena of superconductivity and finding candidates for superconductivity at a higher temperature, the holy grail of superconductivity at room temperature has not yet been found. Even though research has increased the temperature at which superconductivity can be observed by more than 200 kelvins, scientists are still searching intensively for materials that allow current to flow at room temperature without losses.

Promising hydrides

More than 100 years after the ground-breaking discovery by Onnes, computer simulations are essential in the search of high-temperature superconductors, as simulations can provide estimates of promising materials. This saves experimental researchers time and resources. In recent years, the so-called hydrides —a composition of hydrogen with other elements— gave new hope to high-temperature superconductivity. Hundreds of stable hydrogen-rich compounds have been predicted using modern ab initio calculations based on density functional theory (DFT).

In 2017, theoretical predictions pointed out that lanthanum and hydrogen (LaH10) could be a promising candidate for superconductors near room temperature. However, under pressures that are 230 million times higher than those of the atmosphere, 230 gigapascals, lanthanum and hydrogen should arrange themselves into a highly symmetrical crystal, which makes superconductivity possible. Experimentally, researchers succeeded in 2019 in producing this superconducting compound of lanthanum at a pressure of "only" 130 gigapascals, and superconductivity surprisingly took place at a temperature of 250 kelvin (-23°Celsius) —this corresponds to the temperature of a conventional freezer.

An international research group led by José A. Flores-Livas, Assistant Professor at the Department of Physics at the Sapienza University of Rome, has now clarified the discrepancy between theory and experiment using computer simulations. The results of the study are published today in the scientific journal Nature.

For the first time, the scientists considered so-called protonic quantum fluctuations in the optimization of the crystalline structure. Quantum mechanics governs the motion of the electrons and nuclei; usually, for the latter, effects are not considered due to its complexity.  Introducing quantum mechanical fluctuations into the calculations —such as the protonic quantum fluctuation (associated with the energy of the nuclei) and its effect on the zero-point energy; the lowest possible energy that a quantum mechanical system may have— turn out to be essential to describe these types of superconductors accurately. The theoretical models and simulations showed that protonic quantum fluctuations “glue” the symmetric structure of LaH10 in the observed experimental pressure range.

“The calculations show that if the nuclei of atoms are treated as classical particles, that is, as simple points in space, many distortions of the structure tend to lower the energy of the system”, Flores-Livas says.

This means that the classical energy landscape of the simulated LaH10-crystal is very complex, with many local energy minima.

“It looks like a highly deformed mattress, as though many people are standing on it.”

Quantum fluctuations stabilize crystal structures

When the researchers treated the atoms (nuclei and electrons) like quantum objects, which are described with a delocalized wave function, the energy landscape completely reshapes and only one minimum is evident.

“In an illustrative way, quantum effects get rid of everybody on the mattress, and only one foot remains to deform the mattress”, Flores-Livas explains.

And exactly this deformation in the high-dimensional energy landscape corresponds to the highly symmetric structure of LaH10.

Furthermore, the estimations of the critical temperature —where superconductivity took place— using the new quantum energy landscape agree with the experimental evidence. This additionally supports the high-symmetry structure as responsible for the superconducting record.

“The results demonstrate how protonic quantum fluctuations can stabilize crystal structures even at more than 100 gigapascals below their classical instability pressure”, Flores-Livas emphasizes. “Quantum effects stabilize crystal structures with substantial electron-phonon coupling that would otherwise be unstable.”

The scientists are convinced that these results give new hope for the discovery of high-temperature superconducting hydrogen compounds at much lower pressures than expected, maybe even approaching the ambient pressure.

Significant consequences for further research

The results call into question predictions of crystal structures of hydrides that were made without considering quantum fluctuations —this means in figures presumably from more than 100 publications, Flores-Livas says.

In addition, it could mean a setback for some experimental research work, which has tried to synthesise high-temperature superconductors based on classical simulation results.

However, the improved simulations also present a challenge: “The price to be paid to 'add' the quantum mechanical fluctuation into the calculations is still elevated and only affordable for relatively small atomic systems and for a subset of pressures”, Flores-Livas says. “Our study would not be possible without the use of supercomputers”.

He sees a need for action in the improvement of algorithms, as well as further assumptions or different approximations to circumvent the computational overhead.

“Protonic quantum fluctuations, anharmonicity and adiabaticity, among other interesting phenomena, are also present at ambient conditions of pressure and temperature, and for many systems.”, Flores-Livas says.

Therefore, one can argue that all compounds composed of hydrogen will suffer in a more or less dramatic way the consequences of quantum mechanical fluctuations, even at ambient pressure.

Sand dunes can ‘communicate’ with each other Tue, 04 Feb 2020 05:00:12 +0000 nonadult chiefeditor Наука Even though they are inanimate objects, sand dunes can 'communicate' with each other. A team from the University of Cambridge has found that as they move, sand dunes interact with and repel their downstream neighbours.

Using an experimental dune 'racetrack', the researchers observed that two identical dunes start out close together, but over time they get further and further apart. This interaction is controlled by turbulent swirls from the upstream dune, which push the downstream dune away. The results, reported in the journal Physical Review Letters, are key for the study of long-term dune migration, which threatens shipping channels, increases desertification, and can bury infrastructure such as highways.

When a pile of sand is exposed to wind or water flow, it forms a dune shape and starts moving downstream with the flow. Sand dunes, whether in deserts, on river bottoms or sea beds, rarely occur in isolation and instead usually appear in large groups, forming striking patterns known as dune fields or corridors.

It's well-known that active sand dunes migrate. Generally speaking, the speed of a dune is inverse to its size: smaller dunes move faster and larger dunes move slower. What hasn't been understood is if and how dunes within a field interact with each other.

"There are different theories on dune interaction: one is that dunes of different sizes will collide, and keep colliding, until they form one giant dune, although this phenomenon has not yet been observed in nature," said Karol Bacik, a PhD candidate in Cambridge's Department of Applied Mathematics and Theoretical Physics, and the paper's first author. "Another theory is that dunes might collide and exchange mass, sort of like billiard balls bouncing off one another, until they are the same size and move at the same speed, but we need to validate these theories experimentally."

Now, Bacik and his Cambridge colleagues have shown results that question these explanations.

"We've discovered physics that hasn't been part of the model before," said Dr Nathalie Vriend, who led the research.

Most of the work in modelling the behaviour of sand dunes is done numerically, but Vriend and the members of her lab designed and constructed a unique experimental facility which enables them to observe their long-term behaviour. Water-filled flumes are common tools for studying the movement of sand dunes in a lab setting, but the dunes can only be observed until they reach the end of the tank. Instead, the Cambridge researchers have built a circular flume so that the dunes can be observed for hours as the flume rotates, while high-speed cameras allow them to track the flow of individual particles in the dunes.

Bacik hadn't originally meant to study the interaction between two dunes: "Originally, I put multiple dunes in the tank just to speed up data collection, but we didn't expect to see how they started to interact with each other," he said.

The two dunes started with the same volume and in the same shape. As the flow began to move across the two dunes, they started moving.

"Since we know that the speed of a dune is related to its height, we expected that the two dunes would move at the same speed," said Vriend, who is based at the BP Institute for Multiphase Flow. "However, this is not what we observed."

Initially, the front dune moved faster than the back dune, but as the experiment continued, the front dune began to slow down, until the two dunes were moving at almost the same speed.

Crucially, the pattern of flow across the two dunes was observed to be different: the flow is deflected by the front dune, generating 'swirls' on the back dune and pushing it away.

"The front dune generates the turbulence pattern which we see on the back dune," said Vriend. "The flow structure behind the front dune is like a wake behind a boat, and affects the properties of the next dune."

As the experiment continued, the dunes got further and further apart, until they form an equilibrium on opposite sides of the circular flume, remaining 180 degrees apart.

The next step for the research is to find quantitative evidence of large-scale and complex dune migration in deserts, using observations and satellite images. By tracking clusters of dunes over long periods, we can observe whether measures to divert the migration of dunes are effective or not.