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Showing posts with label Physical. Show all posts
Showing posts with label Physical. Show all posts

Monday, 2 December 2019

A new theoretical type of time crystal might not require energy input

A few years ago, physicists presented a new structure of matter with amazing properties: temporal crystals. In these crystals, atomic structures form patterns of arrangement that are repeated in both space and time. However, to induce such a structure, an initial energy supply to the ground state is necessary. But in a recent article, a team of physicists showed, using a model of string theory, that it would be possible to obtain such crystals without this energy input. An attractive theory, but whose experimental realization seems unlikely.

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The method involves inciting entangled particles to influence the spin of others over a certain distance. Temporal crystals may seem like a science fiction concept, but they are a real phenomenon, theorized for the first time in 2012. From the outside, they look like normal crystals. But inside, the atoms - arranged in a repeating network structure otherwise normal - behave in a very special way.

They oscillate, turning in one direction then in the other. These oscillations, called "tics", are blocked on a very regular and particular frequency. Thus, where the structure of ordinary crystals repeats itself in space, in temporal crystals, it is repeated in space and in time.

Quantum intricacy: it would render unnecessary energy input into time crystals

Until now, experimentally produced time crystals have required an external stimulus (such as an electromagnetic radiation pulse) in the ground state, or in the lower energy state, to induce their ticking. This was done in 2016. According to a 2015 article, it seemed that a temporal crystal without adding energy to its ground state was simply physically impossible. In physics, we speak of "no-go theorem".

In the solution proposed by the researchers, each particle interacts with the others in the crystalline structure at long distances, thanks to the phenomenon of quantum entanglement (arrows). Credits: J. Zhang et al.

There is, however, a notable exception to this theorem with respect to time crystals. This is what Valerii Kozin of the University of Iceland in ReykjavĂ­k and Oleksandr Kyriienko of the University of Exeter in the United Kingdom, used to address the problem in their article published in the journal Physical Review Letters.

The 2015 article assumes that interactions between particles decrease with distance. But there is a practical exception. The entangled particles have a relationship that does not weaken with distance.

Measuring the spin of a particle will immediately determine the spin of its entangled partner, regardless of its distance. According to physicists, in time crystals, such a remote interaction could theoretically produce a fundamental state of temporal crystals requiring no energy injection.

A very theoretical solution brought by the string theoryb

In their new article, the researchers propose a system of particles in the temporal crystal, each having a rotation. They demonstrate that there is a way to describe entangled particle spins using a string theory model that corresponds to the definition of a time crystal given in the 2015 article.

Even if the particles were spinning out of sync, the interactions between the particles would produce the ticking of a time crystal, according to the authors. However, this system would be incredibly complicated, each particle can have a spin superposition. In fact, the whole thing might not be feasible in a laboratory. Intricating particles in this way is an idea that works well on paper, but is hardly practical in practice. We therefore look forward to any experiments that will ensue.


Quantum Time Crystals from Hamiltonians with Long-Range Interactions

Valerii K. Kozin and Oleksandr Kyriienko
Phys. Rev. Lett. 123, 210602 –

Published 20 November 2019

Sunday, 24 November 2019

New experimental evidence suggests the existence of a long-theorized particle related to dark matter

Proposed as a solution in 1977 to the problem of CP symmetry in quantum chromodynamics, axions are hypothetical neutrals and very low mass particles, considered today as potential candidates for dark matter. While several experiments aim to detect these particles, a team of physicists has recently discovered clues that axions may well exist. Although these results do not directly prove the existence of axions, they are an important step in the search for particles.

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Physicists have found clues about the existence of the axion, an elusive particle that rarely interacts with normal matter. The axion was first predicted more than 40 years ago, but has never been observed until now.

Physicists have suggested that dark matter could be composed of axions. But rather than searching for axions in space, they discovered the mathematical signatures of an axion in a particular material here on Earth. The results were published in the journal Nature.

The newly discovered axion is not quite a usual particle: it acts as an electron wave in a supercooled material called semi-metal. This strange particle could also help solve a long-standing physical puzzle, known as a strong CP problem. For some reason, the laws of physics seem to act in the same way on particles and their antimatter partners, even when their spatial coordinates are reversed.

This phenomenon is known as charge-parity symmetry, but the Standard Model does not say anything about the origin of this symmetry. Unexpected symmetry can be explained by the existence of a special field (axionic field); to detect an axion would prove that this field exists, thus solving this mystery.

Search for axions as quasi-particles in condensed matter

Because physicists believe that the particle hardly interacts with ordinary matter, they assumed that it would be difficult to detect it using existing space telescopes. The researchers turned to condensed matter.

Condensed matter experiments, such as the one conducted here, have been used to highlight predisposed particles that are elusive in several well-known cases, including that of fermion majorana.

The particles are not detected in the usual sense, but are in the form of collective vibrations in materials that behave and respond exactly as a particle would. It is therefore quasi-particles.

The research team worked with a Weyl (TaSe 4 ) 2 I semi-metal , a special material in which electrons behave as if they had no mass, did not interact and split in two types: right-handed and left-handed.

The property of being right-handed or left-handed is called chirality; the chirality in Weyl's semi-metal is conserved, which means that there is an equal number of left and right electrons. Cooling the semi-metal to minus 11 ° C allowed the electrons to interact and condense to form a crystal of their own.

An important step towards a possible direct detection of axions

Vibration waves propagating through the crystals are called phonons. Since the strange laws of quantum mechanics dictate that particles can also behave in waves, some phonons have the same properties as classical quantum particles, such as electrons and photons.

Gooth and his colleagues observed phonons in the electron crystal, which responded to electric and magnetic fields exactly as predicted for axions.

The behavior of electrons in the Weyl semi-metal showed a dynamic identical to that predicted for axions. Credits: J. Gooth

In addition, these quasi-particles did not have an equal number of right and left particles (physicists also predicted that axions would break the conservation of chirality).

Frank Wilczek (Nobel Prize in Physics), who did not participate in the present study, also suggested that a material such as Weyl's semi-metal could one day be used as a sort of "antenna" to detect fundamental axions, or axions that exist in their own way as particles in the Universe, rather than as collective vibrations.

While the search for the axion as a single, independent particle will continue, experiments like this one are helping more traditional detection experiments by providing boundaries and estimates of the properties of the particle, such as mass. This gives other experimenters a better idea of ​​where to look for these particles. It also convincingly demonstrates that the existence of the particle is possible.


 Article: Axionic charge-density wave in the Weyl semimetal (TaSe4)2I

Authors: J. Gooth, B. Bradlyn, S. Honnali, C. Schindler, N. Kumar, J. Noky, Y. Qi, C. Shekhar, Y. Sun, Z. Wang, B. A. Bernevig & C. Felser

Nature 575, 315–319 (2019) 


Friday, 22 November 2019

New experiments suggest the existence of a potential fifth fundamental force

According to the Standard Model, the Universe is governed by four elementary interactions: electromagnetism, weak and strong nuclear interactions, and gravity. For the first three, mediating particles, the bosons, have been identified. As for gravity, the existence of a graviton potential is still debated. But the Standard Model might not be complete. This is proposed by a team of Hungarian physicists whose recent experimental results suggest, according to their analysis, that there could be a fifth fundamental force. In 2016, the team had already highlighted some experimental evidence for a potential fifth elemental boson.

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The same team has now observed a second example of this potential fifth elemental force and the particle - called X17 - that carries it. If the discovery is confirmed, learning more about X17 could help us better understand the forces that govern our universe, but could also help physicists solve the problem of dark matter. However, these latest results have not yet been validated by peers, so they must be taken with extreme caution.

Attila Krasznahorkay and colleagues at the Hungarian Nuclear Research Institute suspected something strange in 2016 after analyzing how an excited beryllium-8 atom emits light when it disintegrates. If this light is energetic enough, it is transformed into an electron and a positron, which move away from each other at a predictable angle.

A statistical anomaly in the particle separation angle

On the basis of the law of conservation of energy, as the energy of the light producing the two particles increases, the angle between them should decrease. Statistically speaking, at least. But, that's not quite what Krasznahorkay and his team saw. Among the different angles observed, there was an unexpected increase in the number of electrons and positrons separating at an angle of 140 degrees.

In their experiment on beryllium-8, the researchers found an abnormal number of particle pairs separating at an angle of 140 ° (angular correlation). Credits: AJ Krasznahorkay et al. 2019

The study seemed serious enough and quickly attracted the attention of other researchers around the world, who suggested that a whole new particle could be responsible for the anomaly. And its characteristics suggested that it must be a completely new type of fundamental boson.

This new boson can not be one of the fundamental bosons already known, considering its distinctive mass of 17 megaelectronvolts (MeV) - about 33 times that of an electron, and its very short life (10-14 seconds ).

All these data therefore seem to indicate the existence of a fifth force. However, the discovery of a new particle, and a fortiori a new boson, requires several careful examinations and experimental repeatability before being announced.

New experimental evidence for a potential fifth fundamental force

To do this, the Krasznahorkay team decided to hunt down this potential new boson in another experiment, moving from the decay of beryllium-8 to a change in the state of an excited helium nucleus.

Similar to their previous discovery, the researchers found pairs of electrons and positrons separating at an angle that does not fit the currently accepted models. This time the number was closer to 115 degrees.

In their new experiment involving an excited helium nucleus, the researchers again observed an abnormal angular correlation for the electron-positron pairs, but this time around 115 °. Credits: AJ Krasznahorkay et al. 2019

The team calculated that the helium nucleus could also have produced an ephemeral boson with a mass of just under 17 megaelectronvolts. While the 2016 experience has been accepted in the very serious journal Physical Review Letters , this latest study has not yet been peer-reviewed. It is however freely available on the pre-publication server arXiv .

Many other experiments, including those conducted by independent teams, will have to be conducted, and the initial results will have to be corroborated before any official announcement can be made.

The results obtained by the Krasznahorkay team are still far from sufficient. However, if the discovery of a fifth boson was to be confirmed, it could announce a revolution in the field of dark matter. A number of experiments on dark matter actually looking for a 17 MeV peak.


Saturday, 16 November 2019

Discovery of a new state of matter: metallic Cooper pairs

Described for the first time in 1956, the Cooper pairs are low temperature electron bound states responsible for the superconductivity phenomenon. In today's quantum models, these pairs can be either zero electrical resistance or electrical insulation. However, physicists have discovered a new state of matter in which Cooper pairs conduct electricity while generating some resistance, just like ordinary metal. A phenomenon not foreseen by the physics of condensed matter and which could lead to the development of new electronic devices.

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For years, physicists have assumed that the Cooper pairs, the electron pairs that allow superconductors to drive electricity without resistance, were binary systems. The pairs slide freely, creating a superconducting state, or an insulating state by getting stuck in a material, unable to move.

Pairs of Cooper: they can combine conductivity and electrical resistance

But in a new article published in the journal Science , a team of researchers has shown that Cooper's pairs can also conduct electricity with some resistance, as do ordinary metals. The results describe a new state of matter, according to the researchers, which will require a new theoretical explanation.

" It had been proven that this metallic state would form in thin-film superconductors as they cooled down to their superconducting temperature, but the question of whether this state involved Cooper pairs is an open question, " says Jim. Valles, professor of physics at Brown University.

" We have developed a technique that allows us to answer this question and we have shown that, as a result, Cooper pairs are responsible for carrying the charge in this metallic state. What's interesting is that no one is really sure how it works. This discovery will therefore require more theoretical and experimental work to understand exactly what is happening."

Cooper pairs and superconductivity

Cooper's pairs are named after Leon Cooper, professor of physics at Brown, who won the Nobel Prize in 1972 for describing their role in superconductivity. Resistance is created when electrons vibrate in the atomic network of a material as they move. But when the electrons unite to become Cooper pairs, they undergo a remarkable transformation.

The BCS theory explains the phenomenon of superconductivity by the appearance of Cooper pairs. At very low temperatures, the electrons pair together (b), forming many Cooper pairs within the material (c). These pairs occupy the same fundamental quantum state and form a single quantum wave (d). Cooper pairs are treated as bosons, they obey Bose-Einstein statistics and are not subject to the Pauli exclusion principle. Credits: CNRS

The electrons themselves are fermion s, particles that obey the Pauli exclusion principle, which means that each electron tends to maintain its own quantum state. The Cooper pairs, however, act as bosons, which can share with the same state. This bosonic behavior allows Cooper pairs to coordinate their movements with other sets of Cooper pairs, so as to reduce electrical resistance to zero.

A new bosonic state of Cooper pairs

In 2007, Valles, in collaboration with Jimmy Xu, Professor of Engineering and Physics at Brown, showed that Cooper's pairs could also produce insulating states as well as superconductivity. In very thin materials, rather than moving together, couples stay in place, grouped by islands without means to join those around.

Scanning electron microscope observation of the material used for the experiment: a YBCO superconductor with a network of tiny holes to study the dynamics of the Cooper pairs. Credits: Brown University

For this new study, Valles, Xu and his colleagues searched for non-superconducting metallic Cooper pairs using a technique similar to that which revealed insulators of Cooper pairs. This technique consists of modeling a thin-film superconductor, in this case a high-temperature superconducting yttrium, barium and copper oxide (YBCO), with networks of tiny holes.

When the material is traversed by a current and is exposed to a magnetic field, the charge carriers of the material gravitate around the holes like water surrounding a drain. " We can measure the frequency with which these charges revolve around. In this case, we found that the frequency is compatible with the fact that two electrons circulate at once instead of one. So we can conclude that the charge carriers in this state are Cooper pairs and not electrons, "says Valles.

Towards the potential development of new electronics

The fact that this phenomenon has been detected in a high temperature superconductor will make future research more practical. The YBCO type begins its superconductivity at about -181 ° C and the metal phase begins at a temperature just above that. This higher temperature facilitates the use of spectroscopy and other techniques to better understand what is happening in this metallic phase.

According to the researchers, it may be possible to exploit this bosonic metal state for new types of electronic devices. " The problem with bosons is that they tend to be in a wave state more like electrons. We are talking about a phase and interference similar to that of light. So there may be new ways to move the load in the devices by playing with interference between the bosons "concludes Valles.


Intermediate bosonic metallic state in the superconductor-insulator transition

Science 14 Nov 2019:
DOI: 10.1126/science.aax5798 


Sunday, 10 November 2019

Physicists find a way to observe Schrödinger's cat without risk of killing him

To illustrate the principle of quantum superposition and the problem of measurement, in 1935 physicist Erwin Schrödinger invents a thought experiment that later became known as the Schrödinger Cat. In its box, the cat is both alive and dead, and only the observation (the action of looking inside the box) selects one of the two states. However, in a recent study, theoretical physicists have highlighted a way to be able to observe the cat without "risking to kill it".

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" We generally think that the price we pay to observe our environment is nothing, " says the lead author of the study, Holger F. Hofmann, an associate professor of physics at Hiroshima University in Japan. " It's not correct. To look, you must have light, and the light changes the object. Indeed, even a single photon of light transfers energy to the object you are watching.

Hofmann and Kartik Patekar have developed a mathematical framework that separates the initial interaction (looking at the cat) from the result of this interaction (living or dead). " Our main motivation was to look very carefully at how a quantum measurement is done. And the key point is that we separate the measurement in two steps "explains Hofmann. The article was published in the journal New Journal of Physics.

In the experience of Schrödinger's cat, as long as no observer looks inside the box, the cat is simultaneously dead and alive, because the radioactive atom (detected or not by the Geiger counter that triggers the killing mechanism the cat) is simultaneously intact and disintegrated. Credits:

Preservation of information on the condition of the cat

In doing so, Hoffman and Patekar may assume that all the photons involved in the initial interaction are captured without losing information about the condition of the cat. So before reading this information, all there is to know about the status of the cat (and how it has changed) is always available. It is only when we read the information that we lose some of it. " What's interesting is that the reading process selects one of the two types of information and completely erases the other ."

Suppose the cat is still in the box, but rather than looking inward to determine if it's alive or dead, you're putting a camera out of the box, which can somehow take a picture at the inside. Once the photo is taken, the camera has two types of information: how the cat has changed as a result of taking the photo (what researchers call a quantum tag) and whether the cat is alive or dead after the interaction.

None of this information has been lost yet. And depending on how you choose to "develop" the image, you retrieve one or the other information.

Think of a coin. You can choose to find out if a coin has been returned or is currently stacked. But you can not know both. Moreover, if you know how a quantum system has been modified and if this change is reversible, then it is possible to restore its initial state.

A compromise between resolution and disruption

Crucially, the choice of reading comes with a compromise between the resolution of the measure and its perturbation, which are exactly equal. The resolution refers to the amount of information extracted from the quantum system and the disruption to the amount of irreversible changes made to the system. In other words, the more you know about the current status of the cat, the more you have irreversibly changed it.

" What surprised me was that the ability to cancel the disturbance is directly related to the amount of information you get on the observable, " says Hofmann. Although previous work has shown a compromise between resolution and quantum perturbation, this article is the first to quantify the exact relationship, according to Michael Hall, theoretical physicist at the Australian National University.