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

Tuesday, 21 January 2020

Quantum physics: first experimental proof of a link between quantum entanglement and quantum criticality

Illustration showing two entangled particles. | National Institute of Standards and Technology

Quantum physics concerns infinitely small scales (including the interactions between atoms and particles), for which the laws of classical physics are no longer sufficient to describe the different phenomena at play. Quantum entanglement is the phenomenon in which two particles (or groups of particles) form a linked system, with quantum states depending on each other, regardless of the distance between them. Recently, researchers have been able to establish a link between quantum entanglement and quantum criticality (a phenomenon linked to quantum fluctuations ), by entangling billions & billions of electrons through a metallic film.

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To carry out their entanglement experiment, the researchers produced a film from a mixture of ytterbium, rhodium and silicon. They then passed billions & billions of tangled electrons through it. Physicists call this kind of material a “strange metal” because it does not act like real metal at very low temperatures.

"With strange metals, there is an unusual link between electrical resistance and temperature, " said physicist Silke Bühler-Paschen, of the Vienna University of Technology in Austria.

" Unlike simple metals such as copper or gold, this does not seem to be due to the thermal movement of atoms, but to quantum fluctuations at absolute zero temperature, " she adds.

Quantum criticality: a phenomenon linked to quantum fluctuations

These fluctuations represent a quantum criticality. It is more precisely a “point” between the quantum states, which are the equivalent of the transition between liquids, solids and gases in classical physics. The team claims that this cascade of electrons (through the metallic film) has produced the best evidence to date of a link between quantum criticality and entanglement. The results of the study were published in the journal Science .

"When we think of quantum entanglement, we think of small scales,  " says physicist Qimiao Si of Rice University. " We don't associate it with macroscopic objects ". " But at quantum critical point, things are so 'collective' that we are fortunate to see the effects of entanglement, even in a metallic film that contains billions & billions of quantum mechanical objects."

The experiments conducted by Bühler-Paschen, Si and his colleagues, were very difficult to carry out on several levels, from the synthesis of very complex materials required to create the strange metal, to the delicate terahertz spectroscopy required to observe the electrons.

The terahertz spectrometer used to measure entanglement. Credits: Jeff Fitlow / Rice University

After a meticulous measurement process, the team identified what they were looking for: the telltale sign of quantum criticality. " Conceptually, it was truly a dream experience ," says Si. " Probe the charge area at the critical quantum magnetic point, to see if it is indeed critical, if it reveals dynamic scaling ."

“ If nothing collective is observed, no scaling, the critical point is rather easy to describe. But if, on the contrary, something singular is observed (which was the case here), then it is a very direct and new proof concerning the nature of quantum entanglement and quantum criticality, ”he adds

A discovery with high potential and multiple benefits

This discovery may lead to potential advances in quantum computing, telecommunications and more. In the past, researchers had hypothesized a link between quantum entanglement and quantum criticality, but this is the first experimental confirmation.

The study of quantum states is still in its infancy, but it may hold the key to all kinds of exotic quantum phenomena, such as high temperature superconductivity, which is also believed to be supported by quantum criticality.

Understanding how these quantum phases commute gives us more opportunities to manage them in the future. And although we are still far from it, we have come a little closer.

" Our results suggest that the same underlying physics (quantum criticality) can lead to a platform for quantum information and high-temperature superconductivity ," says Si. " When considering this possibility, one cannot refrain from marveling at the wonders of nature ”.


Singular charge fluctuations at a magnetic quantum critical point
L. Prochaska,  X. Li, D. C. MacFarland A. M. Andrews3, M. Bonta. E. F. Bianco, S. Yazdi, W. Schrenk7, H. Detz7,#, A. Limbeck, Q. Si8, E. Ringe6, G. Strasser, J. Kono, S. Paschen

Vol 367, Issue 6475
17 January 2020

Monday, 16 December 2019

Discovery: thermal energy can flow through absolute vacuum through a quantum phenomenon

In a new study at the University of California at Berkeley, researchers have shown that thermal energy can travel through absolute vacuum thanks to quantum fluctuations. To achieve this, the team placed two gold-coated silicon nitride membranes, placed a few hundred nanometers apart in a vacuum chamber. When they heated one of the membranes, the other also warmed up when there was no contact between the two membranes and the electromagnetic radiation was negligible.

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To understand the extent of the discovery, we should give a telling example: you probably know that if you could, placing a vacuum thermos (complete absence of air) to keep your coffee warm would be a great way to keep more heat, the vacuum being a very good insulator because thermal energy has difficulty moving there. The vibrations of atoms or molecules, which carry thermal energy, simply cannot travel if there are no atoms or molecules around.

But researchers at the University of California at Berkeley have successfully shown how the strangeness of quantum mechanics can overturn this basic principle of classical physics.

The study, published this week in the journal Nature , shows that thermal energy can travel through a few hundred nanometers of absolute vacuum, thanks to a phenomenon of quantum mechanics known as the Casimir effect.

Casimir effect and quantum vacuum fluctuations

Although this effect is only significant at very small scales, it could have profound implications for the design of computer chips and other electronic components at the nanoscale, where heat dissipation is essential. It also upsets what many of us have learned about heat transfer in physics.

" Heat is generally conducted in a solid through the vibrations of atoms or molecules, or so-called phonons - but in vacuum there is no physical medium. So, for many years, textbooks have told us that phonons cannot travel through the void, ”said Xiang Zhang, professor of mechanical engineering at UC Berkeley, who led the study. " What we have discovered, surprisingly, is that phonons can indeed be transferred across the void by invisible quantum fluctuations ."

In the experiment, Zhang's team placed two gold-coated silicon nitride membranes a few hundred nanometers apart, placed inside a vacuum chamber. When they heated one of the membranes, the other also warmed up.

To carry out the experiment, the team designed extremely thin silicon nitride membranes, made in a clean room, and then used optical and electronic components to precisely control and monitor the temperature of the membranes when they were locked in the vacuum chamber. Credits: Violet Carter / UC Berkeley

This discovery of a new heat transfer mechanism offers unprecedented opportunities for thermal management at the nanoscale, which is important for high-speed computing and data storage ," says Hao-Kun Li, a former doctoral student in Zhang's group and lead co-author of the study. " Now we can design the quantum vacuum to extract heat from integrated circuits ."

In quantum physics, absolute vacuum does not exist

The feat at first glance impossible to move molecular vibrations through vacuum can be accomplished here because, according to quantum mechanics, absolute vacuum does not exist, said King Yan Fong, postdoctoral researcher at UC Berkeley and second lead co-author of the study.

" Even if you get 'empty' space according to classical physics - no matter, no light - quantum mechanics says it can't be really empty. There are still some fluctuations in the quantum field in a vacuum, ”said Fong. " These fluctuations give rise to a force which can link two objects together, and this is what we call the 'Casimir effect'. So when an object heats up and begins to vibrate and oscillate, this movement can be transmitted to the other object through vacuum, through quantum fluctuations in vacuum . ”

Although theorists have long speculated that the Casimir effect could help molecular vibrations travel through vacuum, proving it experimentally has been a major challenge.

In particular, the researchers discovered that by carefully choosing the size and structure of the membranes, they could transfer thermal energy over a few hundred nanometers of vacuum. This distance was far enough that other possible modes of heat transfer were negligible, such as the energy transported by electromagnetic radiation.

Sound could also travel through the void

Because molecular vibrations are also the basis of the sounds we perceive, this discovery suggests that sound could also travel through vacuum.

" Twenty-five years ago, during my doctoral qualification exam at Berkeley, a professor asked me, 'Why can you hear my voice?'. I replied: 'It is because your sound moves by vibrating molecules in the air'. Then he asked, 'What if we suck all the air molecules out of this room? Would you still hear me? ' I said, 'No, because there would be no more space to vibrate,' ”said Zhang.

" Today, what we have discovered is a surprising new mode of heat conduction through a vacuum without medium, which is reached by the intriguing fluctuations of the quantum vacuum. So, I made a mistake during my 1994 exam! Now you can shout through the void, ”he concludes.


Phonon heat transfer across a vacuum through quantum fluctuations

King Yan Fong, Hao-Kun Li, Rongkuo Zhao, Sui Yang, Yuan Wang & Xiang Zhang

Nature volume 576, pages243–247(2019)


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 Reykjavík 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.