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

Monday, 16 March 2020

Engineers crack 58-year-old puzzle on way to quantum breakthrough

Magnetic nuclear resonance makes it possible to control atomic nuclear spins via the application of a magnetic field. This technique is very often used in many fields such as medicine (MRI), chemistry (characterization of chemical species) or geology. But in 1961, Nobel Prize winner Nicolaas Bloembergen suggested that it is also possible to control nuclear spins via an electric field. About 59 years later, a team of engineers finally confirmed, by serendipity, the existence of an electrical nuclear resonance. A result that will allow the development of much more efficient quantum electronics.

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A happy accident in the laboratory has led to a breakthrough discovery that not only solved a problem that stood for more than half a century, but has major implications for the development of quantum computers and sensors.

In a study published today in Nature, a team of engineers at UNSW Sydney has done what a celebrated scientist first suggested in 1961 was possible, but has eluded everyone since: controlling the nucleus of a single atom using only electric fields.

“This discovery means that we now have a pathway to build quantum computers using single-atom spins without the need for any oscillating magnetic field for their operation,” says UNSW’s Scientia Professor of Quantum Engineering Andrea Morello. “Moreover, we can use these nuclei as exquisitely precise sensors of electric and magnetic fields, or to answer fundamental questions in quantum science.”

More precise atomic control thanks to electrical nuclear resonance

That a nuclear spin can be controlled with electric, instead of magnetic fields, has far-reaching consequences. Generating magnetic fields requires large coils and high currents, while the laws of physics dictate that it is difficult to confine magnetic fields to very small spaces – they tend to have a wide area of influence. Electric fields, on the other hand, can be produced at the tip of a tiny electrode, and they fall off very sharply away from the tip. This will make control of individual atoms placed in nanoelectronic devices much easier.

Professor Morello says the discovery shakes up the paradigm of nuclear magnetic resonance, a widely used technique in fields as disparate as medicine, chemistry, or mining.

“Nuclear magnetic resonance is one of the most widespread techniques in modern physics, chemistry, and even medicine or mining,” he says. “Doctors use it to see inside a patient’s body in great detail while mining companies use it to analyse rock samples. This all works extremely well, but for certain applications, the need to use magnetic fields to control and detect the nuclei can be a disadvantage.”

Diagram explaining the operation of nuclear magnetic resonance on the spin of an atomic nucleus. Credits: HUJI

Professor Morello uses the analogy of a billiard table to explain the difference between controlling nuclear spins with magnetic and electric fields.

“Performing magnetic resonance is like trying to move a particular ball on a billiard table by lifting and shaking the whole table,” he says. “We'll move the intended ball, but we'll also move all the others.

“The breakthrough of electric resonance is like being handed an actual billiards stick to hit the ball exactly where you want it.”

A possibility suggested since 1961

Amazingly, Professor Morello was completely unaware that his team had cracked the longstanding problem of finding a way to control nuclear spins with electric fields, first suggested in 1961 by a pioneer of magnetic resonance and Nobel Laureate, Nicolaas Bloembergen.

“I have worked on spin resonance for 20 years of my life, but honestly, I had never heard of this idea of nuclear electric resonance,” Professor Morello says. “We ‘rediscovered’ this effect by complete accident – it would never have occurred to me to look for it. The whole field of nuclear electric resonance has been almost dormant for more than half a century, after the first attempts to demonstrate it proved too challenging.”

A discovery made entirely by chance

The researchers had originally set out to perform nuclear magnetic resonance on a single atom of antimony – an element that possesses a large nuclear spin. One of the lead authors of the work, Dr Serwan Asaad, explains: “Our original goal was to explore the boundary between the quantum world and the classical world, set by the chaotic behaviour of the nuclear spin. This was purely a curiosity-driven project, with no application in mind.”

“However, once we started the experiment, we realised that something was wrong. The nucleus behaved very strangely, refusing to respond at certain frequencies, but showing a strong response at others,” recalls Dr Vincent Mourik, also a lead author on the paper.

“This puzzled us for a while, until we had a ‘eureka moment’ and realised that we were doing electric resonance instead of magnetic resonance.”

Antenna and antimony atom: the unexpected generation of an electric field

Dr Asaad continued: “What happened is that we fabricated a device containing an antimony atom and a special antenna, optimized to create a high-frequency magnetic field to control the nucleus of the atom. Our experiment demands this magnetic field to be quite strong, so we applied a lot of power to the antenna, and we blew it up!”

Diagram explaining how the electric field allows the control of an atomic nuclear spin. (A): Valence charge density near the Sb + atom (gold) and its 16 closest Si atoms (black), with an isosurface charge density (red). (B): Deformation displacing the Si atoms and the covalent bonds surrounding the nucleus, creating an EFG which results in a quadrupole shift. (D): Electric fields applied via a superposition of voltages distort the charge distribution, which leads to both linear frequency shifts (LQSE) and coherent spin transitions (NER). Credits: Serwan Asaad, et al. 2020

“Normally, with smaller nuclei like phosphorus, when you blow up the antenna it’s ‘game over’ and you have to throw away the device,” says Dr Mourik.

“But with the antimony nucleus, the experiment continued to work. It turns out that after the damage, the antenna was creating a strong electric field instead of a magnetic field. So we ‘rediscovered’ nuclear electric resonance.”

Towards more precise and efficient quantum electronics

After demonstrating the ability to control the nucleus with electric fields, the researchers used sophisticated computer modelling to understand how exactly the electric field influences the spin of the nucleus. This effort highlighted that nuclear electric resonance is a truly local, microscopic phenomenon: the electric field distorts the atomic bonds around the nucleus, causing it to reorient itself.

“This landmark result will open up a treasure trove of discoveries and applications,” says Professor Morello. “The system we created has enough complexity to study how the classical world we experience every day emerges from the quantum realm. Moreover, we can use its quantum complexity to build sensors of electromagnetic fields with vastly improved sensitivity. And all this, in a simple electronic device made in silicon, controlled with small voltages applied to a metal electrode.”


Coherent electrical control of a single high-spin nucleus in silicon

Serwan Asaad, Vincent Mourik, Benjamin Joecker, Mark A. I. Johnson, Andrew D. Baczewski, Hannes R. Firgau, Mateusz T. Mądzik, Vivien Schmitt, Jarryd J. Pla, Fay E. Hudson, Kohei M. Itoh, Jeffrey C. McCallum, Andrew S. Dzurak, Arne Laucht & Andrea Morello

Nature 579, 205–209 (2020).

Friday, 6 March 2020

For the first time, physicists have succeeded in “dividing” one photon into three

The very nature of light fascinates ordinary people as much as scientists. The undulatory properties of the photon are no longer to be proven, just like its corpuscular properties. Light would theoretically be an alternating mixture of these two characteristics, a principle known as wave-particle duality. Recently, researchers from the Institute for Quantum Computing (IQC) at the University of Waterloo (Canada), have achieved an exciting feat with an optical system: the first direct “division” of a photon into three separate photons.

This success could teach us more about the corpuscular nature of the photon and contribute to various technological applications, such as quantum computing.

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The occurrence, the first of its kind, used the spontaneous parametric down-conversion method (SPDC) in quantum optics and created what quantum optics researchers call a non-Gaussian state of light. A non-Gaussian state of light is considered a critical ingredient to gain a quantum advantage.

In a standard SPDC system, the downconversion generates two photons from a “pump” photon. The two photons produced are entangled and have a total energy and momentum equal to that of the original photon. Credits: Wikipedia

"It was understood that there were limits to the type of entanglement generated with the two-photon version, but these results form the basis of an exciting new paradigm of three-photon quantum optics," said Chris Wilson, a principle investigator at IQC faculty member and a professor of Electrical and Computer Engineering at Waterloo. "Given that this research brings us past the known ability to split one photon into two entangled daughter photons, we're optimistic that we've opened up a new area of exploration."

Spontaneous downward parametric conversion for quantum computing

"The two-photon version has been a workhorse for quantum research for over 30 years," said Wilson. "We think three photons will overcome the limits and will encourage further theoretical research and experimental applications and hopefully the development of optical quantum computing using superconducting units."

Chris Wilson's laboratory. Credits: University of Waterloo

Wilson used microwave photons to stretch the known limits of SPDC. The experimental implementation used a superconducting parametric resonator. The result clearly showed the strong correlation among three photons generated at different frequencies. Ongoing work aims to show that the photons are entangled.

"Non-Gaussian states and operations are a critical ingredient for obtaining the quantum advantage," said Wilson. "They are very difficult to simulate and model classically, which has resulted in a dearth of theoretical work for this application."

This laboratory feat brings us closer to ultra-high-performance optical systems, laying the technological foundations for tomorrow's quantum computing and hopefully, mainstream quantum computers.


Observation of Three-Photon Spontaneous Parametric Down-Conversion in a Superconducting Parametric Cavity.

C. W. Sandbo Chang, Carlos Sabín, P. Forn-Díaz, Fernando Quijandría, A. M. Vadiraj, I. Nsanzineza, G. Johansson, C. M. Wilson.

Physical Review X, 2020;

DOI: 10.1103/PhysRevX.10.011011

Wednesday, 4 March 2020

Physicists film the quantum transition of an atom

Measuring a quantum system causes it to change -- one of the strange but fundamental aspects of quantum mechanics. Researchers have now been able to demonstrate how this change happens.

Quantum physics describes the inner world of individual atoms, a world very different from our everyday experience. One of the many strange yet fundamental aspects of quantum mechanics is the role of the observer – measuring the state of a quantum system causes it to change.

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Despite the importance of the measurement process within the theory, it still holds unanswered questions: Does a quantum state collapse instantly during a measurement? If not, how much time does the measurement process take and what is the quantum state of the system at any intermediate step?

A collaboration of researchers from Sweden, Germany and Spain has answered these questions using a single atom – a strontium ion trapped in an electric field. The measurement on the ion lasts only a millionth of a second. By producing a “film” consisting of pictures taken at different times of the measurement they showed that the change of the state happens gradually under the measurement influence.

Superposition preserved

Atoms follow the laws of quantum mechanics which often contradict our normal expectations. The internal quantum state of an atom is formed by the state of the electrons circling around the atomic core. The electron can circle around the core in an orbit close or further away. Quantum mechanics, however, also allows so called superposition states, where the electron occupies both orbits at once, but each orbit only with some probability.

The result of the experiment can be summarized in an animated GIF that shows what happens to the quantum state of the ion during that millionth of a second. The state can be visualized using a three-dimensional board. The heights of the bars indicate the degree of superposition of the possible quantum states. The film shows how during the measurement some of the superpositions are lost – and how this loss is gradual – while others are preserved as they should be in an ideal quantum measurement. Source: F. Pokorny et al., "Tracking the dynamics of an ideal quantum measurement", Physical Review Letters 2020.

“Every time when we measure the orbit of the electron, the answer of the measurement will be that the electron was either in a lower or higher orbit, never something in between. This is true even when the initial quantum state was a superposition of both possibilities. The measurement in a sense forces the electron to decide in which of the two states it is”, says Fabian Pokorny, researcher at the Department of Physics, Stockholm University.

The “film” displays the evolution during the measurement process. The individual pictures show tomography data where the height of the bars reveal the degree of superposition that is still preserved. During the measurement some of the superpositions are lost – and this loss happens gradually – while others are preserved as they should be for an ideal quantum measurement.

Important for quantum computers

“These findings shed new light onto the inner workings of nature and are consistent with the predictions of modern quantum physics”, says Markus Hennrich, group leader of the team in Stockholm.

These results are also important beyond fundamental quantum theory. Quantum measurement are an essential part of quantum computers. The group at Stockholm University is working on computers based on trapped ions, where the measurements are used to read out the result at the end of a quantum calculation.


Tracking the Dynamics of an Ideal Quantum Measurement

Fabian Pokorny, Chi Zhang, Gerard Higgins, Adán Cabello, Matthias Kleinmann, Markus Hennrich.

Physical Review Letters, 2020;

DOI: 10.1103/PhysRevLett.124.080401

Wednesday, 26 February 2020

Researchers create new state of light

Scientists have known for decades that light rotates around a longitudinal axis parallel to the direction in which it travels. However, some specialist researchers are currently trying to establish whether there are other forms and states, and to what extent it would be possible to control this. Recently, researchers from the University of Dayton managed to create a new “state of light”, by making it “rotate” around a transverse axis perpendicular to the displacement.

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After two years dedicated solely to their study, Andy Chong and Qiwen Zhan, researchers from the University of Dayton in the United States, have for the first time managed to create a new “state of light”. As part of their experiment, they show that a beam of light can also rotate around a transverse axis perpendicular to the direction in which it moves, like a vortex. The results of the study were published on February 24 in the specialized journal Nature Photonics.

"The sabbatical allowed us the time to fully concentrate on this research and was very instrumental in putting us in a position to make this discovery," Chong said.

Zhan and Chong didn't go into their research with preconceived notions on what to look for or what they would find.

"It was more of a curiosity. Can we do this or make light do that?," said Zhan, a professor of electro-optics and photonics and managing director of the UD-Fraunhofer Joint Research Center. "Once we discovered we're able to do this, we then asked 'what's next?'"

"What's next?" may be a while off for the researchers and others who will examine the pair's basic research findings for applications, but they surmise this new state of light could be used to improve the transmission of large amounts of data with greater security, among many other potential applications.

a) Experimental device for generating and measuring spatiotemporal vortices (ST) of light; BS: beam splitter. b) Diagram showing the method of measuring the phase of light. The figures in italics represent the relative phases for the vortexes. The numbers in italics represent phases relating to various places. Note that the phase increases clockwise. Credits: Andy Chong, Chenhao Wan / University of Dayton

The researchers demonstrate in particular that a three-dimensional wave packet that is a spatiotemporal (ST) optical vortex with a controllable purely transverse OAM. Contrary to the transverse SAM, the magnitude of the transverse OAM carried by the ST vortex is scalable to a larger value by simple adjustments.

Since the ST vortex carries a controllable OAM uniquely in the transverse dimension, it has strong potential for novel applications that may not be possible otherwise. The scheme reported here can be readily adapted for other spectral regimes and different wave fields, opening opportunities for the study and applications of ST vortices in a wide range of areas.

"We don't know yet? But the sky's the limit," Zhan said. The duo is most interested in how the light interacts with materials. "We want to better understand how this state of light interacts with materials in space and time," said Chong, associate professor of physics and electro-optics and photonics.


Generation of spatiotemporal optical vortices with controllable transverse orbital angular momentum

Andy Chong, Chenhao Wan, Jian Chen & Qiwen Zhan

Nature Photonics (2020)

Tuesday, 18 February 2020

Researchers have discovered new electronic state of matter

In this artist's view, electrons travel in cars in increasing numbers, giving rise to a series of conductances which appears in Pascal's triangle. | Yun-Yi Pai

In a metal or a semiconductor, the electrons move and disperse more or less freely, this movement can be determined by applying an electrical voltage to the material in question. Within ballistic conductors (materials with optimized electrical conductivity, in particular by the absence of collision between electrons), the electrons move almost like vehicles on a highway. The main advantage is that they do not emit heat and can be used in a unique way compared to ordinary electronics. Researchers have previously,  successfully designed this type of ballistic conductor. Recently, scientists have discovered a new electronic state of matter in which electrons move, without dispersing, in groups of two or more at a time rather than individually, as was the case so far in early ballistic conductors.

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" Research is focused on measurements in one-dimensional conductive systems where electrons move without dispersing in groups of two or more at a time, rather than individually, " say Jeremy Levy, professor of condensed matter physics, and Patrick Irvin, Associate Research Professor. Both from the Department of Physics and Astronomy at the University of Pittsburgh are co-authors of the study, the results of which were published on February 14 in the journal Science .

"Normally, the electrons in semiconductors or metals move and disperse, and eventually drift in one direction if you apply voltage. But in ballistic conductors, electrons move more like cars on a highway (see title image). The advantage of this is that they do not give off heat and can be used in very different ways than what is done for ordinary electronics. Researchers before us had successfully created this type of ballistic conductor,” says Levy.

“Electron clusters” giving rise to new forms of electronic matter

"The discovery we made shows that when electrons can be made to attract each other, they can form pairs or clusters of three, four and five electrons, which literally behave like new types of particles, new forms of electronic matter,” he adds.

Levy compared the discovery to the way quarks bond to form neutrons and protons. An important clue to discover this new state of matter was to recognize that these ballistic conductors corresponded to a sequence in the Pascal triangle.

The first three lines of Pascal's triangle. Credits: Wikimedia

"If you look in different directions of Pascal's triangle, you can see different sequences of numbers, including the following: 1, 3, 6, 10, 15, 21. It is a sequence that we noticed in our data, it was therefore a difficult clue as to say what was really going on. The discovery took us a while to understand, but it was because we did not know, at the start, that we were looking at particles composed of an electron, two electrons, three electrons, etc. Together, they correspond to the sequence 1, 3, 6, 10”, explains Levy.

A link with quantum entanglement ...

Also director of the Pittsburgh Quantum Institute, Levy noted that the new particles have properties related to quantum entanglement, which can potentially be used for quantum computing and quantum redistribution. He notably declares that the discovery is an exciting advance towards the next stage of quantum physics.

"This research is part of a larger effort here in Pittsburgh to develop new science and technology related to the second quantum revolution," he said.

During the first quantum revolution, the scientific community realized that the world around us is fundamentally governed by the laws of quantum physics. This discovery made it possible, among other things, to understand the periodic table, the behavior of materials and helped the development of transistors, computers, MRI scanners and information technology in the broad sense.

"Now, in the 21st century, we examine all the strange predictions of quantum physics, we dissect them and use them. When we talk about applications, we think of quantum computing, quantum teleportation, quantum communications, quantum detection ... Ideas that exploit properties of the quantum nature of matter that have been ignored before," concludes Levy.

Directly published by Levy, the video below explains the study at three different levels of complexity:


Pascal conductance series in ballistic one-dimensional LaAlO3/SrTiO3 channels

Megan Briggeman, Michelle Tomczyk, Binbin Tian, Hyungwoo Lee, Jung-Woo Lee, Yuchi H, Anthony Tylan-Tyler, Mengchen Huang, Chang-Beom Eom, David Pekker, Roger S. K. Mong, Patrick Irvin, Jeremy Levy

Science  14 Feb 2020:

Vol. 367, Issue 6479, pp. 769-772

DOI: 10.1126/science.aat6467

Wednesday, 5 February 2020

New Graphene Device Detects and Amplifies Terahertz Waves

In recent years, physicists and engineers have learned to detect and control almost the entire electromagnetic spectrum, from UVs to infrared to gamma rays. However, a range of frequencies still escaped scientists: the terahertz frequency (THz). But recently, a team of researchers has developed a THz graphene detection device capable of amplifying THz waves. A real scientific feat that could lead to a whole new technological era.

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Terahertz (THz) waves lie between microwaves and infrared in the electromagnetic frequency spectrum, but due to their low energy, scientists have not been able to exploit their potential. The riddle is known in scientific circles as the "terahertz divide".

Being able to detect and amplify THz waves (T rays) would open up a new era of medical, communication, satellite, cosmological and other technologies. A major application would be a safe and non-destructive alternative to X-rays. However, so far, the wavelengths involved, which vary between 3 mm and 30 μm, have proven to be impossible to use due to relatively weak signals from from all existing sources.

Place THz waves in the electromagnetic spectrum. Credits: Y. Chassagneux

A THz wave amplifier based on graphene

A team of physicists has created a new type of optical transistor - a functional THz amplifier - using graphene and a high-temperature superconductor. The physics behind the amplifier is based on the properties of graphene, which is transparent and not sensitive to light and whose electrons are “massless”. It is made up of two layers of graphene and a superconductor which trap the “massless” electrons of graphene between them like a sandwich.

THz radiation hits the device and is re-emitted with amplified energy. Credits: Loughborough University

The device is then connected to a power source. When THz radiation hits the outer graphene layer, the trapped particles inside it attach to the outgoing waves, amplifying them. Professor Fedor Kusmartsev of the Loughborough Physics Department explains: “When the THz light hits the sandwich, it is reflected like a mirror. The main point is that there will be more reflected light than that which hit the device."

"It works because external energy is supplied by a battery or by light hitting the surface from other higher frequencies of the electromagnetic spectrum. THz photons are transformed by graphene into massless electrons, which, in turn, are transformed back into reflected and energized THz photons. Because of such a transformation, THz photons get their energy from graphene - or the battery - and weak THz signals are amplified.”

Towards control of the THz frequency and the advent of a new technological era

The study was published in the journal Physical Review Letters . The team continues to develop the device and hopes to have prototypes ready for testing soon. Professor Kusmartsev said he hopes to have an operational amplifier ready to go on the market in about a year. He added that such a device would greatly improve current technology and allow scientists to reveal more about the human brain.

The THz amplifier is small enough to fit into many technologies. Credits: Loughborough University

“The universe is full of terahertz radiation and signals. In fact, all biological organisms absorb and emit them. I hope that with such an amplifier available, we will be able to discover many mysteries of nature. For example, how chemical reactions and biological processes happen, or how our brain works at the thought level,” says Kusmartsev.

"It has properties that would greatly improve vast scientific fields such as imaging, spectroscopy, tomography, medical diagnosis, health monitoring, environmental monitoring and chemical and biological identification. The device we have developed will allow scientists and engineers to exploit this bandwidth and create the next generation of medical equipment, detection equipment and wireless communication technology,” he adds.


Optical transistor for amplification of radiation in a broadband terahertz domain

Phys. Rev. Lett.

K. H. A. Villegas, F. V. Kusmartsev, Y. Luo, and I. G. Savenko

Monday, 27 January 2020

Gravity on a quantum scale would have no symmetry

In physics, symmetry means the conservation of physical laws, or of certain quantities, under transformations or operations. For many years, theorists have been convinced that the fundamental laws describing our Universe, from stars to particles, are necessarily based on symmetries. However, gravity could escape this rule. Indeed, two physicists have shown that on the quantum scale, gravity has no symmetry. If this conception proves to be correct, the current theoretical models which are limited to describing quantum gravity should be modified.

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There are four basic interactions: electromagnetism, strong and weak nuclear interactions, and gravity. Gravity is the only force that does not yet have a description at the quantum level. Its effects on large objects, such as planets or stars, are relatively easy to describe, but things get complicated when the effects of gravity manifest on a quantum scale.

The holographic principle to describe gravity on a quantum scale

To try to understand gravity at the quantum level, Hirosi Ooguri, director of the Kavli Institute for Physics and Mathematics of the Universe in Tokyo, and Daniel Harlow, assistant professor at the Massachusetts Institute of Technology (MIT), started with the holographic principle. This principle explains the three-dimensional phenomena influenced by gravity on a flat two-dimensional space not influenced by gravity.

The researchers have shown that symmetry only affects the hatched areas of the diagram, not the surroundings of the point in the middle, so there can be no overall symmetry. Credits: KAVLI

It is not a real representation of our universe, but it is close enough to help researchers study its fundamental aspects. Earlier work by Harlow and others had found a precise mathematical analogy between the holographic principle and quantum error correction codes, which protect information in a quantum computer.

Ooguri and Harlow have shown that these quantum error correction codes are not compatible with any symmetry, which means that symmetry would not be possible in quantum gravity.

Better understand quantum gravity and its potential lack of symmetry

This work began over four years ago, when Ooguri discovered an article on holography and its relationship to quantum error correction codes by Harlow, who was then a post-doctoral fellow at Harvard University. Shortly after, the two met at the Institute for Advanced Study in Princeton, when Ooguri was on sabbatical and Harlow came to give a seminar.

“ I went to his seminar, prepared with questions. We talked a lot afterwards, then we started to think that the idea he had developed could perhaps be used to explain one of the fundamental properties of quantum gravity, about the lack of symmetry "explains Ooguri.

Their result has several important consequences. In particular, he predicts that protons are stable and do not disintegrate into other elementary particles, and that there are magnetic monopoles.

Although the two theorists have provided theoretical proof of the absence of symmetry in the context of quantum gravity, this preliminary work still needs to be deepened. If these works were generally well received by their peers, the researchers recall that the theoretical framework they used must be developed further.


Constraints on Symmetries from Holography

Daniel Harlow and Hirosi Ooguri

Phys. Rev. Lett. 122, 191601


English nuclear fusion reactor restarted for the first time in 23 years

Mastering nuclear fusion promises clean, unlimited energy. Many countries have already embarked on the fusion race with extremely promising results. Now it is England's turn to restart a prototype fusion reactor that has not been used for 23 years. The researchers hope to create a plasma stable enough to help ITER in its future trials.

In less than a year, researchers will try to create a plasma hotter than the Sun inside a torus-shaped machine in the south-east of England. It will be the country's first nuclear fusion operation in the last century.

The attempt to merge two hydrogen isotopes in November at the Joint European Torus (JET) in Culham, Oxfordshire, will be the first since the facility broke the record for electricity production by nuclear fusion for less than d 'a second in 1997.

Commercial nuclear fusion holds the promise of clean, unlimited energy, but is far from being realized. So far, test projects have consumed more energy (creating the reaction) than they produce. The UK is keen to be a leader in this area, with the government having committed £ 200 million last year for a project to build a commercial power plant based on fusion.

A structurally modified reactor for a more stable plasma

JET will import fuel from Canada for the November return to service: a few grams of each of the hydrogen, deuterium and tritium isotopes over the next few months. Once fused, they will produce a plasma with a temperature of 100 million degrees Celsius, which will be held in place by magnets.

The interior of the reactor has been completely modified to resemble the internal structure of ITER. Credits: JET

There are two key differences between this year's reaction and that of 23 years ago. The most important is that the materials used inside the reactor have been modified, with carbon-based materials such as graphite, replaced by tungsten and beryllium. Carbon acts like a sponge for hydrogen, so the change should mean more hydrogen in the plasma, rather than ending up in the wall.

The second difference is the lifetime of the plasma. In 1997, the maximum output of 16 megawatts lasted only a few milliseconds before the disappearance of the plasma. The group hopes that this time the plasma can be maintained for at least 5 seconds. Whatever the outcome, Wilson says the resulting data will be vital to assist ITER in manufacturing its first plasma, which is currently slated for 2025.

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.


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 


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