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

Tuesday, 7 April 2020

One Step Closer to Quantum Spin Liquids


A recent discovery by University of Arkansas physicists could help researchers establish the existence of quantum spin liquids, a new state of matter. They’ve been a mystery since they were first proposed in the 1970s. If proven to exist, quantum spin liquids would be a step toward much faster, next-generation quantum computing.

Scientists have focused attention and research on the so-called Kitaev-type of spin liquid, named in honor of the Russian scientist, Alexei Kitaev, who first proposed it. In particular, they have looked extensively at two materials – RuCl3  and Na2IrO – as candidates for this type. Both have small quantum spin numbers.

“Traditional candidates are pretty limited to only these two,” said Changsong Xu, a researcher in the Department of Physics and first author of a paper published in the journal Physical Review Letters.

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In their recent work, U of A physicists have greatly expanded the number of materials that might be candidates as Kitaev quantum spin liquids by looking at materials with higher quantum spin numbers, and by putting materials under physical strain to tune their magnetic states.

“Suddenly, we realize there are dozens of candidates we can propose,” said Xu.

Quantum spin liquids are defined by their unusual magnetic arrangement. Magnets have a north and south pole, which combined are called dipoles. These are typically produced by the quantum spin of electrons. Inside a magnetic material, dipoles tend to all be parallel to each other (ferromagnetism) or periodically alternate their up and down direction (antiferromagnetism).

In the case of hypothetical quantum spin liquids, dipoles aren’t as well ordered. Instead, they exhibit unusual ordering within a small distance of each other. Different ordering creates different types of spin liquids.



Quantum spin liquids (QSLs) form an extremely unusual magnetic state in which the spins are highly correlated and fluctuate coherently down to the lowest temperatures, but without symmetry breaking and without the formation of any static long-range-ordered magnetism. Such intriguing phenomena are not only of great fundamental relevance in themselves, but also hold promise for quantum computing and quantum information.

Xu, along with Distinguished Professor of Physics Laurent Bellaiche and colleagues in China and Japan, used computational models to predict a Kitaev quantum spin liquid state in materials such as chromium iodide and chromium germanium telluride. The work, which was supported by grants from the Arkansas Research Alliance and the Department of Energy, will give researchers many more materials to study in a search to prove the existence of quantum spin liquids, said Xu.


Bibliography:

Changsong Xu, Junsheng Feng, Mitsuaki Kawamura, Youhei Yamaji, Yousra Nahas, Sergei Prokhorenko, Yang Qi, Hongjun Xiang, L. Bellaiche.

Possible Kitaev Quantum Spin Liquid State in 2D Materials with S=3/2.

Physical Review Letters, 2020; 124 (8)

DOI: 10.1103/PhysRevLett.124.087205

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.”



Bibliography:

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).

https://doi.org/10.1038/s41586-020-2057-7

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.


Bibliography:

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.




Bibliography:

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.


Bibliography:

Generation of spatiotemporal optical vortices with controllable transverse orbital angular momentum

Andy Chong, Chenhao Wan, Jian Chen & Qiwen Zhan

Nature Photonics (2020)

https://doi.org/10.1038/s41566-020-0587-z

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