Thursday 27 July 2023

Study proposes combining continuum mechanics with Einstein field equations

 



Albert Einstein's general theory of relativity is a landmark in our understanding of the universe. It gave rise to the notion of a spacetime continuum against which all physical phenomena play out. But over the decades, it has inspired many questions that have yet to be answered: How can Einstein's equations describe forces other than gravity? And what are the "dark" forms of energy and matter that cause the universe to expand and galaxies to evolve?

In a new article, author Piotr Ogonowski offers a seemingly simple solution—the theories of spacetime and electromagnetism are describing the same things. Beginning from Einstein's field equations, Ogonowski reveals their ability to describe all known physical interactions, including those described by classical electromagnetism. Spacetime, it appears, may simply be the way we perceive electromagnetic fields.

To support the consistency between spacetime and electromagnetism, the author describes how the cosmological constant in Einstein's theory, which is believed to be responsible for dark energy is actually a description of an electromagnetic field. The conclusions drawn could be far-reaching. If further research confirms these findings, it would mean that Einstein was right from the beginning and that General Relativity explains much more than just gravity.


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Tuesday 25 July 2023

'Quantum avalanche' explains how nonconductors turn into conductors

 




Looking only at their subatomic particles, most materials can be placed into one of two categories.

Metals—like copper and iron—have free-flowing electrons that allow them to conduct electricity, while insulators—like glass and rubbe r— keep their electrons tightly bound and therefore do not conduct electricity.

Insulators can turn into metals when hit with an intense electric field, offering tantalizing possibilities for microelectronics and supercomputing, but the physics behind this phenomenon called resistive switching is not well understood.

Questions, like how large an electric field is needed, are fiercely debated by scientists, like University at Buffalo condensed matter theorist Jong Han.

"I have been obsessed by that," he says.

Han, Ph.D., professor of physics in the College of Arts and Sciences, is the lead author on a study that takes a new approach to answer a long-standing mystery about insulator-to-metal transitions. The study, "Correlated insulator collapse due to quantum avalanche via in-gap ladder states," was published in May in Nature Communications.
Quantum path allows electrons to move between bands

The difference between metals and insulators lies in quantum mechanical principles, which dictate that electrons are quantum particles and their energy levels come in bands that have forbidden gaps, Han says.

Since the 1930s, the Landau-Zener formula has served as a blueprint for determining the size of electric field needed to push an insulator's electrons from its lower bands to its upper bands. But experiments in the decades since have shown materials require a much smaller electric field—approximately 1,000 times smaller—than the Landau-Zener formula estimated.

"So, there is a huge discrepancy, and we need to have a better theory," Han says.

To solve this, Han decided to consider a different question: What happens when electrons already in the upper band of an insulator are pushed?

Jong Han works with his graduate student Xi Chen in his Fronczak Hall office. Chen is one of several graduate students who served as co-authors on the quantum avalanche study. Credit: Douglas Levere/University at Buffalo

Han ran a computer simulation of resistive switching that accounted for the presence of electrons in the upper band. It showed that a relatively small electric field could trigger a collapse of the gap between the lower and upper bands, creating a quantum path for the electrons to go up and down between the bands.

To make an analogy, Han says, "Imagine some electrons are moving on a second floor. When the floor is tilted by an electric field, electrons not only begin to move but previously forbidden quantum transitions open up and the very stability of the floor abruptly falls apart, making the electrons on different floors flow up and down.

"Then, the question is no longer how the electrons on the bottom floor jump up, but the stability of higher floors under an electric field."

This idea helps solve some of the discrepancies in the Landau-Zener formula, Han says. It also provides some clarity to the debate over insulator-to-metal transitions caused by electrons themselves or those caused by extreme heat. Han's simulation suggests the quantum avalanche is not triggered by heat. However, the full insulator-to-metal transition doesn't happen until the separate temperatures of the electrons and phonons—quantum vibrations of the crystal's atoms—equilibrate. This shows that the mechanisms for electronic and thermal switching are not exclusive of each other, Han says, but can instead arise simultaneously.

"So, we have found a way to understand some corner of this whole resistive switching phenomenon," Han says. "But I think it's a good starting point."
Research could improve microelectronics

The study was co-authored by Jonathan Bird, Ph.D., professor and chair of electrical engineering in UB's School of Engineering and Applied Sciences, who provided experimental context. His team has been studying the electrical properties of emergent nanomaterials that exhibit novel states at low temperatures, which can teach researchers a lot about the complex physics that govern electrical behavior.

"While our studies are focused on resolving fundamental questions about the physics of new materials, the electrical phenomena that we reveal in these materials could ultimately provide the basis of new microelectronic technologies, such as compact memories for use in data-intensive applications like artificial intelligence," Bird says.

The research could also be crucial for areas like neuromorphic computing, which tries to emulate the electrical stimulation of the human nervous system. "Our focus, however, is primarily on understanding the fundamental phenomenology," Bird says.

Other authors include UB physics Ph.D. student Xi Chen; Ishiaka Mansaray, who received a Ph.D. in physics and is now a postdoc at the National Institute of Standards and Technology, and Michael Randle, who received a Ph.D. in electrical engineering and is now a postdoc at the Riken research institute in Japan. Other authors include international researchers representing Swiss Federal Institute of Technology in Lausanne, Pohang University of Science and Technology, and the Center for Theoretical Physics of Complex Systems, Institute for Basic Science.

Since publishing the paper, Han has devised an analytic theory that matches the computer's calculation well. Still, there's more for him to investigate, like the exact conditions needed for a quantum avalanche to happen.

"Somebody, an experimentalist, is going to ask me, 'Why didn't I see that before?'" Han says. "Some might have seen it, some might not have. We have a lot of work ahead of us to sort it out."

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Monday 24 July 2023

Unveiling synchronization preferences of quantum thermal machines

 




Researchers from the Center of Theoretical Physics of Complex Systems within the Institute for Basic Science (PCS-IBS) made an important discovery that describes the relationship between synchronization and thermodynamics in quantum systems.

The question of how order arises from disorder has captivated humanity for centuries. One fascinating example of such emergence is synchronization, where multiple oscillators initialized randomly could end up oscillating in harmony. Synchronization exists in our everyday lives—for example, the sound of clapping hands or the simultaneous flashing of fireflies.

Remarkably, scientists have discovered many instances of synchronization in diverse natural and artificial phenomena, including in very small systems governed by quantum mechanics.

At the same time, the study of synchronization must also consider the second law of thermodynamics which only allows the total disorder of the universe to increase. This means that for a spontaneous emergence of order-like synchronization to occur, there has to be a cost of increasing disorder somewhere else (e.g., a wasteful heat in the surrounding environment). Yet, despite these intriguing connections, the precise relationship between synchronization and thermodynamics remains a mystery.

To unravel the underlying connection between synchronization and thermodynamics in the quantum regime, PCS-IBS researchers investigated a novel quantum thermal machine that exhibits synchronization. This machine is capable of acting as a quantum heat engine or as a quantum refrigerator. The study is published in the journal Physical Review Letters.

As a heat engine, it transforms heat flow from hot to cold baths to amplify the intensity of laser light. Conversely, as a refrigerator, it uses energy from laser light to maintain the temperature of the cold bath. Importantly, this machine is able to undergo synchronization simultaneously while performing its task due to its continuous interaction with the laser.

Curiously, the researchers found that as they scaled up the machine, multiple synchronizing actors started to arise within the machine. The synchronization behavior of the machine was not solely influenced by its interaction with lasers but also by the interplay between its various components.

These distinct synchronization actors could both cooperate and compete, much like two individuals jumping on a trampoline—for example, let's call them Jack and Jill. Cooperation arises when both Jack and Jill adjust their jumping rhythm in harmony, reaching their highest and lowest points simultaneously. Conversely, competition occurs when Jack attempts to match Jill's rhythm while Jill deliberately does the opposite, such as aiming to be at the lowest point when Jack reaches his highest.

According to the corresponding author, Dr. Juzar Thingna, "This is the first example in which synchronizing quantum systems are shown to cooperate and compete, opening a path to a richer synchronization landscape like quantum chimeras."

Intriguingly, the cooperation and competition between different synchronization mechanisms are intimately related to the thermodynamic functionality of the machine. Cooperation manifests in the case of the refrigerator, i.e., they have a preference for a system that synchronizes in harmony, like a peaceful orchestra. On the other hand, competition arises in the case of heat engines, as their components thrive in the middle of a crazy party and use all the chaos to perform at their best.

These findings are important because not only do they shed light on the fundamental relation between synchronization and thermodynamics, but they also give us new ideas for designing quantum technologies and relate the abstract notion of synchronization to the performance of quantum devices.

In other words, improving our understanding of how synchronization works in quantum machines, will allow us to make better devices that work coherently together. This could lead to more efficient and powerful quantum machines that will one day ignite the quantum revolution.

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Unveiling the quantum dance: Experiments reveal nexus of vibrational and electronic dynamics

 



Nearly a century ago, physicists Max Born and J. Robert Oppenheimer developed an assumption regarding how quantum mechanics plays out in molecules, which are comprised of intricate systems of nuclei and electrons. The Born-Oppenheimer approximation assumes that the motion of nuclei and electrons in a molecule are independent of each other and can be treated separately.

This model works the vast majority of the time, but scientists are testing its limits. Recently, a team of scientists demonstrated the breakdown of this assumption on very fast time scales, revealing a close relationship between the dynamics of nuclei and electrons. The discovery could influence the design of molecules useful for solar energy conversion, energy production, quantum information science and more.

The team, including scientists from the U.S. Department of Energy's (DOE) Argonne National Laboratory, Northwestern University, North Carolina State University and University of Washington, recently published their discovery in two related papers in Nature and Angewandte Chemie International Edition.

"Our work reveals the interplay between the dynamics of electron spin and the vibrational dynamics of the nuclei in molecules on superfast time scales," said Shahnawaz Rafiq, a research associate at Northwestern University and first author on the Nature paper. "These properties can't be treated independently—they mix together and affect electronic dynamics in complex ways."

A phenomenon called the spin-vibronic effect occurs when changes in the motion of the nuclei within a molecule affect the motion of its electrons. When nuclei vibrate within a molecule—either due to their intrinsic energy or due to external stimuli, such as light—these vibrations can affect the motion of their electrons, which can in turn change the molecule's spin, a quantum mechanical property related to magnetism.

In a process called inter-system crossing, an excited molecule or atom changes its electronic state by flipping its electron spin orientation. Inter-system crossing plays an important role in many chemical processes, including those in photovoltaic devices, photocatalysis and even bioluminescent animals. For this crossing to be possible, it requires specific conditions and energy differences between the electronic states involved.


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Researchers observe strongest quantum contextuality in single system

 



A team led by Prof. Li Chuanfeng and Prof. Xu Jinshi from the University of Science and Technology of China (USTC) of the Chinese Academy of Sciences (CAS), collaborating with Prof. Chen Jingling from Nankai University and Prof. Adán Cabello from the University of Seville, studied the single-system version of multipartite Bell nonlocality, and observed the highest degree of quantum contextuality in single system. Their work was published in Physical Review Letters.

Quantum contextuality refers to the phenomenon that the measurements of quantum observables cannot be simply considered as revealing preexisting properties. It is a distinctive feature in quantum mechanics and a crucial resource for quantum computation. Contextuality defies noncontextuality hidden-variable theories and is closely linked to quantum nonlocality.

In multipartite systems, quantum nonlocality arises as the result of the contradiction between quantum contextuality and noncontextuality hidden-variable theories. The extent of nonlocality can be measured by the violation of Bell inequality and previous researches showed that the violation increases exponentially with the number of quantum bits involved. However, while single-particle high-dimensional system offers more possibilities for measurements compared to multipartite systems, the quest to enhance contextual correlation's robustness remains an ongoing challenge.

To observe more robust quantum contextuality in single-particle system, the researchers adopted a graph-theoretic approach to quantum correlations. They associated the commutation relations between measurements used in nonlocality correlations with a graph of exclusivity, and then looked for another set of measurements in the single high-dimensional system that have commutation relation isomorphic to the graph. This approach fully quantifies the nonclassical properties of quantum correlations using graph parameters.

The researchers found that after transforming the Mermin-Ardehali-Belinskii-Klyshko (MABK) Bell inequality into noncontextuality inequality using the above approach, the maximum violation is the same but the required Hilbert space dimension is smaller compared to the dimension of the original Bell inequality. Further research indicated that this phenomenon of contextuality concentration, wherein contextuality transitions from nonlocality correlations to single-particle high-dimensional correlations, is widely observed within a class of nonlocality correlations previously discovered by the team.

In the experiment, the researchers developed a spatial light modulation technique to achieve high-fidelity quantum state preparation and measurement in a seven-dimensional quantum system based on photon spatial mode encoding.

By ensuring minimal disturbance between the initial and subsequent measurements, they observed a violation exceeding 68 standard deviations in the noncontextuality inequality derived from the three-party MABK inequality. The ratio between the quantum violation value and the classical limit reached 0.274, setting a new record for the highest ratio in single-particle contextuality experiments.

The discovery of quantum contextuality concentration not only lays the foundation for observing more quantum correlations but also holds the potential to advance the realization of quantum computation in various physical systems.


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Friday 21 July 2023

A solid-state quantum microscope that controls the wave functions of atomic quantum dots in silicon

 





Over the past decades, physicists and engineers have been trying to develop various technologies that leverage quantum mechanical effects, including quantum microscopes. These are microscopy tools that can be used to study the properties of quantum particles and quantum states in depth.

Researchers at Silicon Quantum Computing (SQC)/UNSW Sydney and the University of Melbourne recently created a new solid-state quantum microscope that could be used to control and examine the wave functions of atomic qubits in silicon. This microscope, introduced in a paper published in Nature Electronics, was created combining two different techniques, known as ion implantation and atomic precision lithography.


"Qubit device operations often rely on shifting and overlapping the qubit wave functions, which relate to the spatial distribution of the electrons at play, so a comprehensive knowledge of the latter provides a unique insight into building quantum circuits efficiently," Benoit Voisin and Sven Rogge, two researchers who carried out the study, told Phys.org.

"Spatial information about the wave functions is typically not possible during qubit device measurements as these are based on fixed charge sensing of the whole quantum state. Direct access to the full spatial extent of the quantum state can however be accessed using a scanning tunneling microscope (STM), which we have developed to place atoms in silicon with atomic precision. In this paper we have combined the local control of the wave function used for device operation directly within the microscope."

The lab at SQC/UNSW Sydney had been previously manufacturing qubit devices and developing scanning tunneling microscopes to image qubit wave functions in parallel, using individual phosphorus atoms embedded in silicon. In their new paper, Voisin, Rogge and their colleagues tried to merge these two different research efforts into a single platform. More specifically, they set out to realize a quantum microscope that could simultaneously map out and control atomic qubits with local electrodes in a single device.

"A quantum microscope is a tool where arrays of atoms can be engineered with atomic precision, and where each atom, or qubit, can be locally controlled and measured," Voisin and Rogge said. "Comparable microscopes exist in the cold atom community where laser technology is used in a vacuum. Our solid-state version of a quantum microscope very much resembles a transistor, with local electrodes defining the source and gate sides, and the STM tip acting as a drain which can move with picometer resolution from qubit to qubit and scan their wave function."

The new quantum microscope created by the researchers was created by combining two different techniques. More specifically, they used atomic precision lithography to introduce dopant atoms and more conventional ion implantation techniques to create the electrodes for their device. This unique approach to fabricate quantum microscopes was pioneered at UNSW.

"The qubits are defined using the atomic precision manufacturing technique, by incorporating a few phosphorus atoms in small patches of desorbed hydrogen at the silicon surface, close to the source electrode, Voisin and Rogge explained. "Contrary to typical STM experiments performed on conductive substrates, here our microscope operates on insulating silicon, and we had to design a light-assisted protocol to first stabilize the STM tip before being able to map out the qubit wave functions."

The researchers' microscope essentially utilizes the STM tip as a movable electrode, which can have valuable advantages. Most notably, this approach simplifies the collection of large qubit array measurements, without requiring the use of an increasing number of fixed sensors, but instead measuring entire arrays using a single STM tip.

"The ability to map out the qubit wave functions directly during device operation gives us invaluable and predictive insights on how to optimize the device design as we scale, such as distance and orientation between the qubits," Voisin and Rogge said. "As a consequence, with regard to the manufacturing of complete circuits using the atomic qubits we engineer at our SQC/UNSW lab, our quantum microscope will help speed up manufacturing cycles for better device performance."

Atomic precision lithography and ion implantation are two distinct processes typically realized in entirely different laboratory conditions. The integration of these two techniques to create a single device was thus a remarkable achievement for the team.

The recent study by Voisin, Rogge and their colleagues could bring a new wave of innovation in the field of STM and quantum microscopy, as it introduces a new approach for fabricating quantum microscopes. In the future, their proposed approach could be applied to microscopes based on other solid-state systems, such as molecules and magnetic atoms.

The SQC lab at UNSW is now exploring two key research directions. Firstly, they are trying to reach beyond the local electrostatic control demonstrated in their recent paper, by performing microwave coherent operations on the qubits inside their microscope.

"To do this, we need sub-100mK temperatures and finite magnetic fields, and we are currently commissioning a new equipment that will provide these capabilities," Voisin and Rogge added. "The second application we are exploring is to create and probe new correlated states of matter that are challenging to simulate with classical computation techniques or achieve with other experimental platforms such as cold atoms.

"We will fabricate large arrays of qubits strongly coupled to each other, in a regime where topological and superconducting states are expected to emerge. This is a very exciting area where our combination of precision manufacturing and ability to see wave functions directly will open new horizons in our atomic understanding of the world."

#QuantumMicroscope #SolidStateTechnology #AtomicQuantumDots #SiliconNanoscaleImaging #QuantumControl #CuttingEdgeScience #Nanotechnology #QuantumWaveFunctions #FutureTech #Innovation #ScienceBreakthrough

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Tuesday 18 July 2023

What does the Standard Model predict for the magnetic moment of the muon?

 



Predicting the numerical value of the magnetic moment of the muon is one of the most challenging calculations in high-energy physics. Some physicists spend the bulk of their careers improving the calculation to greater precision.

Why do physicists care about the magnetic properties of this particle? Because information from every particle and force is encoded in the numerical value of the muon's magnetic moment. If we can both measure and predict this number to ultra-high precision, we can test whether the Standard Model of Elementary Particles is complete.

Muons are identical to electrons, except they are about 200 times more massive, are not stable, and disintegrate into electrons and neutrinos after a short time. At the simplest level, theory predicts that the muon's magnetic moment, typically represented by the letter g, should equal 2. Any deviation from 2 can be attributed to quantum contributions from the muon's interaction with other—known and unknown—particles and forces. Hence scientists are focused on predicting and measuring g-2.

Several measurements of muon g-2 already exist. Scientists working on the Muon g-2 experiment at the U.S. Department of Energy's Fermi National Accelerator Laboratory expect to announce later this year the result of the most precise measurement ever made of the muon's magnetic moment.

Simultaneously, a large number of scientists are working on improving the Standard Model prediction of the value of muon g-2. Several parts feed into this calculation, related to the electromagnetic force, the weak nuclear force and the strong nuclear force.

The contribution from electromagnetic particles like photons and electrons is considered the most precise calculation in the world. The contribution from weakly interacting particles like neutrinos, W and Z bosons, and the Higgs boson is also well known. The most challenging part of the muon g-2 prediction stems from the contribution from strongly interacting particles like quarks and gluons; the equations governing their contribution are very complex.

Even though the contributions from quarks and gluons are so complex, they are calculable, in principle, and several different approaches have been developed. One of these approaches evaluates the contributions by using experimental data related to the strongly interacting nuclear force. When electrons and positrons collide, they annihilate and can produce particles made of quarks and gluons like pions. Measuring how often pions are produced in these collisions is exactly the data needed to predict the strong nuclear contribution to muon g-2.

For several decades, experiments at electron-positron colliders around the world have measured the contributions from quarks and gluons, including experiments in the US, Italy, Russia, China, and Japan. Results from all these experiments were compiled by a collaboration of experimental and theoretical physicists known as the Muon g-2 Theory Initiative. In 2020, this group announced the best Standard Model prediction for muon g-2 available at that time.

Ten months later, the Muon g-2 collaboration at Fermilab unveiled the result of their first measurement. The comparison of the two indicated a large discrepancy between the experimental result and the Standard Model prediction. In other words, the comparison of the measurement with the Standard Model provided strong evidence that the Standard Model is not complete and muons could be interacting with yet undiscovered particles or forces.

A second approach uses supercomputers to compute the complex equations for the quark and gluon interactions with a numerical approach called lattice gauge theory. While this is a well-tested method to compute the effects of the strong force, computing power has only recently become available to perform the calculations for muon g-2 to the required precision. As a result, lattice calculations published prior to 2021 were not sufficiently precise to test the Standard Model. However, a calculation published by one group of scientists in 2021, the Budapest-Marseille-Wuppertal collaboration, produced a huge surprise. Their prediction using lattice gauge theory was far from the prediction using electron-positron data.

In the last few months, the landscape of predictions for the strong force contribution to muon g-2 has only become more complex. A new round of electron-positron data has come out from the SND and CMD3 collaborations. These are two experiments taking data at the VEPP-2000 electron-positron collider in Novosibirsk, Russia. A result from the SND collaboration agrees with the previous electron-positron data, while a result from the CMD3 collaboration disagrees with the previous data.

What is going on? While there is no simple answer, there are concerted efforts by all the communities involved to better quantify the Standard Model prediction. The Lattice Gauge Theory community is working around the clock towards testing and scrutinizing the BMW collaboration's prediction in independent lattice calculations with improved precision using different methods. The electron-positron collider community is working to identify possible reasons for the differences between the CMD3 result and all previous measurements. More importantly, the community is in the process of repeating these experimental measurements using much larger data sets. Scientists are also introducing new independent techniques to understand the strong-force contribution, such as a new experiment proposed at CERN called MUonE.

What does this mean for muon g-2? The Fermilab Muon g-2 collaboration will release its next result, based on data taken in 2019 and 2020, later this year. Because of the large amount of additional data that is going into the new analysis, the Muon g-2 collaboration expects its result to be twice as precise as the first result from their experiment. But the current uncertainty in the predicted value makes it hard to use the new result to strengthen our previous indication that the Standard Model is incomplete and there are new particles and forces affecting muon g-2.

What is next? The Fermilab Muon g-2 experiment concluded data taking this spring. It will still take a couple of years to analyze the entire data set, and the experiment expects to release its final result in 2025. At the same time, the Muon g-2 Theory Initiative is working to shore up the predicted value using new data and new lattice calculations that should also be available before 2025. It will be a very exciting showdown. In the meantime, the high-energy physics community is eagerly anticipating the announcement of the world's best measurement from the Fermilab Muon g-2 experiment later this year.

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Saturday 15 July 2023

Researchers establish criterion for nonlocal quantum behavior in networks



A new theoretical study provides a framework for understanding nonlocality, a feature that quantum networks must possess to perform operations inaccessible to standard communications technology. By clarifying the concept, researchers determined the conditions necessary to create systems with strong, quantum correlations.

The study, published in Physical Review Letters, adapts techniques from quantum computing theory to create a new classification scheme for quantum nonlocality. This not only allowed the researchers to unify prior studies of the concept into a common framework, but it facilitated a proof that networked quantum systems can only display nonlocality if they possess a particular set of quantum features.

"On the surface, quantum computing and nonlocality in quantum networks are different things, but our study shows that, in certain ways, they are two sides of the same coin," said Eric Chitambar, a professor of electrical and computer engineering at the University of Illinois Urbana-Champaign and the project lead. "In particular, they require the same fundamental set of quantum operations to deliver effects that cannot be replicated with classical technology."

Nonlocality is a consequence of entanglement, in which quantum objects experience strong connections even when separated over vast physical distances. When entangled objects are used to perform quantum operations, the results display statistical correlations that cannot be explained by non-quantum means. Such correlations are said to be nonlocal. A quantum network must possess a degree of nonlocality to ensure that it can perform truly quantum functions, but the phenomenon is still poorly understood.

To facilitate study of nonlocality, Chitambar and physics graduate student Amanda Gatto Lamas applied the formalism of quantum resource theory. By treating nonlocality as a "resource" to manage, the researchers' framework allowed them to view past studies of nonlocality as separate instances of the same concept, just with different restrictions on the resource's availability. This facilitated the proof of their main result, that nonlocality can only be achieved with a limited set of quantum operations.

"Our result is the quantum network analogue to an important quantum computing result called the Gottesman-Knill theorem," Gatto Lamas explained. "While Gottesman-Knill clearly defines what a quantum computer must do to surpass a classical one, we show that a quantum network must be constructed with a particular set of operations to do things that a standard communications network cannot."

Chitambar believes that the framework will not only be useful for developing criteria to assess a quantum network's quality based on the degree of nonlocality it possesses, but that it can be used to expand the concept of nonlocality.

"Right now, there is a relatively good understanding of the type of nonlocality that can emerge between two parties," he said. "However, one can imagine for a quantum network consisting of many connected parties that there might be some kind of global property that you can't reduce to individual pairs on the network. Such a property might depend intimately on the network's overall structure."


5th Edition of International Conference on Quantum Physics and Quantum Technologies

#QuantumNetworks #NonlocalQuantumBehavior #QuantumResearch #QuantumEntanglement #QuantumInformation #QuantumComputing #QuantumPhysics #QuantumTechnology

27-28 July 2023 | Delhi, India.

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Tuesday 11 July 2023

Laser pulse creates exotic order in quantum material

 




Water flows, ice is rigid—this clear difference between the liquid and solid state of substances is part of our everyday experience. It follows from the very regular arrangement of atoms and molecules in crystalline solids, which is lost when they melt. Less clear, however, is the structure of "liquid crystals"—highly interesting states that combine order and disorder in such a way that important applications such as LCDs (liquid crystal displays) are possible.

Researchers from the Max Planck Institute (MPI) for Multidisciplinary Sciences in Göttingen, in collaboration with colleagues from Kiel University (CAU), Deutsches Elektronen Sychrotron DESY and University of Göttingen have now successfully created a state in a crystalline material that—similar to the structure of liquid crystals—can be described as neither clearly liquid nor clearly crystalline.

The studied layered crystal, grown in Kiel by Kai Rossnagel's team, professor at CAU and leading scientist at DESY, is characterized by a minimal distortion of the crystal structure at room temperature. This is due to the special structure of the crystal, in which thin layers of metal and sulfur atoms are stacked on top of each other and only weakly bound.

If these layers are now bombarded with ultrashort laser flashes, the distortion changes its orientation within a trillionth of a second, abruptly increasing the electrical conductivity of the material. Although both types of distortions have an ordered structure and associated crystalline properties, a highly disordered state can be observed during the transition.
Short snapshot: State disappeared after nanosecond fraction

"After exciting the material with light, the atoms in the crystal structure have yet to find their new, slightly different positions. This transforms the material into an unusually disordered, so-called hexatic state," says Till Domröse, Ph.D. student at MPI and first author of the study now published in the journal Nature Materials.

"This state is otherwise mainly observed in liquid crystals. In our experiments, however, it is extremely volatile and has already disappeared after the fractions of a nanosecond." Making the hexatic state visible placed high demands on the measurement technology used. On the one hand, for example, a very fast temporal resolution is required to take a sufficiently short snapshot. On the other hand, the structural changes in the material are so subtle that they can only be seen with a very high sensitivity to atomic positions. Electron microscopes in principle provide the necessary spatial resolution, but are typically not fast enough.

In recent years, the Göttingen team led by Max Planck Director Claus Ropers has closed this gap by developing an "ultrafast" electron microscope capable of imaging even unimaginably rapid processes in the nanocosmos. "This microscope was also used in these experiments and enabled us to capture the unusually ordered phase and its temporal evolution in a series of images," Ropers explains. "At the same time, we developed a new high-resolution diffraction mode that will be essential for studying many other functional nanostructures."



Unique layered crystals

"The highly complex dynamics that take place in this type of layered crystal offer numerous scientific questions and possible applications," says Rossnagel, member of the speaker group of the priority research area KiNSIS (Kiel Nano, Surface and Interface Science) at CAU and lead scientist at the German Electron Synchrotron DESY in Hamburg. "The basis are fascinating network-like structures, which we can only develop and study in close collaboration with state-of-the-art research infrastructures such as at the MPI in Göttingen and DESY in Hamburg. This enables excellent research on quantum materials in northern Germany."

"The highly complex dynamics that take place in this type of layered crystal offer numerous scientific questions and possible applications," says Rossnagel, a member of the speaker group of the research focus KiNSIS (Kiel Nano, Surface and Interface Science) at CAU. "The basis is fascinating network-like structures that we can only develop and investigate in close cooperation with state-of-the-art research infrastructures such as at the MPI in Göttingen and DESY in Hamburg. This enables excellent research on quantum materials in northern Germany."

These special crystals have been grown in Kiel since the early 1980s. Close ties between CAU and DESY have existed for just as long, and are now institutionalized in the Ruprecht-Haensel Laboratory. "DESY's high-precision nanoanalytics with the PETRA III and FLASH facilities have contributed decisively to the high quality of our crystals and helped ensure that we receive enquiries from all over the world," Rossnagel continues. Studies like this one with the MPI in Göttingen, in which a novel state was discovered in a quantum material, also open up perspectives for future collaboration with DESY research groups to better understand new quantum materials.

#QuantumMaterial #ExoticOrder #LaserPulse #QuantumPhysics

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Friday 7 July 2023

Scientists propose an all-optical labeling method for encrypted fiber optic tags

 




Fiber sensing scientists from Shenzhen University have developed an encrypted fiber optic tag that can be used for all-optical labeling and recognition of optical transmission channels such as access networks.

Publishing in the journal International Journal of Extreme Manufacturing, the team led by researchers based at the Guangdong and Hong Kong Joint Research Center for Optical Fiber Sensors proposed an all-optical labeling method with encryption property, which uses the feature information and spatial distribution of fiber Bragg grating arrays to flexibly store different coding sequences.

Unlike traditional optical link labeling methods, the all-fiber tag proposed by the team fully utilizes the characteristics of the optical link to achieve all-optical reading, recognition, and restoration of link information. The findings could have a significant impact on the maintenance of optical distribution networks.

The fiber optic tag is based on a fiber Bragg grating array prepared by femtosecond laser direct writing. By cleverly utilizing the spatial distribution, reflectivity, and reflection wavelength of the gratings, the tag can carry rich information. When using an optical time-domain reflectometer for reading, a specific administrator can perform complete and error-free information recovery.

One of the lead researchers, Professor Changrui Liao, commented, "This study developed a method for all-optical link encryption labeling and recognition. Faced with the increasing number of optical transmission links, traditional physical labels have high labor costs and are prone to waste port resources, making it difficult to meet efficient and stable link labeling requirements. All of these indicate that this fiber optic tag will have broad market application prospects."

One-off printed physical labels or handwritten symbols can be used to number and distinguish links, however, these highly manual methods face significant challenges in the information age.

First author Mr. Zhihao Cai explained, "Our work provides a reliable and efficient encryption labeling method for optical signal transmission link labeling. The use of femtosecond laser direct writing can achieve rapid mass production of tags, which are very helpful for obtaining dumb information for optical network users."

The team prepared a fiber Bragg grating array using femtosecond laser multi-pulse exposure, which can control the characteristics of different grating fragments, such as reflectivity. By utilizing the reflected signal of the grating to increase the number of switch states that the fiber optic tag can represent, the storage capacity of the fiber tag is improved.

Due to the distribution and reflection characteristics of the grating, there are many possibilities for fiber optic tags to read and recover information. Therefore, only specific management personnel can obtain correct optical link information to prove that fiber optic tags have sufficient security.

Professor Yiping Wang said, "This work fully demonstrates the flexibility of the femtosecond laser direct writing technology, by adjusting the number of femtosecond laser pulse exposures to achieve regulation of the reflection characteristics of each encoded grating fragment, ultimately giving the proposed fiber tag greater capacity and application potential. It is a fascinating and practical task for maintaining existing optical networks."

#OpticalLabeling #FiberOpticTags #EncryptionTechnology #SecureCommunication #OpticalCommunication #DataSecurity #ScientificResearch #Innovation #Photonics #OpticalTechnology


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Thursday 6 July 2023

Quantum neural networks: An easier way to learn quantum processes

 






EPFL scientists show that even a few simple examples are enough for a quantum machine-learning model, the "quantum neural networks," to learn and predict the behavior of quantum systems, bringing us closer to a new era of quantum computing.

Imagine a world where computers can unravel the mysteries of quantum mechanics, enabling us to study the behavior of complex materials or simulate the intricate dynamics of molecules with unprecedented accuracy.

Thanks to a pioneering study led by Professor Zoe Holmes and her team at EPFL, we are now closer to that becoming a reality. Working with researchers at Caltech, the Free University of Berlin, and the Los Alamos National Laboratory, they have found a new way to teach a quantum computer how to understand and predict the behavior of quantum systems. The research has been published in Nature Communications.
Quantum neural networks (QNNs)

The researchers worked on "quantum neural networks" (QNNs), a type of machine-learning model designed to learn and process information using principles inspired by quantum mechanics in order to mimic the behavior of quantum systems.

Just like the neural networks used in artificial intelligence, QNNs are made of interconnected nodes, or "neurons," that perform calculations. The difference is that, in QNNs, the neurons operate on the principles of quantum mechanics, allowing them to handle and manipulate quantum information.

"Normally, when we teach a computer something, we need a lot of examples," says Holmes. "But in this study, we show that with just a few simple examples called 'product states' the computer can learn how a quantum system behaves even when dealing with entangled states, which are more complicated and challenging to understand."
Product states

The "product states" that the scientists used refer to a concept in quantum mechanics that describes the specific type of state for a quantum system. For example, if a quantum system is composed of two electrons, then its product state is formed when each individual electron's state is considered independently, and then combined.

Product states are often used as a starting point in quantum computations and measurements because they provide a simpler and more manageable framework for studying and understanding the behavior of quantum systems, before moving on to more complex and entangled states, where the particles are correlated and cannot be described independently.
Better quantum computers ahead

The researchers demonstrated that by training QNNs using only a few of these simple examples, computers can effectively grasp the complex dynamics of entangled quantum systems.

Holmes explains, "This means that [we] might be able to learn about and understand quantum systems using smaller, simpler computers, like the near-term intermediary scale [NISQ] computers we're likely to have in the coming years, instead of needing large and complex ones, which may be decades away."

The work also opens up new possibilities for using quantum computers to solve important problems like studying complex new materials or simulating the behavior of molecules.

Finally, the method improves the performance of quantum computers by enabling the creation of shorter and more error-resistant programs. By learning how quantum systems behave, we can streamline the programming of quantum computers, leading to improved efficiency and reliability. "We can make quantum computers even better by making their programs shorter and less prone to errors," says Holmes.

#QuantumNeuralNetworks #QuantumProcesses #QuantumComputing #MachineLearning #ArtificialIntelligence
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Wednesday 5 July 2023

Scientists discover Rydberg moiré excitons

 


The Rydberg state is widespread in a variety of physical platforms such as atoms, molecules, and solids. In particular, Rydberg excitons are highly excited Coulomb-bound states of electron-hole pairs, first discovered in the semiconductor material Cu2O in the 1950s.

In a study published in Science, Dr. Xu Yang and his colleagues from the Institute of Physics of the Chinese Academy of Sciences (CAS), in collaboration with researchers led by Dr. Yuan Shengjun of Wuhan University, have reported observing Rydberg moiré excitons, which are moiré-trapped Rydberg excitons in the monolayer semiconductor WSe2 adjacent to small-angle twisted bilayer graphene (TBG).

The solid-state nature of Rydberg excitons, combined with their large dipole moments, strong mutual interactions and greatly enhanced interactions with the surroundings, holds promise for a wide range of applications in sensing, quantum optics, and quantum simulation.

However, researchers have not fully exploited the potential of Rydberg excitons. One of the main obstacles lies in the difficulty of efficiently trapping and manipulating Rydberg excitons. The rise of two-dimensional (2D) moiré superlattices with highly tunable periodic potentials provides a possible way forward.

In recent years, Dr. Xu Yang and his collaborators have worked on exploring the application of Rydberg excitons in 2D semiconducting transition metal dichalcogenides (such as WSe2). They have developed a new Rydberg sensing technique that exploits the sensitivity of Rydberg excitons to the dielectric environment to detect the exotic phases in a nearby 2D electronic system.
Spectroscopic evidence of the Rydberg moiré exciton formation in WSe2 adjacent to 0.6° TBG and numerical calculations of the spatial charge distribution in TBG at different doping levels. Credit: IOP

In this study, using low-temperature optical spectroscopy measurements, the researchers first found the Rydberg moiré excitons manifesting as multiple energy splittings, a pronounced red shift, and a narrowed linewidth in the reflectance spectra.

Using numerical calculations performed by the group from Wuhan University, the researchers attributed these observations to the spatially varying charge distribution in TBG, which creates a periodic potential landscape (so-called moiré potential) for interacting with Rydberg excitons.

The strong confinement of Rydberg excitons is achieved by the largely unequal interlayer interactions of the constituent electron and hole of a Rydberg exciton due to the spatially accumulated charges centered in the AA-stacked regions of TBG. The Rydberg moiré excitons thus realize electron–hole separation and exhibit the character of long-lived charge-transfer excitons.

Twist angle dependences and crossover to the strong-coupling regime. Credit: IOThe researcher demonstrated a novel method of manipulating Rydberg excitons, which is difficult to achieve in bulk semiconductors. The long-wavelength (tens of nm) moiré superlattice in this study serves as an analog to the optical lattices created by a standing-wave laser beam or arrays of optical tweezers that are used for Rydberg atom trapping.

In addition, tunable moiré wavelengths, in-situ electrostatic gating, and a longer lifetime all ensure great controllability of the system, with a strong light–matter interaction for convenient optical excitation and readout.

This study may provide new opportunities for realizing the next step in Rydberg–Rydberg interactions and coherent control of Rydberg states, with potential applications in quantum information processing and quantum computation.



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Tuesday 4 July 2023

Novel 'toggle-switch' could lead to more versatile quantum processors with clearer outputs

 


What good is a powerful computer if you can't read its output? Or readily reprogram it to do different jobs? People who design quantum computers face these challenges, and a new device may make them easier to solve.

The device, introduced by a team of scientists at the National Institute of Standards and Technology (NIST), includes two superconducting quantum bits, or qubits, which are a quantum computer's analog to the logic bits in a classical computer's processing chip. The heart of this new strategy relies on a "toggle switch" device that connects the qubits to a circuit called a "readout resonator" that can read the output of the qubits' calculations.

This toggle switch can be flipped into different states to adjust the strength of the connections between the qubits and the readout resonator. When toggled off, all three elements are isolated from each other. When the switch is toggled on to connect the two qubits, they can interact and perform calculations. Once the calculations are complete, the toggle switch can connect either of the qubits and the readout resonator to retrieve the results.

Having a programmable toggle switch goes a long way toward reducing noise, a common problem in quantum computer circuits that makes it difficult for qubits to make calculations and show their results clearly.

"The goal is to keep the qubits happy so that they can calculate without distractions, while still being able to read them out when we want to," said Ray Simmonds, a NIST physicist and one of the paper's authors. "This device architecture helps protect the qubits and promises to improve our ability to make the high-fidelity measurements required to build quantum information processors out of qubits."

The team, which also includes scientists from the University of Massachusetts Lowell, the University of Colorado Boulder and Raytheon BBN Technologies, describes its results in a paper published June 26 in Nature Physics.

Quantum computers, which are still at a nascent stage of development, would harness the bizarre properties of quantum mechanics to do jobs that even our most powerful classical computers find intractable, such as aiding in the development of new drugs by performing sophisticated simulations of chemical interactions.

However, quantum computer designers still confront many problems. One of these is that quantum circuits are kicked around by external or even internal noise, which arises from defects in the materials used to make the computers. This noise is essentially random behavior that can create errors in qubit calculations.

Present-day qubits are inherently noisy by themselves, but that's not the only problem. Many quantum computer designs have what is called a static architecture, where each qubit in the processor is physically connected to its neighbors and to its readout resonator. The fabricated wiring that connects qubits together and to their readout can expose them to even more noise.

Such static architectures have another disadvantage: They cannot be reprogrammed easily. A static architecture's qubits could do a few related jobs, but for the computer to perform a wider range of tasks, it would need to swap in a different processor design with a different qubit organization or layout. (Imagine changing the chip in your laptop every time you needed to use a different piece of software, and then consider that the chip needs to be kept a smidgen above absolute zero, and you get why this might prove inconvenient.)

The team's programmable toggle switch sidesteps both of these problems. First, it prevents circuit noise from creeping into the system through the readout resonator and prevents the qubits from having a conversation with each other when they are supposed to be quiet.

"This cuts down on a key source of noise in a quantum computer," Simmonds said.

Second, the opening and closing of the switches between elements are controlled with a train of microwave pulses sent from a distance, rather than through a static architecture's physical connections. Integrating more of these toggle switches could be the basis of a more easily programmable quantum computer. The microwave pulses can also set the order and sequence of logic operations, meaning a chip built with many of the team's toggle switches could be instructed to perform any number of tasks.

"This makes the chip programmable," Simmonds said. "Rather than having a completely fixed architecture on the chip, you can make changes via software."

One last benefit is that the toggle switch can also turn on the measurement of both qubits at the same time. This ability to ask both qubits to reveal themselves as a couple is important for tracking down quantum computational errors.

The qubits in this demonstration, as well as the toggle switch and the readout circuit, were all made of superconducting components that conduct electricity without resistance and must be operated at very cold temperatures. The toggle switch itself is made from a superconducting quantum interference device, or "SQUID," which is very sensitive to magnetic fields passing through its loop. Driving a microwave current through a nearby antenna loop can induce interactions between the qubits and the readout resonator when needed.

At this point, the team has only worked with two qubits and a single readout resonator, but Simmonds said they are preparing a design with three qubits and a readout resonator, and they have plans to add more qubits and resonators as well. Further research could offer insights into how to string many of these devices together, potentially offering a way to construct a powerful quantum computer with enough qubits to solve the kinds of problems that, for now, are insurmountable.

#QuantumComputing #QuantumProcessors #ToggleSwitch #Versatility #ClearOutputs
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Experimental quantum imaging distillation with undetected light

  It is possible to image an object with an induced coherence effect by making use of photon pairs to gain information on the item of intere...