Physics for You

Effective energy extraction from quantum sources is guided by a new instrument by Institute for Basic Science.

The assumption that energy is a key factor in society evolution has given rise to the notion that a civilization's level of technical advancement can be determined by its capacity for utilising and harnessing energy. On the basis of this, Russian physicist Nikolai Kardashev created the renowned Kardashev Scale in 1964 as a means of categorising civilizations according to their energy use. According to this scale, our civilisation as a whole is currently considered to be at a level of about 0.73.

One of the most important responsibilities for the development of civilisation is energy harvesting. Early civilizations relied on manual labour and animal power to meet their energy demands, but a significant change in how people used energy occurred during the Industrial Revolution in the 18th and 19th centuries. The development of coal mining and the creation of steam power allowed humans to extensively utilise the energy contained in fossil fuels. This facilitated the quick emergence of modern civilization and the corresponding surge in energy consumption.

Our existing way of living makes us exceedingly efficient even if it is very energy-demanding. For instance, it is estimated that the average worker today is 10 times more productive than they were fifty years ago. Thus, the rise in energy consumption played a significant role in the tremendous economic expansion we experienced during the previous century. Therefore, in order for the economy to continue expanding and for us to eventually achieve a post-scarcity society, we must be able to secure ever-increasing quantities of energy sources—ideally sustainable ones.

Technology that has been developed to use these sources efficiently is another essential component. This is true for any energy source: solar cells with different designs have varied efficiencies, while burning fossil fuels at higher temperatures releases more energy. Therefore, the ability to extract this energy and the state of contemporary technology are the two main factors that determine an energy source's output.

Realising there is energy to be extracted is, of course, a necessary component of the ability to extract energy. Imagine a caveman finding a chunk of coal; they would quickly realise that they could use it as a tool to paint pictures within the cave.
But it is by no means obvious that such a lump of coal can be burned, acting as an energy source.

Additionally, there is a distinction between realising that there is energy to be extracted and possessing the tools necessary for doing so. For instance, we are well known that the sun's huge energy output comes from the nuclear fusion of hydrogen atoms into helium. However, achieving this process in a lab is a very different animal, despite the fact that recent technical advancements have brought us closer to this ultimate objective.

The ability to collect energy from quantum sources is becoming a reality as a result of the recent advancements in quantum technologies. Quantum technologies are subject to human constraints just like any other technology. Even now, it is unclear to quantum physicists which quantum systems can act as an energy source and which ones cannot. Furthermore, it is unknown which will be readily available in the future given our current technologies and which will be so in the near future.

Recently, scientists from the Institute for Basic Science's Centre for Theoretical Physics of Complex Systems (PCS) developed observational ergotropy, a measure of a source's extractable energy that takes into account the precise limitations of our existing technology.

The highest amount of work that can be extracted from a system is referred to as ergotropy. Prior assessments of quantum ergotropy were idealistic—they presupposed perfect experimental conditions. This is similar to thinking that our fusion power plant is as effective as the sun without taking into account all the difficulties that come when attempting to intentionally mimic the sun.

Observational ergotropy, in contrast to earlier metrics, offers more accurate predictions and updates in line with our technological capability, letting us know which sources are the best energy sources given our current experimental capabilities.

More information:-
Dominik Šafránek et al, Work extraction from unknown quantum sources, arXiv (2022). DOI: 10.48550/arxiv.2209.11076

Journal information: Physical Review Letters , arXiv

Electron orbital fingerprints are found in supercomputer simulations by University of Texas at Austin.

A perfect sphere is a pure mathematical construction that will never be visible. However, scientists have now observed the signatures of electron orbitals, which are determined by mathematical equations of quantum mechanics and predict where an atom's electron will most likely be. They did this using supercomputer simulations and atomic resolution microscopes.

Two distinct transition-metal atoms, iron (Fe) and cobalt (Co), which are found in metal-phthalocyanines, have their electron orbital fingerprints directly seen by researchers at UT Austin, Princeton University, and ExxonMobil. These hallmarks can be seen in the atomic force microscope measurements of forces, which frequently mirror the underlying orbitals and can be interpreted in this way.

Their research was released in the journal Nature Communications in March 2023 as an Editors' Highlights.

"Our collaborators at Princeton University found that despite Fe and Co being adjacent atoms on the periodic table, which implies similarity, the corresponding force spectra and their measured images show reproducible experimental differences," said study co-author James R. Chelikowsky, the W.A. "Tex" Moncrief, Jr. Chair of Computational Materials and professor in the Departments of Physics, Chemical Engineering, and Chemistry in the College of Natural Sciences at UT Austin. Chelikowsky also serves as the director of the Center for Computational Materials at the Oden Institute for Computational Engineering and Sciences.

Without a theoretical analysis, the Princeton scientists could not determine the source of the differences they spotted using high-resolution non-contact atomic force microscopy (HR-AFM) and spectroscopy that measured molecular-scale forces on the order of piconewtons (pN), one-trillionth of a Newton.

"When we first observed the experimental images, our initial reaction was to marvel at how experiment could capture such subtle differences. These are very small forces," Chelikowsky added.

"By directly observing the signatures of electron orbitals using techniques such as atomic force microscopy, we can gain a better understanding of the behavior of individual atoms and molecules, and potentially even how to design and engineer new materials with specific properties. This is especially important in fields such as materials science, nanotechnology, and catalysis," Chelikowsky said.

The required electronic structure calculations are based on density functional theory (DFT), which starts from basic quantum mechanical equations and serves as a practical approach for predicting the behavior of materials.

"Our main contribution is that we validated through our real-space DFT calculations that the observed experimental differences primarily stem from the different electronic configurations in 3d electrons of Fe and Co near the Fermi level, the highest energy state an electron can occupy in the atom," said study co-first author Dingxin Fan, a former graduate student working with Chelikowsky. Fan is now a postdoctoral research associate at the Princeton Materials Institute.

The DFT calculations included the copper substrate for the Fe and Co atoms, adding a few hundred atoms to the mix and calling for intense computation, for which they were awarded an allocation on the Stampede2 supercomputer at the Texas Advanced Computing Center (TACC).

"In terms of our model, at a certain height, we moved the carbon monoxide tip of the AFM over the sample and computed the quantum forces at every single grid point in real space," Fan said. "This entails hundreds of different computations. The built-in software packages on TACC's Stampede2 helped us to perform data analysis much more easily. For example, the Visual Molecular Dynamics software expedites an analysis of our computational results."

"Stampede2 has provided excellent computational power and storage capacity to support various research projects we have," Chelikowsky added.

By demonstrating that the electron orbital signatures are indeed observable using AFM, the scientists assert that this new knowledge can extend the applicability of AFM into different areas.

What's more, their study, used an inert molecular probe tip to approach another molecule and accurately measured the interactions between the two molecules. This allowed the science team to study specific surface chemical reactions.

For example, suppose that a catalyst can accelerate a certain chemical reaction, but it is unknown which molecular site is responsible for the catalysis. In this case, an AFM tip prepared with the reactant molecule can be used to measure the interactions at different sites, ultimately determining the chemically active site or sites.

Moreover, since the orbital level information can be obtained, scientists can gain a much deeper understanding of what will happen when a chemical reaction occurs. As a result, other scientists could design more efficient catalysts based on this information.

Said Chelikowsky: "Supercomputers, in many ways, allow us to control how atoms interact without having to go into the lab. Such work can guide the discovery of new materials without a laborious 'trial and error' procedure."

According to study co-author James R. Chelikowsky, the W.A. "Tex" Moncrief, Jr. Chair of Computational Materials and professor in the Departments of Physics, Chemical Engineering, and Chemistry in the College of Natural Sciences at UT Australia, "our collaborators at Princeton University found that despite Fe and Co being adjacent atoms on the periodic table, which implies similarity, the corresponding force spectra and their measured images show reproducible experimental differences." The Oden Institute for Computational Engineering and Sciences's Centre for Computational Materials is directed by Chelikowsky as well.

The Princeton researchers could not identify the origin of the discrepancies they discovered using high-resolution non-contact atomic force microscopy (HR-AFM) and spectroscopy, which recorded molecular-scale forces on the order of piconewtons (pN), without conducting a theoretical analysis.

"When we first looked at the experimental photographs, our first thought was how incredible it was that the experiment could pick up such minute variations. These are quite weak forces, Chelikowsky continued.

We can better understand the behaviour of individual atoms and molecules by directly detecting the signatures of electron orbitals using methods like atomic force microscopy, and perhaps even learn how to build and engineer new materials with certain features. This is crucial in areas like materials science, nanotechnology, and catalysis, according to Chelikowsky.

Density functional theory (DFT), which begins with fundamental quantum mechanical equations and acts as a practical method for predicting the behaviour of materials, is the foundation for the electronic structure calculations that are necessary.

Further Reading Link:
https://phys.org/news/2023-05-supercomputing-simulations-electron-orbital-signatures.html

Quantum computers Innovation with fluxonium processors by Monica Hernandez.

The next generation of quantum devices requires high-coherence qubits that are less error-prone. Responding to this need, researchers at the AQT at Berkeley Lab, a state-of-the-art collaborative research laboratory, developed a blueprint for a novel quantum processor based on "fluxonium" qubits. Fluxonium qubits can outperform the most widely used superconducting qubits, offering a promising path toward fault-tolerant universal quantum computing.

In collaboration with researchers from the University of California, Berkeley, and Yale University, the AQT team pioneered a systematic theoretical study of how to engineer fluxonium qubits for higher performance while offering practical suggestions to adapt and build the cutting-edge hardware that will fully harness the potential of quantum computing. Their results were published in the journal PRX Quantum.

On the leading edge of superconducting processors
Superconducting quantum processors consist of multiple qubits designed to have different transition frequencies facilitating precise control of individual qubits and their interactions. The transmon qubit, one of the most widely used in the field for superconducting processors, typically has low anharmonicity. Anharmonicity is the difference between relevant transition frequencies in a qubit. Low anharmonicity contributes to spectral crowding (when qubit frequencies are close to resonating with each other), making the processor more difficult to control since qubit frequencies are arranged tightly together.

In contrast, high anharmonicity allows researchers to have better qubit control because there's less overlap between the frequencies that control the qubits and those that drive any given qubit to higher energy levels. The fluxonium qubit has inherent advantages for complex superconducting processors, such as high anharmonicity, long coherence times, and simple control.

Building on AQT's robust research and development history on superconducting circuits, the team leading the fluxonium-based architecture focused on the scalability and adaptability of the processor's main components, with a set of parameters that researchers can tune to increase the runtime and fidelity of quantum circuits. Some of these adaptations allow simpler operation of the system. Researchers proposed, for example, controlling the fluxonium qubits at low frequency (1-GHz) via microwave pulses directly generated by an electrical arbitrary waveform generator. This straightforward approach allows researchers to design processors and set up multiple qubits flexibly.

Flexible approaches with fluxonium qubits for large-scale devices
Long B. Nguyen is a project scientist at AQT and the paper's lead author. Nguyen started researching alternative superconducting qubits as a University of Maryland graduate student working with Professor Vladimir Manucharyan. Manucharyan introduced fluxonium qubits to the field just a decade earlier, and in 2019 Nguyen demonstrated the possible longer coherence times with fluxonium circuits. The fluxonium circuit is composed of three elements: a capacitor, a Josephson Junction, and a superinductor, which helps suppress magnetic flux noise—a typical source of unwanted interference that affects superconducting qubits and causes decoherence.

"I always wanted to study new physics, and I focused on fluxonium because it appeared to be a better alternative to the transmon at the time. It has three circuit elements that I could play with to get the type of spectra I wanted. It could be designed to evade decoherence due to material imperfections. I also recently realized that scaling up fluxonium is probably more favorable since the estimated fabrication yield is high, and the interactions between individual qubits can be engineered to have high-fidelity," explained Nguyen.

To estimate and validate the performance of the proposed fluxonium blueprint, the team at AQT, in collaboration with the paper's researchers, simulated two types of programmable quantum logic gates—the cross-resonance controlled-NOT (CNOT) and the differential ac-Stark controlled-Z (CZ). The high fidelities resulting from the gates' simulation across the range of proposed qubit parameters validated the team's expectations for the suggested blueprint.

"We provided a potential path towards building fluxonium processors with standard, practical procedures to deploy logic gates with varying frequencies. We hope that more R&D on fluxonium and superconducting qubit alternatives will bring about the next generation of devices for quantum information processing," said Nguyen.

More information: Long B. Nguyen et al, Blueprint for a High-Performance Fluxonium Quantum Processor, PRX Quantum (2023). DOI: 10.1103/PRXQuantum.3.037001. link.aps.org/doi/10.1103/PRXQuantum.3.037001

Journal information: PRX Quantum

Stripes within crystals hint at behavior of electrons in quantum systems
by RIKEN

Hidden stripes in a crystal could help scientists understand the mysterious behavior of electrons in certain quantum systems, including high-temperature superconductors, an unexpected discovery by RIKEN physicists suggests.

The electrons in most materials interact with each other very weakly. But physicists often observe interesting properties in materials in which electrons strongly interact with each other. In these materials, the electrons often collectively behave as particles, giving rise to "quasiparticles."

"A crystal can be thought of like an alternative universe with different laws of physics that allow different fundamental particles to live there," says Christopher Butler of the RIKEN Center for Emergent Matter Science.

Butler and colleagues examined a crystal in which a layer of nickel atoms was arranged in a square lattice, like a chessboard. Individual electrons have a small mass, but within this crystal, they appeared as massless quasiparticles.

The team set out to examine this odd effect using a scanning tunneling microscope, but this proved challenging. The walnut-sized microscope is housed inside a vacuum chamber, surrounded by a roomful of equipment that creates low temperatures and ultralow pressures comparable to that at the surface of the moon.

"To examine the pristine surface of these crystals, we try to cleave off a small flake, much as geologists do," says Butler. "But we have to do this inside the vacuum, and these crystals are so brittle they are prone to explode into dust."

After numerous attempts, they succeeded and used the microscope to scan the flake with a small needle—like a record player—with a voltage across it. Varying the voltage allowed them to probe different features.

The team confirmed the nickel atoms were arranged in a chessboard-like arrangement. But to their surprise, the electrons had broken this pattern and were instead aligned in stripes (Fig. 1). This is called nematicity—where interactions in the system make the electrons display less symmetry than the underlying material.

Butler likens the discovery to standing by a pond and throwing in a pebble. "You'd expect to see circular ripples, so if you saw ripples appearing in parallel lines, you would know something weird is going on," he says. "It demands an explanation."

Such experiments will help physicists test different proposed theories for the behavior of quantum systems with many particle interactions, such as high-temperature superconductors. These new results, for instance, fit with predictions made using a "density-wave" framework proposed by the study's co-authors at Nagoya University in Japan.

"The behavior of many interacting electrons is hard to predict even with supercomputers," says Butler. "But at least we can observe what they are doing under a microscope."

The findings are published in the the Proceedings of the National Academy of Sciences.

More information: Christopher John Butler et al, Correlation-driven electronic nematicity in the Dirac semimetal BaNiS 2, Proceedings of the National Academy of Sciences (2022). DOI: 10.1073/pnas.2212730119

Journal information: Proceedings of the National Academy of Sciences