Category: Physics

by Tejasri Gururaj , Phys.org

Conservation laws are central to our understanding of the universe, and now scientists have expanded our understanding of these laws in quantum mechanics.

A conservation law in physics describes the preservation of certain quantities or properties in isolated physical systems over time, such as mass-energy, momentum, and electric charge.

Conservation laws are fundamental to our understanding of the universe because they define the processes that can or cannot occur in nature. For example, the conservation of momentum reveals that within a closed system, the sum of all momenta remains unchanged before and after an event, such as a collision.

This translates to explaining the behavior of objects in motion, from the motion of planets in space to the complex dynamics of subatomic particles.

However, things get more interesting when we look at the world of quantum mechanics. In quantum mechanics, conservation theorems can be derived from principles like the symmetries of physical systems, unlike classical mechanics, where they are starting points.

Quantum mechanics boasts a repertoire of conservation laws, some of which have classical counterparts, while others are uniquely quantum. Sandu Popescu, one of the authors of a new study published in Proceedings of the National Academy of Sciences, pointed out that despite its incredible success in explaining a multitude of phenomena, quantum mechanics still eludes a deep, intuitive grasp of its underlying principles.

In Dr. Popescu’s own words, “Despite its long-standing presence, the counterintuitive behavior of microscopic particles leaves me with a universal acceptance that a deep, intuitive understanding remains elusive. The ongoing discovery of surprising and paradoxical effects underscores my need to achieve this understanding.”

To do so, the researchers devised a thought experiment.

Thought experiments and conservation laws in quantum mechanics

A thought experiment is a hypothetical scenario used to explore the consequences of theories and principles, providing new perspectives and insights, often challenging prevailing beliefs.

These experiments are intentionally crafted to investigate the consequences of a specific principle. The experiment’s structure may render it impractical to execute, and even if feasible, there may not be an intent to carry it out. To illustrate the significance of thought experiments, let’s delve into a simple example presented by Dr. Popescu.

This thought experiment features two characters, Alice and Bob, each perched on a chair with wheels, positioned to face one another. These chairs glide gracefully across the floor with minimal friction, setting the stage for an intriguing exploration of conservation laws within the quantum realm.

They have the same mass, and when they push each other, they move in opposite directions at the same speed, resulting in a constant sum of speeds equal to zero. As Dr. Popescu elucidated, “The sum of the speeds remains constant, both before and after their interaction.”

He continued, “This is a remarkable finding because it holds regardless of the specific nature of their interaction. You can predict that the sum of their speeds is zero without knowing details of how they pushed each other.”

Understanding the broader significance of this thought experiment requires acknowledging its universal applicability. The principle it illustrates extends to various scenarios, accommodating differences in mass, initial motion, or complex multi-dimensional interactions, highlighting its enduring and invaluable predictive capacity.

Dr. Popescu explained, “At a deeper level, certain conservation laws emerge from the symmetries found in nature. In the case of the example, it’s evident that the location in the universe doesn’t affect the experiment’s outcome. Other conservation laws dictate limitations such as not being able to extract more energy than what was initially invested.”

Challenges with traditional conservational laws in quantum mechanics

In classical physics, the concept of conservation is relatively straightforward. You measure a specific quantity at the beginning of an experiment, and you measure it again at the end. If the values match, the quantity is considered conserved.

“This doesn’t work in quantum mechanics. The reason is that performing a measurement disturbs the system,” explained Dr. Popescu.

Measuring a quantity immediately after preparation disrupts the system, fundamentally altering its subsequent evolution. Despite matching measurement results at the end, it fails to reveal the original state, as the system’s time evolution has been irreversibly changed.

To navigate these challenges, researchers devised a thought experiment. Their experimental setup involved preparing a quantum system in a specific initial state and measuring a conserved quantity—position—immediately after its preparation.

Subsequently, they allowed the system to evolve without any measurement disturbance. “Now the evolution proceeded as desired since you didn’t disturb the system at the beginning,” said Dr. Popescu. Researchers chose the states where the particle was in a superoscillatory region, known for its high-frequency or varied behavior. They then measured the particle’s angular momentum and checked whether it fell within a specific range.

They compared the result of the second measurement, the angular momentum measurement, with the initial measurement, that characterized the particle’s initial state. If these two measurements were the same, it indicated that the measured quantity of interest, in this case, angular momentum, was conserved throughout the experiment.

The conservation is statistical because individual cases can’t definitively prove it due to quantum randomness. It is confirmed by comparing outcome probabilities between experiments measuring the quantity right after preparation and at the end using the same initial state.

The researchers found two key insights. First, they showed that “preparation nonconservation” and “measurement nonconservation” cancel each other out, leading to the conservation of angular momentum even in individual cases. This challenges classical conservation laws.

Second, the paper argued that the proposed pure state, which appeared non-conservative in individual cases, is unrealistic and doesn’t exist in nature. This emphasizes the importance of considering frames of reference and the system-measuring device interaction to understand quantum conservation laws.

This study challenges conventional quantum conservation laws, emphasizing the impact of frames of reference and unphysical states, and calls for a reconsideration of statistical conservation in quantum mechanics.

Dr. Popescu concluded by saying, “We do not claim that there is anything incorrect with the standard way to define conservation laws in quantum mechanics. We claim, however, that one can do better than that.”

More information: Yakir Aharonov et al, Conservation laws and the foundations of quantum mechanics, Proceedings of the National Academy of Sciences (2023). DOI: 10.1073/pnas.2220810120

Journal information: Proceedings of the National Academy of Sciences 

© 2023 Science X Network

by TranSpread

With the growth in artificial intelligence (AI), 5G systems, cloud computation, and the Internet of Things, transmitters with extremely high capacities are required for data communication. Ultrafast optical modulation is an essential technology for high-capacity transmitters. High-speed optical modulators have attracted considerable interest. Significant progress has been achieved using different mechanisms in various material systems.

In a new paper published in Light: Advanced Manufacturing, a team of scientists led by Daoxin Dai from Zhejiang University developed a compact lithium-niobate-on-insulator (LNOI) photonic chip.

Optical modulators on a lithium-niobate-on-insulator (LNOI) platform have exhibited great potential. This arises from a linear electro-optic (EO) effect, low excess loss, and high stability. LNOI EO modulators based on Mach-Zehnder interferometers and microresonators have performed excellently. The length of the phase-shifting arms is typically several millimeters, or even centimeters, to realize a low voltage. Traveling-wave electrodes have been introduced, along with high-speed MZMs. Due to their large footprint, the arraying LNOI MZMs is inconvenient for high-capacity multiplexed systems with multiple channels.

Alternatively, the resonator-based modulators of LNOI can be compact, which may reduce power consumption. Therefore, resonator-based LNOI modulators have attracted significant attention in recent years. The research team has successfully demonstrated high-speed LNOI modulators based on a specific 2 × 2 Fabry-Perot (FP) cavity. It enables an ultra-high 3-dB bandwidth beyond 110 GHz and data capacity up to 140 Gbps. The footprint of the modulation section is much smaller than that of the reported LNOI ring resonator modulators. It is very attractive for arraying wavelength-division multiplexing (WDM) systems.

Meanwhile, advanced multiplexing techniques have been widely studied to expand link capacity by transmitting data through multiple channels in parallel. WDM has been successfully applied in which multiple wavelength channels are introduced. In recent years, various waveguide structures have been developed to realize WDM filters with excellent performance.

Great efforts have been made and impressive progress has been achieved utilizing smart designs without waveguide bends in key regions. A four-channel coarse WDM transmitter chip on LNOI has been demonstrated using a multiplexer, including an angled multimode interferometer. The research team proposed and developed a promising LNOI photonic filter based on a straight multimode waveguide grating (MWG). It enables a box-like spectral response whose central wavelength and bandwidth can be designed flexibly. A four-channel WDM filter with box-like responses was established using cascaded LNOI MWGs for the first time.

Photonic waveguides and circuits of large scale present increased manufacturing requirements. High-quality fabrication is critical for achieving low-loss and low-phase error light propagation in optical waveguides. Several typical fabrication technologies have been established for LNOI photonic waveguides. Dry-etching is preferred because of its process compatibility and fabrication repeatability. Inductively coupled plasma (ICP) etching with Ar gas is one of the most common methods for fabricating LNOI photonic waveguides and devices.

The chip features transport cascading and channel uniformity. Experimentally, the proposed chip enables high-capacity data transmission of 320 Gbps OOK signals, and 400 Gbps PAM4 signals with a low power consumption of 11.9 fJ/bit. This indicates enormous potential for the large-scale photonic integration of LN and the bright prospect of LNOI regarding its widespread use in integrated photonic devices.

The present photonic chip with an ultracompact footprint can be scaled for more channels and thus shows great potential for further realization of ultrahigh-capacity and energy-efficient WDM transmitters on LNOI in the future. Introducing SiO₂, amorphous-Si, or Cr hard masks can improve the fabrication process. With an improved etching process, the propagation loss (mainly originating from the scattering loss at the sidewalls) can be reduced to a wafer scale. It will support the development of high-performance LNOI photonic integrated circuits and satisfy the growing demands for various applications.

More information: Hongxuan Liu et al, Ultra-compact lithium niobate photonic chip for high-capacity and energy-efficient wavelength-division-multiplexing transmitters, Light: Advanced Manufacturing (2023). DOI: 10.37188/lam.2023.013

Provided by TranSpread

by University of Sydney

Ever since Antonie van Leeuwenhoek discovered the world of bacteria through a microscope in the late seventeenth century, humans have tried to look deeper into the world of the infinitesimally small.

There are, however, physical limits to how closely we can examine an object using traditional optical methods. This is known as the diffraction limit and is determined by the fact that light manifests as a wave. It means a focused image can never be smaller than half the wavelength of light used to observe an object.

Attempts to break this limit with “super lenses” have all hit the hurdles of extreme visual losses, making the lenses opaque. Now physicists at the University of Sydney have shown a new pathway to achieve superlensing with minimal losses, breaking through the diffraction limit by a factor of nearly four times. The key to their success was to remove the super lens altogether.

The research is published in Nature Communications.

The work should allow scientists to further improve super-resolution microscopy, the researchers say. It could advance imaging in fields as varied as cancer diagnostics, medical imaging, or archaeology and forensics.

Lead author of the research, Dr. Alessandro Tuniz from the School of Physics and University of Sydney Nano Institute, said, “We have now developed a practical way to implement superlensing, without a super lens. To do this, we placed our light probe far away from the object and collected both high- and low-resolution information. By measuring further away, the probe doesn’t interfere with the high-resolution data, a feature of previous methods.”

Previous attempts have tried to make super lenses using novel materials. However, most materials absorb too much light to make the super lens useful.

Dr. Tuniz said, “We overcome this by performing the superlens operation as a post-processing step on a computer, after the measurement itself. This produces a ‘truthful’ image of the object through the selective amplification of evanescent (or vanishing) light waves.”

Co-author Associate Professor Boris Kuhlmey, also from the School of Physics and Sydney Nano, said, “Our method could be applied to determine moisture content in leaves with greater resolution, or be useful in advanced microfabrication techniques, such as non-destructive assessment of microchip integrity. And the method could even be used to reveal hidden layers in artwork, perhaps proving useful in uncovering art forgery or hidden works.”

Typically, superlensing attempts have tried to home in closely on the high-resolution information. That is because this useful data decays exponentially with distance and is quickly overwhelmed by low-resolution data, which doesn’t decay so quickly. However, moving the probe so close to an object distorts the image.

“By moving our probe further away we can maintain the integrity of the high-resolution information and use a post-observation technique to filter out the low-resolution data,” Associate Professor Kuhlmey said.

The research was done using light at terahertz frequency at millimeter wavelength, in the region of the spectrum between visible and microwave.

Associate Professor Kuhlmey said, “This is a very difficult frequency range to work with, but a very interesting one, because at this range we could obtain important information about biological samples, such as protein structure, hydration dynamics, or for use in cancer imaging.”

Dr. Tuniz said, “This technique is a first step in allowing high-resolution images while staying at a safe distance from the object without distorting what you see. Our technique could be used at other frequency ranges. We expect anyone performing high-resolution optical microscopy will find this technique of interest.”

More information: Subwavelength terahertz imaging via virtual superlensing in the radiating near field, Nature Communications (2023). DOI: 10.1038/s41467-023-41949-5

Journal information: Nature Communications 

Provided by University of Sydney 

by Oak Ridge National Laboratory

Quantum computers process information using quantum bits, or qubits, based on fragile, short-lived quantum mechanical states. To make qubits robust and tailor them for applications, researchers from the Department of Energy’s Oak Ridge National Laboratory sought to create a new material system.

“We are pursuing a new route to create quantum computers using novel materials,” said ORNL materials scientist Robert Moore, who co-led a study published in Advanced Materials with ORNL colleague Matthew Brahlek, who is also a materials scientist.

They coupled a superconductor, which offers no resistance to electrical current, with a topological insulator, which has electrically conductive surfaces but an insulating interior. The result is an atomically sharp interface between crystalline thin films with different symmetric arrangements of atoms. The novel interface that they designed and engineered may give rise to exotic physics and host a unique quantum building block with potential as a superior qubit.

“The idea is to make qubits with materials that have more robust quantum mechanical properties,” Moore said. “What is important is that we have learned how to control the electronic structure of the topological insulator and the superconductor independently, so that we can tailor the electronic structure at that interface. This had never been done.”

Controlling the electronic structure on both sides of an interface may create something called Majorana particles inside the material. “In nature, we have particles and antiparticles, for example electrons and positrons, which annihilate each other when they come in contact. A Majorana particle is its own antiparticle,” Moore said. In 1937 Ettore Majorana predicted the existence of these exotic particles, whose existence remains to be proven.

In 2008, theorical physicists Liang Fu and Charlie Kane of the University of Pennsylvania proposed that creating a novel interface between a topological insulator with a superconductor would generate topological superconductivity, a new phase of matter predicted to host Majorana particles.

“If you have a pair of Majorana particles and move them around each other, there is a memory of this motion. They always know each other’s location,” Moore said. “This process could be used to encode quantum information and compute in new ways.”

However, realization of a new phase of matter that can host Majorana particles depends on finding the right material. Such an achievement takes a diverse team of experts.

When Moore came to ORNL in 2019, he brought a new expertise in angle-resolved photoemission spectroscopy, or ARPES, a technique for probing the electronic structure of materials. ARPES is based on the photoelectric effect, for which Albert Einstein was awarded the 1921 Nobel Prize in physics. It focuses a light source on a sample and characterizes electrons ejected from the material surface when the electrons absorb energy from the photons. The technique helps scientists understand how electrons behave inside a material.

This strategic investment in ARPES expertise helped ORNL win its bid to lead one of five DOE National Quantum Information Science Research Centers, the Quantum Science Center, which launched in 2020. Led by ORNL’s Travis Humble, the QSC aims to realize quantum computing and sensing applications by developing hardware and algorithms and discovering novel materials. Moore and his colleagues focus on topological materials for hardware development. Since April, Moore has also co-directed ORNL’s Interconnected Science Ecosystem, or INTERSECT, with Ben Mintz to develop laboratories of the future—smart, autonomously controlled processes and experiments with the potential to revolutionize research outcomes.

Brahlek, who joined ORNL in 2018 and recently received a DOE Early Career Research award, is an expert in precision synthesis of materials. To make superclean interfaces between a superconductor and topological insulator, he used molecular beam epitaxy, a method industry uses for large-scale fabrication of semiconductors for electronic devices.

With help from former postdoctoral fellow Tyler Smith, Brahlek performed the synthesis under ultrahigh vacuum. “Inside the chamber, there are fewer molecules bouncing around than in outer space. It is a really clean environment. It must be well controlled,” Brahlek said. “You start with little furnaces, each containing one element. Each furnace heats until the element inside starts to sublimate, or pass from solid to vapor state. This creates beams of elements. They all converge on a crystal substrate and adhere.”

He co-deposited iron, selenium and tellurium to make a superconductor that was one atomic layer thick. “If you can get the conditions exactly right, the deposited atoms will chemically bond and assemble, atomic layer by atomic layer, into a crystalline thin film,” Brahlek said.

“A key to getting the results was understanding how to combine bismuth telluride with iron selenide telluride at an atomic interface to gain the desired electronic behavior,” Brahlek said.

That accomplishment was tricky because the superconductor’s lattice of iron, selenium and tellurium comprises ordered square cells, whereas the topological insulator is a network of adjoining triangles. “We’re putting something square on something triangular, but surprisingly, the crystalline film grows nicely,” Brahlek said. “This accomplishment requires understanding the physics and chemistry that happen at these interfaces, which is critical to combining topological and superconducting properties in a single platform.”

That platform is the topological superconductor. To understand its topological properties, Moore used spin-resolved ARPES, with help from ORNL postdoctoral fellow Qiangsheng Lu, to probe quantum spin-dependent electronic structure at the interface of the topological insulator and the superconductor. Meanwhile, to confirm its superconducting behavior, Brahlek and former ORNL postdoctoral fellows Yun-Yi Pai and Michael Chilcote assisted with measurements of electrical resistance.

“We were able to see how the different electronic structures were interacting at the interface, and we were able to control those interactions to ensure all the ingredients for topological superconductivity exist,” Moore said. “We found that the desired topological properties only exist for specific selenium doping ranges. This was a surprise that is crucial for making qubits.”

Meanwhile, Hoyeon Jeon and An-Ping Li at ORNL’s Center for Nanophase Materials Sciences used scanning tunneling microscopy to characterize disorder in the materials. ORNL staff scientists Hu Miao and Satoshi Okamoto provided experimental and theoretical guidance throughout the study.

Crucial challenges remain. “We need to improve and better understand the materials at the atomic level, which is critical to confirming and using Majorana particles for applications,” Moore said. “The next step will be exploring possible Majorana particles using a newly installed ultralow-temperature scanning tunneling microscope instrument at CNMS.”

He added, “Achieving a qubit based on Majorana particles is one of the ultimate goals of the Quantum Science Center. The Majorana particle in materials is such an exotic state. Proving that it exists will require both building and testing a qubit-like device. It is an odd way to think about it, but you have to make a qubit to prove it is a qubit. We now know how to control the materials to the level required to make this happen.”

More information: Robert G. Moore et al, Monolayer Superconductivity and Tunable Topological Electronic Structure at the Fe(Te,Se)/Bi₂Te₃ Interface, Advanced Materials (2023). DOI: 10.1002/adma.202210940

Journal information: Advanced Materials 

Provided by Oak Ridge National Laboratory 

by José Tadeu Arantes, FAPESP

Electrical signals control a vast number of activities in the human body, from exchanges of messages between brain neurons and stimulation of the heart muscle to the impulses that enable hands and feet to move, among many other examples. To monitor or modulate these signals for medical purposes, a biocompatible and biodegradable optical fiber based on agar, a substance extracted from Gracilaria seaweed, has just been developed.

The research was led by Eric Fujiwara, a professor at the State University of Campinas’s School of Mechanical Engineering (FEM-UNICAMP) in São Paulo state, Brazil; Cristiano Monteiro de Barros Cordeiro at the Gleb Wataghin Institute of Physics (IFGW-UNICAMP); and Hiromasa Oku at Gunma University in Japan. An article about it is published in Scientific Reports.

“Biocompatible devices are indispensable when fiber optics is used in medical applications such as monitoring of vitals, phototherapy or optogenetics [a method of controlling and monitoring specific cells by combining optics, genetics and bioengineering], among others. Optical fibers made of biodegradable materials are also an alternative to existing telecommunications technologies based on glass or plastic fibers,” Fujiwara said.

The novel optical fiber was produced from agar, a natural gelatin obtained from red algae. It is transparent, flexible, edible and renewable. The same researchers had previously developed an agar-based biocompatible optical fiber as a sensor to monitor chemical concentration and humidity.

“The production process consists basically of filling cylindrical molds with agar solutions. Our latest research expands the range of applications, proposing a novel type of optical sensor that leverages agar’s electrical conductivity,” Fujiwara explained.

When the fiber is excited by coherent light, it produces granular light patterns that evolve spatially and temporally. As the electrical currents present in the medium traverse the fiber, they modulate the agar’s refraction index and create disturbances in the granular patterns, which are known as speckles.

“Analysis of these disturbances enables us to determine the magnitude and direction of the electrical stimuli with reliable measurements for currents equal to or even smaller than 100 microamperes [μA],” Fujiwara said.

The ability to detect electrical signals as subtle as these could be essential to possible biomedical applications. “The fiber could be used in sensor systems to monitor bioelectrical stimuli produced in the brain or muscles, serving as a biodegradable alternative to conventional electrodes. In this case, the optical signals could be decoded to diagnose disturbances. Another possibility would be to use the fiber as an ancillary interface in human-computer connections for assistive or rehabilitation technologies,” Fujiwara said.

The sensor’s response can be enhanced by adjusting the chemical composition of the material, and agar‘s moldability into various geometries means it can be used to make lenses and other optical devices with sensitivity to electrical current. The most important advantage is that the fiber can be absorbed by the organism after use, avoiding the need for additional surgical interventions.

This is still bench research, Fujiwara stressed, and technological applications are a long way off, but rigorous determination of the physical parameters of the optical response to electrical current lays a sound foundation for future fabrication of biomedical devices using the fiber.

More information: Eric Fujiwara et al, Agar-based optical sensors for electric current measurements, Scientific Reports (2023). DOI: 10.1038/s41598-023-40749-7

Journal information: Scientific Reports 

Provided by FAPESP 

by Chinese Academy of Sciences

Lanthanide-doped upconversion nanoparticles (UCNPs) demonstrate many superior properties of broadly-tunable multicolor emission and long emission lifetimes. This makes them well-suited for many light-emission applications. Despite many advantages, the further progress and practical applications of UCNPs face significant challenges.

Polarization is another important characteristic of fluorescence. It provides orientation and structural information in an additional dimension. Polarization has been extensively employed in the fluorescence polarization imaging technique. The inherent upconversion emission processes associated with the 4f electronic transition in UCNPs are typically weak and do not exhibit polarization characteristics.

In a new paper published in eLight, a team of scientists led by Professors Xiangping Li and Zi-Lan Deng (Jinan University), Junjie Li (Institute of Physics, CAS), Yuri Kivshar (Australian National University) have developed a dielectric metasurface for the amplification of upconversion emissions.

Most previous studies focused on enhancing the upconversion luminescence by coupling UCNPs with plasmonic nanostructures. The concentrated electromagnetic field near these nanostructures can significantly boost the absorption cross-sections and the upconversion radiation rates. Metallic structures supporting surface plasmons effectively localize light and create strong electric fields, enhancing upconversion fluorescence.

Due to intrinsic loss, metallic nanostructures generally have low-quality factor (Q factor) values. They may cause a quenching effect due to direct contact with the emission materials. The Q value is a crucial parameter for resonant excitation. The typical range of Q factor for plasmonic resonances is up to a few tens. Some special designs like metal-insulator-metal (MIM) structures can obtain a high Q factor. The emitters are close to the metallic surface. The quenching effect makes the quantum efficiency drop rapidly.

Compared to plasmonic nanostructures, all-dielectric resonant metasurfaces composed of high-index dielectric nanostructures with low losses at visible frequencies support much richer multipolar Mie-resonances. It provides an excellent alternative to fluorescence enhancement.

Collective high-Q resonances emerge due to coupling these Mie multipoles in periodic arrays. Breaking the symmetry of meta-atoms can transform bound states in the continuum (BICs) into a quasi-BIC with finite but extraordinarily high Q factors. These emerging concepts have recently been well-exploited and demonstrated for fluorescence enhancement and low-threshold lasing applications.

The research team utilized the emerging concept of high-Q collective modes of resonant metasurfaces for polarization-controlled dual-band upconversion bursts. The team designed and fabricated high-Q resonant metasurfaces composed of diatomic nano bricks. The design supports a quasi-BIC mode for the y-polarized incidence and another high-Q Mie resonance mode for the x-polarized incidence. NaYF₄:Yb/Er UCNPs deposited on the metasurface exhibit ultrabright upconversion luminescence at dual bands.

By controlling the rotation of the polarization analyzer, the team demonstrated linearly- and cross-polarized emission at dual bands with ultra-high degrees of polarization (DoPs). The demonstration paves the way for efficient enhancements and polarization control of the UCNP emission with potential applications in low-threshold polarization upconversion lasers and hyperspectral imaging/sensing.

More information: Ziwei Feng et al, Dual-band polarized upconversion photoluminescence enhanced by resonant dielectric metasurfaces, eLight (2023). DOI: 10.1186/s43593-023-00054-2

Journal information: eLight 

Provided by Chinese Academy of Sciences 

by Ingrid Fadelli , Phys.org

The behaviors, physiology and existence of living organisms is supported by countless biological processes, which entail the communication between cells and other molecular components. These molecular components are known to transmit information to each other in various ways, for instance via processes know as diffusion and electrical depolarization or by exchanging mechanical waves.

Researchers at Yale University recently carried out a study aimed at calculating the energetic cost of this transfer of information between cells and molecular components. Their paper, published in Physical Review Letters, introduces a new tool that could be used to study cellular networks and better understand their function.

“We have been thinking about this project for a while now in one form or another,” Benjamin B. Machta, one of the researchers who carried out the study, told Phys.org.

“I first discussed ideas that eventually morphed into this project with my Ph.D. advisor Jim Sethna about a decade ago, but for various reasons that work never quite took off. Sam and I started talking about this when thinking about how to understand the energy costs that biology needs to spend to compute—a theme in much of his Ph.D. work- and maybe more broadly to ensure its parts are coherent and controlled, and he figured out how to do these calculations.”

The recent work by Machta, and his colleague Samuel J. Bryant draws inspiration from earlier papers published in the late 90s, particularly efforts by Simon Laughlin and his collaborators. At the time, this research group had tried to experimentally determine how much energy neurons spend when sending information.

“Laughlin and colleagues found that this energy expenditure ranged between 10⁴-10⁷ K_BT/bit depending on details, which is far higher than the ‘fundamental’ bound of ~ K_BT/bit, sometimes called the the Landauer bound which must be paid to erase a bit of information,” Machta explained.

“In some ways we wanted to understand; was this an example of biology just being wasteful? Or maybe there were other costs that needed be paid; in particular, the Landauer limit makes no reference to geometry or physical details. Applying the Landauer bound is itself subtle, because it is only paid on erasing information it is possible to compute reversibly, never erase anything, and not pay ANY computing cost- but that is not the focus here.”

A further objective of the recent study by Machta and Bryant was to determine whether optimizing these energetic costs could shed light on the reasons why molecular systems communicate with each other using distinct physical mechanisms in different situations. For instance, while neurons typically communicate with each other via electrical signals, other types of tells can communicate via the diffusion of chemicals.

“We wanted to understand in what regime each of these (and others) would be best in terms of an energy cost per bit,” Machta said. “In all our calculations, we consider information that is sent through a physical channel, from a physical sender of information (like a ‘sending’ ion channel that opens and closes to send a signal) to a receiver (a voltage detector in the membrane which could also be an ion channel). The heart of the calculation is a textbook calculation for the information rate through a Gaussian channel, but with a few new twists.”

Firstly, in their estimations, Machta and his colleagues always consider a physical channel, in which currents of physical particles and electrical charges are carried according to a cell’s physics. Secondly, the team always assumed that a channel is corrupted by thermal noise in the cellular environment.

“We can calculate the spectrum of this noise with the ‘fluctuation dissipation theorem’ which relates the spectrum of thermal fluctuations to the near equilibrium response functions,” Machta explained.

A further unique feature of the team’s estimations is that they were performed using relatively simple models. This allowed them the researchers to always place conservative lower bounds on the energy required to power a channel and drive physical currents in a biological system.

“Because the signal must overcome thermal noise, we generally find costs with a geometric prefactor multiplying “K_BT/bit,'” Machta said.

“This geometric factor can have the size of the sender and receiver; a large sender generally decreases costs per bit by allowing a dissipative current to be spread over a larger area. Moreover, a larger receiver allows more averaging over thermal fluctuations, so that a weaker overall signal can still carry the same information.”

“So, for example, for electrical signaling, we get a form for the cost per bit that scales like r²/σ_I σ_O k_BT/bit, where r is the distance between sender and receiver and σ_I,σ_O are the size of the sender and receiver. Importantly, for ion channels which are a few nanometers across, but which send information over microns, this cost could easily be many orders of magnitude larger than k_T/bit that simpler (or more fundamental) arguments suggest as a lower bound.”

Overall, the calculations performed by Machta and his colleagues confirm the high energetic cost associated with the transfer of information between cells. Ultimately, their estimations could be the beginning of an explanation for the high cost of information processing measured in experimental studies.

“Our explanation is less ‘fundamental’ than the Landauer bound, in that it depends on the geometry of neurons and ion channels, and other details,” Machta said. “However, if biology is subject to these details, then it may be that (for example) neurons are efficient and up against real information/energy limitations, and not merely inefficient. These calculations are certainly not enough to yet say that any particular system is efficient, but they do suggest that sending information through space can necessitate very large energy costs.”

In the future, this recent work by Machta and his colleagues could inform new interesting biological studies. In their paper, the researchers also introduced a ‘phase diagram,” representing situations in which the selective use of specific communication strategies (e.g., electrical signaling, chemical diffusion, etc.) is optimal.

This diagram could soon help to better understand the design principles of different cell signaling strategies. For instance, it could shed light on why neurons use chemical diffusion to communicate at synapses, but they use electrical signals when sending information over hundreds of microns from dendrites to the cell body; as well as why E. coli bacteria utilize diffusion to send information about their chemical environment.

“One thing we are working on now is trying to apply this framework towards understanding the energetics of a concrete signal transduction system,” Machta added.

“Our recent work just considered the abstract cost of sending information between two single components—in real systems there are typically information processing networks, and applying our bound requires understanding the flow of information in these networks. This goal also comes with new technical issues—applying our calculations to specific geometries (like a ‘spherical’ neuron or an axon which resembles a tube, each importantly different than the infinite plain we used here).”

More information: Samuel J. Bryant et al, Physical Constraints in Intracellular Signaling: The Cost of Sending a Bit, Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.131.068401

Journal information: Physical Review Letters 

© 2023 Science X Network

by Thamarasee Jeewandara , Phys.org

Magnetic skyrmions have received much attention as promising, topologically protected quasiparticles with applications in spintronics. Skyrmions are small, swirling topological magnetic excitations with particle-like properties. Nevertheless, the lower stability of magnetic skyrmions only allow them to exist in a narrow temperature range, with low density of the particles, thus implying the need for an external magnetic field, which greatly limits their wider applications.

In a new report published in Science Advances, Yuzhu Song and a team of researchers formed high-density, spontaneous magnetic biskyrmions without a magnetic field in ferrimagnets via the thermal expansion of the lattice.

The team noted a strong connection between the atomic-scale ferrimagnetic structure and nanoscale magnetic domains in a ferrimagnet compound by using neutron powder diffraction and Lorentz transmission electron microscopy measurements.

Song and team explored the critical role of negative thermal expansion in generating biskyrmions in the ferrimagnet compound due to magneto-elastic coupling effects, to compare the behavior of the material with positive thermal expansion.

Skyrmions and biskyrmions

Magnetic skyrmions are nanoscale magnetic domain structures with topological protection. Their unique features and small size, and the lower energy consumption alongside electric-current driven behavior make them promising candidates for applications in spintronic storage devices.

Since their discovery in 2009, magnetic skyrmions have entered a period of rapid development. Materials scientists and physicists have found topological spin structures to contain diverse topological charges, which include skyrmions, biskyrmions, anti-skyrmions, merons and antimerons. The competition between magnetic dipole interactions and uniaxial magnetic anisotropy usually determines the generation of biskyrmions.

In this work, the research team proposed the stabilization of high-density, spontaneous magnetic biskyrmions across a wide-range of temperatures by investigating the negative thermal expansion of a lattice, when compared to a bulk metallic ferrimagnetic compound composed of a holmium-cobalt system [Ho(Co,Fe)₃].

The team comparatively studied the compound by characterizing positive thermal expansion and the mechanism of negative thermal expansion, to understand the stability of magnetic biskyrmions within the rare earth magnet (HoCo₃).

The experiments

The research team first obtained crystal and magnetic structures of the magnetic compound by performing variable-temperature dependent neutron-powder diffraction measurements. They noted a distinct variation of the material’s profile intensity across diverse temperature ranges and showed the expression of complex magnetic structural changes.

Song and team determined the crystal structure of the material and explored the magnetic moments of the rare earth element holmium (Ho) constituting the compound, alongside the transition metal atom cobalt (Co).

The magnetic moment of the ferrimagnet rotated with varying temperatures to create a phenomenon known as spin reorientation, allowing them to measure the temperature-dependence of the magnetization process. When temperatures exceeded ~425 K, the magnetic structure assumed a disordered paramagnetic state. The outcomes of the magnetic structure fit the neutron powder diffraction data well at all temperatures.

The magnetic moment

The scientists summed up the temperature-dependent evolution of the magnetic and structural parameters of the ferrimagnet across an entire temperature range. They noted the expansion of a unit cell of a magnetic compound with increased temperature due to anharmonic lattice vibrations. They also conducted additional neutron powder diffraction studies to calculate the magnetic components and total magnetic moments of the constituting holmium and cobalt atoms.

The team explored the complex magnetic ordering in the ferrimagnetic holmium-cobalt system by calculating the band structures and density states of the compound via first principles. As with many rare-earth systems, the Ruderman–Kittel–Kasuya–Yosida (RKKY) interactions underlay the complex magnetism of the ferrimagnet.

Comparing lattice negative thermal expansion and positive thermal expansion

During additional experiments, Song and team conducted neutron powder diffraction analysis to show the rotating magnetic moment of the holmium-cobalt system with lattice negative thermal expansion at cooling.

Under zero magnetic field, the scientists imaged the magnetic domain structures of the ferrimagnet across a temperature range, to show varying magnetic biskyrmions of the compound. They regarded the spin texture of biskyrmions as being composed of two skyrmions with opposite helices.

The spontaneous skyrmions represented very high densities with stability across a wide temperature range. They compared the lattice negative thermal expansion of the holmium-cobalt system and the existence of stable skyrmions by characterizing the outcomes with another compound containing iron to show positive thermal expansion in the latter.

The team did not observe any skyrmions in this latter ferrous-integrated compound, which they investigated at the same temperature range during which biskyrmions appeared in the holmium-cobalt system.

Outlook

In this way, Yuzhu Song and team explored the consistency of lattice expansion and the gradual increase of biskyrmions due to decreasing temperatures, by confirming negative thermal expansion during the stabilization of biskyrmions within a rare earth magnet.

The team obtained high-density, spontaneous magnetic biskyrmions across a wide temperature range, without applying a magnetic field to the bulk holmium-cobalt systems. They determined the complex magnetic and crystal structures of the compounds using neutron powder scattering across the entire experimental temperature range.

The outcomes highlighted an expanded mechanism to generate spontaneous, high-density skyrmions across a broad temperature range in rare earth metal systems.

More information: Yuzhu Song et al, High-density, spontaneous magnetic biskyrmions induced by negative thermal expansion in ferrimagnets, Science Advances (2023). DOI: 10.1126/sciadv.adi1984

X. Z. Yu et al, Real-space observation of a two-dimensional skyrmion crystal, Nature (2010). DOI: 10.1038/nature09124

Journal information: Science Advances  , Nature 

Magnets, those everyday objects we stick to our fridges, all share a unique characteristic: they always have both a north and a south pole. Even if you tried breaking a magnet in half, the poles would not separate—you would only get two smaller dipole magnets. But what if a particle could have a single pole with a magnetic charge?

For over a century, physicists have been searching for such magnetic monopoles. A new study on the preprint server arXiv from the ATLAS collaboration at the Large Hadron Collider (LHC) places new limits on these hypothetical particles, adding new clues for the continuing search.

In 1931, physicist Paul Dirac proved that the existence of magnetic monopoles would be consistent with quantum mechanics and require—as has been observed—the quantization of the electric charge. In the 1970s, magnetic monopoles were also predicted by new theories attempting to unify all the fundamental forces of nature, inspiring physicist Joseph Polchinski to claim that their existence was “one of the safest bets that one can make about physics not yet seen.” Magnetic monopoles might have been present in the early universe but diluted to an unnoticeably tiny density during the early exponential expansion phase known as cosmic inflation.

Researchers at the ATLAS experiment are searching for pairs of point-like magnetic monopoles with masses of up to about 4 teraelectronvolts (TeV). These pairs could be produced in 13 TeV collisions between protons via two different mechanisms: “Drell-Yan,” in which a virtual photon produced in the collisions creates the magnetic monopoles, or “photon-fusion,” in which two virtual photons radiated by the protons interact to create the magnetic monopoles.

The collaboration’s detection strategy relies on Dirac’s theory, which says that the magnitude of the smallest magnetic charge (gD) is equivalent to 68.5 times the fundamental unit of electric charge, the charge of the electron (e). Consequently, a magnetic monopole of charge 1gD would ionize matter in a similar way as a high-electric-charge object (HECO).

When a particle ionizes the detector material, ATLAS records the energy deposited, which is proportional to the square of the particle’s charge. Hence, magnetic monopoles or HECOs would leave large energy deposits along their trajectories in the ATLAS detector. Since the ATLAS detector was designed to record low-charge and neutral particles, the characterization of these high-energy deposits is vital to the search for monopoles and HECOs.

In their new study, the ATLAS researchers combed through the experiment’s full dataset from Run 2 of the LHC (2015–2018) in search of magnetic monopoles and HECOs. The search made use of the detector’s transition radiation tracker and the finely segmented liquid-argon electromagnetic calorimeter. The result places some of the tightest limits yet on the rate of production of magnetic monopoles.

The search targeted monopoles of magnetic charge 1gD and 2gD and HECOs of electric charge 20e, 40e, 60e, 80e and 100e, with masses between 0.2 TeV and 4 TeV. Compared to the previous ATLAS search, the new result benefited from the larger, complete Run-2 dataset. This was also the first ATLAS analysis to consider the photon-fusion production mechanism.

With no evidence of either magnetic monopoles or HECOs in the dataset, the ATLAS researchers established new limits on the production rate and mass of monopoles with a magnetic charge of 1gD and 2gD. ATLAS remains the experiment with the greatest sensitivity to monopoles in this charge range; the smaller LHC experiment MoEDAL-MAPP has previously studied a larger charge range and has also searched for monopoles with a finite size.

ATLAS physicists will continue their quest to find magnetic monopoles and HECOs, further refining their search techniques and developing new strategies to study both Run-2 and Run-3 data.

More information: Search for magnetic monopoles and stable particles with high electric charges in √s=13 TeV_pp collisions with the ATLAS detector, arXiv (2023). DOI: 10.48550/arxiv.2308.04835

Journal information: arXiv 

Provided by CERN 

by SPIE

The past few years have seen a massive surge in the amount of data transferred and processed per second. Rapidly emerging technologies, such as high-dimensional quantum communications, large-scale neural networks, and high-capacity networks, require large bandwidths and high data transfer speeds. One plausible way to achieve them is by replacing the conventional metallic wires between the components in an electronic system with optical interconnects, i.e., using light instead of electricity to establish channels for data transfer.

Optical interconnections can provide incredibly high speeds via a technique known as mode-division multiplexing (MDM). Thanks to precisely designed structures called waveguides, light can propagate in specific patterns called “modes.” Since multiple modes can propagate in the same medium simultaneously without interfering with each other, they effectively act as separate data channels, increasing the overall data transfer rate of the system.

However, the speed of MDM systems reported so far has been limited, mainly due to the imperfections in the device fabrication that cause the refractive index variations of the waveguides. One way to mitigate the imperfections is to carefully engineer the refractive indices of the waveguides by optimizing the structure and composition. Unfortunately, currently available methods are limited by either the choice of materials or the resulting large circuit footprint.

Against this backdrop, a research team including Professor Yikai Su from Shanghai Jiao Tong University in China sought to develop a new approach for coupling (or combining) different light modes. As reported in their study published in Advanced Photonics, the team successfully employed this technique in an MDM system, achieving unprecedented data rates.

The main highlight of the research is an innovative design for a light-mode coupler, a structure that can manipulate a specific light mode traveling in a nearby bus waveguide, such as a nanowire carrying the total multi-mode signal. The coupler can inject a desired light mode into the bus waveguide or extract one from it, sending it towards a different path.

The researchers tailored the refractive index of the coupler such that it interacted strongly with the desired light mode in a wide range of coupling region in the presence of fabrication errors, thus realizing a high coupling coefficient. They achieved this by leveraging a gradient-index metamaterial (GIM) waveguide.

Contrary to usual materials, the GIM exhibited a refractive index that varied continuously along the direction of propagation of light. This, in turn, facilitated a seamless and efficient transition of individual light modes to and from the nanowire bus by mitigating the parameter variations of the waveguides.

By cascading multiple couplers, the researchers created a 16-channel MDM communication system that supported 16 different light modes—TE₀ to TE₁₅—simultaneously. In a data transmission experiment, it achieved a data transfer rate of 2.162 Tbit/s, the highest ever reported value for an on-chip device operating at a single wavelength.

Moreover, the system was fabricated using methods that are compatible in semiconductor device fabrication, such as electron beam lithography, plasma etching, and chemical vapor deposition. This made the design easily scalable and compatible with currently available fabrication technology.

Overall, the proposed coupling strategy using a GIM structure may provide a much-needed boost in data rates, especially in fields where large-scale parallel data transmissions and computations are common. This could translate to new benchmarks in hardware acceleration, large-scale neural networks, and quantum communications.

More information: Yu He et al, On-chip metamaterial-enabled high-order mode-division multiplexing, Advanced Photonics (2023). DOI: 10.1117/1.AP.5.5.056008

Journal information: Advanced Photonics 

Provided by SPIE