Category: Physics

The magnetic moment of the muon is an important precision parameter for putting the Standard Model of particle physics to the test. After years of work, the research group led by Professor Hartmut Wittig of the PRISMA+ Cluster of Excellence at Johannes Gutenberg University Mainz (JGU) has calculated this quantity using the so-called lattice quantum chromodynamics method (lattice QCD method).

Their result agrees with the latest experimental measurements, in contrast to earlier theoretical calculations.

After the experimental measurements had been pushed to ever higher precision in recent years, attention had increasingly turned to the theoretical prediction and the central question of whether it deviates significantly from the experimental results and thus provides evidence for the existence of new physics beyond the Standard Model.

The anomalous magnetic moment is an intrinsic property of elementary particles such as the electron or its heavier brother, the muon. Calculating this quantity with sufficient accuracy within the framework of the Standard Model is an enormous challenge.

With the only exception of gravity, all fundamental interactions contribute to the anomalous magnetic moment. In particular, the contributions of the strong interaction, which describes the forces between the basic building blocks of protons and neutrons, the quarks, cause great difficulties for physicists.

The main source of uncertainty in the theoretical calculation of the anomalous magnetic moment of the muon is the contribution of the so-called hadronic vacuum polarization (HVP). Traditionally, this contribution has been determined using experimental data—this is called the “data-driven” method.

In fact, over many years, this technique provided a significant deviation from the experimental measured value and thus also one of the most promising indications of the existence of new physics.

Result of the PRISMA+ Cluster of Excellence

Wittig’s group has now published a new result for the HVP contribution as a preprint in the open access archive arXiv, which was obtained using the complementary method of lattice QCD.

“Our work confirms earlier evidence suggesting a clear divergence between the data-driven method and lattice QCD calculations,” says Wittig. “At the same time, we have to conclude from our result that the Standard Model has once again been confirmed, because our result agrees with the experimental measurement.”

In 2020, the “Muon g-2 Theory Initiative”—an international group of 130 physicists with strong participation from Mainz—published a reference value for the theoretical prediction of the anomalous magnetic moment of the muon within the framework of the Standard Model, which is based on the data-driven method.

This actually showed a clear deviation from the new direct measurements of this quantity, which have been carried out at Fermilab near Chicago since 2021.

However, since the publication of new results from the CMD-3 experiment in Novosibirsk in February 2023, this reference value has come into question, as the Standard Model prediction varies greatly depending on which data set is used.

In order to overcome the disadvantages of the data-driven method, Wittig’s group has focused on calculations using the lattice QCD method, which allows the contributions of the strong interaction to be calculated numerically using supercomputers. The advantage of such an approach is that, unlike the value published in 2020, it provides results that do not require experimental data.

Agreement with the experimental mean value

Wittig’s group focused on calculating the contribution of the HVP, which provides the largest contribution of the strong interaction to the anomalous magnetic moment of the muon. In their recent work, the team has found a new value for the muon’s anomalous magnetic moment that is consistent with the current experimental mean and far from the 2020 theoretical estimate.

“After years of work on reducing the uncertainties of our calculations and overcoming the computational challenges associated with performing such lattice QCD calculations, we have obtained the HVP contribution with an overall accuracy of just below 1% and a good balance between statistical and systematic uncertainties,” says Wittig. “This allows us to reassess the validity of the Standard Model.”

Even if the new result once again confirms the Standard Model, there are still many puzzles. Where the difference between the lattice QCD and the data-driven method comes from and how the result of the CMD-3 experiment should be evaluated is not yet fully understood.

“We still have a long way to go to achieve our long-term goal of reducing the total error to around 0.2%. No matter how you look at it, we can’t get around the fact that there are discrepancies in the anomalous magnetic moment of the muon that need to be explained. There is still a lot for us to understand,” concludes Wittig.

More information: Dalibor Djukanovic et al, The hadronic vacuum polarization contribution to the muon g-2 at long distances, arXiv (2024). DOI: 10.48550/arxiv.2411.07969

Journal information: arXiv 

Mechanical crystals, also known as phononic crystals, are materials that can control the propagation of vibrations or sound waves, just like photonic crystals control the flow of light. The introduction of defects in these crystals (i.e., intentional disruptions in their periodic structure) can give rise to mechanical modes within the band gap, enabling the confinement of mechanical waves to smaller regions or the materials—a feature that could be leveraged to create new technologies.

Researchers at McGill University recently realized a new mechanical crystal with an optically programmable defect mode. Their paper, published in Physical Review Letters, introduces a new approach to dynamically reprogram mechanical systems, which entails the use of an optical spring to transfer a mechanical mode into a crystal’s band gap.

“Some time ago, our group was thinking a lot about using an optical spring to partially levitate structures and improve their performance,” Jack C. Sankey, principal investigator and co-author of the paper, told Phys.org. “At the same time, we were watching the amazing breakthroughs in our field with mechanical devices that used the band gap of a phononic crystal to insulate mechanical systems from the noisy environment.”

After witnessing recent breakthroughs in the development of mechanical devices, Sankey and his colleagues started exploring the possibility of optically springing up the drumhead-like resonance of a membrane with a periodic array of holes punched in it. They predicted that this would allow them to drag the frequency into a band gap, drawing the vibrational energy inward like a tractor beam and significantly reducing the resonance’s inertial mass.

“We figured this weird situation in which the number of photons present affects how heavy a mechanical system ‘feels’ would present a lot of new opportunities,” said Sankey. “We did some promising calculations, notably finding that larger structures respond more to each photon, and that an average of a single photon in the apparatus could in principle have a measurable effect on the motion of a very feasible, centimeter-scale device.”

To demonstrate their approach, the team, led by Ph.D. student Tommy Clark, first patterned and released a membrane using standard photolithography techniques. They then aligned the fiber cavity near this membrane’s center, using tight-tolerance guide ferrules.

“We mounted the whole thing on a vibration-isolating stage in ultrahigh vacuum and used additional active feedback to stabilize the cavity mirrors to within the ~10s of picometers required for the laser light to enter the cavity near its natural resonance frequency,” explained Sankey. “Once the system is assembled and stabilized, we used the cavity’s resonant enhancement to create an intense optical field that applies a spring-like pressure to a small section of the membrane.”

Using this optical spring, the researchers deliberately disrupted their membrane’s periodic pattern, generating a defect. By adjusting the laser’s intensity, they could then dynamically and reversibly modify the properties of the defect they introduced.

“I have always loved the idea of coupling light to the shape and mass of a mechanical resonance, but there are a host of interesting applications as well, from new studies of mechanical dissipation to simulations of condensed matter systems,” said Sankey.

“There is also currently a great deal of interest in employing mechanical systems to store and transport quantum information on chip, and to connect nominally disparate quantum systems to each other. Mechanical systems are versatile tools, and Tommy’s (incredible) work demonstrates a qualitatively new way to manipulate motion with light.”

The team’s new approach for the in situ reconfiguration of mechanical defects could open new interesting possibilities for the creation of reprogrammable mechanical systems. For instance, arrays of such defects they generated could be used to program waveguides or other structures designed to route and reroute the flow of mechanical information.

“In the near future, we are most looking forward to exploring the idea that each photon interacts with many similar mechanical resonances simultaneously, while also connecting them all to each other through the same radiation force,” added Sankey. “This creates a dense ‘web’ of interactions that enhances the influence of each photon, and I am interested in leveraging this to generate increasingly macroscopic quantum states of motion.”

More information: Thomas J. Clark et al, Optically Defined Phononic Crystal Defect, Physical Review Letters (2024). DOI: 10.1103/PhysRevLett.133.226904

Journal information: Physical Review Letters 

© 2024 Science X Network

Quantum walks are a powerful theoretical model using quantum effects such as superposition, interference and entanglement to achieve computing power beyond classical methods.

A research team at the National Innovation Institute of Defense Technology from the Academy of Military Sciences (China) recently published a review article that thoroughly summarizes the theories and characteristics, physical implementations, applications and challenges of quantum walks and quantum walk computing. The review was published Nov. 13 in Intelligent Computing in an article titled “Quantum Walk Computing: Theory, Implementation, and Application.”

As quantum mechanical equivalents of classical random walks, quantum walks use quantum phenomena to design advanced algorithms for applications such as database search, network analysis and navigation, and quantum simulations. Different types of quantum walks include discrete-time quantum walks, continuous-time quantum walks, discontinuous quantum walks, and nonunitary quantum walks. Each model presents unique features and computational advantages.

Discrete-time quantum walks involve step-by-step transitions without a time factor, using coin-based models like Hadamard and Grover walks or coinless models such as Szegedy and staggered quantum walks for graph-based movement. In contrast, continuous-time quantum walks operate on graphs using time-independent Hamiltonians, making them particularly useful for spatial searches and traversal problems.

Discontinuous quantum walks combine the properties of both discrete-time and continuous-time models, enabling universal computation through perfect state transfers. Meanwhile, nonunitary quantum walks, including stochastic quantum walks and open quantum walks, act as open quantum systems and find applications in simulating photosynthesis and quantum Markov processes.

The two original branches, discrete-time and continuous-time quantum walks, achieve faster diffusion than classical random walk models and exhibit similar probability distributions. To some extent, discrete-time and continuous-time models are interchangeable. In addition, various discrete models can be interchanged based on the graph structure, highlighting the versatility of quantum walk models.

According to the authors, quantum walks not only have evolutionary merits, but also improve sampling efficiency, solving problems previously considered computationally difficult for classical systems.

The wide variety of physical quantum systems used to implement quantum walks demonstrates the utility of discrete-time and continuous-time quantum walk models and quantum-walk-based algorithms. There are two different approaches to physically implementing quantum walks:

• Analog physical simulation primarily uses solid-state, optical and photonic systems to directly implement specific Hamiltonians without translation into quantum logic. This approach enables scalability by increasing particle numbers and dimensions but lacks error correction and fault tolerance. It faces challenges in efficiently simulating large graphs.
• Digital physical simulation constructs quantum circuits to simulate quantum walks, offering error correction and fault tolerance. Designing efficient circuits remains difficult, but digital implementations can achieve quantum speedup and simulate a variety of graphs.

Quantum walk applications are categorized into four main categories: quantum computing, quantum simulation, quantum information processing and graph-theoretic applications.

• Quantum Computing: Quantum walks enable universal quantum computation and accelerate computations in algebraic and number-theoretic problems. They are also being explored for applications in machine learning and optimization.
• Quantum Simulation: Quantum walks are an important tool for simulating the behavior of uncontrollable quantum systems, providing insight into complex quantum phenomena that are difficult or impossible to analyze classically. Applications include simulating multi-particle systems, solving complex physics problems, and modeling biochemical processes.
• Quantum Information Processing: Quantum walks are used for the preparation, manipulation, characterization and transmission of quantum states, as well as in quantum cryptography and security applications.
• Graph-Theoretic Applications: Quantum walks, associated with graph structures, provide promising solutions for graph-theoretic problems and various network applications. They are used to explore graph characteristics, rank vertex centrality and identify structural differences between graphs.

Despite rapid progress, practical quantum walk computing faces challenges, including devising effective algorithms, scaling up the physical implementations and implementing quantum walks with error correction or fault tolerance. These challenges, however, provide a roadmap for future innovations and advancements in the field.

More information: Xiaogang Qiang et al, Quantum Walk Computing: Theory, Implementation, and Application, Intelligent Computing (2024). DOI: 10.34133/icomputing.0097

Provided by Intelligent Computing 

Results from a 2021 experiment led by a University of Alaska Fairbanks scientist have begun to reveal the particle-level processes that create the type of auroras that dance rapidly across the sky.

The Kinetic-scale Energy and momentum Transport experiment—KiNET-X—lifted off from NASA’s Wallops Flight Facility in Virginia on May 16, 2021, in the final minutes of the final night of the nine-day launch window.

UAF professor Peter Delamere’s analysis of the experiment’s results was published Nov. 19 in Physics of Plasmas.

“The dazzling lights are extremely complicated,” Delamere said. “There’s a lot happening in there, and there’s a lot happening in the Earth’s space environment that gives rise to what we observe.

“Understanding causality in the system is extremely difficult, because we don’t know exactly what’s happening in space that’s giving rise to the light that we observe in the aurora,” he said. “KiNET-X was a highly successful experiment that will reveal more of the aurora’s secrets.”

One of NASA’s largest sounding rockets soared over the Atlantic Ocean into the ionosphere and released two canisters of barium thermite. The canisters were then detonated, one at about 249 miles high and one 90 seconds later on the downward trajectory at about 186 miles, near Bermuda. The resulting clouds were monitored on the ground at Bermuda and by a NASA research aircraft.

The experiment aimed to replicate, on a minute scale, an environment in which the low energy of the solar wind becomes the high energy that creates the rapidly moving and shimmering curtains known as the discrete aurora. Through KiNET-X, Delamere and colleagues on the experiment are closer to understanding how electrons are accelerated.

“We generated energized electrons,” Delamere said. “We just didn’t generate enough of them to make an aurora, but the fundamental physics associated with electron energization was present in the experiment.”

The experiment aimed to create an Alfvén wave, a type of wave that exists in magnetized plasmas such as those found in the sun’s outer atmosphere, Earth’s magnetosphere and elsewhere in the solar system. Plasmas—a form of matter composed largely of charged particles—can also be created in laboratories and experiments such as KiNET-X.

Alfvén waves originate when disturbances in plasma affect the magnetic field. Plasma disturbances can be caused in a variety of ways, such as through the sudden injection of particles from solar flares or the interaction of two plasmas with different densities.

KiNET-X created an Alfvén wave by disturbing the ambient plasma with the injection of barium into the far upper atmosphere.

Sunlight converted the barium into an ionized plasma. The two plasma clouds interacted, creating the Alfvén wave.

That Alfvén wave instantly created electric field lines parallel to the planet’s magnetic field lines. And, as theorized, that electric field significantly accelerated the electrons on the magnetic field lines.

“It showed that the barium plasma cloud coupled with, and transferred energy and momentum to, the ambient plasma for a brief moment,” Delamere said.

The transfer manifested as a small beam of accelerated barium electrons heading toward Earth along the magnetic field line. The beam is visible only in the experiment’s magnetic field line data.

“That’s analogous to an auroral beam of electrons,” Delamere said.

He calls it the experiment’s “golden data point.”

Analysis of the beam, visible only as varying shades of green, blue and yellow pixels in Delamere’s data imagery, can help scientists learn what is happening to the particles to create the dancing northern lights.

The results so far show a successful project, one that can even allow more information to be gleaned from its predecessor experiments.

“It’s a question of trying to piece together the whole picture using all of the data products and numerical simulations,” Delamere said.

Three UAF students doing their doctoral research at the UAF Geophysical Institute also participated. Matthew Blandin supported optical operations at Wallops Flight Facility, Kylee Branning operated cameras on a NASA Gulfstream III aircraft out of Langley Research Center, also in Virginia, and Nathan Barnes assisted with computer modeling in Fairbanks.

The experiment also included researchers and equipment from Dartmouth College, the University of New Hampshire and Clemson University.

More information: P. A. Delamere et al, Alfvén wave generation and electron energization in the KiNET-X sounding rocket mission, Physics of Plasmas (2024). DOI: 10.1063/5.0228435

Read the story of the KiNET-X mission in 12 short installments that include videos, animations and additional photographs.

Journal information: Physics of Plasmas 

Provided by University of Alaska Fairbanks 

A team of physicists and engineers at China’s Hefei National Laboratory has succeeded in conducting the first instance of precise absolute distance measurement over a path exceeding 100 km. The group has written a paper describing how they achieved such a feat and posted it on the arXiv preprint server.

As scientists develop ever more sophisticated technology, the need for more precise measurement grows. One such application is satellite formation flying. For it to be done as precisely as needed, new ways to very accurately measure long distances are needed—such as from a satellite to the ground, and back. In this new effort, the research team in China has found a way to measure such distances with unprecedented precision.

The work by the team involved adding an improvement to a measuring technique involving the use of an optical comb—a device that allows for averaging the time it takes for multiple wavelengths of light to travel to a target and bounce back—while also analyzing interference patterns that may have arisen during the trip.

Such technology has proven to be very precise when measuring relatively short distances. It has not worked very well over long distances, unfortunately, because the light is subject to noise and environmental factors such as humidity and temperature. In this new effort, the team overcame these problems to extend the use of optical combs to distances over 100 kilometers.

The solution, the team found, was to add a second comb at the other end of the space to be measured. Doing so caused the beams of light to interfere with one another, resulting in patterns that could be used to measure the distance between them. The technique works, the researchers note, because while the combs are indistinguishable, the interference patterns created at each end are not.

Because the beams are traveling in reverse directions relative to each other, comparison of the differences in interference patterns can be resolved, resulting in mitigating the impacts of environmental noise. The result is an extremely precise way to measure objects that are more than a kilometer apart.

The team proved the effectiveness of their approach by using it to measure objects 113 kilometers apart with a precision of 82 nm. They also note that their test is the first to achieve such precision over such a long distance.

More information: Yan-Wei Chen et al, 113 km absolute ranging with nanometer precision, arXiv (2024). DOI: 10.48550/arxiv.2412.05542

Journal information: arXiv 

Plasmons are collective oscillations of electrons in a solid and are important for a wide range of applications, such as sensing, catalysis, and light harvesting. Plasmonic waves that travel along the surface of a metal, called surface plasmon polaritons, have been studied for their ability to enhance electromagnetic fields.

One of the most powerful tools for studying these waves is time-resolved electron microscopy, which uses ultrashort laser pulses to observe how these plasmonic waves behave. An international research team recently pushed the boundaries of this technique.

As reported in Advanced Photonics, the researchers used multiple time-delayed laser pulses of four different polarizations to capture the full electric field of these waves. This method allowed them to achieve a level of accuracy previously not possible.

To test their technique, the team investigated a specific spin texture known as a meron pair. A meron is a topological structure where the direction of the spin texture only covers half of a sphere, which distinguishes it from other similar structures, like skyrmions, whose spin covers the entire sphere.

To reconstruct the spin texture from the experiment, the researchers needed the electric and magnetic field vectors of the surface plasmon polaritons. While the electric field vectors could be directly measured, the magnetic field vectors had to be calculated based on the electric field’s behavior over time and space.

By using their precise method, the researchers were able to reconstruct the spin texture and determine its topological properties, such as the Chern number, which describes the number of times the spin texture maps onto a sphere. In this case, the Chern number was found to be one, indicating the presence of a meron pair.

The study also demonstrated that the spin texture remains stable throughout the duration of the plasmonic pulse, despite the fast rotation of the electric and magnetic field vectors. This new approach is not limited to meron pairs and can be applied to other complex surface plasmon polariton fields.

Understanding these fields and their topological properties is important, especially at the nanoscale, where topological protection can help maintain the stability of materials and devices.

This research shows that it is now possible to study complex spin textures with high precision on extremely short timescales. The ability to accurately reconstruct the full electric and magnetic fields of surface plasmon polaritons opens new possibilities for exploring the topological properties of electromagnetic near fields, which may have important implications for future technologies at the nanoscale.

More information: Pascal Dreher et al, Spatiotemporal topology of plasmonic spin meron pairs revealed by polarimetric photo-emission microscopy, Advanced Photonics (2024). DOI: 10.1117/1.AP.6.6.066007

Journal information: Advanced Photonics

Researchers at Rice University have made a meaningful advance in the simulation of molecular electron transfer—a fundamental process underpinning countless physical, chemical and biological processes. The study, published in Science Advances, details the use of a trapped-ion quantum simulator to model electron transfer dynamics with unprecedented tunability, unlocking new opportunities for scientific exploration in fields ranging from molecular electronics to photosynthesis.

Electron transfer, critical to processes such as cellular respiration and energy harvesting in plants, has long posed challenges to scientists due to the complex quantum interactions involved. Current computational techniques often fall short of capturing the full scope of these processes. The multidisciplinary team at Rice, including physicists, chemists and biologists, addressed these challenges by creating a programmable quantum system capable of independently controlling the key factors in electron transfer: donor-acceptor energy gaps, electronic and vibronic couplings and environmental dissipation.

Using an ion crystal trapped in a vacuum system and manipulated by laser light, the researchers demonstrated the ability to simulate real-time spin dynamics and measure transfer rates across a range of conditions. The findings not only validate key theories of quantum mechanics but also pave the way for novel insights into light-harvesting systems and molecular devices.

“This is the first time that this kind of model was simulated on a physical device while including the role of the environment and even tailoring it in a controlled way,” said lead researcher Guido Pagano, assistant professor of physics and astronomy. “It represents a significant leap forward in our ability to use quantum simulators to investigate models and regimes that are relevant for chemistry and biology. The hope is that by harnessing the power of quantum simulation, we will eventually be able to explore scenarios that are currently inaccessible to classical computational methods.”

The team achieved a significant milestone by successfully replicating a standard model of molecular electron transfer using a programmable quantum platform. Through the precise engineering of tunable dissipation, the researchers explored both adiabatic and nonadiabatic regimes of electron transfer, demonstrating how these quantum effects operate under varying conditions. Additionally, their simulations identified optimal conditions for electron transfer, which parallel the energy transport mechanisms observed in natural photosynthetic systems.

“Our work is driven by the question: Can quantum hardware be used to directly simulate chemical dynamics?” Pagano said. “Specifically, can we incorporate environmental effects into these simulations as they play a crucial role in processes essential to life such as photosynthesis and electron transfer in biomolecules? Addressing this question is significant as the ability to directly simulate electron transfer in biomolecules could provide valuable insights for designing new light-harvesting materials.”

The implications for practical applications are far-reaching. Understanding electron transfer processes at this level could lead to breakthroughs in renewable energy technologies, molecular electronics and even the development of new materials for quantum computing.

“This experiment is a promising first step to gain a deeper understanding of how quantum effects influence energy transport, particularly in biological systems like photosynthetic complexes,” said Jose N. Onuchic, study co-author, the Harry C. and Olga K. Wiess Chair of Physics and professor of physics and astronomy, chemistry and biosciences. “The insights we gain in this type of experiment could inspire the design of more efficient light-harvesting materials.”

Peter G. Wolynes, study co-author, the D.R. Bullard-Welch Foundation Professor of Science and professor of chemistry, biosciences and physics and astronomy, emphasized the broader significance of the findings: “This research bridges the gap between theoretical predictions and experimental verification, offering an exquisitely tunable framework for exploring quantum processes in complex systems.”

The team plans to extend its simulations to include more complex molecular systems such as those involved in photosynthesis and DNA charge transport. The researchers also hope to investigate the role of quantum coherence and delocalization in energy transfer, leveraging the unique capabilities of their quantum platform.

“This is just the beginning,” said Han Pu, co-lead author of the study and professor of physics and astronomy. “We are excited to explore how this technology can help unravel the quantum mysteries of life and beyond.”

The study’s other co-authors include graduate students Visal So, Midhuna Duraisamy Suganthi, Abhishek Menon, Mingjian Zhu and research scientist Roman Zhuravel.

More information: Visal So et al, Trapped-ion quantum simulation of electron transfer models with tunable dissipation, Science Advances (2024). DOI: 10.1126/sciadv.ads8011

Journal information: Science Advances 

Provided by Rice University 

Northwestern University engineers are the first to successfully demonstrate quantum teleportation over a fiberoptic cable already carrying internet traffic.

The discovery introduces the new possibility of combining quantum communication with existing internet cables—greatly simplifying the infrastructure required for distributed quantum sensing or computing applications.

The study is published on the arXiv preprint server and is due to appear in the journal Optica.

“This is incredibly exciting because nobody thought it was possible,” said Northwestern’s Prem Kumar, who led the study. “Our work shows a path towards next-generation quantum and classical networks sharing a unified fiberoptic infrastructure. Basically, it opens the door to pushing quantum communications to the next level.”

An expert in quantum communication, Kumar is a professor of electrical and computer engineering at Northwestern’s McCormick School of Engineering, where he directs the Center for Photonic Communication and Computing.

Only limited by the speed of light, quantum teleportation could make communications nearly instantaneous. The process works by harnessing quantum entanglement, a technique in which two particles are linked, regardless of the distance between them. Instead of particles physically traveling to deliver information, entangled particles exchange information over great distances—without physically carrying it.

“In optical communications, all signals are converted to light,” Kumar explained. “While conventional signals for classical communications typically comprise millions of particles of light, quantum information uses single photons.”

Before Kumar’s new study, conventional wisdom suggested that individual photons would drown in cables filled with the millions of light particles carrying classical communications. It would be like a flimsy bicycle trying to navigate through a crowded tunnel of speeding heavy-duty trucks.

Kumar and his team, however, found a way to help the delicate photons steer clear of the busy traffic. After conducting in-depth studies of how light scatters within fiberoptic cables, the researchers found a less crowded wavelength of light to place their photons. Then, they added special filters to reduce noise from regular internet traffic.

“We carefully studied how light is scattered and placed our photons at a judicial point where that scattering mechanism is minimized,” Kumar said. “We found we could perform quantum communication without interference from the classical channels that are simultaneously present.”

To test the new method, Kumar and his team set up a 30 kilometer-long fiberoptic cable with a photon at either end. Then, they simultaneously sent quantum information and regular internet traffic through it. Finally, they measured the quality of the quantum information at the receiving end while executing the teleportation protocol by making quantum measurements at the mid-point. The researchers found the quantum information was successfully transmitted—even with busy internet traffic whizzing by.

Next, Kumar plans to extend the experiments over longer distances. He also plans to use two pairs of entangled photons—rather than one pair—to demonstrate entanglement swapping, another important milestone leading to distributed quantum applications. Finally, his team is exploring the possibility of carrying out experiments over real-world inground optical cables rather than on spools in the lab. But, even with more work to do, Kumar is optimistic.

“Quantum teleportation has the ability to provide quantum connectivity securely between geographically distant nodes,” Kumar said. “But many people have long assumed that nobody would build specialized infrastructure to send particles of light. If we choose the wavelengths properly, we won’t have to build new infrastructure. Classical communications and quantum communications can coexist.”

More information: Quantum teleportation coexisting with classical communications in optical fiber, Optica (2024).

Preprint: Jordan M. Thomas et al, Quantum teleportation coexisting with classical communications in optical fiber, arXiv (2024). DOI: 10.48550/arxiv.2404.10738

Journal information: Optica  , arXiv 

Provided by Northwestern University 

Colloidal gels are complex systems made up of microscopic particles dispersed in a liquid, ultimately producing a semi-solid network. These materials have unique and advantageous properties that can be tuned using external forces, which have been the focus of various physics studies.

Researchers at University of Copenhagen in Denmark and the UGC-DAE Consortium for Scientific Research in India recently ran simulations and performed analyses aimed at understanding how the injection of active particles, such as swimming bacteria, would influence colloidal gels.

Their paper, published in Physical Review Letters, shows that active particles can influence the structure of 3D colloidal gels, kneading them into porous and denser structures.

“Traditionally, much of physics focuses on systems that evolve toward their most stable or ‘favorable’ state, referred to as equilibrium,” Kristian Thijssen, senior author of the paper, told Phys.org.

“For instance, a gas or liquid that spreads evenly to fill its container is considered to be in equilibrium. However, in the physical world we inhabit, many systems do not reach equilibrium within the timescales of practical interest, or they remain continually energized in some way.”

An example of systems that remain continually energized to some extent is glass. The arrangement of particles is known to prevent the material from relaxing into its most thermodynamically stable state, which translates into a high sensitivity to its formation history.

“This is evident in glassblowing, where the process of shaping the material directly influences its internal structure,” explained Thijssen. “Colloidal gels, which consist of networks of particles with large voids, exhibit similar behavior. Their structure is not only influenced by their initial formation but also by the forces exerted on them.”

An emerging research field, known as active matter, has been trying to understand how living systems behave as far-from-equilibrium systems. This entails studying the behavior of living organisms, such as bacteria, when they are introduced into various environments.

These organisms introduce energy into their surroundings, by moving or swimming with the energy they acquire from food or other energy sources. This injection of energy prevents a system from reaching a state of equilibrium, continuously influencing their behavior.

“In our research, we sought to investigate what occurs when these two systems combine,” said Thijssen. “Specifically, we explored the dynamics of a gel, which is normally dependent on its history, when subjected to active particles that locally inject energy into their surroundings.”

Thijssen and his colleagues initially predicted that active particles would simply compress a gel into a more compact state, as this is what was observed in two-dimensional (2D) systems. Surprisingly, however, they found that their effect on 3D colloidal gels was far more intriguing.

“Instead of merely compacting the gel, the active particles reorganized the gel into a denser structure while preserving sufficient pathways for particle movement,” said Thijssen. “In this way, the gel is adapted to facilitate the transport of the active particles, resulting in a dynamic and efficient structure that continuously evolves as the active particles interact with it.”

To investigate the effects of injected active particles on 3D gels, the researchers ran a series of computer simulations using the open-source platform LAMMPS, which modeled the dynamics of gel particles and active particles. To simulate the gel particles, they used a model known as “short-range sticky potential” that captures the formation of colloidal gels.

“When colloidal particles are mixed with smaller particles in a liquid, the polymers around the colloids tend to spread evenly throughout the fluid,” said Thijssen.

“However, when two colloidal particles approach each other closely, the polymers can no longer fit between them, leading to a repulsive force that pushes the particles together. This results in attractive forces strong enough to drive the formation of a gel structure.”

To simulate the active particles, the team drew inspiration from a model describing the behavior of swimming bacteria called active Brownian particles (ABPs). These particles are known to self-propel in one direction, which they periodically change, mimicking the ‘run-and-tumble’ motion of bacteria.

“To understand how the gel responds to these active particles, we applied a technique called topological data analysis (TDA),” explained Thijssen.

“Although TDA has been used in other fields, it has not been widely applied to gels or active matter systems. TDA allows us to analyze the gel’s structure based on its topology, or overall shape. For example, a sphere would be classified as a single connected component, a ring would have one hole, and a shell would have a cavity in the center.”

Using this technique, the researchers characterized the structure of the colloidal gel in ways that unveiled crucial mechanical properties. They particularly focused on the connections between the empty spaces within a gel, which active particles use to move through the material.

“This connectivity is crucial because the active particles can alter the gel’s structure, creating more accessible pathways for movement,” said Thijssen.

The simulations and analyses carried out by the researchers yielded very interesting results. Firstly, they revealed that when injected with active particles, 3D colloidal gels restructure themselves into more compact and energetically favorable structures, while retaining several spaces that the particles can traverse.

This adaptation was only identifiable using TDA, thus demonstrating the potential of this analytical tool. In this case, TDA allowed the researchers to unveil the dynamic adaptation of colloidal gels in response to the movement of active particles.

“Our study demonstrates how Topological Data Analysis (TDA) can be leveraged to quantify gel structures,” said Thijssen. “This innovative approach offers new insights into the mechanical properties of gels and other porous materials, which have long posed challenges to comprehensive understanding.”

This recent work also demonstrates that there is a fundamental topological difference between 2D and 3D systems in adaptable materials. In 2D materials, empty regions can only form enclosed spaces that trap any particles within them.

In 3D systems, on the other hand, empty regions form both enclosed and interconnected spaces, which allow particles to move freely through networks of spaces.

“This distinction has profound implications for understanding the behavior of porous media—beyond just gels—in response to reconfigurations driven by living organisms,” said Thijssen.

“By bridging this gap, our work paves the way for more accurate models and predictions of how a diverse range of materials—ranging from biological tissues to engineered systems—respond to dynamic changes in their environments.”

This study could soon pave the way for further investigations focusing on the impact of active particles on both colloidal gels and other porous materials. In their next studies, the team plan to build on their findings to carry out additional simulations and analysis that integrate models of other materials or more complex living organisms.

“In this project, we used relatively simple active particles as models for living organisms,” said Thijssen. “However, in densely packed living systems—such as swarming bacteria or flocks of birds—collective motion often emerges from the interactions between individual agents. This motion is a defining characteristic of active systems, but it is also strongly influenced by the surrounding environment.”

A further interesting aspect of the evolution of porous media observed by the researchers is that it could also produce feedback loops. In other words, the motion of the active particles could adjust in response to the evolving porous structures, which could produce dynamic interactions with even more complex outcomes.

“Exploring these feedback mechanisms is a promising direction for future research,” added Thijssen.

“Understanding these dynamics could have practical applications in areas such as regulating bacterial movement to enhance biodegradation, preventing contamination in industrial piping systems, or managing bacterial infections by disrupting their ability to penetrate mucosal membranes.”

More information: Martin Cramer Pedersen et al, Active Particles Knead Three-Dimensional Gels into Porous Structures, Physical Review Letters (2024). DOI: 10.1103/PhysRevLett.133.228301. On arXiv: arxiv.org/html/2404.07767v1

Journal information: Physical Review Letters  , arXiv 

Bright, twisted light can be produced with technology similar to an Edison light bulb, researchers at the University of Michigan have shown. The finding adds nuance to fundamental physics while offering a new avenue for robotic vision systems and other applications for light that traces out a helix in space.

“It’s hard to generate enough brightness when producing twisted light with traditional ways like electron or photon luminescence,” said Jun Lu, an adjunct research investigator in chemical engineering at U-M and first author of the study on the cover of this week’s Science.

“We gradually noticed that we actually have a very old way to generate these photons—not relying on photon and electron excitations, but like the bulb Edison developed.”

Every object with any heat to it, including yourself, is constantly sending out photons (particles of light) in a spectrum tied to its temperature. When the object is the same temperature as its surroundings, it is also absorbing an equivalent amount of photons—this is idealized as “blackbody radiation” because the color black absorbs all photon frequencies.

While a tungsten lightbulb’s filament is much warmer than its surroundings, the law defining blackbody radiation—Planck’s law—offers a good approximation of the spectrum of photons it sends out. All together, the visible photons look like white light, but when you pass the light through a prism, you can see the rainbow of different photons within it.

This radiation is also why you show up brightly in a thermal image, but even room-temperature objects are constantly emitting and receiving blackbody photons, making them dimly visible as well.

Typically, the shape of the object emitting the radiation doesn’t get much consideration—for most purposes (as so often in physics), the object can be imagined as a sphere. But while shape doesn’t affect the spectrum of wavelengths of the different photons, it can affect a different property: their polarization.

Usually, photons from a blackbody source are randomly polarized—their waves may oscillate along any axis. The new study revealed that if the emitter was twisted at the micro or nanoscale, with the length of each twist similar to the wavelength of the emitted light, the blackbody radiation would be twisted too. The strength of the twisting in the light, or its elliptical polarization, depended on two main factors: how close the wavelength of the photon was to the length of each twist and the electronic properties of the material—nanocarbon or metal, in this case.

Twisted light is also called “chiral” because the clockwise and counterclockwise rotations are mirror images of one another. The study was undertaken to demonstrate the premise of a more applied project that the Michigan team would like to pursue: using chiral blackbody radiation to identify objects. They envision robots and self-driving cars that can see like mantis shrimp, differentiating among light waves with different directions of twirl and degrees of twistedness.

“The advancements in physics of blackbody radiation by chiral nanostructures is central to this study. Such emitters are everywhere around us,” said Nicholas Kotov, the Irving Langmuir Distinguished Professor of Chemical Sciences and Engineering, director of NSF Center of Complex Particles and Particle Systems (COMPASS) and corresponding author of the study.

“These findings, for example, could be important for an autonomous vehicle to tell the difference between a deer and a human, which emit light with similar wavelengths but different helicity because deer fur has a different curl from our fabric.”

While brightness is the main advantage of this method for producing twisted light—up to 100 times brighter than other approaches—the light includes a broad spectrum of both wavelengths and twists. The team has ideas about how to address this, including exploring the possibility of building a laser that relies on twisted light-emitting structures.

Kotov also wants to explore further into the infrared spectrum. The peak wavelength of blackbody radiation at room temperature is roughly 10,000 nanometers or 0.01 millimeters.

“This is an area of the spectrum with a lot of noise, but it may be possible to enhance contrast through their elliptical polarization,” Kotov said.

Kotov is also the Joseph B. and Florence V. Cejka Professor of Engineering, a professor of macromolecular science and engineering and a member of U-M’s Biointerfaces Institute. Lu is an incoming assistant professor of chemistry and physics at the National University of Singapore.

The device was built in the COMPASS Lab located at the North Campus Research Complex of U-M and studied at the Michigan Center for Materials Characterization.

More information: Jun Lu et al, Bright, circularly polarized black-body radiation from twisted nanocarbon filaments, Science (2024). DOI: 10.1126/science.adq4068

Journal information: Science 

Provided by University of Michigan