Category: Physics

Just as the sound of a guitar depends on its strings and the materials used for its body, the performance of a quantum computer depends on the composition of its building blocks. Arguably the most critical components are the devices that encode information in quantum computers.

One such device is the transmon qubit—a patterned chip made of metallic niobium layers on top of a substrate, such as silicon. Between the two materials resides an ultrathin layer that contains both niobium and silicon. The compounds of this layer are known as silicides (Nb_xSi_y). Their impact on the performance of transmon qubits has not been well understood—until now.

Silicides form when elemental niobium is deposited onto silicon during the fabrication process of a transmon qubit. They need to be well understood to make devices that reliably and efficiently store quantum information for as long as possible.

Researchers at the Superconducting Quantum Materials and Systems Center, hosted by the U.S. Department of Energy’s Fermi National Accelerator Laboratory, have discovered how silicides impact the performance of transmon qubits. Their research has been published in Physical Review Materials.

An unexpected signal

Carlos Torres-Castanedo was analyzing the materials of a transmon qubit using X-rays, when he came across a peculiar signal.

“I thought the signal came from a surface oxide, because that’s just what usually happens,” said Torres-Castanedo, a doctoral candidate in materials science at Northwestern University. “After spending a day trying to fit the data to match an oxide, the only possibility was to introduce a niobium silicide layer. When the data beautifully fit the model, I showed the results to my co-workers, and we all became excited about what this could mean for transmon qubit performance.”

The SQMS Center researchers dug deeper. They identified the types of silicides present, the thickness of the layer—typically only a few nanometers thick—and its physical and chemical structure. After completing these measurements, they focused on figuring out how these compounds affect the performance of qubits.

The researchers simulated different types of silicides. Not only did they find that silicides are detrimental to the performance of transmon qubits, but they also found that some are more detrimental than others.

Impact on coherence time

Qubits are the basic and fragile units of information that a quantum computer uses to perform calculations. They are physically encoded through transmon qubits.

Similar to a street performer plucking an A note on a guitar string and allowing the tone to ring out before it becomes obscured by street noise, quantum information in a transmon qubit exists for a limited time before it dissipates or is obscured by environmental noise. This time span is known as the coherence time. The longer the coherence time, the better the performance of the transmon qubit.

“This interface will never be like silicon stop, niobium start,” said SQMS Center researcher James Rondinelli, Walter Dill Scott Professor of Materials Science and Engineering at Northwestern University. “The first observation was that there is not an atomically sharp interface, but rather a compositional gradient between the silicon substrate —which is the platform for the system—and the niobium.”

With that observation, Rondinelli and his group began a detailed computational study as part of a greater SQMS Center effort to improve qubit coherence times.

Simulations with a supercomputer

With a newfound curiosity about what the presence of silicides could mean for transmon qubits, the researchers used a supercomputer at the National Energy Research Scientific Computing Center, located at the DOE’s Lawrence Berkley National Laboratory.

Think of silicides as a thin material inside the street performer’s guitar that affects the sound of the guitar string. Researchers studying transmon qubits are essentially trying to isolate an A note and seeing to what extent the hidden material interferes.

Some silicides, for example, have magnetic properties that can interfere with the quantum information that rings out from the transmon qubit. The stronger the magnetism, the more the quantum information is obscured.

Through simulations, researchers found that the silicide compound Nb₆Si₅ does not have any magnetic properties, while Nb₅Si₃ introduces magnetic noise. If silicides will always be present in transmon qubits, whether researchers like it or not, Nb₆Si₅ is less detrimental, and scientists will have to make do.

“I find it interesting how the research on the properties of these silicides have been studied since the ’80s, but never have been understood in a nanometer-sized film,” said Torres-Castanedo. “I feel proud that I was able to work alongside my fellow researchers to conduct this important study.”

These findings by themselves are significant. In the greater context of the SQMS Center’s aim to develop a state-of-the-art quantum computer, however, the results have much further implications than just understanding the properties of materials.

“The community who’s worked on superconducting qubits has traditionally been quantum physicists and engineers. The reason the SQMS Center has been so successful is they’ve embraced material scientists,” said Rondinelli. “To really push the field forward, you have to embrace a little bit of an outsider perspective to make an advancement, and we’re optimistic our multidisciplinary approach will solve this challenge.”

More information: Xuezeng Lu et al, Stability, metallicity, and magnetism in niobium silicide nanofilms, Physical Review Materials (2022). DOI: 10.1103/PhysRevMaterials.6.064402

Provided by Fermi National Accelerator Laboratory 

A team of researchers has developed the first chip-scale titanium-doped sapphire laser—a breakthrough with applications ranging from atomic clocks to quantum computing and spectroscopic sensors.

The work was led by Hong Tang, the Llewellyn West Jones, Jr. Professor of Electrical Engineering, Applied Physics & Physics. The results are published in Nature Photonics.

When the titanium-doped sapphire laser was introduced in the 1980s, it was a major advance in the field of lasers. Critical to its success was the material used as its gain medium—that is, the material that amplifies the laser’s energy. Sapphire doped with titanium ions proved to be particularly powerful, providing a much wider laser emission bandwidth than conventional semiconductor lasers. The innovation led to fundamental discoveries and countless applications in physics, biology, and chemistry.

The table-top titanium-sapphire laser is a must-have for many academic and industrial labs. However, the large bandwidth of this laser comes at the cost of a relatively high threshold—that is, the amount of power that it requires. As a result, these lasers are costly and take up a lot of space, largely limiting their use to laboratory research. Without overcoming this limitation, said Yubo Wang, lead author of the study and a graduate student in Tang’s lab, titanium-sapphire lasers will remain limited to niche customers.

The combination of the performance of titanium-sapphire lasers with the small size of a chip could drive applications that are limited by how much power or space they can consume, such as atomic clocks, portable sensors, visible light communication devices, and even quantum computing chips.

To that end, the Tang lab has demonstrated the world’s first titanium-doped sapphire laser integrated with a chip-scale photonic circuit, which provides the widest gain spectrum yet seen on a chip—paving the way for numerous new applications.

The key is in the laser‘s low threshold. While conventional titanium-doped sapphire lasers have a threshold of more than 100 milliwatts, the Tang lab’s system had a threshold of about 6.5 milliwatts. With further tweaking, they believe they can further reduce it to 1 milliwatt. The system they developed is also compatible with the family of gallium nitride optoelectronics, which are widely used in blue LEDs and lasers.

More information: Yubo Wang et al, Photonic-circuit-integrated titanium:sapphire laser, Nature Photonics (2023). DOI: 10.1038/s41566-022-01144-2

Journal information: Nature Photonics 

Provided by Yale University 

University of Central Florida College of Optics and Photonics researchers achieved the first observation of de Broglie-Mackinnon wave packets by exploiting a loophole in a 1980s-era laser physics theorem.

The research paper by CREOL and Florida Photonics Center of Excellence professor Ayman Abouraddy and research assistant Layton Hall has been published in the journal Nature Physics.

Observation of optical de Broglie–Mackinnon wave packets highlights the team’s research using a class of pulsed laser beams they call space-time wave packets.

In an interview with Dr. Abouraddy, he provides more insight into his team’s research and what it may hold for the future.

You accomplished several ‘firsts’ during this phase of your research. Will you provide some history of the theoretical ideas that brought you here?

In the early days of the development of quantum mechanics almost 100 years ago, Louis de Broglie made the crucial conceptual breakthrough of identifying waves with particles, sometimes called wave-particle duality. However, a crucial dilemma was not resolved. Particles are spatially stable: their size does not change as they travel, however waves do change, spreading in space and time. How can one construct a model out of the waves suggested by de Broglie that nevertheless correspond accurately to a particle?

In the 1970s, L. Mackinnon proposed a solution by combining Einstein’s special theory of relativity with de Broglie’s waves to construct a stable ‘wave packet’ that does not spread and can thus accompany a traveling particle. This proposal went unnoticed because there was no methodology for producing such a wave packet. In recent years, my group has been working on a new class of pulsed laser beams that we have called ‘space-time wave packets,’ which travel rigidly in free space.

In our recent research, Layton extended this behavior to propagation in dispersive media, which normally stretch optical pulses—except for space-time wave packets that resist this stretching. He recognized that the propagation of space-time wave packets in a medium endowed with a special kind of dispersion (so-called ‘anomalous’ dispersion) corresponds to Mackinnon’s proposal. In other words, space-time wave packets hold the key to finally achieving de Broglie’s dream. By carrying out laser experiments along these lines, we observed for the first time what we have called de Broglie-Mackinnon wave packets and verified their predicted properties.

What is unique about your results?

There are several unique aspects of this paper. This is the first example of a pulse propagating invariantly in a medium with anomalous dispersion. In fact, a well-known theorem in laser physics from the 1980’s purports to prove that such a feat is impossible. We found a loophole in that theorem that we exploited in designing our optical fields.

Also, all previous pulsed fields that propagate without change have been X-shaped. It has long been theorized that O-shaped propagation-invariant wave packets should exist, but they have never been observed. Our results reveal the first observed O-shaped propagation-invariant wave packets.

The U.S. Office of Naval Research is supporting your research. How are your findings useful to them and others?

We don’t know yet exactly. However, these findings have practical consequences in terms of the propagation of optical pulses in dispersive media without suffering the deleterious impact of dispersion.

These results may pave the way to optical tests of the solutions of the Klein-Gordon equation for massive particles, and may even lead to the synthesis of non-dispersive wave packets using matter waves. This would also enable new sensing and microscope techniques.

What are the next steps?

This work is a part of a larger study of the propagation characteristics of space-time wave packets. This includes long-distance propagation of space-time wave packets that we are testing at UCF’s Townes Institute Science and Technology Experimentation Facility (TISTEF) on Florida’s space coast. From a fundamental perspective, the optical spectrum that we have used in our experiments lies on a closed trajectory. This has never been achieved before, and it opens the path to studying topological structures of light on closed surfaces.

More information: Layton A. Hall et al, Observation of optical de Broglie–Mackinnon wave packets, Nature Physics (2023). DOI: 10.1038/s41567-022-01876-6

Journal information: Nature Physics 

Provided by University of Central Florida 

Microchip fab plants in the United States can cram billions of data processing transistors onto a tiny silicon chip, but a critical device, in essence a “clock,” to time the operation of those transistors must be made separately—creating a weak point in chip security and the supply line. A new approach uses commercial chip fab materials and techniques to fabricate specialized transistors that serve as the building block of this timing device, addressing the weak point and enabling new functionality through enhanced integration.

“You would have one chip that does everything instead of multiple chips, multiple fabrication methods and multiple material sets that must be integrated—often overseas,” said Dana Weinstein, a Purdue University professor of electrical and computer engineering, who is developing acoustic resonators with the processes used to produce industry-standard fin field-effect transistors (FinFETs).

“There’s a need for America to advance its capabilities in chip manufacturing, and an advance of this nature addresses multiple concerns in supply chain, national security and hardware security. By moving the whole clock inside the processor, you harden the device against clock-glitching attacks, and you enable new functionality such as acoustic fingerprinting of the packaged chip for tamper detection.”

Like all transistors—the devices that undergird modern microelectronics—FinFETs are a voltage-activated on/off gate. As its name suggests, a FinFET passes a current along a fin of semiconducting material that runs through the gate. In the closed, or off, state, the fin does not conduct electricity. A voltage applied to the top of the gate builds an electric charge in the fin, allowing electricity to flow in an open, or on, state.

But transistors must be synchronized to perform operations for microprocessors, sensors and radios used in all electronic devices. The devices that do this are built on sound, the resonant frequency that some structures emit, much as a glass bowl may sound a specific note when pinged.

The regular repeating wave of this so-called acoustic resonator serves as a cadence that is incorporated into a larger microelectromechanical system and used to mark time. Current commercial microelectromechanical resonators cannot be fabricated in a standard chip fabrication process and must be made separately and later bundled with microchips for use.

Weinstein’s innovation is to build an acoustic resonator with the existing repertoire of materials and fabrication techniques available in a standard complementary metal oxide semiconductor chip fab. In a recent paper in Nature Electronics, her research team reports its most advanced design to date.

Using a commercial process run at the GlobalFoundries Fab 8 facility in New York and described in the GlobalFoundries 14LPP FinFET technology design manual, team members fabricated a specialized set of FinFETs capable of producing a frequency in the range of 8-12 gigahertz, which exceeds the typical native clock rates of microprocessors.

The elegant solution essentially repurposes data processing transistors into a timing device.

“With our approach, the chip fab runs this device through the same process they would use for a computer’s central processing unit or other application,” said Jackson Anderson, a Purdue graduate student in electrical and computer engineering and first author on the Nature Electronics paper. “When the microprocessor and other components are done, so is the resonator. It doesn’t have to undergo further fabrication or be sent somewhere else for integration with a separate microprocessor chip.”

Although the on or off state of a transistor ordinarily directs current to serve as the 0s and 1s of binary code, all transistors can also be used as capacitors to store and release a charge. Weinstein’s team does exactly that with arrays of “drive” transistors, squeezing and releasing a thin layer of dielectric materials between the fin and the gate.

“We’re squeezing those layers between the gate and the semiconductor, pushing and pulling on that thin region between the gate and the fin,” Jackson said. “We do this alternately on adjacent transistors—one compressing, one stretching—building vibrations laterally in the device.”

The drive transistors are sized to guide and amplify the vibrations into building upon themselves into a specific resonant frequency. This, in turn, stretches and compresses the semiconductor material in an adjacent group of “sense” transistors, which alters the characteristics of a current across those transistors, translating the vibration into an electrical signal.

“Every single piece of high-performance electronics that you have uses FinFETs,” Weinstein said. “Integrating these functions advances our microelectronics capabilities beyond just digital microprocessors. If the technology changes, we can adapt, but we would be moving forward with an integrated microprocessor system.”

More information: Jackson Anderson et al, Integrated acoustic resonators in commercial fin field-effect transistor technology, Nature Electronics (2022). DOI: 10.1038/s41928-022-00827-6

Journal information: Nature Electronics 

Provided by Purdue University 

The ability to see invisible structures in our bodies, like the inner workings of cells, or the aggregation of proteins, depends on the quality of one’s microscope. Ever since the first optical microscopes were invented in the 17th century, scientists have pushed for new ways to see more things more clearly, at smaller scales and deeper depths.

Randy Bartels, professor in the Department of Electrical Engineering at Colorado State University, is one of those scientists. He and a team of researchers at CSU and Colorado School of Mines are on a quest to invent some of the world’s most powerful light microscopes—ones that can resolve large swaths of biological material in unimaginable detail.

The name of the game is super–resolution microscopy, which is any optical imaging technique that can resolve things smaller than half the wavelength of light. The discipline was the subject of the 2014 Nobel Prize in Chemistry, and Bartels and others are in a race to keep circumventing that diffraction limit to illuminate biologically important structures inside the body.

Bartels, together with Jeff Squier, a professor of physics at Colorado School of Mines, have theorized a new super–resolution technique that uniquely fuses quantum and classical information derived from light to drastically improve imaging resolution. The math and physics behind their idea, and why they think it will work, is detailed in the journal Intelligent Computing. The paper includes Jeff Field, former director of CSU’s Center for Imaging and Surface Science, as co-author.

Counting photons

Their new computational imaging method works by precisely counting the arrival of photons, or quantum particles of light, emitted from a biological sample. The photons are excited by laser pulses, and by counting them one by one with detectors, a set of quantum and classical images emerge. The researchers then apply an algorithm to produce images that resolve the details of small structures, like cells, over large regions.

“Right now, people can do very high-resolution optical imaging, but it’s kind of like flying over the Rocky Mountains and being able to see all the trees, but not the fine details of the trees,” explained Bartels. Other super–resolution techniques exist that allow someone to zoom way in, say, on an individual leaf, he continued. But to look in fine detail across an entire forest is what Bartels and the team are going for.

“The idea here is to illuminate a large region of many cells, at high resolution, high speed, and across a high volume,” Bartels said.

Bartels has worked with Mines’ Squier for several years. Their research partnership combines Bartels’ expertise in computation with Squier’s expertise in optical engineering, creating custom optics for their shared goals.

“While our paper represents a fusion of classical and quantum physics, it is fair to say it also represents a fusion of ideas from both Mines and CSU,” Squier said. “The fundamental ideas in the paper simply would not have manifested without the close collaboration that has been built up over the past several years by our groups.”

More information: Randy A. Bartels et al, Super-Resolution Imaging by Computationally Fusing Quantum and Classical Optical Information, Intelligent Computing (2022). DOI: 10.34133/icomputing.0003

Provided by Colorado State University 

Researchers have experimentally demonstrated, for the first time, a mechanically flexible silver mesh that is visibly transparent, allows high-quality infrared wireless optical communication and efficiently shields electromagnetic interference in the X band portion of the microwave radio region. Optical communication channels are important to the operation of many devices and are often used for remote sensing and detection.

Electronic devices are now found throughout our homes, on factory floors and in medical facilities. Electromagnetic interference shielding is often used to prevent electromagnetic radiation from these devices from interfering with each other and affecting device performance.

Electromagnetic shielding, which is also used in the military to keep equipment and vehicles hidden from the enemy, can also block the optical communication channels needed for remote sensing, detection or operation of the devices. A shield that can block interference but allow for optical communication channels could help to optimize device performance in a variety of civilian and military settings.

“Many conventional transparent electromagnetic interference shields allow only visible light signals through,” said research team leader Liu Yang from Zhejiang University in China. “However, visible wavelengths are not well suited for optical communication, especially free-space—or wireless—optical communication, because of the huge amount of background noise.”

In the journal Optical Materials Express, the researchers describe their new mesh. They show that when combined with transparent silicone and polyethylene, it can achieve a high average electromagnetic shielding effectiveness of 26.2 dB in the X band with good optical transmittance at a wide range of wavelengths, including those in the infrared.

“We take the advantage of the ultrabroad transparency and low haze of a metallic micromesh to demonstrate efficient electromagnetic shielding, visible transparency and high-quality free-space optical communication,” said Yang. “Sandwiching the mesh between transparent materials improves the chemical stability and mechanical flexibility of the silver mesh while also imparting a self-cleaning quality. These properties will enable our silver mesh to be applied widely both indoors and outdoors, even on corrosive and free-form surfaces.”

A flexible and transparent mesh

The researchers designed the new silver mesh with a very simple structure—a repeating square grid pattern applied to a transparent and flexible polyethylene substrate. The continuous grid structure makes the silver mesh very flexible by releasing stress during bending. Because the transparency of the silver mesh is primarily determined by the opening ratio, a measure of the size of the holes in the mesh, it is independent of the incident light wavelength.

“A large opening ratio, for example, is beneficial for a high, broadband transparency and low haze but is detrimental to high conductivity and thus electromagnetic shielding performance,” said Yang. “Because the physical parameters for our mesh can be easily optimized by changing the grid period, line width and thickness, it is easier to achieve well-balanced optical, electrical and electromagnetic properties compared with what is possible with other kinds of transparent conductive films such as silver nanowire networks, ultrathin metallic films and carbon-based materials.”

To demonstrate their new technology, the researchers fabricated a silver mesh onto a polyethylene substrate. The mesh had a grid period of approximately 150 μm, a grid line width of approximately 6 μm and a thickness that ranged from 59 to 220 nm. This was then covered with a layer of 60-μm thick polydimethylsiloxane. The resulting film showed high transmission for a broad wavelength range from 400 nm to 2000 nm and sheet resistance as low as 7.12 Ω/sq, allowing a high electromagnetic shield effectiveness up to 26.2 dB in the X band. The researchers also showed that the film could shield low-frequency mobile phone signals.

The researchers caution that this work is only a prototype demonstration, so there is much room for improvement. For example, using more conductive materials would improve the electromagnetic shielding effectiveness, and materials that are more transparent and have a lower haze could improve not only the visible transparency but also the free-space optical communication quality.

They are also exploring mid-infrared transparent conductive materials, which would extend the FSO communication to longer wavelengths where atmospheric interference is reduced and higher communication quality can be achieved. For commercialization, the mesh would also have to be more practical to install and less expensive.

More information: Qiyun Lei et al, Broadband transparent and flexible silver mesh for efficient electromagnetic interference shielding and high-quality free-space optical communication, Optical Materials Express (2022). DOI: 10.1364/OME.478830

Journal information: Optical Materials Express 

Provided by Optica 

Approximately 85% of the mass of our galaxy is comprised by dark matter, matter that does not emit, absorb or reflect light and thus cannot be directly observed. While several studies have hinted at or theorized about its composition, it remains one of the greatest unresolved physics problems.

Physicists all over the world have been conducting dark matter searches or trying to come up with new methods to directly observe different dark matter candidates. One hypothetical form of dark matter that has so far eluded detection is dark-photon dark matter.

An intriguing possibility is that dark matter is comprised of dark photons, which resemble photons (i.e., the particles that make up visible light), but interact with charges with feeble strength. These dark photons could theoretically have masses in the milli-electrovolt range, approximately a million times lighter than those of electrons and thus notoriously difficult to detect.

Researchers at Northwestern University, Stanford University and Fermilab have recently introduced a new method that could be used to search for meV dark photons. The validity of method, outlined in a paper published in Physical Review Letters, was demonstrated in a short proof-of-principle trial, which also helped to set new constraints on dark-photon dark matter.

“The idea for our study arose from discussions between experimenters and theorists facilitated by the DOE SQMS Center,” Gabriel Gabrielse, one of the researchers who carried out the study, told Phys.org. “At Northwestern we were looking for BSM applications for an unusual one-electron detector that was background free. We had developed the novel detector to measure the electron electric dipole moment and test the Standard Model’ss most precise prediction.”

After learning about the work carried out by Gabrielse and his colleagues at Northeastern, a team of theoretical physicists at Stanford reached out and pointed out the potential of their detector for conducting meV dark photon searches. This sparked a series of interactions and collaborations between the two research groups, also including Roni Harnik, a theorist at Fermilab.

The new method introduced by the researchers is based on the use of trapped electrons as high-Q resonators for detecting meV dark photon dark matter. Its underlying hypothesis is that when the rest energy of a dark photon matches the energy splitting of the two lowest cyclotron levels, the first state of the electron cyclotron will be excited.

“If a meV dark photon enters the trap in which a single electron is suspended, then the electron can be excited from the ground state to the first excited state of its cyclotron motion,” Gabrielse explained. “There is no background and the single excitation of a single trapped electron can be detected without ambiguity. Failing to see any such excitations for several days allowed us to set a limit on the strength of the dark photon field that passed through, based upon theoretical calculations of the efficiency with which a dark photon could produce such an excitation.”

To demonstrate the practicality of their proposed method, Gabrielse and his colleagues used it to collect an initial measurement, using a single electron. This trial showed that their strategy was background-free for a search that lasted just over 7 days.

The researchers were also able to set a new limit on dark photon dark matter, specifically at 148 GHz (0.6 meV). In the future, their work could pave the way for new studies aimed at evaluating and using their proposed strategy to search for meV dark photons.

“The most notable achievement of our work is the concrete demonstration of an entirely new method for searching for meV dark matter,” Gabrielse added. “We are now planning to do a broad search for meV dark photons in an apparatus that is designed for this. The apparatus in which the demonstration measurement took place was optimized for narrow band electron magnetic moment measurements, while the new apparatus and new ideas we are developing will allow broad searches.”

More information: Xing Fan et al, One-Electron Quantum Cyclotron as a Milli-eV Dark-Photon Detector, Physical Review Letters (2022). DOI: 10.1103/PhysRevLett.129.261801

Journal information: Physical Review Letters 

© 2023 Science X Network

In a new field trial, researchers show that a real-time coherent transceiver prototype can be used for continuous sensing over a 524-km live network aerial fiber wound around high-voltage power cables suspended from outdoor poles.

By monitoring polarization changes in the light traveling through a fiber link, this approach could enable new environmental or current sensing types. It can also be used to improve network integrity by continuously monitoring the health of the fiber link, for example, by detecting increases in fiber length that might indicate a pole is starting to tilt.

Mikael Mazur from Nokia Bell Labs will present the new findings at the Optical Fiber Communication Conference (OFC), which will take place 05—09 March 2023 in San Diego, California, U.S..

“Optical fibers are everywhere, and if we can expand the use of this infrastructure to create a dense worldwide spanning mesh of environmental sensors, these communication systems can play an even bigger role in our daily life,” said Mazur. “Sensing transceivers can prevent service interruption and improve our understanding of the environment by significantly scaling the number of sensors without the cost of a dedicated sensing system. Most importantly, this can be done without any loss in data throughput, enabling full use of the communication system for its intended purpose.”

In the new work, the researchers use a field-programmable gate array coherent transceiver sensing prototype for continuous fiber sensing by using information extracted from coherent digital signal processing (DSP). They used the DSP-based timing recovery module as a time-of-flight sensor.

Using this approach, the researchers continuously monitored a 524-km length of aerial fiber for 70 hours using time-of-flight measurements. They correlated the sensing measurements with temperatures acquired from stations along the network link. The analysis revealed strong oscillations driven by polarization changes over 50 Hz, likely from the Faraday effect induced by the spun fiber. They demonstrated polarization sensing of various wind conditions by filtering out the low-frequency portion of these polarization changes. The results show that coherent transceivers could potentially be used to perform continuous sensing over aerial fibers, making it possible to perform both environmental and network sensing using existing aerial fibers.

“We are just scratching the surface of potential applications and will continues to perform field trials over various networks in different environments,” said Mazur. “Our goal is to better understand how this sensor can be used in future smart cities to improve the resilience of both communication systems and infrastructure, while getting a better understanding of the environment around us. We are also actively looking at algorithms for real-time analytics and autonomous decision-making based on transceiver sensing data, enabling early-warning applications.”

More information: The 2023 Optical Fiber Communication Conference (OFC): www.ofcconference.org/en-us/home/about/

Provided by Optica 

Inspired by the bubbles bacteria create inside their cells, researchers developed a similar system by coating tiny gas vesicles with protein. The resulting bubbles are safe, highly stable, and function as contrast agent in medical applications. They could be used to diagnose, for example, cardiological issues, blood flow, and liver lesions.

Bacteria produce gas vesicles, tiny thin-walled sacs filled with air or fluid, to help them float. This phenomenon has captured the attention of scientists who see potential for similar bubble-based designs in fields like medicine.

A team of researchers at Aalto University’s Department of Applied Physics, led by Professor Robin Ras, have now used the same idea to create a new kind of contrast agent for use in medical applications such as ultrasound imaging. The research was recently published in the Proceedings of the National Academy of Sciences.

Natural materials and biological inspiration

The researchers created bubbles, referred to as giant gas vesicles, ranging from 10 to 100 micrometers in length, and measured their mechanical properties with a technique called micropipette aspiration. The bubbles were coated with proteins called hydrophobins, which come from fungi. In addition, the team developed a theory to better understand the intricacies of compressibility and porosity in micro-scale physics.

“By studying the mechanical properties of gas vesicles and developing our own micropipette technique, we were able to make the bubbles stable enough to withstand pressures like the ones you would find in the human body. The bubbles function as a contrast agent, and potentially could be used to diagnose things like cardiological issues, blood flow, and liver lesions with ultrasound in the future,” says Doctoral Researcher Hedar Al-Terke.

“We have significantly extended the theoretical framework of the pipette aspiration technique. It can now be used to fully characterize the mechanical properties, including porosity, of compressible gas-filled systems such as the hydrophobin-coated bubbles used in this study,” says Research Fellow Grégory Beaune.

The research on giant gas vesicles is part of the team’s focus on researching the medical applications of micro-scale physics.

More information: Hedar H. Al-Terke et al, Compressibility and porosity modulate the mechanical properties of giant gas vesicles, Proceedings of the National Academy of Sciences (2023). DOI: 10.1073/pnas.2211509120

Journal information: Proceedings of the National Academy of Sciences 

Provided by Aalto University 

It’s not at every university that laser pulses powerful enough to burn paper and skin are sent blazing down a hallway. But that’s what happened in UMD’s Energy Research Facility, an unremarkable looking building on the northeast corner of campus. If you visit the utilitarian white and gray hall now, it seems like any other university hall—as long as you don’t peak behind a cork board and spot the metal plate covering a hole in the wall.

But for a handful of nights in 2021, UMD Physics Professor Howard Milchberg and his colleagues transformed the hallway into a laboratory: The shiny surfaces of the doors and a water fountain were covered to avoid potentially blinding reflections; connecting hallways were blocked off with signs, caution tape and special laser-absorbing black curtains; and scientific equipment and cables inhabited normally open walking space.

As members of the team went about their work, a snapping sound warned of the dangerously powerful path the laser blazed down the hall. Sometimes the beam’s journey ended at a white ceramic block, filling the air with louder pops and a metallic tang. Each night, a researcher sat alone at a computer in the adjacent lab with a walkie-talkie and performed requested adjustments to the laser.

Their efforts were to temporarily transfigure thin air into a fiber optic cable—or, more specifically, an air waveguide—that would guide light for tens of meters. Like one of the fiber optic internet cables that provide efficient highways for streams of optical data, an air waveguide prescribes a path for light. These air waveguides have many potential applications related to collecting or transmitting light, such as detecting light emitted by atmospheric pollution, long-range laser communication or even laser weaponry. With an air waveguide, there is no need to unspool solid cable and be concerned with the constraints of gravity; instead, the cable rapidly forms unsupported in the air. In a paper accepted for publication in the journal Physical Review X the team described how they set a record by guiding light in 45-meter-long air waveguides and explained the physics behind their method.

The researchers conducted their record-setting atmospheric alchemy at night to avoid inconveniencing (or zapping) colleagues or unsuspecting students during the workday. They had to get their safety procedures approved before they could repurpose the hallway.

“It was a really unique experience,” says Andrew Goffin, a UMD electrical and computer engineering graduate student who worked on the project and is a lead author on the resulting journal article. “There’s a lot of work that goes into shooting lasers outside the lab that you don’t have to deal with when you’re in the lab—like putting up curtains for eye safety. It was definitely tiring.”

All the work was to see to what lengths they could push the technique. Previously Milchberg’s lab demonstrated that a similar method worked for distances of less than a meter. But the researchers hit a roadblock in extending their experiments to tens of meters: Their lab is too small and moving the laser is impractical. Thus, a hole in the wall and a hallway becoming lab space.

“There were major challenges: the huge scale-up to 50 meters forced us to reconsider the fundamental physics of air waveguide generation, plus wanting to send a high-power laser down a 50-meter-long public hallway naturally triggers major safety issues,” Milchberg says. “Fortunately, we got excellent cooperation from both the physics and from the Maryland environmental safety office!”

Without fiber optic cables or waveguides, a light beam—whether from a laser or a flashlight—will continuously expand as it travels. If allowed to spread unchecked, a beam’s intensity can drop to un-useful levels. Whether you are trying to recreate a science fiction laser blaster or to detect pollutant levels in the atmosphere by pumping them full of energy with a laser and capturing the released light, it pays to ensure efficient, concentrated delivery of the light.

Milchberg’s potential solution to this challenge of keeping light confined is additional light—in the form of ultra-short laser pulses. This project built on previous work from 2014 in which his lab demonstrated that they could use such laser pulses to sculpt waveguides in the air.

The short pulse technique utilizes the ability of a laser to provide such a high intensity along a path, called a filament, that it creates a plasma—a phase of matter where electrons have been torn free from their atoms. This energetic path heats the air, so it expands and leaves a path of low-density air in the laser’s wake. This process resembles a tiny version of lighting and thunder where the lightning bolt’s energy turns the air into a plasma that explosively expands the air, creating the thunderclap; the popping sounds the researchers heard along the beam path were the tiny cousins of thunder.

But these low-density filament paths on their own weren’t what the team needed to guide a laser. The researchers wanted a high-density core (the same as internet fiber optic cables). So, they created an arrangement of multiple low-density tunnels that naturally diffuse and merge into a moat surrounding a denser core of unperturbed air.

The 2014 experiments used a set arrangement of just four laser filaments, but the new experiment took advantage of a novel laser setup that automatically scales up the number of filaments depending on the laser energy; the filaments naturally distribute themselves around a ring.

The researchers showed that the technique could extend the length of the air waveguide, increasing the power they could deliver to a target at the end of the hallway. At the conclusion of the laser’s journey, the waveguide had kept about 20% of the light that otherwise would have been lost from their target area. The distance was about 60 times farther than their record from previous experiments. The team’s calculations suggest that they are not yet near the theoretical limit of the technique, and they say that much higher guiding efficiencies should be easily achievable with the method in the future.

“If we had a longer hallway, our results show that we could have adjusted the laser for a longer waveguide,” says Andrew Tartaro, a UMD physics graduate student who worked on the project and is an author on the paper. “But we got our guide right for the hallway we have.”

The researchers also did shorter eight-meter tests in the lab where they investigated the physics playing out in the process in more detail. For the shorter test they managed to deliver about 60% of the potentially lost light to their target.

The popping sound of the plasma formation was put to practical use in their tests. Besides being an indication of where the beam was, it also provided the researchers with data. They used a line of 64 microphones to measure the length of the waveguide and how strong the waveguide was along its length (more energy going into making the waveguide translates to a louder pop).

The team found that the waveguide lasted for just hundredths of a second before dissipating back into thin air. But that’s eons for the laser bursts the researchers were sending through it: Light can traverse more than 3,000 km in that time.

Based on what the researchers learned from their experiments and simulations, the team is planning experiments to further improve the length and efficiency of their air waveguides. They also plan to guide different colors of light and to investigate if a faster filament pulse repetition rate can produce a waveguide to channel a continuous high-power beam.

“Reaching the 50-meter scale for air waveguides literally blazes the path for even longer waveguides and many applications”, Milchberg says. “Based on new lasers we are soon to get, we have the recipe to extend our guides to one kilometer and beyond.”

More information: Optical guiding in 50-meter-scale air waveguides, Physical Review X (2023). journals.aps.org/prx/accepted/ … e56bdc44a53b2267be80

Journal information: Physical Review X 

Provided by University of Maryland