A study published today in the journal Science provides the first glimpse into how the brains of social mammals process these types of complex group interactions.

In the study, neuroscientists at the University of California, Berkeley, used wireless neural recording devices to track the brain activity of Egyptian fruit bats as they freely interacted in groups and occasionally vocalized to each other through high-pitched screeches and grunts.

“Most studies of communication, particularly vocalization, are typically performed with single animals or with pairs of animals, but basically none have been conducted in actual group settings,” said study co-first author Maimon Rose, a graduate student in the NeuroBat Lab at UC Berkeley. “However, many social mammals, including humans, typically interact in groups. Egyptian fruit bats, specifically, like to interact within large colonies.”

By tracking which of the bats vocalized, while simultaneously measuring the real-time neural activity in both the vocalizing and the listening bats, the researchers were able to decode how neurons in the bats’ frontal cortices distinguished among vocalizations made by themselves and by others, as well how the bats distinguished among different individuals in the group.

When they compared the neural recordings among the different bats, they also found that brain activity became highly correlated when a bat made a vocalization. Surprisingly, they found that communication produced by bats that were “friendlier” — those that spent more time in close proximity to others — induced a higher degree of correlations across the brains of the group members.

“Other neuroscience studies have tried to examine small pieces of these interactions individually. For example, one study might examine how neurons respond when somebody else speaks, and then a separate study might look at how neurons respond when that individual speaks,” said study senior author Michael Yartsev,an assistant professor of neurobiology and bioengineering at UC Berkeley. “This study is the first to really put all of these pieces together to get a full picture of communication within a social group.”

Thousands of squabbling roommates

Like humans, Egyptian fruit bats are highly social creatures. After long nights spent flying 10 miles or more in search of ripe fruit, these nocturnal animals pass the daylight hours packed into tight caves and crevices alongside hundreds or thousands of other bats. Not surprisingly, studies suggest that these bats typically vocalize to squabble over food, sleeping space and mating attempts.

“These bats are very long-lived — they live about 25 years — and basically their entire lives are spent in this group social living,” Yartsev said. “So, the ability to live together in a group and communicate with each other is an inherent feature of their lives.”

Even in laboratory settings, bats seem to prefer the comfort of a crowd, typically spending most of their time physically pressed against each other in a tight cluster. Notably, aside from making clicking noises for echolocation, Egyptian fruit bats do not engage in any long-distance form of communication and appear to vocalize to other bats only when clustered together.

“If you visit these bat caves, you can just look up and see tens of thousands of animals,” Yartsev said. “So, it really wouldn’t make sense for a bat to shout across to the cave to another bat.”

Bats’ habit of only vocalizing within tight social clumps makes them ideal subjects for studying group communication because, if a bat does call out while in a cluster, that call is most likely an indicator that social communication is taking place. However, this behavior also posed one of many technical challenges for the research team, said study co-first author Boaz Styr, a postdoctoral researcher in the NeuroBat Lab.

“One big problem was trying to identify which bat made a vocalization, because they spend their time in tight clusters and sometimes obscure each other,” Styr said. “Even though we had high resolution cameras recording at different angles, and lots of microphones around, it could be hard to pinpoint which bat was making a call at exactly which point.”

During the experiments, four to eight bats were allowed to freely interact in a darkened enclosure in the lab, and allowed to spontaneously vocalize. To accurately identify which bat made each vocalization, the team developed wireless vibration sensors that the bats could wear around their necks, almost like necklaces, and which could detect the vibrations created when a bat made a call.

“These vibration sensors, paired with our ability to wirelessly record neural data from multiple bats at the same time, allowed us to create this experiment in which the bats could freely behave and spontaneously communicate,” Styr said. “Getting all of these technical things to work together was extremely challenging, but it allowed us to ask these very important questions.”

Neurons for self and others

In one set of experiments, the researchers allowed groups of four or five bats to freely interact within a darkened enclosure in the lab, while carefully monitoring each bat’s vocalizations and brain activity.

They found that, within each bat’s frontal cortex — an area known to be involved in mediating social behaviors in animals and humans — separate sets of neurons were activated, depending on which bat in the group vocalized; in other words, a vocalization from one bat would stimulate activity in one set of neurons, while a vocalization from a different bat would stimulate a different set of neurons. These correlations were so strong that, after identifying which sets of neurons corresponded to which bat, the researchers could identify which bat had vocalized purely by looking at the neural activity of the other bats.

“What these individual neurons cared about was, ‘Am I making the call? Or is somebody else making the call?’ no matter what type of vocalization it was,” Styr said. “Other neurons were only sensitive to when one specific bat within the group was talking.”

Earlier work from the NeuroBat Lab has demonstrated that the brains of bat pairs tend to sync up when they socialize. In this study, the authors discovered that during vocal communication, the whole group syncs up together. This effect was not observed when the bats simply heard playback of the same sounds, suggesting that this phenomenon was specific to active communication taking place among the group members.

Intriguingly, the degree of correlation among the group members’ brains appeared to depend on which bat was talking, with some bats having stronger synchronization with specific individuals. Remarkably, these inter-brain patterns lasted for weeks, presumably representing stable social relationships among the individuals.

To better understand how social dynamics impact brain activity, the researchers conducted a separate set of experiments in which eight bats were allowed to freely interact in a larger enclosure. In addition to monitoring the vocalizations and neural activity of each bat, they also tracked each bat’s spatial position relative to the other bats in the group.

“Bats can recognize and have stable social relationships with other individual bats, even over long periods of time and in different circumstances,” Rose said. “And because we had this group of bats, we decided to track their positions in a larger area to see if that would tell us anything about their social relationships — who likes whom, and who are more sociable bats and the less sociable bats.”

They found that, while most “in-cluster” bats spent nearly all their time clumped together with other bats, a couple of “out-of-cluster” bats spent more time off to the side, separate from the group. Surprisingly, the team also found that the in-cluster or out-of-cluster status of a bat impacted the neural activity of the other bats during vocalizations.

“We found that when the in-cluster bats vocalized, they elicited a much more accurate neural representation of their identity in the other bats and also elicited a much higher level of brain synchrony within the group,” Rose said. “So, while its not entirely clear what exactly is going on, it seems that the behavior of the out-of-cluster bats really shifts their neural representation in the brains of the other bats.”

Understanding the neural underpinnings of why some individuals can navigate almost any social situation with ease, while others are consistently ostracized or misunderstood, could have major implications for improving human mental health, Yartsev said. He hopes the study inspires neuroscientists to take a more comprehensive look at group communication within other social mammals.

“Often, in neuroscience, we like to take a simplified approach and focus on one component of a complex process at a time,” Yartsev said. “But in reality, the social world is complex. When we spend time with our friends, there’s a lot of relationship history and baggage that comes with each interaction: what happened yesterday, who that person is friends with, how each person feels in the moment. And so, breaking things down and looking at them individually can give an illusion of control but, in fact, make it very difficult to get the complete picture.”

“Our brains, and those of animals, have evolved for and constantly struggle with the complexity of real life,” Yartsev added. “I personally believe that to truly understand the brain, we need to embrace this complexity, rather than fear it, and, indeed, every time we did so, we found out something new and exciting. I hope that this, as well as our other studies, demonstrate that we need to study the brain in all its complexity.”

Co-authors of the paper include Tobias A. Schmid and Julie E. Elie of UC Berkeley. This research was supported by the National Institutes of Health (Award DP2-DC016163), the National Institute of Mental Health (Award 1-R01MH25387-01), the New York Stem Cell Foundation (Award NYSCF-R-NI40), the Alfred P. Sloan Foundation (Award FG-2017-9646), the Brain Research Foundation (Award BRFSG-2017-09), the Packard Fellowship (Award 2017-66825), the Klingenstein Simons Fellowship, the Human Frontiers Science Program, the Pew Charitable Trust (Award 00029645), the McKnight Foundation, the Dana Foundation and the Human Frontiers Science Program postdoctoral fellowship.

In research published in the journal Nature, the team demonstrates a solid ion conductor that combines copper with cellulose nanofibrils — polymer tubes derived from wood. The paper-thin material has an ion conductivity that is 10 to 100 times better than other polymer ion conductors, the researchers say. It could be used as either a solid battery electrolyte or as an ion-conducting binder for the cathode of an all-solid-state battery.

“By incorporating copper with one-dimensional cellulose nanofibrils, we demonstrated that the normally ion-insulating cellulose offers a speedier lithium-ion transport within the polymer chains,” said Liangbing Hu, a professor in the University of Maryland’s Department of Materials Science and Engineering. “In fact, we found this ion conductor achieved a record high ionic conductivity among all solid polymer electrolytes.”

The work was a collaboration between Hu’s lab and the lab of Yue Qi, a professor at Brown’s School of Engineering.

Today’s lithium ion batteries, which are widely used in everything from cellphones to cars, have electrolytes made from lithium salt dissolved in a liquid organic solvent. The electrolyte’s job is to conduct lithium ions between a battery’s cathode and anode. Liquid electrolytes work pretty well, but they have some downsides. At high currents, tiny filaments of lithium metal, called dendrites, can form in the electrolyte leading to short circuits. In addition, liquid electrolytes are made with flammable and toxic chemicals, which can catch fire.

Solid electrolytes have the potential to prevent dendrite penetration and can be made from non-flammable materials. Most of the solid electrolytes investigated so far are ceramic materials, which are great at conducting ions but they’re also thick, rigid and brittle. Stresses during manufacturing as well as charging and discharging can lead to cracks and breaks.

The material introduced in this study, however, is thin and flexible, almost like a sheet of paper. And its ion conductivity is on par with ceramics.

Qi and Qisheng Wu, a senior research associate at Brown, performed computer simulations of the microscopic structure of the copper-cellulose material to understand why it is able to conduct ions so well. The modeling study revealed that the copper increases the space between cellulose polymer chains, which normally exist in tightly packed bundles. The expanded spacing creates what amount to ion superhighways through which lithium ions can zip by relatively unimpeded.

“The lithium ions move in this organic solid electrolyte via mechanisms that we typically found in inorganic ceramics, enabling the record high ion conductivity,” Qi said. “Using materials nature provides will reduce the overall impact of battery manufacture to our environment.”

In addition to working as a solid electrolyte, the new material can also act as a cathode binder for a solid-state battery. In order to match the capacity of anodes, cathodes need to be substantially thicker. That thickness, however, can compromise ion conduction, reducing efficiency. In order for thicker cathodes to work, they need to be encased in an ion-conducting binder. Using their new material as a binder, the team demonstrated what they believe to be one of the thickest functional cathodes ever reported.

The researchers are hopeful that the new material could be a step toward making bringing solid state battery technology to the mass market.

The research at Brown University was supported by the National Science Foundation (DMR-2054438).

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The research is published in the Proceedings of the National Academy of Sciences.

“Today’s shape-shifting materials and structures can only transition between a few stable configurations but we have shown how to create structural materials that have an arbitrary range of shape-morphing capabilities,” said L Mahadevan, the Lola England de Valpine Professor of Applied Mathematics, of Organismic and Evolutionary Biology, and of Physics and senior author of the paper. “These structures allow for independent control of the geometry and mechanics, laying the foundation for engineering functional shapes using a new type of morphable unit cell.”

One of the biggest challenges in designing shape-morphing materials is balancing the seemingly contradictory needs of conformability and rigidity. Conformability enables transformation to new shapes but if it’s too conformal, it can’t stably maintain the shapes. Rigidity helps lock the material into place but if it’s too rigid, it can’t take on new shapes.

The team started with a neutrally stable unit cell with two rigid elements, a strut and a lever, and two stretchable elastic springs. If you’ve ever seen the beginning of a Pixar movie, you’ve seen a neutrally stable material. The Pixar lamp head is stable in any position because the force of gravity is always counteracted by springs that stretch and compress in a coordinated way, regardless of the lamp configuration. In general, neutrally stable systems, a combination of rigid and elastic elements balances the energy of the cells, making each neutrally stable, meaning that they can transition between an infinite number of positions or orientations and be stable in any of them.

“By having a neutrally stable unit cell we can separate the geometry of the material from its mechanical response at both the individual and collective level,” said Gaurav Chaudhary, a postdoctoral fellow at SEAS and co-first author of the paper. “The geometry of the unit cell can be varied by changing both its overall size as well as the length of the single movable strut, while its elastic response can be changed by varying either the stiffness of the springs within the structure or the length of the struts and links.”

The researchers dubbed the assembly as “totimorphic materials” because of their ability to morph into any stable shape. The researchers connected individual unit cells with naturally stable joints, building 2D and 3D structures from individual totimorphic cells.

The researchers used both mathematical modeling and real-world demonstrations to show the material’s shape-shifting ability. The team demonstrated that one single sheet of totimorphic cells can curve up, twist into a helix, morph into the shape of two distinct faces and even bear weight.

“We show that we can assemble these elements into structures that can take on any shape with heterogeneous mechanical responses,” said S. Ganga Prasath, a postdoctoral fellow at SEAS and co-first author of the paper. “Since these materials are grounded in geometry, they could be scaled down to be used as sensors in robotics or biotechnology or could be scaled up to be used at the architectural scale.

“All together, these totimorphs pave the way for a new class of materials whose deformation response can be controlled at multiple scales,” said Mahadevan.

The research was co-authored by Edward Soucy.

“Accumulating evidence indicates that memory, reasoning, decision-making and other higher-level functions take place across people,” the researchers wrote in a review in the journal Frontiers in Systems Neuroscience. “Cognition extends into the physical world and the brains of others.”

The co-authors — neuroscientist Aron Barbey, a professor of psychology at the University of Illinois Urbana-Champaign; Richard Patterson, a professor emeritus of philosophy at Emory University; and Steven Sloman, a professor of cognitive, linguistic and psychological sciences at Brown University — wanted to address the limitations of studying brains in isolation, out of the context in which they operate and stripped of the resources they rely on for optimal function.

“In cognitive neuroscience, the standard approach is essentially to assume that knowledge is represented in the individual brain and transferred between individuals,” Barbey said. “But there are, we think, important cases where those assumptions begin to break down.”

Take, for instance, the fact that people often “outsource” the task of understanding or coming to conclusions about complex subject matter, using other people’s expertise to guide their own decision-making.

“Most people will agree that smoking contributes to the incidence of lung cancer — without necessarily understanding precisely how that occurs,” Barbey said. “And when doctors diagnose and treat disease, they don’t transfer all of their knowledge to their patients. Instead, patients rely on doctors to help them decide the best course of action.

“Without relying on experts in our community, our beliefs would become untethered from the social conventions and scientific evidence that are necessary to support them,” he said. “It would become unclear, for example, whether ‘smoking causes lung cancer,’ bringing into question the truth of our beliefs, the motivation for our actions.”

To understand the role that knowledge serves in human intelligence, the researchers wrote that it is necessary to look beyond the individual and to study the community.

“Cognition is, to a large extent, a group activity, not an individual one,” Sloman said. “People depend on others for their reasoning, judgment and decision-making. Cognitive neuroscience is not able to shed light on this aspect of cognitive processing.”

The limitations of individual knowledge and human dependence on others for understanding are the themes of “The Knowledge Illusion: Why We Never Think Alone,” a book Sloman wrote with Phil Fernbach, a cognitive scientist and professor of marketing at the University of Colorado.

“The challenge for cognitive neuroscience becomes how to capture knowledge that does not reside in the individual brain but is outsourced to the community,” Barbey said.

Neuroscientific methods such as functional MRI were designed to track activity in one brain at a time and have limited capacity for capturing the dynamics that occur when individuals interact in large communities, he said.

Some neuroscientists are trying to overcome this limitation. In a recent study, researchers placed two people face-to-face in a scanner and tracked their brain activity and eye movements while they interacted. Other teams use a technique called “hyperscanning,” which allows the simultaneous recording of brain activity in people who are physically distant from each another but interacting online.

Such efforts have found evidence suggesting that the same brain regions are activated in people who are effectively communicating with one another or cooperating on a task, Barbey said. These studies are also showing how brains operate differently from one another, depending on the type of interaction and the context.

Several fields of research are ahead of neuroscience in understanding and embracing the collective, collaborative nature of knowledge, Patterson said. For example, “social epistemology” recognizes that knowledge is a social phenomenon that depends on community norms, a shared language and a reliable method for testing the trustworthiness of potential sources.

“Philosophers studying natural language also illustrate how knowledge relies on the community,” Patterson said. “For example, according to ‘externalism,’ the meaning of words depends on how they are used and represented within a social context. Thus, the meaning of the word and its correct use depends on collective knowledge that extends beyond the individual.”

To address these shortfalls, neuroscientists can look to other social science fields, Barbey said.

“We need to incorporate not only neuroscience evidence, but also evidence from social psychology, social anthropology and other disciplines that are better positioned to study the community of knowledge,” he said.

“We humans, with all our access to technology, can’t make something with a nanoscale architecture as intricate as a pearl,” said Robert Hovden, U-M assistant professor of materials science and engineering and an author on the paper. “So we can learn a lot by studying how pearls go from disordered nothingness to this remarkably symmetrical structure.”

The analysis was done in collaboration with researchers at the Australian National University, Lawrence Berkeley National Laboratory, Western Norway University and Cornell University.

Published in the Proceedings of the National Academy of Sciences, the study found that a pearl’s symmetry becomes more and more precise as it builds, answering centuries-old questions about how the disorder at its center becomes a sort of perfection.

Layers of nacre, the iridescent and extremely durable organic-inorganic composite that also makes up the shells of oysters and other mollusks, build on a shard of aragonite that surrounds an organic center. The layers, which make up more than 90% of a pearl’s volume, become progressively thinner and more closely matched as they build outward from the center.

Perhaps the most surprising finding is that mollusks maintain the symmetry of their pearls by adjusting the thickness of each layer of nacre. If one layer is thicker, the next tends to be thinner, and vice versa. The pearl pictured in the study contains 2,615 finely matched layers of nacre, deposited over 548 days.

“These thin, smooth layers of nacre look a little like bed sheets, with organic matter in between,” Hovden said. “There’s interaction between each layer, and we hypothesize that that interaction is what enables the system to correct as it goes along.”

The team also uncovered details about how the interaction between layers works. A mathematical analysis of the pearl’s layers show that they follow a phenomenon known as “1/f noise,” where a series of events that seem to be random are connected, with each new event influenced by the one before it. 1/f noise has been shown to govern a wide variety of natural and human-made processes including seismic activity, economic markets, electricity, physics and even classical music.

“When you roll dice, for example, every roll is completely independent and disconnected from every other roll. But 1/f noise is different in that each event is linked,” Hovden said. “We can’t predict it, but we can see a structure in the chaos. And within that structure are complex mechanisms that enable a pearl’s thousands of layers of nacre to coalesce toward order and precision.”

The team found that pearls lack true long-range order — the kind of carefully planned symmetry that keeps the hundreds of layers in brick buildings consistent. Instead, pearls exhibit medium-range order, maintaining symmetry for around 20 layers at a time. This is enough to maintain consistency and durability over the thousands of layers that make up a pearl.

The team gathered their observations by studying Akoya “keshi” pearls, produced by the Pinctada imbricata fucata oyster near the Eastern shoreline of Australia. They selected these particular pearls, which measure around 50 millimeters in diameter, because they form naturally, as opposed to bead-cultured pearls, which have an artificial center. Each pearl was cut with a diamond wire saw into sections measuring three to five millimeters in diameter, then polished and examined under an electron microscope.

Hovden says the study’s findings could help inform next-generation materials with precisely layered nanoscale architecture.

“When we build something like a brick building, we can build in periodicity through careful planning and measuring and templating,” he said. “Mollusks can achieve similar results on the nanoscale by using a different strategy. So we have a lot to learn from them, and that knowledge could help us make stronger, lighter materials in the future.”

While previous studies mostly focused on individual worlds classified as “hot Jupiters” due to their superficial similarity to the gas giant in our own solar system, the new study is the first to look at a broader population of the strange worlds. Published in Nature Astronomy, the study, led by a University of Arizona researcher, provides astronomers with an unprecedented “field guide” to hot Jupiters and offers insight into planet formation in general.

Although astronomers think that only about 1 in 10 stars host an exoplanet in the hot Jupiter class, the peculiar planets make up a sizeable portion of exoplanets discovered to date, due to the fact that they are bigger and brighter than other types of exoplanets, such as rocky, more Earthlike planets or smaller, cooler gas planets. Ranging in size from about one-third the size of Jupiter to 10 Jupiter masses, all hot Jupiters orbit their host star at an extremely close range, usually much closer than Mercury, the innermost planet in our solar system, is to the sun. A “year” on a typical hot Jupiter lasts hours, or at most a few days. For comparison, Mercury takes almost three months to complete a trip around the sun.

Because of their close orbits, most, if not all, hot Jupiters are thought to be locked in a high-speed embrace with their host stars, with one side eternally exposed to the star’s radiation and the other shrouded in perpetual darkness. The surface of a typical hot Jupiter can get as hot as almost 5,000 degrees Fahrenheit, with “cooler” specimens reaching 1,400 degrees — hot enough to melt aluminum.

The research, which was led by Megan Mansfield, a NASA Sagan Fellow at the University of Arizona’s Steward Observatory, used observations made with the Hubble Space Telescope that allowed the team to directly measure emission spectra from hot Jupiters, despite the fact that Hubble can’t image any of these planets directly.

“These systems, these stars and their hot Jupiters, are too far away to resolve the individual star and its planet,” Mansfield said. “All we can see is a point — the combined light source of the two.”

Mansfield and her team used a method known as secondary eclipsing to tease out information from the observations that allowed them to peer deep into the planets’ atmospheres and gain insights into their structure and chemical makeup. The technique involves repeated observations of the same system, catching the planet at various places in its orbit, including when it dips behind the star.

“We basically measure the combined light coming from the star and its planet and compare that measurement with what we see when the planet is hidden behind its star,” Mansfield said. “This allows us to subtract the star’s contribution and isolate the light emitted by the planet, even though we can’t see it directly.”

The eclipse data provided the researchers with insight into the thermal structure of the atmospheres of hot Jupiters and allowed them to construct individual profiles of temperatures and pressures for each one. The team then analyzed near-infrared light, which is a band of wavelengths just beyond the range humans can see, coming from each hot Jupiter system for so-called absorption features. Because each molecule or atom has its own specific absorption profile, like a fingerprint, looking at different wavelengths allows researchers to obtain information about the chemical makeup of hot Jupiters. For example, if water is present in the planet’s atmosphere, it will absorb light at 1.4 microns, which falls into the range of wavelengths that Hubble can see very well.

“In a way, we use molecules to scan through the atmospheres on these hot Jupiters,” Mansfield said. “We can use the spectrum we observe to get information on what the atmosphere is made of, and we can also get information on what the structure of the atmosphere looks like.”

The team went a step further by quantifying the observational data and comparing it to models of the physical processes believed to be at work in the atmospheres of hot Jupiters. The two sets matched very well, confirming that many predictions about the planets’ nature based on theoretical work appear to be correct, according to Mansfield, who said the findings are “exciting because they were anything but guaranteed.”

The results suggest that all hot Jupiters, not just the 19 included in the study, are likely to contain similar sets of molecules, like water and carbon monoxide, along with smaller amounts of other molecules. The differences among individual planets should mostly amount to varying relative amounts of these molecules. The findings also revealed that the observed water absorption features varied slightly from one hot Jupiter to the next.

“Taken together, our results tell us there is a good chance we have the big picture items figured out that are happening in the chemistry of these planets,” Mansfield said. “At the same time, each planet has its own chemical makeup, and that also influences what we see in our observations.”

According to the authors, the results can be used to guide expectations of what astronomers might be able to see when looking at a hot Jupiter that hasn’t been studied before. The launch of NASA’s news flagship telescope, the James Webb Space Telescope, slated for Dec. 18, has exoplanet hunters excited because Webb can see in a much broader range of infrared light, and will allow a much more detailed look at exoplanets, including hot Jupiters.

“There is a lot that we still don’t know about how planets form in general, and one of the ways we try to understand how that could happen is by looking at the atmospheres of these hot Jupiters and figuring out how they got to be where they are,” Mansfield said. “With the Hubble data, we can look at trends by studying the water absorption, but when we are talking about the composition of the atmosphere as a whole, there are many other important molecules you want to look at, such as carbon monoxide and carbon dioxide, and JWST will give us a chance to actually observe those as well.”

Given its broad utility, it’s no surprise that light plays a critical role in enabling 21^st century quantum information applications. For example, scientists use laser light to precisely control atoms, turning them into ultra-sensitive measures of time, acceleration, and even gravity. Currently, such early quantum technology is limited by size — state-of-the-art systems would not fit on a dining room table, let alone a chip. For practical use, scientists and engineers need to miniaturize quantum devices, which requires re-thinking certain components for harnessing light.

Now IQUIST member Gaurav Bahl and his research group have designed a simple, compact photonic circuit that uses sound waves to rein in light. The new study, published in the October 21 issue of the journal Nature Photonics, demonstrates a powerful way to isolate, or control the directionality of light. The team’s measurements show that their approach to isolation currently outperforms all previous on-chip alternatives and is optimized for compatibility with atom-based sensors.

“Atoms are the perfect references anywhere in nature and provide a basis for many quantum applications,” said Bahl, a professor in Mechanical Science and Engineering (MechSe) at the University of Illinois at Urbana-Champaign. “The lasers that we use to control atoms need isolators that block undesirable reflections. But so far the isolators that work well in large-scale experiments have proved tough to miniaturize.”

Even in the best of circumstances, light is difficult to control — it will reflect, absorb, and refract when encountering a surface. A mirror sends light back where it came from, a shard of glass bends light while letting it through, and dark rocks absorb light and converts it to heat. Essentially, light will gladly scatter every which way off anything in its path. This unwieldy behavior is why even a smidgen of light is beneficial for seeing in the dark.

Controlling light within large quantum devices is normally an arduous task that involves a vast sea of mirrors, lenses, fibers, and more. Miniaturization requires a different approach to many of these components. In the last several years, scientists and engineers have made significant advances in designing various light-controlling elements on microchips. They can fabricate waveguides, which are channels for transporting light, and can even change its color using certain materials. But forcing light, which is made from tiny blips called photons, to move in one direction while suppressing undesirable backwards reflections is tricky.

“An isolator is a device that allows light to pass uninterrupted one way and blocks it completely in the opposite direction,” said the study’s first author Benjamin Sohn, a former graduate student and postdoctoral researcher in Mechse who is now at NIST, Boulder. “This unidirectionality cannot be achieved using just any common dielectric materials or glasses, and so we need to be a little more innovative. We also want the isolator to operate at wavelengths of light tuned to atomic sensors, which can be hard even at large scales.”

In typical experiments, the best tool for achieving unidirectionality uses magnets. For example, nearly every laser has a magneto-optic isolator that lets light exit the laser but prevents it from traveling backwards, which would mess with laser functionality. While even lasers can be miniaturized, shrinking conventional isolators is problematic for two reasons. First, in compact devices, magnetic fields would negatively affect nearby atoms. Second, even if there was a way to get around this, the materials that are inside the isolator do not work as well on the smaller length scales on a chip.

Bahl’s team demonstrated a new non-magnetic isolator that turns out to be simple in design, uses common optical materials, and is easily adaptable for different wavelengths of light.

“We wanted to design a device that naturally avoids loss, and the best way to do that is to have light propagate through nothing. The simplest bit of ‘nothing’ that can still guide photons along a controlled path is a waveguide, which is a very basic component in photonic circuits,” said Bahl.

In a complete atom-based system, the waveguide would steer laser light through a series of elements to a small chamber containing atoms. With this in mind, the team has optimized their chip for use with 780 nanometer light, which is the wavelength needed to configure common rubidium-based sensors.

That is only the first half of the design because for isolation, the light must be simultaneously blocked in the opposite direction. Previously, the team showed that they could launch sound waves into a photonic circuit to break the symmetric flow of light. In the new study the team turned this idea into a demonstration of a functional chip element.

The complete photonic isolator contains a waveguide and an adjacent ring resonator, which looks like an oblong racetrack. Normally, incoming light would just pass from the waveguide into the resonator, irrespective of its direction, thus blocking all light flow. But when the team applied sound waves to the ring, the resonator only captured light that was moving backwards through the waveguide. In the forward direction, light passed through the waveguide unimpeded, as if the resonator was simply not there.

The team’s measurements revealed that nearly every photon moves through the waveguide in the forward direction, while having only one-in-ten-thousand chance of making it through backwards. This means that the design reduced losses, or undesirable light absorption, to nearly zero, which has been a long-standing problem with previous on-chip isolators. The data show that the new devices exhibit record-breaking performance for on-chip isolation and operate as well as the larger magnet-based devices. In addition, the approach is flexible and can used for multiple wavelengths without changing the starting material.

“The simplicity in fabrication is key — with our approach, you could print photonic isolators that work well for whatever wavelength you need, all on the same chip at the same time. This is just not possible with other approaches today,” said co-author Ogulcan Orsel, graduate student in Electrical Engineering at the U of I.

This could make the new design useful for other applications, like quantum computing, where stray, uncontrolled magnetic fields as well as unwanted light can erode overall device performance.

The work was supported by the Defense Advanced Research Projects Agency (DARPA), the Air Force Office of Scientific Research (AFOSR), the National Science Foundation (NSF), and the Office of Naval Research (ONR).

But that’s not just true for people (or Muppets), it’s also true for lasers.

According to new research from the USC Viterbi School of Engineering, recently published in Nature Photonics, adding two lasers together as a sort of optical “it couple” promises to make wireless communications faster and more secure than ever before. But first, a little background. Most laser-based communications — think fiber optics, commonly used for things like high-speed internet — is transmitted in the form of a laser (optical) beam traveling through a cable. Optical communications is exceptionally fast but is limited by the fact that it must travel through physical cables. Bringing the high-capacity capabilities of lasers to untethered and roving applications — such as to airplanes, drones, submarines, and satellites — is truly exciting and potentially game-changing.

The USC Viterbi researchers have gotten us one step closer to that feat by focusing on something called Free Space Optical Communication (FSOC). This is no small feat, and it is a challenge researchers have been working on for some time. One major roadblock has been something called “atmospheric turbulence.”

As a single optical laser beam carrying information travels through the air, it experiences natural turbulence, much like a plane does. Wind and temperature changes in the atmosphere around it cause the beam to become less stable. Our inability to control that turbulence is what has prevented FSOC from advancing in performance similar to radio and optical fiber systems. Leaving us stuck with slower old radio waves for most wireless communication.

“While FSOC has been around a while, it has been a fundamental challenge to efficiently recover information from an optical beam that has been affected by atmospheric turbulence,” said Runzhou Zhang, the lead author and a Ph.D. student at USC Viterbi’s Optical Communications Laboratory in the Ming Hsieh Department of Electrical and Computer Engineering.

The researchers made an advance to solving this problem by sending a second laser beam (called a “pilot” beam) traveling along with the first to act as a partner. Traveling as a couple, the two beams are sent through the same air, experience the same turbulence, and have the same distortion. If only one beam is sent, the receiver must calculate all the distortion the beam experienced along the way before it can decode the data. This severely limits the system’s performance.

But, when the pilot beam travels alongside the original beam, the distortion is automatically removed. Like Kermit duetting “Rainbow Connection” with Miss Piggy, the information in that beam arrives at its destination clear, crisp and easy to understand. From an engineering perspective, this accomplishment is no small feat. “The problem with radio waves, our current best bet for most wireless communication, is that it is much slower in data rate and much less secure than optical communications,” said Alan Willner, team lead on the paper and USC Viterbi professor of electrical and computer engineering. “With our new approach, we are one step closer to mitigating turbulence in high-capacity optical links.”

Perhaps most impressively, the researchers did not solve this problem with a new device or material. They simply looked at the physics and changed their perspective. “We used the underlying physics of a well-known device called a photo detector, usually used for detecting intensity of light, and realized it could be used in a new way to make an advance towards solving the turbulence problem for laser communication systems,” said Zhang.

Think about it this way: When Kermit and Miss Piggy sing their song, both their voices get distorted through the air in a similar way. That makes sense; they’re standing right next to each other, and their sound is traveling through the same atmosphere. What this photo detector does is turn the distortion of Kermit’s voice into the opposite of the distortion for Miss Piggy’s voice. Now, when they are mixed back together, the distortion is automatically canceled in both voices and we hear the song clearly and crisply.

With this newly realized application of physics, the team plans to continue exploring how to make the performance even better. “We hope that our approach will one day enable higher-performance and secure wireless links,” said Willner. Such links may be used for anything from high-resolution imaging to high-performance computing.

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Materials provided by University of Southern California. Original written by Ben Paul. Note: Content may be edited for style and length.