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      Drones could help create a quantum internet

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      The quantum internet may be coming to you via drone.
      Scientists have now used drones to transmit particles of light, or photons, that share the quantum linkage called entanglement. The photons were sent to two locations a kilometer apart, researchers from Nanjing University in China report in a study to appear in Physical Review Letters.
      Entangled quantum particles can retain their interconnected properties even when separated by long distances. Such counterintuitive behavior can be harnessed to allow new types of communication. Eventually, scientists aim to build a global quantum internet that relies on transmitting quantum particles to enable ultrasecure communications by using the particles to create secret codes to encrypt messages. A quantum internet could also allow distant quantum computers to work together, or perform experiments that test the limits of quantum physics.
      Quantum networks made with fiber-optic cables are already beginning to be used (SN: 9/28/20). And a quantum satellite can transmit photons across China (SN: 6/15/17). Drones could serve as another technology for such networks, with the advantages of being easily movable as well as relatively quick and cheap to deploy.
      The researchers used two drones to transmit the photons. One drone created pairs of entangled particles, sending one particle to a station on the ground while relaying the other to the second drone. That machine then transmitted the particle it received to a second ground station a kilometer away from the first. In the future, fleets of drones could work together to send entangled particles to recipients in a variety of locations. More

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      Galileo’s famous gravity experiment holds up, even with individual atoms

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      According to legend, Galileo dropped weights off of the Leaning Tower of Pisa, showing that gravity causes objects of different masses to fall with the same acceleration. In recent years, researchers have taken to replicating this test in a way that the Italian scientist probably never envisioned — by dropping atoms.
      A new study describes the most sensitive atom-drop test so far and shows that Galileo’s gravity experiment still holds up — even for individual atoms. Two different types of atoms had the same acceleration within about a part per trillion, or 0.0000000001 percent, physicists report in a paper in press in Physical Review Letters.
      Compared with a previous atom-drop test, the new research is a thousand times as sensitive. “It represents a leap forward,” says physicist Guglielmo Tino of the University of Florence, who was not involved with the new study.
      Researchers compared rubidium atoms of two different isotopes, atoms that contain different numbers of neutrons in their nuclei. The team launched clouds of these atoms about 8.6 meters high in a tube under vacuum. As the atoms rose and fell, both varieties accelerated at essentially the same rate, the researchers found.

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      In confirming Galileo’s gravity experiment yet again, the result upholds the equivalence principle, a foundation of Albert Einstein’s theory of gravity, general relativity. That principle states that an object’s inertial mass, which determines how much it accelerates when force is applied, is equivalent to its gravitational mass, which determines how strong a gravitational force it feels. The upshot: An object’s acceleration under gravity doesn’t depend on its mass or composition.
      So far, the equivalence principle has withstood all tests. But atoms, which are subject to the strange laws of quantum mechanics, could reveal its weak points. “When you do the test with atoms … you’re testing the equivalence principle and stressing it in new ways,” says physicist Mark Kasevich of Stanford University.
      Kasevich and colleagues studied the tiny particles using atom interferometry, which takes advantage of quantum mechanics to make extremely precise measurements. During the atoms’ flight, the scientists put the atoms in a state called a quantum superposition, in which particles don’t have one definite location. Instead, each atom existed in a superposition of two locations, separated by up to seven centimeters. When the atoms’ two locations were brought back together, the atoms interfered with themselves in a way that precisely revealed their relative acceleration.
      Many scientists think that the equivalence principle will eventually falter. “We have reasonable expectations that our current theories … are not the end of the story,” says physicist Magdalena Zych of the University of Queensland in Brisbane, Australia, who was not involved with the research. That’s because quantum mechanics — the branch of physics that describes the counterintuitive physics of the very small — doesn’t mesh well with general relativity, leading scientists on a hunt for a theory of quantum gravity that could unite these ideas. Many scientists suspect that the new theory will violate the equivalence principle by an amount too small to have been detected with tests performed thus far.
      But physicists hope to improve such atom-based tests in the future, for example by performing them in space, where objects can free-fall for extended periods of time. An equivalence principle test in space has already been performed with metal cylinders, but not yet with atoms (SN: 12/4/17).
      So there’s still a chance to prove Galileo wrong. More

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      The diabolical ironclad beetle can survive getting run over by a car. Here’s how

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      The diabolical ironclad beetle is like a tiny tank on six legs.
      This insect’s rugged exoskeleton is so tough that the beetle can survive getting run over by cars, and many would-be predators don’t stand a chance of cracking one open. Phloeodes diabolicus is basically nature’s jawbreaker.
      Analyses of microscope images, 3-D printed models and computer simulations of the beetle’s armor have now revealed the secrets to its strength. Tightly interlocked and impact-absorbing structures that connect pieces of the beetle’s exoskeleton help it survive enormous crushing forces, researchers report in the Oct. 22 Nature. Those features could inspire new, sturdier designs for things such as body armor, buildings, bridges and vehicles.
      The diabolical ironclad beetle, which dwells in desert regions of western North America, has a distinctly hard-to-squish shape. “Unlike a stink beetle, or a Namibian beetle, which is more rounded … it’s low to the ground [and] it’s flat on top,” says David Kisailus, a materials scientist at the University of California, Irvine. In compression experiments, Kisailus and colleagues found that the beetle could withstand around 39,000 times its own body weight. That would be like a person shouldering a stack of about 40 M1 Abrams battle tanks.
      Within the diabolical ironclad beetle’s own tanklike physique, two key microscopic features help it withstand crushing forces. The first is a series of connections between the top and bottom halves of the exoskeleton. “You can imagine the beetle’s exoskeleton almost like two halves of a clamshell sitting on top of each other,” Kisailus says. Ridges along the outer edges of the top and bottom latch together.
      This slice of a diabolical ironclad beetle’s back shows the jigsaw-shaped links that connect the left and right sides of its exoskeleton. These protrusions are tightly interlocked and highly damage-resistant, helping give the beetle its incredible durability.David Kisailus
      But those ridged connections have different shapes across the beetle’s body. Near the front of the beetle, around its vital organs, the ridges are highly interconnected — almost like zipper teeth. Those connections are stiff and resist bending under pressure.
      The connective ridges near the back of the beetle, on the other hand, are not as intricately interlocked, allowing the top and bottom halves of the exoskeleton to slide past each other slightly. That flexibility helps the beetle absorb compression in a region of its body that is safer to squish.
      The second key feature is a rigid joint, or suture, that runs the length of the beetle’s back and connects its left and right sides. A series of protrusions, called blades, fit together like jigsaw puzzle pieces to join the two sides. These blades contain layers of tissue glued together by proteins, and are highly damage-resistant. When the beetle is squashed, tiny cracks form in the protein glue between the layers of each blade. Those small, healable fractures allow the blades to absorb impacts without completely snapping, explains Jesus Rivera, an engineer at UC Irvine.

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      This toughness makes the diabolical ironclad beetle pretty predator-proof. An animal might be able to make a meal out of the beetle by swallowing it whole, Kisailus says. “But the way it’s built, in terms of other predation — let’s say like a bird that’s pecking at it, or a lizard that’s trying to chew on it — the exoskeleton would be really hard” to crack.
      That hard exterior is also a nuisance for insect collectors. The diabolical ironclad beetle is notorious among entomologists for being so fantastically durable that it bends the steel pins usually used to mount insects for display, says entomologist Michael Caterino of Clemson University in South Carolina. But “the basic biology of this thing is not particularly well-known,” he says. “I found it fascinating” to learn what makes the beetle so indestructible.
      The possibility of using beetle-inspired designs for sturdier airplanes and other structures is intriguing, Caterino adds. And with the splendid variety of insects all over the world, who knows what other critters might someday inspire clever engineering designs. More

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      The first room-temperature superconductor has finally been found

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      It’s here: Scientists have reported the discovery of the first room-temperature superconductor, after more than a century of waiting.
      The discovery evokes daydreams of futuristic technologies that could reshape electronics and transportation. Superconductors transmit electricity without resistance, allowing current to flow without any energy loss. But all superconductors previously discovered must be cooled, many of them to very low temperatures, making them impractical for most uses.
      Now, scientists have found the first superconductor that operates at room temperature — at least given a fairly chilly room. The material is superconducting below temperatures of about 15° Celsius (59° Fahrenheit), physicist Ranga Dias of the University of Rochester in New York and colleagues report October 14 in Nature.
      The team’s results “are nothing short of beautiful,” says materials chemist Russell Hemley of the University of Illinois at Chicago, who was not involved with the research.
      However, the new material’s superconducting superpowers appear only at extremely high pressures, limiting its practical usefulness.

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      Dias and colleagues formed the superconductor by squeezing carbon, hydrogen and sulfur between the tips of two diamonds and hitting the material with laser light to induce chemical reactions. At a pressure about 2.6 million times that of Earth’s atmosphere, and temperatures below about 15° C, the electrical resistance vanished.
      That alone wasn’t enough to convince Dias. “I didn’t believe it the first time,” he says. So the team studied additional samples of the material and investigated its magnetic properties.
      Superconductors and magnetic fields are known to clash — strong magnetic fields inhibit superconductivity. Sure enough, when the material was placed in a magnetic field, lower temperatures were needed to make it superconducting. The team also applied an oscillating magnetic field to the material, and showed that, when the material became a superconductor, it expelled that magnetic field from its interior, another sign of superconductivity.
      The scientists were not able to determine the exact composition of the material or how its atoms are arranged, making it difficult to explain how it can be superconducting at such relatively high temperatures. Future work will focus on describing the material more completely, Dias says.
      When superconductivity was discovered in 1911, it was found only at temperatures close to absolute zero (−273.15° C). But since then, researchers have steadily uncovered materials that superconduct at higher temperatures. In recent years, scientists have accelerated that progress by focusing on hydrogen-rich materials at high pressure.
      In 2015, physicist Mikhail Eremets of the Max Planck Institute for Chemistry in Mainz, Germany, and colleagues squeezed hydrogen and sulfur to create a superconductor at temperatures up to −70° C (SN: 12/15/15). A few years later, two groups, one led by Eremets and another involving Hemley and physicist Maddury Somayazulu, studied a high-pressure compound of lanthanum and hydrogen. The two teams found evidence of superconductivity at even higher temperatures of −23° C and −13° C, respectively, and in some samples possibly as high as 7° C (SN: 9/10/18).
      The discovery of a room-temperature superconductor isn’t a surprise. “We’ve been obviously heading toward this,” says theoretical chemist Eva Zurek of the University at Buffalo in New York, who was not involved with the research. But breaking the symbolic room-temperature barrier is “a really big deal.”
      If a room-temperature superconductor could be used at atmospheric pressure, it could save vast amounts of energy lost to resistance in the electrical grid. And it could improve current technologies, from MRI machines to quantum computers to magnetically levitated trains. Dias envisions that humanity could become a “superconducting society.”
      But so far scientists have created only tiny specks of the material at high pressure, so practical applications are still a long way off.
      Still, “the temperature is not a limit anymore,” says Somayazulu, of Argonne National Laboratory in Lemont, Ill., who was not involved with the new research. Instead, physicists now have a new aim: to create a room-temperature superconductor that works without putting on the squeeze, Somayazulu says. “That’s the next big step we have to do.”

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      Fundamental constants place a new speed limit on sound

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      Sound has a speed limit. Under normal circumstances, its waves can travel no faster than about 36 kilometers per second, physicists propose October 9 in Science Advances.
      Sound zips along at different rates in different materials — moving faster in water than in air for example. But under conditions found naturally on Earth, no material can host sound waves that outpace this ultimate limit, which is about 100 times the typical speed of sound traveling in air.
      The team’s reasoning rests on well-known equations of physics and mathematical relationships.  “Given the simplicity of the argument, it suggests that [the researchers] are putting their finger on something very deep,” says condensed matter physicist Kamran Behnia of École Supérieure de Physique et de Chimie Industrielles in Paris.
      The equation for the speed limit rests on fundamental constants, special numbers that rule the cosmos. One such number, the speed of light, sets the universe’s ultimate speed limit — nothing can go faster. Another, known as the fine-structure constant, determines the strength with which electrically charged particles push and pull one another. When combined in the right arrangement with another constant — the ratio of the masses of the proton and electron — these numbers yield sound’s speed limit.

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      Sound waves, which consist of the vibrations of atoms or molecules, travel through a material as one particle jostles another. The wave’s speed depends on various factors, including the types of chemical bonds holding the material together and how massive its atoms are.
      None of the sound speeds previously measured in a variety of liquids and solids surpass the proposed limit, condensed matter physicist Kostya Trachenko and colleagues found. The fastest speed measured, in diamond, was only about half the theoretical maximum.  
      The limit applies only to solids and liquids at pressures typically found on Earth. At pressures millions of times that of Earth’s atmosphere, sound waves move faster and could surpass the limit.
      One material expected to boast a high sound speed exists only at such high pressures: hydrogen squeezed hard enough to turn into a solid metal (SN: 6/28/19). That metal has never been convincingly created, so the researchers calculated the expected speed instead of using a measurement. Above about 6 million times Earth’s atmospheric pressure, the sound speed limit would be broken, the calculations suggest.
      The role of the fundamental constants in sound’s maximum speed results from how the waves move through materials. Sound travels thanks to the electromagnetic interactions of neighboring atoms’ electrons, which is where the fine-structure constant comes into play. And the proton-electron mass ratio is important because, although the electrons are interacting, the nuclei of the atoms move as a result.
      The fine-structure constant and the proton-electron mass ratio are dimensionless constants, meaning there are no units attached to them (so their value does not depend on any particular system of units). Such dimensionless constants fascinate physicists, because the values are crucial to the existence of the universe as we know it (SN: 11/2/16). For example, if the fine-structure constant were significantly altered, stars, planets and life couldn’t have formed. But no one can explain why these all-important numbers have the values they do.
      “When I have sleepless nights, I sometimes think about this,” says Trachenko, of Queen Mary University of London. So he and colleagues are extending this puzzle from the cosmic realm to more commonplace concepts like the speed of sound. Trachenko and coauthor Vadim Veniaminovich Brazhkin of the Institute for High Pressure Physics, in Troitsk, Russia, also reported a minimum possible viscosity for liquids in the April 24 Science Advances.
      That viscosity limit depends on the Planck constant, a number at the heart of quantum mechanics, the math that governs physics on very small scales. If the Planck constant were 100 times larger, Trachenko says, “water would be like honey, and that probably would be the end of life because the processes in cells would not flow as efficiently.” More

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      A measurement of positronium’s energy levels confounds scientists

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      Positronium is positively puzzling.
      A new measurement of the exotic “atom” — consisting of an electron and its antiparticle, a positron — disagrees with theoretical calculations, scientists report in the Aug. 14 Physical Review Letters. And physicists are at a loss to explain it.
      A flaw in either the calculations or the experiment seems unlikely, researchers say. And new phenomena, such as undiscovered particles, also don’t provide an easy answer, adds theoretical physicist Jesús Pérez Ríos of the Fritz Haber Institute of the Max Planck Society in Berlin. “Right now, the best I can tell you is, we don’t know,” says Pérez Ríos, who was not involved with the new research.
      Positronium is composed of an electron, with a negative charge, circling in orbit with a positron, with a positive charge — making what’s effectively an atom without a nucleus (SN: 9/12/07). With just two particles and free from the complexities of a nucleus, positronium is appealingly simple. Its simplicity means it can be used to precisely test the theory of quantum electrodynamics, which explains how electrically charged particles interact.

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      A team of physicists from University College London measured the separation between two specific energy levels of positronium, what’s known as its fine structure. The researchers formed positronium by colliding a beam of positrons with a target, where they met up with electrons. After manipulating the positronium atoms with a laser to put them in the appropriate energy level, the team hit them with microwave radiation to induce some of them to jump to another energy level.
      The researchers pinpointed the frequency of radiation needed to make the atoms take the leap, which is equivalent to finding the size of the gap between the energy levels. While the frequency predicted from calculations was about 18,498 megahertz, the researchers measured about 18,501 megahertz, a difference of about 0.02 percent. Given that the estimated experimental error was only about 0.003 percent, that’s a wide gap.
      The team searched for experimental issues that could explain the result, but came up empty. Additional experiments are now needed to help investigate the mismatch, says physicist Akira Ishida of the University of Tokyo, who was not involved with the study. “If there is still significant discrepancy after further precise measurements, the situation becomes much more exciting.”
      The theoretical prediction also seems solid. In quantum electrodynamics, making predictions involves calculating to a certain level of precision, leaving out terms that are less significant and more difficult to calculate. Those additional terms are expected to be too small to account for the discrepancy. But, “it’s conceivable that you could be surprised,” says theoretical physicist Greg Adkins of Franklin & Marshall College in Lancaster, Pa., also not involved with the research.
      If the experiments and the theoretical calculations check out, the discrepancy might be due to a new particle, but that explanation also seems unlikely. A new particle’s effects probably would have shown up in earlier experiments. For example, says Pérez Ríos, positronium’s energy levels could be affected by a hypothetical axion-like particle. That’s a lightweight particle that has the potential to explain dark matter, an invisible type of matter thought to permeate the universe. But if that type of particle was causing this mismatch, researchers would also have seen its effects in measurements of the magnetic properties of the electron and its heavier cousin, the muon.
      That leaves scientists still searching for an answer, says physicist David Cassidy, a coauthor of the study. “It’s going to be something surprising. I just don’t know what.­” More