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  • richardmitnick 5:04 am on March 4, 2015 Permalink | Reply
    Tags: , , , , Physics   

    From AAAS: “Physicists gear up to catch a gravitational wave” 

    AAAS

    AAAS

    3 March 2015
    Adrian Cho

    1
    The twin 4-kilometer arms of LIGO Livingston embrace a working forest, where logging generates vibrations that the instrument must damp out.

    This patch of woodland just north of Livingston, Louisiana, population 1893, isn’t the first place you’d go looking for a breakthrough in physics. Standing on a small overpass that crosses an odd arching tunnel, Joseph Giaime, a physicist at Louisiana State University (LSU), 55 kilometers west in Baton Rouge, gestures toward an expanse of spindly loblolly pine, parts of it freshly reduced to stumps and mud. “It’s a working forest,” he says, “so they come in here to harvest the logs.” On a quiet late fall morning, it seems like only a logger or perhaps a hunter would ever come here.

    Yet it is here that physicists may fulfill perhaps the most spectacular prediction of Albert Einstein’s theory of gravity, or general relativity. The tunnel runs east to west for 4 kilometers and meets a similar one running north to south in a nearby warehouselike building. The structures house the Laser Interferometer Gravitational-Wave Observatory (LIGO), an ultrasensitive instrument that may soon detect ripples in space and time set off when neutron stars or black holes merge.

    Einstein himself predicted the existence of such gravitational waves nearly a century ago. But only now is the quest to detect them coming to a culmination. The device in Livingston and its twin in Hanford, Washington, ran from 2002 to 2010 and saw nothing. But those Initial LIGO instruments aimed only to prove that the experiment was technologically feasible, physicists say. Now, they’re finishing a $205 million rebuild of the detectors, known as Advanced LIGO, which should make them 10 times more sensitive and, they say, virtually ensure a detection. “It’s as close to a guarantee as one gets in life,” says Peter Saulson, a physicist at Syracuse University in New York, who works on LIGO.

    Detecting those ripples would open a new window on the cosmos. But it won’t come easy. Each tunnel contains a pair of mirrors that form an “optical cavity,” within which infrared light bounces back and forth. To look for the stretching of space, physicists will compare the cavities’ lengths. But they’ll have to sense that motion through the din of other vibrations. Glancing at the pavement on the overpass, Giaime says that the ground constantly jiggles by about a millionth of a meter, shaken by seismic waves, the rumble of nearby trains, and other things. LIGO physicists have to shield the mirrors from such vibrations so that they can see the cavities stretch or shorten by distances 10 trillion times smaller—just a billionth the width of an atom.

    IN 1915, Einstein explained that gravity arises when mass and energy warp space and time, or spacetime. A year later, he predicted that massive objects undergoing the right kind of oscillating motion should emit ripples in spacetime—gravitational waves that zip along at light speed.

    For decades that prediction remained controversial, in part because the mathematics of general relativity is so complicated. Einstein himself at first made a technical error, says Rainer Weiss, a physicist at the Massachusetts Institute of Technology (MIT) in Cambridge. “Einstein had it right,” he says, “but then he [messed] up.” Some theorists argued that the waves were a mathematical artifact and shouldn’t actually exist. In 1936, Einstein himself briefly took that mistaken position.

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    Rainer Weiss of the Massachusetts Institute of Technology laid out the basic plan for LIGO 43 years ago. © MATT WEBER

    Even if the waves were real, detecting them seemed impossible, Weiss says. At a time when scientists knew nothing of the cosmos’s gravitational powerhouses—neutron stars and black holes—the only obvious source of waves was a pair of stars orbiting each other. Calculations showed that they would produce a signal too faint to be detected.

    By the 1950s, theorists were speculating about neutron stars and black holes, and they finally agreed that the waves should exist. In 1969, Joseph Weber, a physicist at the University of Maryland, College Park, even claimed to have discovered them. His setup included two massive aluminum cylinders 1.5 meters long and 0.6 meters wide, one of them in Illinois. A gravitational wave would stretch a bar and cause it to vibrate like a tuning fork, and electrical sensors would then detect the stretching. Weber saw signs of waves pinging the bars together. But other experimenters couldn’t reproduce Weber’s published results, and theorists argued that his claimed signals were implausibly strong.

    Still, Weber’s efforts triggered the development of LIGO. In 1969, Weiss, a laser expert, had been assigned to teach general relativity. “I knew bugger all about it,” he says. In particular, he couldn’t understand Weber’s method. So he devised his own optical method, identifying the relevant sources of noise. “I worked it out for myself, and I gave it to the students as a homework problem,” he says.

    Weiss’s idea, which he published in 1972 in an internal MIT publication, was slow to catch on. “It was obvious to me that this was pie in the sky and it would never work,” recalls Kip Thorne, a theorist at the California Institute of Technology (Caltech) in Pasadena, California. Thorne recorded his skepticism in Gravitation, the massive textbook that he co-wrote and published in 1973. “I had an exercise that said ‘Show that this technology will never work to detect gravitational waves,’ ” Thorne says.

    But by 1978 Thorne had warmed to the idea, and he persuaded Caltech to put up $2 million to build a 40-meter prototype interferometer. “It wasn’t a hard sell at all,” Thorne says, “which was a contrast to the situation at MIT.” Weiss says that Thorne played a vital role in winning support for a full-scale detector from the National Science Foundation in 1990. Construction in Livingston and Hanford finally began in 1994.

    Now, many physicists say Advanced LIGO is all but a sure winner. On a bright Monday morning in December, researchers at Livingston are embarking on a 10-day stint that will mark their first attempt to run as if making observations. LIGO Livingston has the feel of an outpost. Roughly 30 physicists, engineers, technicians, and operators gather in the large room that serves as the facility’s foyer, auditorium, and—with a table-tennis table in one corner—rec room. “Engineering run 6 began 8 minutes ago,” announces Janeen Romie, an engineer from Caltech. It seems odd that so few people can run such a big rig.

    But in principle, LIGO is simple. Within the interferometer’s sewer pipe–like vacuum chamber, at the elbow of the device, a laser beam shines on a beam splitter, which sends half the light down each of the interferometer’s arms. Within each arm, the light builds up as it bounces between the mirrors at either end. Some of the light leaks through the mirrors at the near ends of the arms and shines back on the beam splitter. If the two arms are exactly the same length, the merging waves will overlap and interfere with each other in a way that directs the light back toward the laser.

    The ultimate motion sensor

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    In a LIGO interferometer, light waves leaking out of the two storage arms ordinarily interfere to send light back to the laser. By stretching the two arms by different amounts, a gravitational wave would alter the interference and send light toward a photodetector. G. GRULLÓN/SCIENCE

    But if the lengths are slightly different, then the recombining waves will be out of sync and light will emerge from the beam splitter perpendicular to the original beam. From that “dark port” output, physicists can measure any difference in the arms’ lengths to an iota of the light’s wavelength. Because a gravitational wave sweeping across the apparatus would generally stretch one arm more than the other, it would cause light to warble out of the dark port at the frequency at which the wave ripples. That light would be the signal of the gravitational wave.

    In practice, LIGO is a monumental challenge in sifting an infinitesimal signal from a mountain of vibrational noise. Sources of gravitational waves should “sing” at frequencies ranging from 10 to 1000 cycles per second, or hertz. But at frequencies of hundreds or thousands of hertz the individual photons in the laser beam produce noise as they jostle the mirrors. To smooth out such noise, researchers crank up the amount of light and deploy massive mirrors. At frequencies of tens of hertz and lower, seismic vibrations dominate, so researchers dangle the mirrors from elaborate suspension systems and actively counteract that motion. Still, a large earthquake anywhere in the world or even the surf pounding the distant coast can knock the interferometer off line.

    To boost the Hanford and Livingston detectors’ sensitivity 10-fold, to a ten-billionth of a nanometer, physicists have completely rebuilt the devices. Each of the original 22-kilogram mirrors hung like a pendulum from a single steel fiber; the new 40-kilogram mirrors hang on silica fibers at the end of a four-pendulum chain. Instead of LIGO’s original 10 kilowatts of light power, researchers aim to circulate 750 kilowatts. They will collect 100,000 channels of data to monitor the interferometer. Comparing the new and old LIGO is “like comparing a car to a wheel,” says Frederick Raab, a Caltech physicist who leads the Hanford site.

    The new Livingston machine has already doubled Initial LIGO’s sensitivity. “In 6 months they’ve made equivalent progress to what Initial LIGO made in 3 or 4 years,” says Raab, who adds that the Hanford site is about 6 months behind. But Valery Frolov, a Caltech physicist in charge of commissioning the Livingston detector, cautions that machine isn’t running anywhere close to specs. The seismic isolation was supposed to be better, he says, and researchers haven’t been able to keep the interferometer “locked” and running for long periods. As for reaching design sensitivity, “I don’t know whether it will take 1 year or whether it will take 5 years like Initial LIGO did,” he warns.

    Still, LIGO researchers plan to make a first observing run this year and hope to reach design sensitivity next year. “We will have detections that we will be able to stand up and defend, if not in 2016, then in 2017 or 2018,” says Gabriela González, a physicist at LSU and spokesperson for the more than 900-member LIGO Science Collaboration.

    That forecast is based on the statistics of the stars. LIGO’s prime target is the waves generated by a pair of neutron stars—the cores of exploded stars that weigh more than the sun but measure tens of kilometers across—whirling into each other in a death spiral lasting several minutes. Initial LIGO could sense such a pair up to 50 million light-years way. Given the rarity of neutron-star pairs, that search volume was too small to guarantee seeing one. Advanced LIGO should see 10 times as far and probe 1000 times as much space, enough to contain about 10 sources per year, González says. However, Clifford Will, a theorist at the University of Florida in Gainesville, notes that the number of sources is the most uncertain part of the experiment. “If it’s less than one per year, that’s not going to be too good,” he says.

    Enlarging the search

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    Compared with Initial LIGO, Advanced LIGO will be able to detect gravitational wave sources up to 10 times as far away, probing 1000 times as much space. Such a volume will likely yield multiple sources. ADAPTED FROM NSF BY G. GRULLÓN/SCIENCE

    The hunt will be global. As well as combining data from the two LIGO detectors, researchers will share data with their peers working on the VIRGO detector, an interferometer with 3-kilometer arms near Pisa, Italy, that is undergoing upgrades, and on GEO600, one with 600-meter arms near Hannover, Germany.

    VIRGO interferometer EGO
    VIRGO interferometer EGO Campus
    VIRGO

    GEO600
    GEO600

    By comparing data, collaborators can better sift signals from noise and can pinpoint sources on the sky. Japanese researchers are also building a detector, and LIGO leaders hope to add a third detector, in India.

    FOR THEORISTS—if not for the rest of the world—seeing gravitational waves for the first time will be something of an anti-climax. “We are so confident that gravitational waves exist that we don’t actually need to see one,” says Marc Kamionkowski, a theorist at Johns Hopkins University in Baltimore, Maryland. That’s because in 1974 American astrophysicists Russell Hulse and Joseph Taylor Jr. found indirect but convincing evidence of the waves. They spotted two pulsars—neutron stars that emit radio signals with clockwork regularity—orbiting each other. From the timing of the radio pulses, Hulse and Taylor could monitor the pulsars’ orbit. They found it is decaying at exactly the rate expected if the pulsars were radiating energy in the form of gravitational waves.

    LIGO’s real payoff will come in opening a new frontier in astronomy, says Robert Wald, a gravitational theorist at the University of Chicago in Illinois. “It’s kind of like after being able to see for a while, being able to hear, too,” Wald says. For example, if a black hole tears apart a neutron star, then details of the gravitational waves may reveal the properties of matter in neutron stars.

    All told, detecting gravitational waves would merit science’s highest accolade, physicists say. “As soon as they detect a gravitational wave, it’s a Nobel Prize,” Kamionkowski predicts. “It’s such an extraordinary experimental accomplishment.” But the prize can be shared by at most three people, so the question is who should get it.

    Weiss is a shoo-in, many say, but he demurs. “I don’t want to deny that there was some innovation [in my work], but it didn’t come out of the blue,” he says. “The lone crazy man working in a box, that just doesn’t hold true.” In 1962 two Russian physicists published a paper on detecting gravitational waves with an interferometer, as Weiss says he learned long after his 1972 work. In the 1970s, Robert Forward of the Hughes Aircraft Company in Malibu, California, ran a small interferometer. Key design elements of LIGO came from Ronald Drever, project director at Caltech from 1979 to 1987, who, Thorne says, “has to be recognized as one of the fathers of the LIGO idea.”

    But to make that prize-winning discovery, physicists must get Advanced LIGO up and running. At 8 a.m. on Tuesday morning, LIGO operator Gary Traylor comes off the night shift. “Last night was a total washout,” he says in his soft Southern accent, swiveling in a chair in the brightly lit control room. “There’s a low pressure area moving over the Atlantic that’s causing 20-foot waves to crash into the coast,” Traylor says, and that distant drumming overwhelmed the detector. So in the small hours, LIGO did sense waves. But not the ones everybody is hoping to see.

    See the full article here.

    The American Association for the Advancement of Science is an international non-profit organization dedicated to advancing science for the benefit of all people.

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  • richardmitnick 11:48 am on March 3, 2015 Permalink | Reply
    Tags: , , , Physics   

    From FNAL: “Detecting something with nothing” 

    FNAL Home


    Fermilab is an enduring source of strength for the US contribution to scientific research world wide.

    Tuesday, March 3, 2015
    Lauren Biron

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    From left: Jason Bono (Rice University), Dan Ambrose (University of Minnesota) and Richie Bonventre (Lawrence Berkeley National Laboratory) work on the Mu2e straw chamber tracker unit at Lab 3. Photo: Reidar Hahn

    Researchers are one step closer to finding new physics with the completion of a harp-shaped prototype detector element for the Mu2e experiment.

    FNAL Mu2e experiment
    Mu2e

    Mu2e will look for the conversion of a muon to only an electron (with no other particles emitted) — something predicted but never before seen. This experiment will help scientists better understand how these heavy cousins of the electron decay. A successful sighting would bring us nearer to a unifying theory of the four forces of nature.

    The experiment will be 10,000 times as sensitive as other experiments looking for this conversion, and a crucial part is the detector that will track the whizzing electrons. Researchers want to find one whose sole signature is its energy of 105 MeV, indicating that it is the product of the elusive muon decay.

    In order to measure the electron, scientists track the helical path it takes through the detector. But there’s a catch. Every interaction with detector material skews the path of the electron slightly, disturbing the measurement. The challenge for Mu2e designers is thus to make a detector with as little material as possible, says Mu2e scientist Vadim Rusu.

    “You want to detect the electron with nothing — and this is as close to nothing as we can get,” he said.

    So how to detect the invisible using as little as possible? That’s where the Mu2e tracker design comes in. Panels made of thin straws of metalized Mylar, each only 15 microns thick, will sit inside a cylindrical magnet. Rusu says that these are the thinnest straws that people have ever used in a particle physics experiment.

    These straws, filled with a combination of argon and carbon dioxide gas and threaded with a thin wire, will wait in vacuum for the electrons. Circuit boards placed on both ends of the straws will gather the electrical signal produced when electrons hit the gas inside the straw. Scientists will measure the arrival times at each end of the wire to help accurately plot the electron’s overall trajectory.

    “This is another tricky thing that very few have attempted in the past,” Rusu said.

    The group working on the Mu2e tracker electronics have also created the tiny, low-power circuit boards that will sit at the end of each straw. With limited space to run cooling lines, necessary features that whisk away heat that would otherwise sit in the vacuum, the electronics needed to be as cool and small as possible.

    “We actually spent a lot of time designing very low-power electronics,” Rusu said.

    This first prototype, which researchers began putting together in October, gives scientists a chance to work out kinks, improve design and assembly procedures, and develop the necessary components.

    One lesson already learned? Machining curved metal with elongated holes that can properly hold the straws is difficult and expensive. The solution? Using 3-D printing to make a high-tech, transparent plastic version instead.

    Researchers also came up with a system to properly stretch the straws into place. While running a current through the straw, they use a magnet to pluck the straw — just like strumming a guitar string — and measure the vibration. This lets them set the proper tension that will keep the straw straight throughout the lifetime of the experiment.

    Although the first prototype of the tracker is complete, scientists are already hard at work on a second version (using the 3D-printed plastic), which should be ready in June or July. The prototype will then be tested for leaks and to see if the electronics pick up and transmit signals properly.

    A recent review of Mu2e went well, and Rusu expects work on the tracker construction to begin in 2016.

    See the full article here.

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    Fermilab Campus

    Fermi National Accelerator Laboratory (Fermilab), located just outside Batavia, Illinois, near Chicago, is a US Department of Energy national laboratory specializing in high-energy particle physics.

     
  • richardmitnick 8:39 am on March 3, 2015 Permalink | Reply
    Tags: , Physics,   

    From AAAS: “A step closer to explaining high-temperature superconductivity?” 

    AAAS

    AAAS

    27 February 2015
    Adrian Cho

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    In the new experiment, scientists glimpsed a pattern of up- and down-spinning atoms, which mimics the up-and-down pattern of magnetism seen in high-temperature superconductors. R. A. Hart et al., Nature (2015)

    For years some physicists have been hoping to crack the mystery of high-temperature superconductivity—the ability of some complex materials to carry electricity without resistance at temperatures high above absolute zero—by simulating crystals with patterns of laser light and individual atoms. Now, a team has taken—almost—the next-to-last step in such “optical lattice” simulation by reproducing the pattern of magnetism seen in high-temperature superconductors from which the resistance-free flow of electricity emerges.

    “It’s a very big improvement over previous results,” says Tilman Esslinger, an experimentalist at the Swiss Federal Institute of Technology in Zurich, who was not involved in the work. “It’s very exciting to see steady progress.”

    An optical lattice simulation is essentially a crystal made of light. A real crystal contains a repeating 3D pattern of ions, and electrons flow from ion to ion. In the simulation, spots of laser light replace the ions, and ultracold atoms moving among spots replace the electrons. Physicists can adjust the pattern of spots, how strongly the spots attract the atoms, and how strongly the atoms repel one another. That makes the experiments ideal for probing physics such as high-temperature superconductivity, in which materials such as mercury barium calcium copper oxide carry electricity without resistance at temperatures up to 138 K, far higher above absolute zero than ordinary superconductors such as niobium can.

    Just how the copper-and-oxygen, or cuprate, superconductors work remains unclear. The materials contain planes of copper and oxygen ions with the coppers arranged in a square pattern. Repelling one another, the electrons get stuck in a one-to-a-copper traffic jam called a Mott insulator state. They also spin like tops, and at low temperatures neighboring electrons spin in opposite directions, creating an up-down-up-down pattern of magnetism called antiferromagnetism. Superconductivity sets in when impurities soak up a few electrons and ease the traffic jam. The remaining electrons then pair to glide freely along the planes.

    Theorists do not yet agree how that pairing occurs. Some think that wavelike ripples in the antiferromagnetic pattern act as a glue to attract one electron to the other. Others argue that the pairing arises, paradoxically, from the repulsion among the electrons alone. Theorists can write down a mathematical model of electrons on a checkerboard plane, known as the Fermi-Hubbard model, but it is so hard to “solve” that nobody has been able to show whether it produces superconductivity.

    Experimentalists hope to reproduce the Fermi-Hubbard model in laser light and cold atoms to see if it yields superconductivity. In 2002, Immanuel Bloch, a physicist at the Max Planck Institute for Quantum Optics (MPQ) in Garching, Germany, and colleagues realized a Mott insulator state in an optical lattice. Six years later, Esslinger and colleagues achieved the Mott state with atoms with the right amount of spin to mimic electrons. Now, Randall Hulet, a physicist at Rice University in Houston, Texas, and colleagues have nearly achieved the next-to-last step along the way: antiferromagnetism.

    Hulet and colleagues trapped between 100,000 and 250,000 lithium-6 atoms in laser light. They then ramped up the optical lattice and ramped it back down to put them in order. Shining laser light of a specific wavelength on the atoms, they observed evidence of an emerging up-down-up-down spin pattern. The laser light was redirected, or diffracted, at a particular angle by the rows of atoms—just as x-rays diffract off the ions in a real crystal. Crucially, the light probed the spin of the atoms: The light wave flipped if it bounced off an atom spinning one way but not the other. Without that flipping, the diffraction wouldn’t have occurred, so observation confirms the emergence of the up-down-up-down pattern, Hulet says.

    Hulet’s team solved a problem that has plagued other efforts. Usually, turning the optical lattice on heats the atoms. To avoid that, the researchers added another laser that slightly repelled the atoms, so that the most energetic ones were just barely held by the trap. Then, as the atoms heated, the most energetic ones “evaporated” like steam from hot soup to keep the other ones cool, the researchers report online this week in Nature. They didn’t quite reach a full stable antiferromagnetic pattern: The temperature was 40% too high. But the technique might get there and further, Hulet says. “We don’t have a good sense of what the limit of this method is,” he says. “We could get a factor of two lower, we could get a factor of 10 lower.”

    “It is indeed very promising,” says Tin-Lun “Jason” Ho, a theorist at Ohio State University, Columbus. Reducing the temperature by a factor of two or three might be enough to reach the superconducting state, he says. However, MPQ’s Bloch cautions that it may take still other techniques to get that cold. “There are several cooling techniques that people are developing and interesting experiments coming up,” he says.

    Physicists are also exploring other systems and problems with optical lattices. The approach is still gaining steam, Hulet says: “It’s an exciting time.”

    See the full article here.

    The American Association for the Advancement of Science is an international non-profit organization dedicated to advancing science for the benefit of all people.

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  • richardmitnick 7:54 pm on March 2, 2015 Permalink | Reply
    Tags: , , Light studies, Physics,   

    From EPFL: “The first ever photograph of light as both a particle and wave” 

    EPFL bloc

    Ecole Polytechnique Federale Lausanne

    1

    March 2, 2015

    Light behaves both as a particle and as a wave. Since the days of [Albert] Einstein, scientists have been trying to directly observe both of these aspects of light at the same time. Now, scientists at EPFL have succeeded in capturing the first-ever snapshot of this dual behavior.

    Quantum mechanics tells us that light can behave simultaneously as a particle or a wave. However, there has never been an experiment able to capture both natures of light at the same time; the closest we have come is seeing either wave or particle, but always at different times. Taking a radically different experimental approach, EPFL scientists have now been able to take the first ever snapshot of light behaving both as a wave and as a particle. The breakthrough work is published in Nature Communications.

    When UV light hits a metal surface, it causes an emission of electrons. Albert Einstein explained this “photoelectric” effect by proposing that light – thought to only be a wave – is also a stream of particles. Even though a variety of experiments have successfully observed both the particle- and wave-like behaviors of light, they have never been able to observe both at the same time.

    A research team led by Fabrizio Carbone at EPFL has now carried out an experiment with a clever twist: using electrons to image light. The researchers have captured, for the first time ever, a single snapshot of light behaving simultaneously as both a wave and a stream of particles particle.

    The experiment is set up like this: A pulse of laser light is fired at a tiny metallic nanowire. The laser adds energy to the charged particles in the nanowire, causing them to vibrate. Light travels along this tiny wire in two possible directions, like cars on a highway. When waves traveling in opposite directions meet each other they form a new wave that looks like it is standing in place. Here, this standing wave becomes the source of light for the experiment, radiating around the nanowire.

    This is where the experiment’s trick comes in: The scientists shot a stream of electrons close to the nanowire, using them to image the standing wave of light. As the electrons interacted with the confined light on the nanowire, they either sped up or slowed down. Using the ultrafast microscope to image the position where this change in speed occurred, Carbone’s team could now visualize the standing wave, which acts as a fingerprint of the wave-nature of light.

    While this phenomenon shows the wave-like nature of light, it simultaneously demonstrated its particle aspect as well. As the electrons pass close to the standing wave of light, they “hit” the light’s particles, the photons. As mentioned above, this affects their speed, making them move faster or slower. This change in speed appears as an exchange of energy “packets” (quanta) between electrons and photons. The very occurrence of these energy packets shows that the light on the nanowire behaves as a particle.

    “This experiment demonstrates that, for the first time ever, we can film quantum mechanics – and its paradoxical nature – directly,” says Fabrizio Carbone. In addition, the importance of this pioneering work can extend beyond fundamental science and to future technologies. As Carbone explains: “Being able to image and control quantum phenomena at the nanometer scale like this opens up a new route towards quantum computing.”

    This work represents a collaboration between the Laboratory for Ultrafast Microscopy and Electron Scattering of EPFL, the Department of Physics of Trinity College (US) and the Physical and Life Sciences Directorate of the Lawrence Livermore National Laboratory. The imaging was carried out EPFL’s ultrafast energy-filtered transmission electron microscope – one of the two in the world.

    See the full article here.

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    EPFL is Europe’s most cosmopolitan technical university. It receives students, professors and staff from over 120 nationalities. With both a Swiss and international calling, it is therefore guided by a constant wish to open up; its missions of teaching, research and partnership impact various circles: universities and engineering schools, developing and emerging countries, secondary schools and gymnasiums, industry and economy, political circles and the general public.

     
  • richardmitnick 1:48 pm on March 1, 2015 Permalink | Reply
    Tags: , , , Physics   

    From Perimeter: “Pioneering Women of Physics” 

    Perimeter Institute
    Perimeter Institute

    February 25, 2015

    For more information, contact:
    Lisa Lambert
    Manager, External Relations and Public Affairs
    llambert@perimeterinstitute.ca
    (519) 569-7600 x5051

    They discovered pulsars, found the first evidence of dark matter, pioneered mathematics, radioactivity, nuclear fission, elasticity, and computer programming, and have even stopped light.
    Perimeter celebrates women who made pioneering contributions to physics, often overcoming tremendous challenges to do so.

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    See the full article here.

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    About Perimeter

    Perimeter Institute is a leading centre for scientific research, training and educational outreach in foundational theoretical physics. Founded in 1999 in Waterloo, Ontario, Canada, its mission is to advance our understanding of the universe at the most fundamental level, stimulating the breakthroughs that could transform our future. Perimeter also trains the next generation of physicists through innovative programs, and shares the excitement and wonder of science with students, teachers and the general public.

     
  • richardmitnick 4:32 am on February 25, 2015 Permalink | Reply
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    From phys.org: “How can space travel faster than the speed of light?” 

    physdotorg
    phys.org

    Feb 23, 2015
    Vanessa Janek

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    Light speed is often spoken of as a cosmic speed limit… but not everything plays by these rules. In fact, space itself can expand faster than a photon could ever hope to travel.

    Cosmologists are intellectual time travelers. Looking back over billions of years, these scientists are able to trace the evolution of our Universe in astonishing detail. 13.8 billion years ago, the Big Bang occurred. Fractions of a second later, the fledgling Universe expanded exponentially during an incredibly brief period of time called inflation. Over the ensuing eons, our cosmos has grown to such an enormous size that we can no longer see the other side of it.

    But how can this be? If light’s velocity marks a cosmic speed limit, how can there possibly be regions of spacetime whose photons are forever out of our reach? And even if there are, how do we know that they exist at all?

    The Expanding Universe

    Like everything else in physics, our Universe strives to exist in the lowest possible energy state possible. But around 10-36 seconds after the Big Bang, inflationary cosmologists believe that the cosmos found itself resting instead at a “false vacuum energy” – a low-point that wasn’t really a low-point. Seeking the true nadir of vacuum energy, over a minute fraction of a moment, the Universe is thought to have ballooned by a factor of 1050.

    Since that time, our Universe has continued to expand, but at a much slower pace. We see evidence of this expansion in the light from distant objects. As photons emitted by a star or galaxy propagate across the Universe, the stretching of space causes them to lose energy. Once the photons reach us, their wavelengths have been redshifted in accordance with the distance they have traveled.

    This is why cosmologists speak of redshift as a function of distance in both space and time. The light from these distant objects has been traveling for so long that, when we finally see it, we are seeing the objects as they were billions of years ago.

    The Hubble Volume

    Redshifted light allows us to see objects like galaxies as they existed in the distant past; but we cannot see all events that occurred in our Universe during its history. Because our cosmos is expanding, the light from some objects is simply too far away for us ever to see.

    The physics of that boundary rely, in part, on a chunk of surrounding spacetime called the Hubble volume. Here on Earth, we define the Hubble volume by measuring something called the Hubble parameter (H0), a value that relates the apparent recession speed of distant objects to their redshift. It was first calculated in 1929, when Edwin Hubble discovered that faraway galaxies appeared to be moving away from us at a rate that was proportional to the redshift of their light.

    Dividing the speed of light by H0, we get the Hubble volume. This spherical bubble encloses a region where all objects move away from a central observer at speeds less than the speed of light. Correspondingly, all objects outside of the Hubble volume move away from the center faster than the speed of light.

    Yes, “faster than the speed of light.” How is this possible?

    2
    Two sources of redshift: Doppler and cosmological expansion; modeled after Koupelis & Kuhn. Bottom: Detectors catch the light that is emitted by a central star. This light is stretched, or redshifted, as space expands in between. Credit: Brews Ohare

    The answer has to do with the difference between special relativity and general relativity. Special relativity requires what is called an “inertial reference frame” – more simply, a backdrop. According to this theory, the speed of light is the same when compared in all inertial reference frames. Whether an observer is sitting still on a park bench on planet Earth or zooming past Neptune in a futuristic high-velocity rocketship, the speed of light is always the same. A photon always travels away from the observer at 300,000,000 meters per second, and he or she will never catch up.

    General relativity, however, describes the fabric of spacetime itself. In this theory, there is no inertial reference frame. Spacetime is not expanding with respect to anything outside of itself, so the the speed of light as a limit on its velocity doesn’t apply. Yes, galaxies outside of our Hubble sphere are receding from us faster than the speed of light. But the galaxies themselves aren’t breaking any cosmic speed limits. To an observer within one of those galaxies, nothing violates special relativity at all. It is the space in between us and those galaxies that is rapidly proliferating and stretching exponentially.

    The Observable Universe

    Now for the next bombshell: The Hubble volume is not the same thing as the observable Universe.

    To understand this, consider that as the Universe gets older, distant light has more time to reach our detectors here on Earth. We can see objects that have accelerated beyond our current Hubble volume because the light we see today was emitted when they were within it.

    Strictly speaking, our observable Universe coincides with something called the particle horizon. The particle horizon marks the distance to the farthest light that we can possibly see at this moment in time – photons that have had enough time to either remain within, or catch up to, our gently expanding Hubble sphere.

    And just what is this distance? A little more than 46 billion light years in every direction – giving our observable Universe a diameter of approximately 93 billion light years, or more than 500 billion trillion miles.

    (A quick note: the particle horizon is not the same thing as the cosmological event horizon. The particle horizon encompasses all the events in the past that we can currently see. The cosmological event horizon, on the other hand, defines a distance within which a future observer will be able to see the then-ancient light our little corner of spacetime is emitting today.

    In other words, the particle horizon deals with the distance to past objects whose ancient light that we can see today; the cosmological event horizon deals with the distance that our present-day light that will be able to travel as faraway regions of the Universe accelerate away from us.)

    3
    Fit of redshift velocities to Hubble’s law. Credit: Brews Ohare

    Dark Energy

    Thanks to the expansion of the Universe, there are regions of the cosmos that we will never see, even if we could wait an infinite amount of time for their light to reach us. But what about those areas just beyond the reaches of our present-day Hubble volume? If that sphere is also expanding, will we ever be able to see those boundary objects?

    This depends on which region is expanding faster – the Hubble volume or the parts of the Universe just outside of it. And the answer to that question depends on two things: 1) whether H0 is increasing or decreasing, and 2) whether the Universe is accelerating or decelerating. These two rates are intimately related, but they are not the same.

    In fact, cosmologists believe that we are actually living at a time when H0 is decreasing; but because of dark energy, the velocity of the Universe’s expansion is increasing.

    That may sound counterintuitive, but as long as H0 decreases at a slower rate than that at which the Universe’s expansion velocity is increasing, the overall movement of galaxies away from us still occurs at an accelerated pace. And at this moment in time, cosmologists believe that the Universe’s expansion will outpace the more modest growth of the Hubble volume.

    4
    The observable universe, more technically known as the particle horizon.

    So even though our Hubble volume is expanding, the influence of dark energy appears to provide a hard limit to the ever-increasing observable Universe.

    Our Earthly Limitations

    Cosmologists seem to have a good handle on deep questions like what our observable Universe will someday look like and how the expansion of the cosmos will change. But ultimately, scientists can only theorize the answers to questions about the future based on their present-day understanding of the Universe. Cosmological timescales are so unimaginably long that it is impossible to say much of anything concrete about how the Universe will behave in the future. Today’s models fit the current data remarkably well, but the truth is that none of us will live long enough to see whether the predictions truly match all of the outcomes.

    Disappointing? Sure. But totally worth the effort to help our puny brains consider such mind-bloggling science – a reality that, as usual, is just plain stranger than fiction.

    See the full article here.

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  • richardmitnick 2:23 pm on February 14, 2015 Permalink | Reply
    Tags: , Laser cooling, Physics,   

    From physicsworld: “Physicists reveal new way of cooling large objects with light” 

    physicsworld
    physicsworld.com

    Feb 13, 2015
    Hamish Johnston

    1
    Cold light: Dispersive and dissipative coupling join forces

    A new technique for cooling a macroscopic object with laser light has been demonstrated by a team of physicists in Germany and Russia. Making clever use of the noise in an optical cavity, which normally heats an object up, the technique could lead to the development of “stable optical springs” that would boost the sensitivity of gravitational-wave detectors. It could also be used to create large quantum-mechanical oscillators for studying the quantum properties of macroscopic objects or to create components for quantum computers.

    Physicists already have ways of cooling tiny mirrors by placing them in an optical cavity containing laser light. When the mirror is warm, it vibrates – creating a series of “sidebands” that resonate with light at certain frequencies. The first lower sideband has a frequency equal to the difference between the resonant frequency of the cavity and the vibrational frequency of the mirror. So when a photon at that frequency enters the cavity, it can be absorbed and re-emitted with an extra quantum of vibrational energy. As a result of this “dispersive coupling” process, the mirror cools because energy from it is removed.

    Dispersive coupling works best when the bandwidth of the cavity is much smaller than the vibrational frequency of the mirror. This is possible for relatively small mirrors with vibrational frequencies in the hundreds of megahertz. However, for more massive mirrors with vibrational frequencies in the hundreds of kilohertz, optical cavities with sufficiently narrow bandwidths are simply not available.

    Cooling with noise

    In this latest work, a large object was cooled using a new technique that involves “dissipative coupling” as well as dispersive coupling. Dissipative coupling was first proposed in 2009 by Florian Elste and Aashish Clerk of McGill University and Steven Girvin at Yale University. It makes clever use of quantum “shot noise” in laser light, which would normally be absorbed by the mirror and cause it to heat up.

    However, if the mirror is in an optical cavity and its motion couples to the mirror’s reflectivity in just the right way, then there are two ways that the noise can reach the mirror: it can travel directly from the laser to the mirror or it can bounce around the cavity before driving the mirror. Just as in an interferometer, noise taking these two paths can interfere destructively or constructively.

    Clerk and colleagues realized that the system can be set up so that destructive interference stops this quantum noise from heating the mirror but does not prevent the mirror from losing energy to the noise. The net effect is a strong cooling of the mirror’s motion, which could in principle take the mirror to its quantum ground state. “Unlike standard cavity cooling schemes, this interference doesn’t rely on having a very large mechanical frequency,” explains Clerk – meaning that it can be used to cool large mirrors that have low vibrational frequencies.

    Couplings working together

    In the latest work, Roman Schnabel and colleagues at the Max Planck Institute for Gravitational Physics in Hannover, Moscow State University and the Leibniz University of Hannover have now shown that dissipative and dispersive coupling can work together to cool relatively large mirrors. Based on an idea first proposed by the researchers in 2013, the technique uses a cavity created by a Michelson–Sagnac interferometer (see figure).

    What they have done is to fire laser light at a beamsplitter to create two beams that go off at right angles to each other. These beams then bounce off two mirrors, making their paths form a right-angled triangle. Light from the output port of the interferometer is sent to a “signal-recycling mirror“, or SRM, where some of the light is reflected back into the interferometer and some is sent to a detector. The optical cavity is fine-tuned by adjusting the position of the SRM, while the cavity properties are monitored using a frequency analyser connected to the detector.

    The object to be cooled is a silicon-nitride mirror just 40 nm thick, which is placed at the centre of the cavity. The mirror is about 1.2 mm across, weighs 80 ng and has a fundamental vibrational frequency of 136 kHz. The vibrational motion of the mirror changes not only the resonant frequency of the cavity – leading to the emergence of sidebands and dispersive cooling – but also the bandwidth of the cavity. When the rate of change of the bandwidth is large, energy can be exchanged between the cavity and the mirror. By carefully adjusting the phase between the vibrating mirror and the light, energy alone will flow from the mirror to the cavity, thereby cooling the mirror.

    Sub-kelvin cooling

    The researchers monitored the temperature of the mirror by using the laser light to measure its motion. They found that by using a combination of dispersive and dissipative cooling, they could cool the mirror from room temperature to 126 mK. Commenting on the experiment, Clerk told physicsworld.com that “Schnabel’s is the first experimental system where you have the special kind of dissipative optomechanical coupling that can let you do something truly new”.

    One possible application of the technique is to use it to cool relatively large objects to their quantum ground states of vibration. Such quantum oscillators would comprise billions of atoms and could be used as “Schrödinger’s cats” to study quantum mechanics on a macroscopic scale. Other applications include using such quantum oscillators as components in quantum computers and other quantum-information systems.
    Stabilizing an optical spring

    However, it is not the cooling power of the technique that most interests Schnabel and colleagues. Schnabel told physicsworld.com that the demonstration is a proof-of-principle of their model of how light interacts with an oscillating mirror within a gravitational-wave detector. Their goal is to create a “stable optical spring” whereby a mirror in a huge interferometer undergoes a stable oscillation when laser light is shone on it. A gravitational wave travelling through the mirror would cause a tiny disruption in the oscillation, which would be detected by the interferometer. The problem is that noise in the system heats the mirror and causes it to vibrate erratically. This makes the measurement extremely difficult in existing set-ups.

    “Our goal is to avoid uncontrolled heating of the mirror,” explains Schnabel, who says that the team will now use the model to try to create a stable optical spring using a 100 g pendulum as a mirror in a small interferometer. The ultimate goal of the research is use a mirror of about 40 kg for use in gravitational-wave detectors of the future.

    The research is reported in Physical Review Letters.

    See the full article here.

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    We engage with policymakers and the general public to develop awareness and understanding of the value of physics and, through IOP Publishing, we are world leaders in professional scientific communications.
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  • richardmitnick 1:44 pm on February 14, 2015 Permalink | Reply
    Tags: , , Physics, ,   

    From Rutgers: “Rutgers-Led Research Team Makes Major Stride in Explaining 30-Year-Old ‘Hidden Order’ Physics Mystery” 

    Rutgers University
    Rutgers University

    February 12, 2015

    Findings may lead to new kinds of materials for electronics and superconducting magnets.

    A new explanation for a type of order, or symmetry, in an exotic material made with uranium may lead to enhanced computer displays and data storage systems, and more powerful superconducting magnets for medical imaging and levitating high-speed trains, according to a Rutgers-led team of research physicists.

    The team’s findings are a major step toward explaining a puzzle that physicists worldwide have been struggling with for 30 years, when scientists first noticed a change in the material’s electrical and magnetic properties but were unable to describe it fully. This subtle change occurs when the material is cooled to 17.5 degrees above absolute zero or lower (a bone-chilling minus 428 degrees Fahrenheit).

    1
    Physicists Hsiang-Hsi Kung and Girsh Blumberg with instrumentation they used to examine hidden order. Photo: Carl Blesch

    “This ‘hidden order’ has been the subject of nearly a thousand scientific papers since it was first reported in 1985 at Leiden University in the Netherlands,” said Girsh Blumberg, professor in the Department of Physics and Astronomy in the School of Arts and Sciences.

    Collaborators from Rutgers University, the Los Alamos National Laboratory in New Mexico, and Leiden University published their findings this week in the web-based journal Science Express, which features selected research papers in advance of their appearance in the journal Science. Blumberg and two Rutgers colleagues, graduate student Hsiang-Hsi Kung and professor Kristjan Haule, led the collaboration.

    Changes in order are what make liquid crystals, magnetic materials and superconductors work and perform useful functions. While the Rutgers-led discovery won’t transform high-tech products overnight, this kind of knowledge is vital to ongoing advances in electronic technology.

    “The Los Alamos collaborators produced a crystalline sample of the uranium, ruthenium and silicon compound with unprecedented purity, a breakthrough we needed to make progress in solving the puzzle of hidden order,” said Blumberg. Uranium is commonly known as an element in nuclear reactor fuel or weapons material, but in this case, physicists value it as a heavy metal with electrons that behave differently than those in common metals.

    2
    Below the hidden order temperature of 17.5 degrees Kelvin, uranium electron orbital patterns in adjacent crystal layers become mirror images of each other (right side of illustration). Above that temperature, uranium electron orbitals are the same (left side of illustration).Image: Hsiang-Hsi Kung

    Under these cold conditions, the orbital patterns made by electrons in uranium atoms from adjacent crystal layers become mirror images of each other. Above the hidden order temperature, these electron orbitals are the same. The Rutgers researchers discovered this so-called “broken mirror symmetry” using instrumentation they developed – based on a principle known as Raman scattering – to distinguish the pattern of the mirror images in the electron orbitals.

    Blumberg also credits two theoretical physics professors at Rutgers for predicting the phenomenon that his team discovered.

    “In this field, it’s rare to have such predictive power,” he said, noting that Gabriel Kotliar developed a computational technique that led to the prediction of the hidden order symmetry. Haule and Kotliar applied this technique to predict the changes in electron orbitals that Kung and Blumberg detected.

    At still colder temperatures of 1.5 degrees above absolute zero, the material becomes superconducting – losing all resistance to the flow of electricity. While not practical for today’s products and systems that rely on superconductivity, the material provides new insights into ways that materials can become superconducting.

    3
    Kristjan Haule, left, reviews prediction of hidden order symmetry with Hsiang-Hsi Kung and Girsh Blumberg. Photo: Carl Blesch

    The hidden order puzzle has also been a focus of other Rutgers researchers. Two years ago, professors Premala Chandra and Piers Coleman, along with alumna Rebecca Flint, published another theoretical explanation of the phenomenon in the journal Nature.

    The Leiden University collaborator, John Mydosh, is a member of the laboratory that discovered hidden order in 1985.

    “The work of Blumberg and his team is an important and viable step towards the understanding of hidden order,” Mydosh said. “We are well on our way after 30 years towards the final solution.”

    Working with Kung, Blumberg and Haule at Rutgers were Verner Thorsmølle and Weilu Zhang. The Los Alamos National Laboratory collaborators are Ryan Baumbach and Eric Bauer.

    The research was funded by the National Science Foundation and the U.S. Department of Energy’s Office of Basic Energy Sciences, Division of Materials Sciences and Engineering.

    See the full article here.

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  • richardmitnick 7:39 pm on February 12, 2015 Permalink | Reply
    Tags: , , Physics, Quantum spin Hall effect   

    From phys.org: “Exotic states materialize with supercomputers” 

    physdotorg
    phys.org

    Feb 12, 2015
    Jorge Salazar

    Scientists used supercomputers to find a new class of materials that possess an exotic state of matter known as the quantum spin Hall effect. The researchers published their results in the journal Science in December 2014, where they propose a new type of transistor made from these materials.

    The science team included Ju Li, Liang Fu, Xiaofeng Qian, and Junwei Liu, experts in topological phases of matter and two-dimensional materials research at the Massachusetts Institute of Technology (MIT). They calculated the electronic structures of the materials using the Stampede and Lonestar supercomputers of the Texas Advanced Computing Center.

    Texas Stampede Supercomputer
    Stampede

    U Texas Lonestar supercomputer
    Lonestar

    The computational allocation was made through XSEDE, the Extreme Science and Engineering Discovery Environment, a single virtual system funded by the National Science Foundation (NSF) that scientists use to interactively share computing resources, data and expertise. The study was funded by the U.S. Department of Energy and the NSF.

    “To me, national computing resources like XSEDE, or specifically the Stampede and Lonestar supercomputers, are extremely helpful to computational scientists,” Xiaofeng Qian said. In January 2015, Qian left MIT to join Texas A&M University as the first tenure-track assistant professor at its newly formed Department of Materials Science and Engineering.

    What Qian and colleagues did was purely theoretical work, using Stampede for part of the calculations that modeled the interactions of atoms in the novel materials, two-dimensional transition metal dichalcogenides (TMDC). Qian used the molecular dynamics simulation software Vienna Ab initio Simulation Package to model a unit cell of atoms, the basic building block of the crystal lattice of TMDC.

    1
    Qian and colleagues found that the topological phases in the TMDC materials can be turned on and off by simply applying a vertical electric field that is perpendicular to the atomic plane of the material. That’s shown here in calculations by the red crossing lines that conduct electricity along the edges of the material when the electric field is turned off. When the electric field is turned on the red lines are broken and a black gap appears between the valence and conducting bands of TMDC, which indicate the edges no longer conduct. Credit: Qian et. al.

    2
    This picture tells quite a story to scientists. It’s a portrait of what they call a topological insulator, materials that conduct only at their edges. Technically it shows the edge density of states calculated for a monolayer transition metal dichalcogenide in the 1T’-MoS2 structural phase. There’s a black gap between the purple blobs at the bottom and top. What’s more, there’s crisscrossing reddish lines that bridge the gap. The lines indicate the edge state of the material, allowing electrons to cross the gap and conduct electricity. Credit: Qian et. al.

    “If you look at the unit cell, it’s not large. They are just a few atoms. However, the problem is that we need to predict the band structure of charge carriers in their excited states in the presence of spin coupling as accurately as possible,” Qian said.

    Scientists diagram the electronic band structure of materials to show the energy ranges an electron is allowed, with the band gap showing forbidden zones that basically block the flow of current. Spin coupling accounts for the electromagnetic interactions between electron’s spin and magnetic field generated from the electron’s motion around the nucleus.

    The complexity lies in the details of these interactions, for which Qian applied many-body perturbation theory with the GW approximation, a state-of-the-art first principles method, to calculate the quasiparticle electronic structures for electrons and holes. The ‘G’ is short for Green’s Function and ‘W’ for screened Coulomb interaction, Qian explained.

    This diagram illustrates the concept behind the MIT team’s vision of a new kind of electronic device based on 2-D materials. The 2-D material is at the middle of a layered “sandwich,” with layers of another material, boron nitride, at top and bottom (shown in gray). When an electric field is applied to the material, by way of the rectangular areas at top, it switches the quantum state of the middle layer (yellow areas). The boundaries of these “switched” regions act as perfect quantum wires, potentially leading to new electronic devices with low losses. (Credit: Yan Lian, MIT.) “In order to carry out these calculations to obtain reasonable convergence in the results, we have to use 96 cores, sometimes even more,” Qian said. “And then we need them for 24 hours. The Stampede computer is very efficient and powerful. The work that we have been showing is not just one material; we have several other materials as well as different conditions. In this sense, access to the resources, especially Stampede, is very helpful to our project.”

    The big picture for Qian and his colleagues is the hunt for new kinds of materials with extraordinarily useful properties. Their target is room-temperature quantum spin Hall insulators, which are basically near-two-dimensional materials that block current flow everywhere except along their edges. “Along the edges you have the so-called spin up electron flow in one direction, and at the same time you have spin down electrons and flows away in the opposite direction,” Qian explained. “Basically, you can imagine, by controlling the injection of charge carriers, one can come up with spintronics, or electronics.”

    The scientists in this work proposed a topological field-effect transistor, made of sheets of hexagonal boron interlaced with sheets of TMDC. “We found a very convenient method to control the topological phase transition in these quantum spin Hall interlayers,” Qian said. “This is very important because once we have this capability to control the phase transition, we can design some electronic devices that can be controlled easily through electrical fields.”

    Qian stressed that this work lays the theoretical ground for future real experiments in the lab. He hopes it might develop into an actual transistor suitable for a quantum computer, basically an as-yet-unrealized machine that manipulates data beyond just the binary of ones and zeros.

    “So far, we haven’t looked into the detailed applications for quantum computing yet,” Qian said. “However, it is possible to combine these materials with superconductors and come up with the so-called Majorana fermion zero mode for quantum computing.”

    Read more at: http://phys.org/news/2015-02-exotic-states-materialize-supercomputers.html#jCp

    See the full article here.

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    About Phys.org in 100 Words

    Phys.org™ (formerly Physorg.com) is a leading web-based science, research and technology news service which covers a full range of topics. These include physics, earth science, medicine, nanotechnology, electronics, space, biology, chemistry, computer sciences, engineering, mathematics and other sciences and technologies. Launched in 2004, Phys.org’s readership has grown steadily to include 1.75 million scientists, researchers, and engineers every month. Phys.org publishes approximately 100 quality articles every day, offering some of the most comprehensive coverage of sci-tech developments world-wide. Quancast 2009 includes Phys.org in its list of the Global Top 2,000 Websites. Phys.org community members enjoy access to many personalized features such as social networking, a personal home page set-up, RSS/XML feeds, article comments and ranking, the ability to save favorite articles, a daily newsletter, and other options.

     
  • richardmitnick 5:34 am on February 12, 2015 Permalink | Reply
    Tags: , , CERN COMPASS, Physics   

    From phys.org: “Experiment brings precision to a cornerstone of particle physics” 

    physdotorg
    phys.org

    Feb 11, 2015

    1

    In a paper published yesterday in the journal Physical Review Letters, the COMPASS experiment at CERN reports a key measurement on the strong interaction. The strong interaction binds quarks into protons and neutrons, and protons and neutrons into the nuclei of all the elements from which matter is built.

    2
    The nucleus of a Helium atom. The two protons have the same charge but still stay together due to the residual nuclear force

    Inside those nuclei, particles called pions made up of a quark and an antiquark mediate the interaction. Strong interaction theory makes a precise prediction on the polarisability of pions – the degree to which their shape can be stretched. This polarisability has baffled scientists since the 1980s, when the first measurements appeared to be at odds with the theory. Today’s result is in close agreement with theory.

    “The theory of the strong interaction is one of the cornerstones of our understanding of nature at the level of the fundamental particles,” said Fabienne Kunne and Andrea Bressan, spokespersons of the COMPASS experiment, “so this result, in perfect agreement with the theory, is a very important one.”

    “Despite the high energies available at CERN, the experiment is a big challenge, as the pion polarisability is tiny and its effect hard to isolate,” said Jan Friedrich, researcher at the Technische Universität München and leading scientist in the project.

    Everything we see in the universe is made up of fundamental particles called quarks and leptons. Quarks are bound together in groups of three to make up the building blocks of the nuclei of elements – protons and neutrons. The hydrogen nucleus, for example, consists of a single proton, whereas the nucleus of a gold atom consists of 79 protons and 118 neutrons. Flitting between the protons and neutrons in a nucleus are pions, which mediate the strong force binding the nucleus together. These pions are made up of a quark and an antiquark, themselves held tightly bound by the strong force. This makes their deformability, or polarisability, a direct measure of the strong binding force between the quarks.

    To measure the polarisability of the pion, the COMPASS experiment shot a beam of pions at a target of nickel. As the pions approached the nickel on average at distances twice the radius of the particles themselves, they experienced the very strong electric field of the nickel nucleus, which caused them to deform, and change trajectory, in the process emitting a particle of light called a photon. It is by measuring the photon energy and the deflection of the pion for a large sample of 63000 pions that the polarisability could be measured. The result reveals that the pion is significantly stiffer than shown by previous measurements, as expected from strong interaction theory.

    “This result is admirably complementary to the studies of fundamental interactions performed at the LHC and a testimony to the diversity and strength of CERN’s research programme,” said CERN Director General Rolf Heuer. “While the Higgs boson – proposed by Brout, Englert and Higgs – accounts for the masses of the fundamental particles, thereby allowing composite objects such as us to exist, the bulk of our mass comes from the binding energy of the strong interaction holding them together.”

    See the full article here.

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    Phys.org™ (formerly Physorg.com) is a leading web-based science, research and technology news service which covers a full range of topics. These include physics, earth science, medicine, nanotechnology, electronics, space, biology, chemistry, computer sciences, engineering, mathematics and other sciences and technologies. Launched in 2004, Phys.org’s readership has grown steadily to include 1.75 million scientists, researchers, and engineers every month. Phys.org publishes approximately 100 quality articles every day, offering some of the most comprehensive coverage of sci-tech developments world-wide. Quancast 2009 includes Phys.org in its list of the Global Top 2,000 Websites. Phys.org community members enjoy access to many personalized features such as social networking, a personal home page set-up, RSS/XML feeds, article comments and ranking, the ability to save favorite articles, a daily newsletter, and other options.

     
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