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  • richardmitnick 7:35 am on September 7, 2018 Permalink | Reply
    Tags: , Fish-eye lens may entangle pairs of atoms, , , , Quantum entanglement,   

    From MIT News: “Fish-eye lens may entangle pairs of atoms” 

    MIT News
    MIT Widget

    From MIT News

    September 5, 2018
    Jennifer Chu

    1
    James Maxwell was the first to realize that light is able to travel in perfect circles within the fish-eye lens because the density of the lens changes, with material being thickest at the middle and gradually thinning out toward the edges. No image credit.

    Scientists find a theoretical optical device may have uses in quantum computing.

    Nearly 150 years ago, the physicist James Maxwell proposed that a circular lens that is thickest at its center, and that gradually thins out at its edges, should exhibit some fascinating optical behavior. Namely, when light is shone through such a lens, it should travel around in perfect circles, creating highly unusual, curved paths of light.

    He also noted that such a lens, at least broadly speaking, resembles the eye of a fish. The lens configuration he devised has since been known in physics as Maxwell’s fish-eye lens — a theoretical construct that is only slightly similar to commercially available fish-eye lenses for cameras and telescopes.

    Now scientists at MIT and Harvard University have for the first time studied this unique, theoretical lens from a quantum mechanical perspective, to see how individual atoms and photons may behave within the lens. In a study published Wednesday in Physical Review A, they report that the unique configuration of the fish-eye lens enables it to guide single photons through the lens, in such a way as to entangle pairs of atoms, even over relatively long distances.

    Entanglement is a quantum phenomenon in which the properties of one particle are linked, or correlated, with those of another particle, even over vast distances. The team’s findings suggest that fish-eye lenses may be a promising vehicle for entangling atoms and other quantum bits, which are the necessary building blocks for designing quantum computers.

    “We found that the fish-eye lens has something that no other two-dimensional device has, which is maintaining this entangling ability over large distances, not just for two atoms, but for multiple pairs of distant atoms,” says first author Janos Perczel, a graduate student in MIT’s Department of Physics. “Entanglement and connecting these various quantum bits can be really the name of the game in making a push forward and trying to find applications of quantum mechanics.”

    The team also found that the fish-eye lens, contrary to recent claims, does not produce a perfect image. Scientists have thought that Maxwell’s fish-eye may be a candidate for a “perfect lens” — a lens that can go beyond the diffraction limit, meaning that it can focus light to a point that is smaller than the light’s own wavelength. This perfect imaging, scientist predict, should produce an image with essentially unlimited resolution and extreme clarity.

    However, by modeling the behavior of photons through a simulated fish-eye lens, at the quantum level, Perczel and his colleagues concluded that it cannot produce a perfect image, as originally predicted.

    “This tells you that there are these limits in physics that are really difficult to break,” Perczel says. “Even in this system, which seemed to be a perfect candidate, this limit seems to be obeyed. Perhaps perfect imaging may still be possible with the fish eye in some other, more complicated way, but not as originally proposed.”

    Perczel’s co-authors on the paper are Peter Komar and Mikhail Lukin from Harvard University.

    A circular path

    Maxwell was the first to realize that light is able to travel in perfect circles within the fish-eye lens because the density of the lens changes, with material being thickest at the middle and gradually thinning out toward the edges. The denser a material, the slower light moves through it. This explains the optical effect when a straw is placed in a glass half full of water. Because the water is so much denser than the air above it, light suddenly moves more slowly, bending as it travels through water and creating an image that looks as if the straw is disjointed.

    In the theoretical fish-eye lens, the differences in density are much more gradual and are distributed in a circular pattern, in such a way that it curves rather bends light, guiding light in perfect circles within the lens.

    In 2009, Ulf Leonhardt, a physicist at the Weizmann Institute of Science in Israel was studying the optical properties of Maxwell’s fish-eye lens and observed that, when photons are released through the lens from a single point source, the light travels in perfect circles through the lens and collects at a single point at the opposite end, with very little loss of light.

    “None of the light rays wander off in unwanted directions,” Perczel says. “Everything follows a perfect trajectory, and all the light will meet at the same time at the same spot.”

    Leonhardt, in reporting his results, made a brief mention as to whether the fish-eye lens’ single-point focus might be useful in precisely entangling pairs of atoms at opposite ends of the lens.

    “Mikhail [Lukin] asked him whether he had worked out the answer, and he said he hadn’t,” Perczel says. “That’s how we started this project and started digging deeper into how well this entangling operation works within the fish-eye lens.”

    Playing photon ping-pong

    To investigate the quantum potential of the fish-eye lens, the researchers modeled the lens as the simplest possible system, consisting of two atoms, one at either end of a two-dimensional fish-eye lens, and a single photon, aimed at the first atom. Using established equations of quantum mechanics, the team tracked the photon at any given point in time as it traveled through the lens, and calculated the state of both atoms and their energy levels through time.

    They found that when a single photon is shone through the lens, it is temporarily absorbed by an atom at one end of the lens. It then circles through the lens, to the second atom at the precise opposite end of the lens. This second atom momentarily absorbs the photon before sending it back through the lens, where the light collects precisely back on the first atom.

    “The photon is bounced back and forth, and the atoms are basically playing ping pong,” Perczel says. “Initially only one of the atoms has the photon, and then the other one. But between these two extremes, there’s a point where both of them kind of have it. It’s this mind-blowing quantum mechanics idea of entanglement, where the photon is completely shared equally between the two atoms.”

    Perczel says that the photon is able to entangle the atoms because of the unique geometry of the fish-eye lens. The lens’ density is distributed in such a way that it guides light in a perfectly circular pattern and can cause even a single photon to bounce back and forth between two precise points along a circular path.

    “If the photon just flew away in all directions, there wouldn’t be any entanglement,” Perczel says. “But the fish-eye gives this total control over the light rays, so you have an entangled system over long distances, which is a precious quantum system that you can use.”

    As they increased the size of the fish-eye lens in their model, the atoms remained entangled, even over relatively large distances of tens of microns. They also observed that, even if some light escaped the lens, the atoms were able to share enough of a photon’s energy to remain entangled. Finally, as they placed more pairs of atoms in the lens, opposite to one another, along with corresponding photons, these atoms also became simultaneously entangled.

    “You can use the fish eye to entangle multiple pairs of atoms at a time, which is what makes it useful and promising,” Perczel says.

    Fishy secrets

    In modeling the behavior of photons and atoms in the fish-eye lens, the researchers also found that, as light collected on the opposite end of the lens, it did so within an area that was larger than the wavelength of the photon’s light, meaning that the lens likely cannot produce a perfect image.

    “We can precisely ask the question during this photon exchange, what’s the size of the spot to which the photon gets recollected? And we found that it’s comparable to the wavelength of the photon, and not smaller,” Perczel says. “Perfect imaging would imply it would focus on an infinitely sharp spot. However, that is not what our quantum mechanical calculations showed us.”

    Going forward, the team hopes to work with experimentalists to test the quantum behaviors they observed in their modeling. In fact, in their paper, the team also briefly proposes a way to design a fish-eye lens for quantum entanglement experiments.

    “The fish-eye lens still has its secrets, and remarkable physics buried in it,” Perczel says. “But now it’s making an appearance in quantum technologies where it turns out this lens could be really useful for entangling distant quantum bits, which is the basic building block for building any useful quantum computer or quantum information processing device.”

    See the full article here .


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  • richardmitnick 9:49 am on August 20, 2018 Permalink | Reply
    Tags: , Quantum entanglement, ,   

    From MIT News: “Light from ancient quasars helps confirm quantum entanglement” 

    MIT News
    MIT Widget

    From MIT News

    August 19, 2018
    Jennifer Chu

    1
    The quasar dates back to less than one billion years after the big bang. Image: NASA/ESA/G.Bacon, STScI

    2
    Courtesy of the researchers.

    Results are among the strongest evidence yet for “spooky action at a distance.”

    3
    One of the two units for mobile receiving station entangled photons, which was operated by quantum physics Academy of Sciences Austria (OeAW) La Palma to measure the quantum entanglement. Copyright: Dominick Rauch / OeAW

    Last year, physicists at MIT, the University of Vienna, and elsewhere provided strong support for quantum entanglement, the seemingly far-out idea that two particles, no matter how distant from each other in space and time, can be inextricably linked, in a way that defies the rules of classical physics.

    Take, for instance, two particles sitting on opposite edges of the universe. If they are truly entangled, then according to the theory of quantum mechanics their physical properties should be related in such a way that any measurement made on one particle should instantly convey information about any future measurement outcome of the other particle — correlations that Einstein skeptically saw as “spooky action at a distance.”

    In the 1960s, the physicist John Bell calculated a theoretical limit beyond which such correlations must have a quantum, rather than a classical, explanation.

    But what if such correlations were the result not of quantum entanglement, but of some other hidden, classical explanation? Such “what-ifs” are known to physicists as loopholes to tests of Bell’s inequality, the most stubborn of which is the “freedom-of-choice” loophole: the possibility that some hidden, classical variable may influence the measurement that an experimenter chooses to perform on an entangled particle, making the outcome look quantumly correlated when in fact it isn’t.

    Last February, the MIT team and their colleagues significantly constrained [Physical Review Letters] the freedom-of-choice loophole, by using 600-year-old starlight to decide what properties of two entangled photons to measure. Their experiment proved that, if a classical mechanism caused the correlations they observed, it would have to have been set in motion more than 600 years ago, before the stars’ light was first emitted and long before the actual experiment was even conceived.

    Now, in a paper published today in Physical Review Letters, the same team has vastly extended the case for quantum entanglement and further restricted the options for the freedom-of-choice loophole. The researchers used distant quasars, one of which emitted its light 7.8 billion years ago and the other 12.2 billion years ago, to determine the measurements to be made on pairs of entangled photons. They found correlations among more than 30,000 pairs of photons, to a degree that far exceeded the limit that Bell originally calculated for a classically based mechanism.

    “If some conspiracy is happening to simulate quantum mechanics by a mechanism that is actually classical, that mechanism would have had to begin its operations — somehow knowing exactly when, where, and how this experiment was going to be done — at least 7.8 billion years ago. That seems incredibly implausible, so we have very strong evidence that quantum mechanics is the right explanation,” says co-author Alan Guth, the Victor F. Weisskopf Professor of Physics at MIT.

    “The Earth is about 4.5 billion years old, so any alternative mechanism — different from quantum mechanics — that might have produced our results by exploiting this loophole would’ve had to be in place long before even there was a planet Earth, let alone an MIT,” adds David Kaiser, the Germeshausen Professor of the History of Science and professor of physics at MIT. “So we’ve pushed any alternative explanations back to very early in cosmic history.”

    Guth and Kaiser’s co-authors include Anton Zeilinger and members of his group at the Austrian Academy of Sciences and the University of Vienna, as well as physicists at Harvey Mudd College and the University of California at San Diego.

    A decision, made billions of years ago

    In 2014, Kaiser and two members of the current team, Jason Gallicchio and Andrew Friedman, proposed an experiment to produce entangled photons on Earth — a process that is fairly standard in studies of quantum mechanics. They planned to shoot each member of the entangled pair in opposite directions, toward light detectors that would also make a measurement of each photon using a polarizer. Researchers would measure the polarization, or orientation, of each incoming photon’s electric field, by setting the polarizer at various angles and observing whether the photons passed through — an outcome for each photon that researchers could compare to determine whether the particles showed the hallmark correlations predicted by quantum mechanics.

    The team added a unique step to the proposed experiment, which was to use light from ancient, distant astronomical sources, such as stars and quasars, to determine the angle at which to set each respective polarizer. As each entangled photon was in flight, heading toward its detector at the speed of light, researchers would use a telescope located at each detector site to measure the wavelength of a quasar’s incoming light. If that light was redder than some reference wavelength, the polarizer would tilt at a certain angle to make a specific measurement of the incoming entangled photon — a measurement choice that was determined by the quasar. If the quasar’s light was bluer than the reference wavelength, the polarizer would tilt at a different angle, performing a different measurement of the entangled photon.

    In their previous experiment, the team used small backyard telescopes to measure the light from stars as close as 600 light years away. In their new study, the researchers used much larger, more powerful telescopes to catch the incoming light from even more ancient, distant astrophysical sources: quasars whose light has been traveling toward the Earth for at least 7.8 billion years — objects that are incredibly far away and yet are so luminous that their light can be observed from Earth.

    Tricky timing

    On Jan. 11, 2018, “the clock had just ticked past midnight local time,” as Kaiser recalls, when about a dozen members of the team gathered on a mountaintop in the Canary Islands and began collecting data from two large, 4-meter-wide telescopes: the William Herschel Telescope and the Telescopio Nazionale Galileo, both situated on the same mountain and separated by about a kilometer.

    One telescope focused on a particular quasar, while the other telescope looked at another quasar in a different patch of the night sky. Meanwhile, researchers at a station located between the two telescopes created pairs of entangled photons and beamed particles from each pair in opposite directions toward each telescope.

    In the fraction of a second before each entangled photon reached its detector, the instrumentation determined whether a single photon arriving from the quasar was more red or blue, a measurement that then automatically adjusted the angle of a polarizer that ultimately received and detected the incoming entangled photon.

    “The timing is very tricky,” Kaiser says. “Everything has to happen within very tight windows, updating every microsecond or so.”

    Demystifying a mirage

    The researchers ran their experiment twice, each for around 15 minutes and with two different pairs of quasars. For each run, they measured 17,663 and 12,420 pairs of entangled photons, respectively. Within hours of closing the telescope domes and looking through preliminary data, the team could tell there were strong correlations among the photon pairs, beyond the limit that Bell calculated, indicating that the photons were correlated in a quantum-mechanical manner.

    Guth led a more detailed analysis to calculate the chance, however slight, that a classical mechanism might have produced the correlations the team observed.

    He calculated that, for the best of the two runs, the probability that a mechanism based on classical physics could have achieved the observed correlation was about 10 to the minus 20 — that is, about one part in one hundred billion billion, “outrageously small,” Guth says. For comparison, researchers have estimated the probability that the discovery of the Higgs boson was just a chance fluke to be about one in a billion.

    “We certainly made it unbelievably implausible that a local realistic theory could be underlying the physics of the universe,” Guth says.

    And yet, there is still a small opening for the freedom-of-choice loophole. To limit it even further, the team is entertaining ideas of looking even further back in time, to use sources such as cosmic microwave background photons that were emitted as leftover radiation immediately following the Big Bang, though such experiments would present a host of new technical challenges.

    “It is fun to think about new types of experiments we can design in the future, but for now, we are very pleased that we were able to address this particular loophole so dramatically. Our experiment with quasars puts extremely tight constraints on various alternatives to quantum mechanics. As strange as quantum mechanics may seem, it continues to match every experimental test we can devise,” Kaiser says.

    This research was supported in part by the Austrian Academy of Sciences, the Austrian Science Fund, the U.S. National Science Foundation, and the U.S. Department of Energy.

    See the full article here .


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  • richardmitnick 8:19 am on July 9, 2018 Permalink | Reply
    Tags: , , , , Quantum entanglement   

    From Quanta Magazine: “Physicists Find a Way to See the ‘Grin’ of Quantum Gravity” 

    Quanta Magazine
    From Quanta Magazine

    March 6, 2018
    Natalie Wolchover

    Re-released 7.8.18

    A recently proposed experiment would confirm that gravity is a quantum force.

    1
    Two microdiamonds would be used to test the quantum nature of gravity. Olena Shmahalo/Quanta Magazine

    In 1935, when both quantum mechanics and Albert Einstein’s general theory of relativity were young, a little-known Soviet physicist named Matvei Bronstein, just 28 himself, made the first detailed study of the problem of reconciling the two in a quantum theory of gravity. This “possible theory of the world as a whole,” as Bronstein called it, would supplant Einstein’s classical description of gravity, which casts it as curves in the space-time continuum, and rewrite it in the same quantum language as the rest of physics.

    Bronstein figured out how to describe gravity in terms of quantized particles, now called gravitons, but only when the force of gravity is weak — that is (in general relativity), when the space-time fabric is so weakly curved that it can be approximated as flat. When gravity is strong, “the situation is quite different,” he wrote. “Without a deep revision of classical notions, it seems hardly possible to extend the quantum theory of gravity also to this domain.”

    His words were prophetic. Eighty-three years later, physicists are still trying to understand how space-time curvature emerges on macroscopic scales from a more fundamental, presumably quantum picture of gravity; it’s arguably the deepest question in physics.

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    To Solve the Biggest Mystery in Physics, Join Two Kinds of Law. Robbert Dijkgraaf . James O’Brien for Quanta Magazine.Reductionism breaks the world into elementary building blocks. Emergence finds the simple laws that arise out of complexity. These two complementary ways of viewing the universe come together in modern theories of quantum gravity. September 7, 2017

    Perhaps, given the chance, the whip-smart Bronstein might have helped to speed things along. Aside from quantum gravity, he contributed to astrophysics and cosmology, semiconductor theory, and quantum electrodynamics, and he also wrote several science books for children, before being caught up in Stalin’s Great Purge and executed in 1938, at the age of 31.

    The search for the full theory of quantum gravity has been stymied by the fact that gravity’s quantum properties never seem to manifest in actual experience. Physicists never get to see how Einstein’s description of the smooth space-time continuum, or Bronstein’s quantum approximation of it when it’s weakly curved, goes wrong.

    The problem is gravity’s extreme weakness. Whereas the quantized particles that convey the strong, weak and electromagnetic forces are so powerful that they tightly bind matter into atoms, and can be studied in tabletop experiments, gravitons are individually so weak that laboratories have no hope of detecting them. To detect a graviton with high probability, a particle detector would have to be so huge and massive that it would collapse into a black hole. This weakness is why it takes an astronomical accumulation of mass to gravitationally influence other massive bodies, and why we only see gravity writ large.

    Not only that, but the universe appears to be governed by a kind of cosmic censorship: Regions of extreme gravity — where space-time curves so sharply that Einstein’s equations malfunction and the true, quantum nature of gravity and space-time must be revealed — always hide behind the horizons of black holes.

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    Mike Zeng for Quanta Magazine. Where Gravity Is Weak and Naked Singularities Are Verboten. Natalie Wolchover Recent calculations tie together two conjectures about gravity, potentially revealing new truths about its elusive quantum nature.

    “Even a few years ago it was a generic consensus that, most likely, it’s not even conceivably possible to measure quantization of the gravitational field in any way,” said Igor Pikovski, a theoretical physicist at Harvard University.

    Now, a pair of papers recently published in Physical Review Letters has changed the calculus.

    Spin Entanglement Witness for Quantum Gravity https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.119.240401
    Gravitationally Induced Entanglement between Two Massive Particles is Sufficient Evidence of Quantum Effects in Gravity https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.119.240402

    The papers contend that it’s possible to access quantum gravity after all — while learning nothing about it. The papers, written by Sougato Bose at University College London and nine collaborators and by Chiara Marletto and Vlatko Vedral at the University of Oxford, propose a technically challenging, but feasible, tabletop experiment that could confirm that gravity is a quantum force like all the rest, without ever detecting a graviton. Miles Blencowe, a quantum physicist at Dartmouth College who was not involved in the work, said the experiment would detect a sure sign of otherwise invisible quantum gravity — the “grin of the Cheshire cat.”

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    A levitating microdiamond (green dot) in Gavin Morley’s lab at the University of Warwick, in front of the lens used to trap the diamond with light. Gavin W Morley

    The proposed experiment will determine whether two objects — Bose’s group plans to use a pair of microdiamonds — can become quantum-mechanically entangled with each other through their mutual gravitational attraction. Entanglement is a quantum phenomenon in which particles become inseparably entwined, sharing a single physical description that specifies their possible combined states. (The coexistence of different possible states, called a “superposition,” is the hallmark of quantum systems.) For example, an entangled pair of particles might exist in a superposition in which there’s a 50 percent chance that the “spin” of particle A points upward and B’s points downward, and a 50 percent chance of the reverse. There’s no telling in advance which outcome you’ll get when you measure the particles’ spin directions, but you can be sure they’ll point opposite ways.

    The authors argue that the two objects in their proposed experiment can become entangled with each other in this way only if the force that acts between them — in this case, gravity — is a quantum interaction, mediated by gravitons that can maintain quantum superpositions. “If you can do the experiment and you get entanglement, then according to those papers, you have to conclude that gravity is quantized,” Blencowe explained.

    To Entangle a Diamond

    Quantum gravity is so imperceptible that some researchers have questioned whether it even exists. The venerable mathematical physicist Freeman Dyson, 94, has argued since 2001 that the universe might sustain a kind of “dualistic” description, where “the gravitational field described by Einstein’s theory of general relativity is a purely classical field without any quantum behavior,” as he wrote that year in The New York Review of Books, even though all the matter within this smooth space-time continuum is quantized into particles that obey probabilistic rules.

    Dyson, who helped develop quantum electrodynamics (the theory of interactions beween matter and light) and is professor emeritus at the Institute for Advanced Study in Princeton, New Jersey, where he overlapped with Einstein, disagrees with the argument that quantum gravity is needed to describe the unreachable interiors of black holes. And he wonders whether detecting the hypothetical graviton might be impossible, even in principle. In that case, he argues, quantum gravity is metaphysical, rather than physics.

    He is not the only skeptic. The renowned British physicist Sir Roger Penrose and, independently, the Hungarian researcher Lajos Diósi have hypothesized that space-time cannot maintain superpositions. They argue that its smooth, solid, fundamentally classical nature prevents it from curving in two different possible ways at once — and that its rigidity is exactly what causes superpositions of quantum systems like electrons and photons to collapse. This “gravitational decoherence,” in their view, gives rise to the single, rock-solid, classical reality experienced at macroscopic scales.

    The ability to detect the “grin” of quantum gravity would seem to refute Dyson’s argument. It would also kill the gravitational decoherence theory, by showing that gravity and space-time do maintain quantum superpositions.

    Bose’s and Marletto’s proposals appeared simultaneously mostly by chance, though experts said they reflect the zeitgeist. Experimental quantum physics labs around the world are putting ever-larger microscopic objects into quantum superpositions and streamlining protocols for testing whether two quantum systems are entangled. The proposed experiment will have to combine these procedures while requiring further improvements in scale and sensitivity; it could take a decade or more to pull it off. “But there are no physical roadblocks,” said Pikovski, who also studies how laboratory experiments might probe gravitational phenomena. “I think it’s challenging, but I don’t think it’s impossible.”

    The plan is laid out in greater detail in the paper by Bose and co-authors — an Ocean’s Eleven cast of experts for different steps of the proposal. In his lab at the University of Warwick, for instance, co-author Gavin Morley is working on step one, attempting to put a microdiamond in a quantum superposition of two locations. To do this, he’ll embed a nitrogen atom in the microdiamond, next to a vacancy in the diamond’s structure, and zap it with a microwave pulse. An electron orbiting the nitrogen-vacancy system both absorbs the light and doesn’t, and the system enters a quantum superposition of two spin directions — up and down — like a spinning top that has some probability of spinning clockwise and some chance of spinning counterclockwise. The microdiamond, laden with this superposed spin, is subjected to a magnetic field, which makes up-spins move left while down-spins go right. The diamond itself therefore splits into a superposition of two trajectories.

    In the full experiment, the researchers must do all this to two diamonds — a blue one and a red one, say — suspended next to each other inside an ultracold vacuum. When the trap holding them is switched off, the two microdiamonds, each in a superposition of two locations, fall vertically through the vacuum. As they fall, the diamonds feel each other’s gravity. But how strong is their gravitational attraction?

    If gravity is a quantum interaction, then the answer is: It depends. Each component of the blue diamond’s superposition will experience a stronger or weaker gravitational attraction to the red diamond, depending on whether the latter is in the branch of its superposition that’s closer or farther away. And the gravity felt by each component of the red diamond’s superposition similarly depends on where the blue diamond is.

    In each case, the different degrees of gravitational attraction affect the evolving components of the diamonds’ superpositions. The two diamonds become interdependent, meaning that their states can only be specified in combination — if this, then that — so that, in the end, the spin directions of their two nitrogen-vacancy systems will be correlated.

    3
    Lucy Reading-Ikkanda/Quanta Magazine

    After the microdiamonds have fallen side by side for about three seconds — enough time to become entangled by each other’s gravity — they then pass through another magnetic field that brings the branches of each superposition back together. The last step of the experiment is an “entanglement witness” protocol developed by the Dutch physicist Barbara Terhal and others: The blue and red diamonds enter separate devices that measure the spin directions of their nitrogen-vacancy systems. (Measurement causes superpositions to collapse into definite states.) The two outcomes are then compared. By running the whole experiment over and over and comparing many pairs of spin measurements, the researchers can determine whether the spins of the two quantum systems are correlated with each other more often than a known upper bound for objects that aren’t quantum-mechanically entangled. In that case, it would follow that gravity does entangle the diamonds and can sustain superpositions.

    “What’s beautiful about the arguments is that you don’t really need to know what the quantum theory is, specifically,” Blencowe said. “All you have to say is there has to be some quantum aspect to this field that mediates the force between the two particles.”

    Technical challenges abound. The largest object that’s been put in a superposition of two locations before is an 800-atom molecule. Each microdiamond contains more than 100 billion carbon atoms — enough to muster a sufficient gravitational force. Unearthing its quantum-mechanical character will require colder temperatures, a higher vacuum and finer control. “So much of the work is getting this initial superposition up and running,” said Peter Barker, a member of the experimental team based at UCL who is improving methods for laser-cooling and trapping the microdiamonds. If it can be done with one diamond, Bose added, “then two doesn’t make much of a difference.”

    Why Gravity Is Unique

    Quantum gravity researchers do not doubt that gravity is a quantum interaction, capable of inducing entanglement. Certainly, gravity is special in some ways, and there’s much to figure out about the origin of space and time, but quantum mechanics must be involved, they say. “It doesn’t really make much sense to try to have a theory in which the rest of physics is quantum and gravity is classical,” said Daniel Harlow, a quantum gravity researcher at the Massachusetts Institute of Technology. The theoretical arguments against mixed quantum-classical models are strong (though not conclusive).

    On the other hand, theorists have been wrong before, Harlow noted: “So if you can check, why not? If that will shut up these people” — meaning people who question gravity’s quantumness — “that’s great.”

    Dyson wrote in an email, after reading the PRL papers, “The proposed experiment is certainly of great interest and worth performing with real quantum systems.” However, he said the authors’ way of thinking about quantum fields differs from his. “It is not clear to me whether [the experiment] would settle the question whether quantum gravity exists,” he wrote. “The question that I have been asking, whether a single graviton is observable, is a different question and may turn out to have a different answer.”

    In fact, the way Bose, Marletto and their co-authors think about quantized gravity derives from how Bronstein first conceived of it in 1935. (Dyson called Bronstein’s paper “a beautiful piece of work” that he had not seen before.) In particular, Bronstein showed that the weak gravity produced by a small mass can be approximated by Newton’s law of gravity. (This is the force that acts between the microdiamond superpositions.) According to Blencowe, weak quantized-gravity calculations haven’t been developed much, despite being arguably more physically relevant than the physics of black holes or the Big Bang. He hopes the new experimental proposal will spur theorists to find out whether there are any subtle corrections to the Newtonian approximation that future tabletop experiments might be able to probe.

    Leonard Susskind, a prominent quantum gravity and string theorist at Stanford University, saw value in carrying out the proposed experiment because “it provides an observation of gravity in a new range of masses and distances.” But he and other researchers emphasized that microdiamonds cannot reveal anything about the full theory of quantum gravity or space-time. He and his colleagues want to understand what happens at the center of a black hole, and at the moment of the Big Bang.

    Perhaps one clue as to why it is so much harder to quantize gravity than everything else is that other force fields in nature exhibit a feature called “locality”: The quantum particles in one region of the field (photons in the electromagnetic field, for instance) are “independent of the physical entities in some other region of space,” said Mark Van Raamsdonk, a quantum gravity theorist at the University of British Columbia. But “there’s at least a bunch of theoretical evidence that that’s not how gravity works.”

    In the best toy models of quantum gravity (which have space-time geometries that are simpler than those of the real universe), it isn’t possible to assume that the bendy space-time fabric subdivides into independent 3-D pieces, Van Raamsdonk said. Instead, modern theory suggests that the underlying, fundamental constituents of space “are organized more in a 2-D way.” The space-time fabric might be like a hologram, or a video game: “Even though the picture is three-dimensional, the information is stored in some two-dimensional computer chip,” he said. In that case, the 3-D world is illusory in the sense that different parts of it aren’t all that independent. In the video-game analogy, a handful of bits stored in the 2-D chip might encode global features of the game’s universe.

    The distinction matters when you try to construct a quantum theory of gravity. The usual approach to quantizing something is to identify its independent parts — particles, say — and then apply quantum mechanics to them. But if you don’t identify the correct constituents, you get the wrong equations. Directly quantizing 3-D space, as Bronstein did, works to some extent for weak gravity, but the method fails when space-time is highly curved.

    Witnessing the “grin” of quantum gravity would help motivate these abstract lines of reasoning, some experts said. After all, even the most sensible theoretical arguments for the existence of quantum gravity lack the gravitas of experimental facts. When Van Raamsdonk explains his research in a colloquium or conversation, he said, he usually has to start by saying that gravity needs to be reconciled with quantum mechanics because the classical space-time description fails for black holes and the Big Bang, and in thought experiments about particles colliding at unreachably high energies. “But if you could just do this simple experiment and get the result that shows you that the gravitational field was actually in a superposition,” he said, then the reason the classical description falls short would be self-evident: “Because there’s this experiment that suggests gravity is quantum.”

    Correction March 6, 2018: An earlier version of this article referred to Dartmouth University. Despite the fact that Dartmouth has multiple individual schools, including an undergraduate college as well as academic and professional graduate schools, the institution refers to itself as Dartmouth College for historical reasons.

    See the full article here .


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    Formerly known as Simons Science News, Quanta Magazine is an editorially independent online publication launched by the Simons Foundation to enhance public understanding of science. Why Quanta? Albert Einstein called photons “quanta of light.” Our goal is to “illuminate science.” At Quanta Magazine, scientific accuracy is every bit as important as telling a good story. All of our articles are meticulously researched, reported, edited, copy-edited and fact-checked.

     
  • richardmitnick 2:27 pm on June 27, 2018 Permalink | Reply
    Tags: Quantum entanglement,   

    From University of Heidelberg: “Quantum Mechanics: Entanglements In Ultracold Atomic Clouds” 

    U Heidelberg bloc

    From University of Heidelberg

    27 June 2018

    Heidelberg researchers verify non-local correlations in clouds of rubidium atoms.

    1
    Schematic representation of the experimental implementation: A cigar-shaped cloud of rubidium atoms (blue dots) is cooled to ultra-cold temperatures. Due to collisions between atoms, quantum correlations, also called entanglement, build up (yellow compounds). The atomic cloud is finally imaged onto a camera with the aid of laser light. Due to the high spatial resolution of the camera, correlations between different parts (A and B) of the condensate, and in particular their quantum mechanical character, can be detected. Photo: Philipp Kunkel, SynQS

    A system’s state is characterised as entangled or quantum correlated if two or more particles cannot be described as a combination of separate, independent states but only as a whole. Researchers at the Kirchhoff Institute for Physics of Heidelberg University recently succeeded in verifying so-called non-local quantum correlations between ultracold clouds of rubidium atoms. Under the direction of Prof. Dr Markus Oberthaler und Prof. Dr Thomas Gasenzer, the researchers were able to gain important new insights into the character of quantum mechanical many-body systems.

    The correlations that the theory of quantum mechanics predicts are counter-intuitive. These quantum correlations seem to contradict the Heisenberg uncertainty principle, which states that two properties of an object, such as position and speed, can never be precisely determined at the same time. In quantum mechanical systems, however, two particles can be prepared so as to accurately predict the position of particle two by localising the position of particle one. Similarly, measuring the speed of one particle allows predicting the speed of the other. “In this case, the position and speed of particle two do need to be precisely determined prior to measurement,” explains Prof. Oberthaler. “The measurement result for particle one cannot be immediately present at particle two’s position if the two are spatially separate.”

    The uncertainty principle actually does not support this simultaneous determination of position and speed. But in quantum mechanics, two objects are not considered separate if they are correlated, i.e., entangled, hence resolving the apparent contradiction. “If we can prove that measurement results of different observables in one system can actually be predicted by measuring a second, remote system, then we can use this evidence to substantiate entanglement as well – and that’s exactly what we did in our experiment,” states Philipp Kunkel, the study’s primary author.

    In their experiment, the researchers used a cloud of approximately 11,000 rubidium atoms, which they cooled to extremely low temperatures. Using laser light, they kept the atoms suspended in a vacuum chamber, which allowed them to exclude any disturbing effects, such as collisions with air molecules. Because quantum effects are detectable only at very low temperatures, working with ultracold atoms is required. Like when measuring position and speed, these extreme conditions allow the internal state of the particles, often called spin, to be measured as well. “By measuring the spin in one half of the cloud, we were able to predict the spin in the other more accurately than the local uncertainty principle would allow,” explains Philipp Kunkel.

    The characterisation of quantum mechanical many-body systems is important for future applications such as quantum computers and quantum communication, among others. The most recent Heidelberg research results were published in Science.

    See the full article here .

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    Founded in 1386, Heidelberg University, a state university of BadenWürttemberg, is Germany’s oldest university. In continuing its timehonoured tradition as a research university of international standing the Ruprecht-Karls-University’s mission is guided by the following principles:
    Firmly rooted in its history, the University is committed to expanding and disseminating our knowledge about all aspects of humanity and nature through research and education. The University upholds the principle of freedom of research and education, acknowledging its responsibility to humanity, society, and nature.

     
  • richardmitnick 12:50 pm on April 28, 2018 Permalink | Reply
    Tags: , , , , , , , Quantum entanglement, , Thermodynamics   

    From Kavli Institute for the Physics and Mathematics of the Universe: “Study Finds Way to Use Quantum Entanglement to Study Black Holes” 

    KavliFoundation

    The Kavli Foundation

    Kavli IPMU
    Kavli IMPU

    April 23, 2018

    A team of researchers has found a relationship between quantum physics, the study of very tiny phenomena, to thermodynamics, the study of very large phenomena, reports a new study this week in Nature Communications.

    “Our function can describe a variety of systems from quantum states in electrons to, in principle, black holes,” says study author Masataka Watanabe.

    Quantum entanglement is a phenomenon fundamental to quantum mechanics, where two separated regions share the same information. It is invaluable to a variety of applications including being used as a resource in quantum computation, or quantifying the amount of information stored in a black hole.

    Quantum mechanics is known to preserve information, while thermal equilibrium seems to lose some part of it, and so understanding the relationship between these microscopic and macroscopic concepts is important. So a group of graduate students and a researcher at the University of Tokyo, including the Kavli Institute for the Physics and Mathematics of the Universe, investigated the role of the quantum entanglement in thermal equilibrium in an isolated quantum system.

    1
    Figure 1: Graph showing quantum entanglement and spatial distribution. When separating matter A and B, the vertical axis shows how much quantum entanglement there is, while the horizontal axis shows the length of matter A. (Credit: Nakagawa et al.)

    “A pure quantum state stabilizing into thermal equilibrium can be compared to water being poured into a cup. In a quantum-mechanical system, the colliding water molecules create quantum entanglements, and these quantum entanglements will eventually lead a cup of water to thermal equilibrium. However, it has been a challenge to develop a theory which predicts how much quantum entanglement was inside because lots of quantum entanglements are created in complicated manners at thermal equilibrium,” says Watanabe.

    In their study, the team identified a function predicting the spatial distribution of information stored in an equilibrated system, and they revealed that it was determined by thermodynamic entropy alone. Also, by carrying out computer simulations, they found that the spatial distribution remained the same regardless of what systems were used and regardless of how they reached thermal equilibrium.

    See the full article here .

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    Kavli IPMU (Kavli Institute for the Physics and Mathematics of the Universe) is an international research institute with English as its official language. The goal of the institute is to discover the fundamental laws of nature and to understand the Universe from the synergistic perspectives of mathematics, astronomy, and theoretical and experimental physics. The Institute for the Physics and Mathematics of the Universe (IPMU) was established in October 2007 under the World Premier International Research Center Initiative (WPI) of the Ministry of Education, Sports, Science and Technology in Japan with the University of Tokyo as the host institution. IPMU was designated as the first research institute within the University of Tokyo Institutes for Advanced Study (UTIAS) in January 2011. It received an endowment from The Kavli Foundation and was renamed the “Kavli Institute for the Physics and Mathematics of the Universe” in April 2012. Kavli IPMU is located on the Kashiwa campus of the University of Tokyo, and more than half of its full-time scientific members come from outside Japan. http://www.ipmu.jp/
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    The Kavli Foundation, based in Oxnard, California, is dedicated to the goals of advancing science for the benefit of humanity and promoting increased public understanding and support for scientists and their work.

    The Foundation’s mission is implemented through an international program of research institutes, professorships, and symposia in the fields of astrophysics, nanoscience, neuroscience, and theoretical physics as well as prizes in the fields of astrophysics, nanoscience, and neuroscience.

     
  • richardmitnick 10:19 am on April 5, 2018 Permalink | Reply
    Tags: A New State of Quantum Matter Has Been Found in a Material Scientists Thought Was All Chaos, , Photoemission electron microscopy, Quantum entanglement, , , , Shakti geometry, Spin ice   

    From Science Alert: “A New State of Quantum Matter Has Been Found in a Material Scientists Thought Was All Chaos” 

    ScienceAlert

    Science Alert

    5 APR 2018
    MIKE MCRAE

    1
    (enot-poloskun/istock)

    What else is lurking in there?

    Experiments carried out on a complex arrangement of magnetic particles have identified a completely new state of matter, and it can only be explained if scientists turn to quantum physics.

    The messy structures behind the research show strange properties that could allow us to study the chaos of exotic particles – if researchers can find order in there, it could help us understand these particles in greater detail, opening up a whole new landscape for quantum technology.

    Physicists from the US carried out their research on the geometrical arrangements of particles in a weird material known as spin ice.

    Like common old water ice, the particles making up spin ice sort themselves into geometric patterns as the temperature drops.

    There are a number of compounds that can be used to build this kind of material, but they all share the same kind of quantum property – their individual magnetic ‘spin’ sets up a bias in how the particles point to one another, creating complex structures.

    So, unlike the predictable crystalline patterns in water ice, the nanoscale magnetic particles making up spin ice can look disordered and chaotic under certain conditions, flipping back and forth wildly.

    The researchers focussed on one particular structure called a Shakti geometry, and measured how its magnetic arrangements fluctuated with changes in temperature.

    States of matter are usually broken down into categories such as solid, liquid, and gas. We’re taught on a fundamental level that a material’s volume and fluidity can change with shifts in its temperature and pressure.

    But there’s another way to think of a state of matter – by considering the points at which there’s a dramatic change in the way particles arrange themselves as they gain or lose energy.

    For example, the freezing of water is one such dramatic change – a sudden restructuring that occurs as pure water is chilled below 0 degrees Celsius (32 degrees Fahrenheit), where its molecules lose the energy they need to remain free and adopt another stable configuration.

    When researchers slowly lowered the temperature on spin ice arranged in a Shakti geometry, they got it to produce a similar behaviour – one that has never been seen before in other forms of spin ice.

    Using a process called photoemission electron microscopy, the team was then able to image the changes in pattern based on how their electrons emitted light.

    They were noticing points at which a specific arrangement persisted even as the temperature continued to drop.

    “The system gets stuck in a way that it cannot rearrange itself, even though a large-scale rearrangement would allow it to fall to a lower energy state,” says senior researcher Peter Schiffer, currently at Yale University.

    Such a ‘sticking point’ is a hallmark of a state of matter, and one that wasn’t expected in the flip-flopping madness of spin ice.

    Most states of matter can be described fairly efficiently using classical models of thermodynamics, with jiggling particles overcoming binding forces as they swap heat energy.

    In this case there was no clear model describing what was balancing the changes in energy with the material’s stable arrangement.

    So the team applied a quantum touch, looking at how entanglement between particles aligned to give rise to a particular topology, or pattern within a changing space.

    “Our research shows for the first time that classical systems such as artificial spin ice can be designed to demonstrate topological ordered phases, which previously have been found only in quantum conditions,” says physicist Cristiano Nisoli from Los Alamos National Laboratory.

    Ten years ago, quasiparticles that behaved like magnetic monopoles [Nature] were observed in another type of spin ice, also pointing at a weird kind of phase transition.

    Quasiparticles are becoming big things in our search for new kinds of matter that behaves in odd but useful ways, as they have pontential to be used in quantum computing. So having better models for understanding this quantum landscape will no doubt come in handy.

    This research was published in Nature.

    See the full article here .

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  • richardmitnick 2:15 pm on March 12, 2018 Permalink | Reply
    Tags: A possible experiment to prove that gravity and quantum mechanics can be reconciled, , , , , Quantum entanglement,   

    From phys.org: “A possible experiment to prove that gravity and quantum mechanics can be reconciled” 

    physdotorg
    phys.org

    March 12, 2018
    Bob Yirka

    1
    Credit: G. W. Morley/University of Warwick and APS/Alan Stonebraker

    Two teams of researchers working independently of one another have come up with an experiment designed to prove that gravity and quantum mechanics can be reconciled. The first team is a pairing of Chiara Marletto of the University of Oxford and Vlatko Vedral of National University of Singapore. The second is an international collaboration. In the papers, both published in Physical Review Letters, the teams describe their experiment and how it might be carried out Spin Entanglement Witness for Quantum Gravity and Gravitationally Induced Entanglement between Two Massive Particles is Sufficient Evidence of Quantum Effects in Gravity.

    Gravity is a tough nut to crack, there is just no doubt about it. In comparison, the strong, weak and electromagnetic forces are a walk in the park. Scientists still can’t explain the nature of gravity, though how it works is rather well understood. The current best theory regarding gravity goes all the way back to Einstein’s general theory of relativity, but there has been no way to reconcile it with quantum mechanics. Some physicists suggest it could be a particle called the graviton. But proving that such a particle exists has been frustrating, because it would be so weak that it would be very nearly impossible to measure its force. In this new effort, neither team is suggesting that their experiment could reconcile gravity and quantum mechanics. Instead, they are claiming that if such an experiment is successful, it would very nearly prove that it should be possible to do it.

    The experiment essentially involves attempting to entangle two particles using their gravitational attraction as a means of confirming quantum gravity. In practice, it would consist of levitating two tiny diamonds a small distance from one another and putting each of them into a superposition of two spin directions. After that, a magnetic field would be applied to separate the spin components. At this point, a test would be made to see if each of the components is gravitationally attracted. If they are, the researchers contend, that will prove that gravity is quantum; if they are not, then it will not. The experiment would have to run many times to get an accurate assessment. And while a first look might suggest such an experiment could be conducted very soon, the opposite is actually true. The researchers suggest it will likely be a decade before such an experiment could be carried out due to the necessity of improving scale and the sensitivity involved in such an experiment.

    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.

     
  • richardmitnick 8:42 am on March 9, 2018 Permalink | Reply
    Tags: , , MIT’s interdisciplinary Quantum Engineering Group (QEG), , Quantum entanglement, Scientists gain new visibility into quantum information transfer   

    From MIT: “Scientists gain new visibility into quantum information transfer” 

    MIT News

    MIT Widget

    MIT News

    March 8, 2018
    Peter Dunn | Department of Nuclear Science and Engineering

    1
    The NMR spectrometer in the Quantum Engineering Group (QEG) lab. Image: Paola Cappellaro.

    2
    Quantum many-body correlations in a spin chain grow from an initial localized state in the absence of disorder, but are restricted to a finite size by disorder, as measured by the average correlation length. Image: Paola Cappellaro.

    Advance holds promise for “wiring” of quantum computers and other systems, and opens new avenues for understanding basic workings of the quantum realm.

    When we talk about “information technology,” we generally mean the technology part, like computers, networks, and software. But information itself, and its behavior in quantum systems, is a central focus for MIT’s interdisciplinary Quantum Engineering Group (QEG) as it seeks to develop quantum computing and other applications of quantum technology.

    A QEG team has provided unprecedented visibility into the spread of information in large quantum mechanical systems, via a novel measurement methodology and metric described in a new article in Physics Review Letters. The team has been able, for the first time, to measure the spread of correlations among quantum spins in fluorapatite crystal, using an adaptation of room-temperature solid-state nuclear magnetic resonance (NMR) techniques.

    Researchers increasingly believe that a clearer understanding of information spreading is not only essential to understanding the workings of the quantum realm, where classical laws of physics often do not apply, but could also help engineer the internal “wiring” of quantum computers, sensors, and other devices.

    One key quantum phenomenon is nonclassical correlation, or entanglement, in which pairs or groups of particles interact such that their physical properties cannot be described independently, even when the particles are widely separated.

    That relationship is central to a rapidly advancing field in physics, quantum information theory. It posits a new thermodynamic perspective in which information and energy are linked — in other words, that information is physical, and that quantum-level sharing of information underlies the universal tendency toward entropy and thermal equilibrium, known in quantum systems as thermalization.

    QEG head Paola Cappellaro, the Esther and Harold E. Edgerton Associate Professor of Nuclear Science and Engineering, co-authored the new paper with physics graduate student Ken Xuan Wei and longtime collaborator Chandrasekhar Ramanathan of Dartmouth College.

    Cappellaro explains that a primary aim of the research was measuring the quantum-level struggle between two states of matter: thermalization and localization, a state in which information transfer is restricted and the tendency toward higher entropy is somehow resisted through disorder. The QEG team’s work centered on the complex problem of many-body localization (MBL) where the role of spin-spin interactions is critical.

    The ability to gather this data experimentally in a lab is a breakthrough, in part because simulation of quantum systems and localization-thermalization transitions is extremely difficult even for today’s most powerful computers. “The size of the problem becomes intractable very quickly, when you have interactions,” says Cappellaro. “You can simulate perhaps 12 spins using brute force but that’s about it — far fewer than the experimental system is capable of exploring.”

    NMR techniques can reveal the existence of correlations among spins, as correlated spins rotate faster under applied magnetic fields than isolated spins. However, traditional NMR experiments can only extract partial information about correlations. The QEG researchers combined those techniques with their knowledge of the spin dynamics in their crystal, whose geometry approximately confines the evolution to linear spin chains.

    “That approach allowed us to figure out a metric, average correlation length, for how many spins are connected to each other in a chain,” says Cappellaro. “If the correlation is growing, it tells you that interaction is winning against the disorder that’s causing localization. If the correlation length stops growing, disorder is winning and keeping the system in a more quantum localized state.”

    In addition to being able to distinguish between different types of localization (such as MBL and the simpler Anderson localization), the method also represents a possible advance toward the ability to control of these systems through the introduction of disorder, which promotes localization, Cappellaro adds. Because MBL preserves information and prevents it from becoming scrambled, it has potential for memory applications.

    The research’s focus “addresses a very fundamental question about the foundation of thermodynamics, the question of why systems thermalize and even why the notion of temperature exists at all,” says former MIT postdoc Iman Marvian, who is now an assistant professor in Duke University’s departments of Physics and Electrical and Computer Engineering. “Over the last 10 years or so there’s been mounting evidence, from analytical arguments to numerical simulations, that even though different parts of the system are interacting with each other, in the MBL phase systems don’t thermalize. And it is very exciting that we can now observe this in an actual experiment.”

    “People have proposed different ways to detect this phase of matter, but they’re difficult to measure in a lab,” Marvian explains. “Paola’s group studied it from a new point of view and introduced quantities that can be measured. I’m really impressed at how they’ve been able to extract useful information about MBL from these NMR experiments. It’s great progress, because it makes it possible to experiment with MBL on a natural crystal.”

    The research was able to leverage NMR-related capabilities developed under a previous grant from the US Air Force, says Cappellaro, and some additional funding from the National Science Foundation. Prospects for this research area are promising, she adds. “For a long time, most many-body quantum research was focused on equilibrium properties. Now, because we can do many more experiments and would like to engineer quantum systems, there’s much more interest in dynamics, and new programs devoted to this general area. So hopefully we can get more funding and continue the work.”

    See the full article here .

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    The mission of MIT is to advance knowledge and educate students in science, technology, and other areas of scholarship that will best serve the nation and the world in the twenty-first century. We seek to develop in each member of the MIT community the ability and passion to work wisely, creatively, and effectively for the betterment of humankind.

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  • richardmitnick 12:02 pm on February 15, 2018 Permalink | Reply
    Tags: , , , , Quantum entanglement, ,   

    From U Vienna: “Fingerprints of quantum entanglement” 

    University of Vienna

    15. February 2018

    Dr. Borivoje Dakic
    Fakultät für Physik
    Universität Wien & IQOQI Wien, ÖAW
    1090 – Wien, Boltzmanngasse 5
    + 43-4277-725 80
    borivoje.dakic@univie.ac.at

    Rückfragehinweis
    Mag. Alexandra Frey
    Pressebüro der Universität Wien
    Forschung und Lehre
    Universität Wien
    1010 – Wien, Universitätsring 1
    +43-1-4277-175 33
    +43-664-60277-175 33
    alexandra.frey@univie.ac.at

    Dipl.-Soz. Sven Hartwig
    Leitung Öffentlichkeit & Kommunikation
    Österreichische Akademie der Wissenschaften
    1010 – Wien, Dr. Ignaz Seipel-Platz 2
    +43 1 51581-13 31
    sven.hartwig@oeaw.ac.at

    1
    Entangled qubits are sent to measurement devices which output a sequence of zeroes and ones. This pattern heavily depends on the type of measurements performed on individual qubits. If we pick the set of measurements in a peculiar way, entanglement will leave unique fingerprints in the measurement patterns (Copyright: Juan Palomino).

    Quantum entanglement is a key feature of a quantum computer. Yet, how can we verify that a quantum computer indeed incorporates a large-scale entanglement? Using conventional methods is hard since they require a large number of repeated measurements. Aleksandra Dimić from the University of Belgrade and Borivoje Dakić from the Austrian Academy of Sciences and the University of Vienna have developed a novel method where in many cases even a single experimental run suffices to prove the presence of entanglement. Their surprising results will be published in the online open access journal npj Quantum Information of the Nature Publishing group.

    The ultimate goal of quantum information science is to develop a quantum computer, a fully-fledged controllable device which makes use of the quantum states of subatomic particles to store information. As with all quantum technologies, quantum computing is based on a peculiar feature of quantum mechanics, quantum entanglement. The basic units of quantum information, the qubits, need to correlate in this particular way in order for the quantum computer to achieve its full potential.

    One of the main challenges is to make sure that a fully functional quantum computer is working as anticipated. In particular, scientists need to show that the large number of qubits are reliably entangled. Conventional methods require a large number of repeated measurements on the qubits for reliable verification. The more often a measurement run is repeated the more certain one can be about the presence of entanglement. Therefore, if one wants to benchmark entanglement in large quantum systems it will require a lot of resources and time, which is practically difficult or simply impossible. The main question arises: can we prove entanglement with only a low number of measurement trials?

    Now researchers from the University of Belgrade, the University of Vienna and the Austrian Academy of Sciences have developed a novel verification method which requires significantly fewer resources and, in many cases, even only a single measurement run to prove large-scale entanglement with a high confidence. For Aleksandra Dimić from the University of Belgrade, the best way to understand this phenomenon is to use the following analogy: “Let us consider a machine which simultaneously tosses, say, ten coins. We manufactured the machine such that it should produce correlated coins. We now want to validate whether the machine produces the anticipated result. Imagine a single trial revealing all coins landing on tails. This is a clear signature of correlations, as ten independent coins have 0.01% chance to land on the same side simultaneously. From such an event, we certify the presence of correlations with more than 99.9% confidence. This situation is very similar to quantum correlations captured by entanglement.” Borivoje Dakić says: “In contrast to classical coins, qubits can be measured in many, many different ways. The measurement result is still a sequence of zeros and ones, but its structure heavily depends on how we choose to measure individual qubits”, he continues. “We realized that, if we pick these measurements in a peculiar way, entanglement will leave unique fingerprints in the measured pattern”, he concludes.

    The developed method promises a dramatic reduction in time and resources needed for reliable benchmark of future quantum devices.

    Publication in npj Quantum Information:
    A.Dimić and B.Dakić, “Single-copy enntaglement detection”, npj Quantum Information, 2018.
    DOI: 10.1038/s41534-017-0055-x
    http://www.nature.com/articles/s41534-017-0055-x

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    The University of Vienna (German: Universität Wien) is a public university located in Vienna, Austria. It was founded by Duke Rudolph IV in 1365 and is one of the oldest universities in the German-speaking world. With its long and rich history, the University of Vienna has developed into one of the largest universities in Europe, and also one of the most renowned, especially in the Humanities. It is associated with 15 Nobel prize winners and has been the academic home to a large number of scholars of historical as well as of academic importance.

     
  • richardmitnick 5:33 pm on November 7, 2017 Permalink | Reply
    Tags: A way to link a group of atoms’ quantum mechanical properties among themselves far more quickly than is currently possible potentially providing a tool for highly precise sensing and quantum compute, , Dipolar interaction, Getting the atoms into an entangled state more quickly would be a potential advantage in any practical application not least because entanglement can be fleeting, Need Entangled Atoms? Get 'Em FAST! With NIST’s New Patent-Pending Method, , , Quantum entanglement, Uncertainty is the key here   

    From NIST: “Need Entangled Atoms? Get ‘Em FAST! With NIST’s New Patent-Pending Method” 


    NIST

    November 07, 2017

    Chad Boutin
    boutin@nist.gov
    (301) 975-4261

    1
    While quantum entanglement usually spreads through the atoms in an optical lattice via short-range interactions with the atoms’ immediate neighbors (left), new theoretical research shows that taking advantage of long-range dipolar interactions among the atoms could enable it to spread more quickly (right), a potential advantage for quantum computing and sensing applications.
    Credit: Gorshkov and Hanacek/NIST

    Physicists at the National Institute of Standards and Technology (NIST) have come up with a way to link a group of atoms’ quantum mechanical properties among themselves far more quickly than is currently possible, potentially providing a tool for highly precise sensing and quantum computer applications. NIST has applied for a patent on the method, which is detailed in a new paper in Physical Review Letters.

    The method, which has not yet been demonstrated experimentally, essentially would speed up the process of quantum entanglement in which the properties of multiple particles become interconnected with one other. Entanglement would propagate through a group of atoms in dramatically less time, allowing scientists to build an entangled system exponentially faster than is common today.

    Arrays of entangled atoms suspended in laser light beams, known as optical lattices, are one approach to creating the logic centers of prototype quantum computers, but an entangled state is difficult to maintain more than briefly. Applying the method to these arrays could give scientists precious time to do more with these arrays of atoms before entanglement is lost in a process known as decoherence.

    The method takes advantage of a physical relationship among the atoms called dipolar interaction, which allows atoms to influence each other over greater distances than previously possible. The research team’s Alexey Gorshkov compares it to sharing tennis balls among a group of people. While previous methods essentially allowed people to pass tennis balls only to a person standing next to them, the new approach would allow an individual to toss them to people across the room.

    “It is these long-range dipolar interactions in 3-D that enable you to create entanglement much faster than in systems with short-range interactions,” said Gorshkov, a theoretical physicist at NIST and at both the Joint Center for Quantum Information and Computer Science and the Joint Quantum Institute, which are collaborations between NIST and the University of Maryland. “Obviously, if you can throw stuff directly at people who are far away, you can spread the objects faster.”

    Applying the technique would center around adjusting the timing of laser light pulses, turning the lasers on and off in particular patterns and rhythms to quick-change the suspended atoms into a coherent entangled system.

    The approach also could find application in sensors, which might exploit entanglement to achieve far greater sensitivity than classical systems can. While entanglement-enhanced quantum sensing is a young field, it might allow for high-resolution scanning of tiny objects, such as distinguishing slight temperature differences among parts of an individual living cell or performing magnetic imaging of its interior.

    Gorshkov said the method builds on two studies from the 1990s in which different NIST researchers considered the possibility of using a large number of tiny objects—such as a group of atom—as sensors. Atoms could measure the properties of a nearby magnetic field, for example, because the field would change their electrons’ energy levels. These earlier efforts showed that the uncertainty in these measurements would be advantageously lower if the atoms were all entangled, rather than merely a bunch of independent objects that happened to be near one another.

    “Uncertainty is the key here,” said Gorshkov. “You want that uncertainty as low as possible. If the atoms are entangled, you have less uncertainty about that magnetic field’s magnitude.”

    Getting the atoms into an entangled state more quickly would be a potential advantage in any practical application, not least because entanglement can be fleeting.

    When a group of atoms is entangled, the quantum state of each one is bound up with the others so that the entire system possesses a single quantum state. This connection can exist even if the atoms are separated and completely isolated from one another (giving rise to Einstein’s famous description of it as “spooky action at a distance”), but entanglement is also quite a fragile condition. The difficulty of maintaining it among large numbers of atoms has slowed the development of entanglement-based technologies such as quantum computers.

    “Entangled states tend to decohere and go back to being a bunch of ordinary independent atoms,” Gorshkov said. “People knew how to create entanglement, but we looked for a way to do it faster.”

    If the method can be experimentally demonstrated, it could give a quantum computer’s processor additional time so it can outpace decoherence, which threatens to make a computation fall apart before the qubits can finish their work. It would also reduce the uncertainty if used in sensing applications.

    “We think this is a practical way to increase the speed of entanglement,” Gorshkov said. “It was cool enough to patent, so we hope it proves commercially useful to someone.”

    See the full article here.

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