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  • richardmitnick 1:22 pm on January 21, 2019 Permalink | Reply
    Tags: , , , , Quantum entanglement,   

    From Max Planck Gesellschaft: “Flying optical cats for quantum communication” 

    MPG bloc

    From Max Planck Gesellschaft

    January 21, 2019

    An entangled atom-light state realizes a paradoxical thought experiment by Erwin Schrödinger.

    1
    Dead and alive: Schrödinger’s cat is entangled with an atom. If the atom is excited, the cat is alive. If it has decayed, the cat is dead. In the experiment, a light pulse represents the two states (peaks) and may be in a superposition of both, just like the cat. © Christoph Hohmann, Nanosystems Initiative Munich (NIM)

    An old thought experiment now appears in a new light. In 1935 Erwin Schrödinger formulated a thought experiment designed to capture the paradoxical nature of quantum physics. A group of researchers led by Gerhard Rempe, Director of the Department of Quantum Dynamics at the Max Planck Institute of Quantum Optics, has now realized an optical version of Schrödinger’s thought experiment in the laboratory. In this instance, pulses of laser light play the role of the cat. The insights gained from the project open up new prospects for enhanced control of optical states, that can in the future be used for quantum communications.

    “According to Schrödinger‘s idea, it is possible for a microscopic particle, such as a single atom, to exist in two different states at once. This is called a superposition. Moreover, when such a particle interacts with a macroscopic object, they can become ‘entangled’, and the macroscopic object may end up in superposition state. Schrödinger proposed the example of a cat, which can be both dead and alive, depending on whether or not a radioactive atom has decayed – a notion which is in obvious conflict with our everyday experience,” Professor Rempe explains.

    In order to realize this philosophical gedanken experiment in the laboratory, physicists have turned to various model systems. The one implemented in this instance follows a scheme proposed by the theoreticians Wang and Duan in 2005. Here, the superposition of two states of an optical pulse serves as the cat. The experimental techniques required to implement this proposal – in particular an optical resonator – have been developed in Rempe’s group over the past few years.

    A test for the scope of quantum mechanics

    The researchers involved in the project were initially skeptical as to whether it would be possible to generate and reliably detect such quantum mechanically entangled cat states with the available technology. The major difficulty lay in the need to minimize optical losses in their experiment. Once this was achieved, all measurements were found to confirm Schrödinger’s prediction. The experiment allows the scientists to explore the scope of application of quantum mechanics and to develop new techniques for quantum communication.

    The laboratory at the Max Planck Institute in Garching is equipped with all the tools necessary to perform state-of-the-art experiments in quantum optics. A vacuum chamber and high-precision lasers are used to isolate a single atom and manipulate its state. At the core of the set-up is an optical resonator, consisting of two mirrors separated by a slit only 0.5 mm wide, where an atom can be trapped. A laser pulse is fed into the resonator and reflected, and thereby interacts with the atom. As a result, the reflected light gets entangled with the atom. By performing a suitable measurement on the atom, the optical pulse can be prepared in a superposition state, just like that of Schrödinger’s cat. One special feature of the experiment is that the entangled states can be generated deterministically. In other words, a cat state is produced in every trial.

    “We have succeeded in generating flying optical cat states, and demonstrated that they behave in accordance with the predictions of quantum mechanics. These findings prove that our method for creating cat states works, and allowed us to explore the essential parameters,” says PhD student Stephan Welte.

    A whole zoo of states for future quantum communication

    “In our experimental setup, we have succeeded not only in creating one specific cat state, but arbitrarily many such states with different superposition phases – a whole zoo, so to speak. This capability could in the future be utilized to encode quantum information,” adds Bastian Hacker.

    “Schrödinger‘s cat was originally enclosed in a box to avoid any interaction with the environment. Our optical cat states are not enclosed in a box. They propagate freely in space. Yet they remain isolated from the environment and retain their properties over long distances. In the future we could use this technology to construct quantum networks, in which flying optical cat states transmit information,” says Gerhard Rempe. This underlines the significance of his group’s latest achievement.

    See the full article here .


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    MPG campus

    The Max Planck Society for the Advancement of Science (German: Max-Planck-Gesellschaft zur Förderung der Wissenschaften e. V.; abbreviated MPG) is a formally independent non-governmental and non-profit association of German research institutes founded in 1911 as the Kaiser Wilhelm Society and renamed the Max Planck Society in 1948 in honor of its former president, theoretical physicist Max Planck. The society is funded by the federal and state governments of Germany as well as other sources.

    According to its primary goal, the Max Planck Society supports fundamental research in the natural, life and social sciences, the arts and humanities in its 83 (as of January 2014)[2] Max Planck Institutes. The society has a total staff of approximately 17,000 permanent employees, including 5,470 scientists, plus around 4,600 non-tenured scientists and guests. Society budget for 2015 was about €1.7 billion.

    The Max Planck Institutes focus on excellence in research. The Max Planck Society has a world-leading reputation as a science and technology research organization, with 33 Nobel Prizes awarded to their scientists, and is generally regarded as the foremost basic research organization in Europe and the world. In 2013, the Nature Publishing Index placed the Max Planck institutes fifth worldwide in terms of research published in Nature journals (after Harvard, MIT, Stanford and the US NIH). In terms of total research volume (unweighted by citations or impact), the Max Planck Society is only outranked by the Chinese Academy of Sciences, the Russian Academy of Sciences and Harvard University. The Thomson Reuters-Science Watch website placed the Max Planck Society as the second leading research organization worldwide following Harvard University, in terms of the impact of the produced research over science fields.

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  • richardmitnick 4:47 pm on November 6, 2018 Permalink | Reply
    Tags: Physicists Race to Demystify Einstein’s ‘Spooky’ Science, Quantum entanglement,   

    From UC San Diego: “Physicists Race to Demystify Einstein’s ‘Spooky’ Science” 

    UC San Diego bloc

    From UC San Diego

    August 20, 2018

    Cynthia Dillon
    858-822-0142
    cdillon@ucsd.edu

    International research team recasts timeline, dating from the Big Bang, of possible quantum theory alternatives.

    When it comes to fundamental physics, things can get spooky. At least that’s what Albert Einstein said when describing the phenomenon of quantum entanglement—the linkage of particles in such a way that measurements performed on one particle seem to affect the other, even when separated by great distances. “Spooky action at a distance” is how Einstein described what he couldn’t explain.

    1
    Schematic of the 2018 “Cosmic Bell” experiment at the Roque de Los Muchachos Observatory in the Canary Islands, where two large telescopes observed the fluctuating color of light from distant quasars (red and blue galaxies). The green beams indicate polarization-entangled photons sent through the open air between stations separated by about one kilometer. Image by Andrew S. Friedman and Dominik Rauch.

    While quantum mechanics includes many mysterious phenomena like entanglement, it remains the best fundamental physical theory describing how matter and light behave at the smallest scales. Quantum theory has survived numerous experimental tests in the past century while enabling many advanced technologies: modern computers, digital cameras and the displays of TVs, laptops and smartphones. Quantum entanglement itself is also the key to several next-generation technologies in computing, encryption and telecommunications. Yet, there is no clear consensus on how to interpret what quantum theory says about the true nature of reality at the subatomic level, or to definitively explain how entanglement actually works.

    2
    Diagram of a run of the Cosmic Bell test. The regions of space and time where an alternative, non-quantum mechanism could still have acted (limited to the red and/or blue regions) corresponds to at least 7.78 billion years ago (blue region). Light from the more distant quasar was emitted 12.21 billion years ago (red region). Compared to the gray region, representing all of space and time prior to the experiment, the alternatives are limited to within four percent of the space-time volume since the Big Bang. Image by Andrew Friedman and David Leon.

    According to Andrew Friedman, a research scientist at the University of California San Diego Center for Astrophysics and Space Sciences (CASS), “the race is on” around the globe to identify and experimentally close potential loopholes that could still allow alternative theories, distinct from quantum theory, to explain perplexing phenomena like quantum entanglement. Such loopholes could potentially allow future quantum encryption schemes to be hacked. So, Friedman and his fellow researchers conducted a “Cosmic Bell” test with polarization-entangled photons designed to further close the “freedom-of-choice” or “free will” loophole in tests of Bell’s inequality, a famous theoretical result derived by physicist John S. Bell in the 1960s. Published in the Aug. 20 issue of Physical Review Letters, their findings are consistent with quantum theory and push back to at least 7.8 billion years ago the most recent time by which any causal influences from alternative, non-quantum mechanisms could have exploited the freedom-of-choice loophole.

    “Our findings imply that any such mechanism is excluded from explaining the results from within a whopping 96% of the space-time volume in the causal past of our experiment, stretching all the way from the Big Bang until today,” said Friedman. “While these alternatives to quantum theory have not been completely ruled out, we are pushing them into a progressively smaller corner of space and time.”

    In their experiment, the researchers outsourced the choice of entangled photon measurements to the universe. They did this by using the color of light that has been traveling to Earth for billions of years from distant galaxies—quasars—as a “cosmic random number generator.”

    “This is a rare experiment that comes along only very seldomly in a scientist’s career: a pioneering experiment that sets the bar so high few, if any, competitors can ever match it,” noted UC San Diego astrophysicist Brian Keating. “I’m so proud that my graduate student David Leon had the chance to make a significant contribution to this fascinating research, co-led by CASS research scientist, Dr. Andrew Friedman.”

    Besides UC San Diego’s Friedman and Leon, the full research team included lead author and Ph.D. student Dominik Rauch, along with Anton Zeilinger and his experimental quantum optics group from the University of Vienna; theoretical physicists David Kaiser and Alan Guth at MIT; Jason Gallicchio and his experimental physics group at Harvey Mudd College, and others. Expanding upon their previous quantum entanglement experiments [Physical Review Letters], Friedman and colleagues went to great effort to choose entangled particle measurements using 3.6 and 4.2 meter telescopes in the Canary Islands, allowing them to collect sufficient light from the much fainter, distant quasars.

    To conduct their test, they shined laser light into a special crystal that generated pairs of entangled photons, which the scientists repeatedly sent through the open air toward both telescopes. From the quasar light collected, the scientists could choose polarization measurement settings while each entangled photon was in mid-flight. The group was allotted three nights and a few hours at the Roque de los Muchachos Observatory in La Palma, amidst operationally challenging conditions including freezing rain, high winds, and uncertainty about whether they would have enough time to complete the experiment. Additionally, Friedman and colleagues had to write software that could choose the best quasars to observe on-the-fly—from a database of more than 1.5 million—and predict the observation time needed to obtain a statistically significant result.

    “We pushed to the limit what could be done within the time constraints,” said Friedman. “The experiment would not have been possible without an amazing international collaboration. It was a roller coaster experience to see it actually work.”

    The research was funded by the Austrian Academy of Sciences; The Austrian Science Fund with SFB F40 (FOQUS) and project COQuS (W1210-N16); the Austrian Federal Ministry of Education, Science and Research; the University of Vienna (via the project QUESS); the National Science Foundation INSPIRE Grant (PHY-1541160); the U.S. Department of Energy (DE-SC0012567); the U.S. Department of Defense, through the National Defense Science & Engineering Graduate Fellowship Program, and UC San Diego’s Ax Center for Experimental Cosmology.

    See the full article here .

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    UC San Diego Campus

    The University of California, San Diego (also referred to as UC San Diego or UCSD), is a public research university located in the La Jolla area of San Diego, California, in the United States.[12] The university occupies 2,141 acres (866 ha) near the coast of the Pacific Ocean with the main campus resting on approximately 1,152 acres (466 ha).[13] Established in 1960 near the pre-existing Scripps Institution of Oceanography, UC San Diego is the seventh oldest of the 10 University of California campuses and offers over 200 undergraduate and graduate degree programs, enrolling about 22,700 undergraduate and 6,300 graduate students. UC San Diego is one of America’s Public Ivy universities, which recognizes top public research universities in the United States. UC San Diego was ranked 8th among public universities and 37th among all universities in the United States, and rated the 18th Top World University by U.S. News & World Report ‘s 2015 rankings.

     
  • richardmitnick 3:56 pm on November 6, 2018 Permalink | Reply
    Tags: “Cosmic Bell” experiment at the Roque de Los Muchachos Observatory in the Canary Islands, Quantum entanglement,   

    From Symmetry: “The quest to test quantum entanglement” 

    Symmetry Mag
    From Symmetry

    11/06/18
    Laura Dattaro

    Quantum entanglement, doubted by Einstein, has passed increasingly stringent tests.

    Quantum entanglement and spatial distribution Credit Nakagawa et al

    Quantum entanglement By Ishdasrox (Own work) [CC BY-SA 4.0 (via Wikimedia Commons)]

    Over 12 billion years ago, speeding particles of light left an extremely luminous celestial object called a quasar and began a long journey toward a planet that did not yet exist. More than 4 billion years later, more photons left another quasar for a similar trek. As Earth and its solar system formed, life evolved, and humans began to study physics, the particles continued on their way. Ultimately, they landed in the Canary Island of La Palma in a pair of telescopes set up for an experiment testing the very nature of reality.

    1
    Schematic of the 2018 “Cosmic Bell” experiment at the Roque de Los Muchachos Observatory in the Canary Islands, where two large telescopes observed the fluctuating color of light from distant quasars (red and blue galaxies). The green beams indicate polarization-entangled photons sent through the open air between stations separated by about one kilometer. Credit: Andrew S. Friedman and Dominik Rauch

    The experiment was designed to study quantum entanglement, a phenomenon that connects quantum systems in ways that are impossible in our macro-sized, classical world. When two particles, like a pair of electrons, are entangled, it’s impossible to measure one without learning something about the other. Their properties, like momentum and position, are inextricably linked.

    “Quantum entanglement means that you can’t describe your joint quantum system in terms of just local descriptions, one for each system,” says Michael Hall, a theoretical physicist at the Australian National University.

    Entanglement first arose in a thought experiment worked out by none other than Albert Einstein. In a 1935 paper, Einstein and two colleagues showed that if quantum mechanics fully described reality, then conducting a measurement on one part of an entangled system would instantaneously affect our knowledge about future measurements on the other part, seemingly sending information faster than the speed of light, which is impossible according to all known physics. Einstein called this effect “spooky action at a distance,” implying something fundamentally wrong with the budding science of quantum mechanics.

    Decades later, quantum entanglement has been experimentally confirmed time and again. While physicists have learned to control and study quantum entanglement, they’ve yet to find a mechanism to explain it or to reach consensus on what it means about the nature of reality.

    “Entanglement itself has been verified over many, many decades,” says Andrew Friedman, an astrophysicist at University of California, San Diego, who worked on the quasar experiment, also known as a “cosmic Bell test.” “The real challenge is that even though we know it’s an experimental reality, we don’t have a compelling story of how it actually works.”

    Bell’s assumptions

    The world of quantum mechanics—the physics that governs the behavior of the universe at the very smallest scales—is often described as exceedingly weird. According to its laws, nature’s building blocks are simultaneously waves and particles, with no definite location in space. It takes an outside system observing or measuring them to push them to “choose” a definitive state. And entangled particles seem to affect one another’s “choices” instantaneously, no matter how far apart they are.

    Einstein was so dissatisfied with these ideas that he postulated classical “hidden variables,” outside our understanding of quantum mechanics, that, if we understood them, would make entanglement not so spooky. In the 1960s, physicist John Bell devised a test for models with such hidden variables, known as “Bell’s inequality.”

    Bell outlined three assumptions about the world, each with a corresponding mathematical statement: realism, which says objects have properties they maintain whether they are being observed or not; locality, which says nothing can influence something far enough away that a signal between them would need to travel faster than light; and freedom of choice, which says physicists can make measurements freely and without influence from hidden variables. Probing entanglement is the key to testing these assumptions. If experiments show that nature obeys these assumptions, then we live in a world we can understand classically, and hidden variables are only creating the illusion of quantum entanglement. If experiments show that the world does not follow them, then quantum entanglement is real and the subatomic world is truly as strange as it seems.

    “What Bell showed is that if the world obeys these assumptions, there’s an upper limit to how correlated entangled particle measurements can be,” Friedman says.

    Physicists can measure properties of particles, such as their spin, momentum or polarization. Experiments have shown that when particles are entangled, the outcome of these measurements are more statistically correlated than would be expected in a classical system, violating Bell’s inequalities.

    In one type of Bell test, scientists send two entangled photons to detectors far apart from one another. Whether the photons reach the detectors depends on their polarization; if they are perfectly aligned, they will pass through, but otherwise, there is some probability they will be blocked, depending on the angle of alignment. Scientists look to see whether the entangled particles wind up with the same polarization more often than could be explained by classical statistics. If they do, at least one of Bell’s assumptions can’t be true in nature. If the world does not obey realism, then properties of particles aren’t well defined before measurements. If the particles could influence one another instantaneously, then they would somehow be communicating to one another faster than the speed of light, violating locality and Einstein’s theory of special relativity.

    Scientists have long speculated that previous experimental results can be explained best if the world does not obey one or both of the first two of Bell’s assumptions—realism and locality. But recent work has shown that the culprit could be his third assumption—the freedom of choice. Perhaps the scientists’ decision about the angle at which to let the photons in is not as free and random as they thought.

    The quasar experiment was the latest to test the freedom of choice assumption. The scientists determined the angle at which they would allow photons into their detectors based on the wavelength of the light they detected from the two distant quasars, something determined 7.8 and 12.2 billion years ago, respectively. The long-traveling photons took the place of physicists or conventional random number generators in the decision, eliminating earthbound influences on the experiment, human or otherwise.

    At the end of the test, the team found far higher correlations among the entangled photons than Bell’s theorem would predict if the world were classical.

    That means that, if some hidden classical variable were actually determining the outcomes of the experiment, in the most extreme scenario, the choice of measurement would have to have been laid out long before human existence—implying that quantum “weirdness” is really the result of a universe where everything is predetermined.

    “That’s unsatisfactory to a lot of people,” Hall says. “They’re really saying, if it was set up that long ago, you would have to try and explain quantum correlations with predetermined choices. Life would lose all meaning, and we’d stop doing physics.”

    Of course, physics marches on, and entanglement retains many mysteries to be probed. In addition to lacking a causal explanation for entanglement, physicists don’t understand how measuring an entangled system suddenly reverts it to a classical, unentangled state, or whether entangled particles are actually communicating in some way, mysteries that they continue to explore with new experiments.

    “No information can go from here to there instantaneously, but different interpretations of quantum mechanics will agree or disagree that there’s some hidden influence,” says Gabriela Barreto Lemos, a postdoctoral researcher at the International Institute of Physics in Brazil. “But something we all agree upon is this definition in terms of correlation and statistics.”

    Looking for something strange

    Developing a deeper understanding of entanglement can help solve problems both practical and fundamental. Quantum computers rely on entanglement. Quantum encryption, a theoretical security measure that is predicted to be impossible to break, also requires a full understanding of quantum entanglement. If hidden variables are valid, quantum encryption might actually be hackable.

    And entanglement may hold the key to some of the most fundamental questions in physics. Some researchers have been studying materials with large numbers of particles entangled, rather than simply pairs. When this many-body entanglement happens, physicists observe new states of matter beyond the familiar solid, liquid and gas, as well as new patterns of entanglement not seen anywhere else.

    “One thing it tells you is that the universe is richer than you previously suspected,” says Brian Swingle, a University of Maryland physicist researching such materials. “Just because you have a collection of electrons does not mean that the resulting state of matter has to be electron-like.”

    Such interesting properties are emerging from these materials that physicists are starting to realize that entanglement may actually stitch together space-time itself—a somewhat ironic twist, as Einstein, who first connected space and time in his relativity theory, disliked quantum mechanics so much. But if the theory proves correct, entanglement could help physicists finally reach one of their ultimate goals: achieving a theory of quantum gravity that unites Einstein’s relativistic world with the enigmatic and seemingly contradictory quantum world.

    “It’s important to do these experiments even if we don’t believe we’re going to find anything strange,” Lemos says. “In physics, the revolution comes when we think we’re not going to find something strange, and then we do. So you have to do it.”

    See the full article here .


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    Symmetry is a joint Fermilab/SLAC publication.


     
  • richardmitnick 1:47 pm on November 5, 2018 Permalink | Reply
    Tags: , Griffith precision measurement takes it to the limit, Griffith University, Heisenberg limit, , Quantum computing algorithms, Quantum entanglement,   

    From Griffith University via phys.org: “Griffith precision measurement takes it to the limit” 

    Griffith U bloc

    From Griffith University

    via

    phys.org

    November 5, 2018

    1
    Griffith University researchers have demonstrated a procedure for making precise measurements of speed, acceleration, material properties and even gravity waves possible, approaching the ultimate sensitivity allowed by laws of quantum physics. Credit: Griffith University

    Published in Nature Communications, the work saw the Griffith team, led by Professor Geoff Pryde, working with photons (single particles of light) and using them to measure the extra distance travelled by the light beam, compared to its partner reference beam, as it went through the sample being measured—a thin crystal.

    The researchers combined three techniques—entanglement (a kind of quantum connection that can exist between the photons), passing the beams back and forth along the measurement path, and a specially-designed detection technique.

    “Every time a photon passes through the sample, it makes a kind of mini-measurement. The total measurement is the combination of all of these mini-measurements,” said Griffith’s Dr. Sergei Slussarenko, who oversaw the experiment. “The more times the photons pass through, the more precise the measurement becomes.

    “Our scheme will serve as a blueprint for tools that can measure physical parameters with precision that is literally impossible to achieve with the common measurement devices.

    Lead author of the paper Dr. Shakib Daryanoosh said this method can be used to investigate and measure other quantum systems.

    “These can be very fragile, and every probe photon we send it would disturb it. In this case, using few photons but in the most efficient way possible is critical and our scheme shows how do exactly that,” he said.

    While one strategy is to just use as many photons as possible, that’s not enough to reach the ultimate performance. For that, it is necessary to also extract the maximum amount of measurement information per photon pass, and that is what the Griffith experiment has achieved, coming far closer?to the so-called Heisenberg limit of precision than any comparable experiment.

    The remaining error is due experimental imperfection, as the scheme designed by Dr. Daryanoosh and Professor Howard Wiseman, is capable of achieving the exact Heisenberg limit, in theory.

    “The really nice thing about this technique is that it works even when you don’t have a good starting guess for the measurement,” Prof. Wiseman said. “Previous work has mostly focused a lot on the case where it’s possible to make a very good starting approximation, but that’s not always possible.”

    A few extra steps are required before this proof-of-principle demonstration can be harnessed outside the lab.

    Producing entangled photons is not simple with current technology, and this means it is still much easier to use many photons inefficiently, rather than each set of entangled photons in the best way possible.

    However, according to the team, the ideas behind this approach can find immediate applications in quantum computing algorithms and research in fundamental science.

    The scheme can ultimately be extended to a larger number of entangled photons, where the difference of the Heisenberg limit over the usually achievable limit is more significant.

    See the full article here .

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    Griffith U Campus

    In 1971, Griffith was created to be a new kind of university—one that offered new degrees in progressive fields such as Asian studies and environmental science. At the time, these study areas were revolutionary—today, they’re more important than ever.

    Since then, we’ve grown into a comprehensive, research-intensive university, ranking in the top 5% of universities worldwide. Our teaching and research spans five campuses in South East Queensland and all disciplines, while our network of more than 120,000 graduates extends around the world.

    Griffith continues the progressive traditions of its namesake, Sir Samuel Walker Griffith, who was twice the Premier of Queensland, the first Chief Justice of the High Court of Australia, and the principal author of the Australian Constitution.

     
  • richardmitnick 11:27 am on September 27, 2018 Permalink | Reply
    Tags: , Atom-based quantum computer, , , , Quantum entanglement, , Rubidium atoms, Rydberg state,   

    From Science Magazine: “Arrays of atoms emerge as dark horse candidate to power quantum computers” 

    AAAS
    From Science Magazine

    Sep. 26, 2018
    Sophia Chen

    1
    Lasers are used to trap arrays of atoms within glass chambers made by ColdQuanta, a neutral atom quantum computing startup.
    COLDQUANTA INC.

    In a small basement laboratory, Harry Levine, a Harvard University graduate student in physics, can assemble a rudimentary computer in a fraction of a second. There isn’t a processor chip in sight; his computer is powered by 51 rubidium atoms that reside in a glass cell the size of a matchbox. To create his computer, he lines up the atoms in single file, using a laser split into 51 beams. More lasers—six beams per atom—slow the atoms until they are nearly motionless. Then, with yet another set of lasers, he coaxes the atoms to interact with each other, and, in principle, perform calculations.

    It’s a quantum computer, which manipulates “qubits” that can encode zeroes and ones simultaneously in what’s called a superposition state. If scaled up, it might vastly outperform conventional computers at certain tasks. But in the world of quantum computing, Levine’s device is somewhat unusual. In the race to build a practical quantum device, investment has largely gone to qubits that can be built on silicon, such as tiny circuits of superconducting wire and small semiconductors structures known as quantum dots. Now, two recent studies have demonstrated the promise of the qubits Levine works with: neutral atoms. In one study, a group including Levine showed a quantum logic gate made of two neutral atoms could work with far fewer errors than ever before. And in another, researchers built 3D structures of carefully arranged atoms, showing that more qubits can be packed into a small space by taking advantage of the third dimension.

    The advances, along with the arrival of venture capital funding, suggest neutral atoms could be on the upswing, says Dana Anderson, CEO of ColdQuanta, a Boulder, Colorado–based company that is developing an atom-based quantum computer. “We’ve done our homework,” Anderson says. “This is really in the engineering arena now.”

    Because neutral atoms lack electric charge and interact reluctantly with other atoms, they would seem to make poor qubits. But by using specifically timed laser pulses, physicists can excite an atom’s outermost electron and move it away from the nucleus, inflating the atom to billions of times its usual size. Once in this so-called Rydberg state, the atom behaves more like an ion, interacting electromagnetically with neighboring atoms and preventing them from becoming Rydberg atoms themselves.

    Physicists can exploit that behavior to create entanglement—the quantum state of interdependence needed to perform a computation. If two adjacent atoms are excited into superposition, where both are partially in a Rydberg state and partially in their ground state, a measurement will collapse the atoms to one or the other state. But because only one of the atoms can be in its Rydberg state, the atoms are entangled, with the state of one depending on the state of the other.

    Once entangled, neutral atoms offer some inherent advantages. Atoms need no quality control: They are by definition identical. They’re much smaller than silicon-based qubits, which means, in theory, more qubits can be packed into a small space. The systems operate at room temperature, whereas superconducting qubits need to be placed inside a bulky freezer. And because neutral atoms don’t interact easily, they are more immune to outside noise and can hold onto quantum information for a relatively long time. “Neutral atoms have great potential,” says Mark Saffman, a physicist at the University of Wisconsin in Madison. “From a physics perspective, [they could offer] easier scalability and ultimately better performance.”

    Entangled atoms

    The two new studies bolster these claims. By engineering better quality lasers, Levine and his colleagues, led by physicist Mikhail Lukin at Harvard, were able to accurately program a two-rubidium atom logic gate 97% of the time, they report in a paper published on 20 September in Physical Review Letters. That puts the method closer to the performance of superconducting qubits, which already achieve fidelity rates above 99%. In a second study, published in Nature on 5 September, Antoine Browaeys of the Charles Fabry Laboratory near Paris and his colleagues demonstrated an unprecedented level of control over a 3D array of 72 atoms. To show off their control, they even arranged the atoms into the shape of the Eiffel Tower. Another popular qubit type, ions, are comparably small. But they can’t be stacked this densely because they repel each other, acknowledges Crystal Senko, a physicist at the University of Waterloo in Canada who works on ion quantum computers.

    Not everyone is convinced. Compared with other qubits, neutral atoms tend not to stay put, says Varun Vaidya, a physicist at Xanadu, a quantum computing company in Toronto, Canada, that builds quantum devices with photon qubits. “The biggest issue is just holding onto the atoms,” he says. If an atom falls out of place, Lukin’s automated laser system can reassemble the atoms in less than a second, but Vaidya says this may still prohibit the devices from performing longer tasks. “Right now, nobody knows what’s going to be the best qubit,” Senko says. “The bottom line is, they all have their problems.”

    Still, ColdQuanta has recently received $6.75 million in venture funding. Another startup, Atom Computing, based in Berkeley, California, has raised $5 million. CEO Ben Bloom says the company will pursue qubits made of atoms with two valence electrons instead of rubidium’s one, such as calcium and strontium. Bloom believes these atoms will allow for longer-lived qubits. Lukin says he’s also interested in commercializing his group’s technology.

    The startups, as well as Saffman’s group, are aiming to build fully programmable quantum computers. For now, Lukin wants his group to focus on building quantum simulators, a more limited kind of computer that specializes in solving specific optimization problems by preparing the qubits a certain way and letting them evolve naturally. Levine says his group’s device could, for example, help telecommunications engineers figure out where to put radio towers to minimize cost and maximize coverage. “We’re going to try to do something useful with these devices,” Levine says. “People still don’t know yet what quantum systems can do.”

    In the next year or two, he and his colleagues think neutral atom devices could deliver an answer.

    See the full article here .


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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.

    2
    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.

    3
    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.”

    2
    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.

     
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