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  • richardmitnick 12:12 pm on May 21, 2016 Permalink | Reply
    Tags: , Quantum Mechanics,   

    From WIRED: “New Evidence Could Overthrow the Standard View of Quantum Mechanics” 

    Wired logo

    WIRED

    05.21.16
    Dan Falk

    1
    Olena Shmahalo/Quanta Magazine

    Of the many counterintuitive features of quantum mechanics, perhaps the most challenging to our notions of common sense is that particles do not have locations until they are observed. This is exactly what the standard view of quantum mechanics, often called the Copenhagen interpretation, asks us to believe. Instead of the clear-cut positions and movements of Newtonian physics, we have a cloud of probabilities described by a mathematical structure known as a wave function. The wave function, meanwhile, evolves over time, its evolution governed by precise rules codified in something called the Schrödinger equation. The mathematics are clear enough; the actual whereabouts of particles, less so. Until a particle is observed, an act that causes the wave function to “collapse,” we can say nothing about its location. Albert Einstein, among others, objected to this idea. As his biographer Abraham Pais wrote: “We often discussed his notions on objective reality. I recall that during one walk Einstein suddenly stopped, turned to me and asked whether I really believed that the moon exists only when I look at it.”

    But there’s another view—one that’s been around for almost a century—in which particles really do have precise positions at all times. This alternative view, known as pilot-wave theory or Bohmian mechanics, never became as popular as the Copenhagen view, in part because Bohmian mechanics implies that the world must be strange in other ways. In particular, a 1992 study claimed to crystalize certain bizarre consequences of Bohmian mechanics and in doing so deal it a fatal conceptual blow. The authors of that paper concluded that a particle following the laws of Bohmian mechanics would end up taking a trajectory that was so unphysical—even by the warped standards of quantum theory—that they described it as “surreal.”

    Nearly a quarter-century later, a group of scientists has carried out an experiment in a Toronto laboratory that aims to test this idea. And if their results, first reported* earlier this year, hold up to scrutiny, the Bohmian view of quantum mechanics—less fuzzy but in some ways more strange than the traditional view—may be poised for a comeback.

    Saving Particle Positions

    Bohmian mechanics was worked out by Louis de Broglie in 1927 and again, independently, by David Bohm in 1952, who developed it further until his death in 1992. (It’s also sometimes called the de Broglie–Bohm theory.) As with the Copenhagen view, there’s a wave function governed by the Schrödinger equation. In addition, every particle has an actual, definite location, even when it’s not being observed. Changes in the positions of the particles are given by another equation, known as the “pilot wave” equation (or “guiding equation”). The theory is fully deterministic; if you know the initial state of a system, and you’ve got the wave function, you can calculate where each particle will end up.

    That may sound like a throwback to classical mechanics, but there’s a crucial difference. Classical mechanics is purely “local”—stuff can affect other stuff only if it is adjacent to it (or via the influence of some kind of field, like an electric field, which can send impulses no faster than the speed of light). Quantum mechanics, in contrast, is inherently nonlocal. The best-known example of a nonlocal effect—one that Einstein himself considered, back in the 1930s—is when a pair of particles are connected in such a way that a measurement of one particle appears to affect the state of another, distant particle. The idea was ridiculed by Einstein as “spooky action at a distance.” But hundreds of experiments, beginning in the 1980s, have confirmed that this spooky action is a very real characteristic of our universe.

    In the Bohmian view, nonlocality is even more conspicuous. The trajectory of any one particle depends on what all the other particles described by the same wave function are doing. And, critically, the wave function has no geographic limits; it might, in principle, span the entire universe. Which means that the universe is weirdly interdependent, even across vast stretches of space. The wave function “combines—or binds—distant particles into a single irreducible reality,” as Sheldon Goldstein, a mathematician and physicist at Rutgers University, has written.

    The differences between Bohm and Copenhagen become clear when we look at the classic “double slit” experiment, in which particles (let’s say electrons) pass through a pair of narrow slits, eventually reaching a screen where each particle can be recorded. When the experiment is carried out, the electrons behave like waves, creating on the screen a particular pattern called an “interference pattern.” Remarkably, this pattern gradually emerges even if the electrons are sent one at a time, suggesting that each electron passes through both slits simultaneously.

    Those who embrace the Copenhagen view have come to live with this state of affairs—after all, it’s meaningless to speak of a particle’s position until we measure it. Some physicists are drawn instead to the Many Worlds interpretation of quantum mechanics, in which observers in some universes see the electron go through the left slit, while those in other universes see it go through the right slit—which is fine, if you’re comfortable with an infinite array of unseen universes.

    By comparison, the Bohmian view sounds rather tame: The electrons act like actual particles, their velocities at any moment fully determined by the pilot wave, which in turn depends on the wave function. In this view, each electron is like a surfer: It occupies a particular place at every specific moment in time, yet its motion is dictated by the motion of a spread-out wave. Although each electron takes a fully determined path through just one slit, the pilot wave passes through both slits. The end result exactly matches the pattern one sees in standard quantum mechanics.

    2
    Lucy Reading-Ikkanda for Quanta Magazine

    For some theorists, the Bohmian interpretation holds an irresistible appeal. “All you have to do to make sense of quantum mechanics is to say to yourself: When we talk about particles, we really mean particles. Then all the problems go away,” said Goldstein. “Things have positions. They are somewhere. If you take that idea seriously, you’re led almost immediately to Bohm. It’s a far simpler version of quantum mechanics than what you find in the textbooks.” Howard Wiseman, a physicist at Griffith University in Brisbane, Australia, said that the Bohmian view “gives you a pretty straightforward account of how the world is…. You don’t have to tie yourself into any sort of philosophical knots to say how things really are.”

    But not everyone feels that way, and over the years the Bohm view has struggled to gain acceptance, trailing behind Copenhagen and, these days, behind Many Worlds as well. A significant blow came with the paper known as “ESSW,”** an acronym built from the names of its four authors. The ESSW paper claimed that particles can’t follow simple Bohmian trajectories as they traverse the double-slit experiment. Suppose that someone placed a detector next to each slit, argued ESSW, recording which particle passed through which slit. ESSW showed that a photon could pass through the left slit and yet, in the Bohmian view, still end up being recorded as having passed through the right slit. This seemed impossible; the photons were deemed to follow “surreal” trajectories, as the ESSW paper put it.

    The ESSW argument “was a striking philosophical objection” to the Bohmian view, said Aephraim Steinberg, a physicist at the University of Toronto. “It damaged my love for Bohmian mechanics.”

    But Steinberg has found a way to rekindle that love. In a paper published*** in Science Advances, Steinberg and his colleagues—the team includes Wiseman, in Australia, as well as five other Canadian researchers—describe what happened when they actually performed the ESSW experiment. They found that the photon trajectories aren’t surrealistic after all—or, more precisely, that the paths may seem surrealistic, but only if one fails to take into account the nonlocality inherent in Bohm’s theory.

    The experiment that Steinberg and his team conducted was analogous to the standard two-slit experiment. They used photons rather than electrons, and instead of sending those photons through a pair of slits, they passed through a beam splitter, a device that directs a photon along one of two paths, depending on the photon’s polarization. The photons eventually reach a single-photon camera (equivalent to the screen in the traditional experiment) that records their final position. The question “Which of two slits did the particle pass through?” becomes “Which of two paths did the photon take?”

    Importantly, the researchers used pairs of entangled photons rather than individual photons. As a result, they could interrogate one photon to gain information about the other. When the first photon passes through the beam splitter, the second photon “knows” which path the first one took. The team could then use information from the second photon to track the first photon’s path. Each indirect measurement yields only an approximate value, but the scientists could average large numbers of measurements to reconstruct the trajectory of the first photon.

    The team found that the photon paths do indeed appear to be surreal, just as ESSW predicted: A photon would sometimes strike one side of the screen, even though the polarization of the entangled partner said that the photon took the other route.

    But can the information from the second photon be trusted? Crucially, Steinberg and his colleagues found that the answer to the question “Which path did the first photon take?” depends on when it is asked.

    At first—in the moments immediately after the first photon passes through the beam splitter—the second photon is very strongly correlated with the first photon’s path. “As one particle goes through the slit, the probe [the second photon] has a perfectly accurate memory of which slit it went through,” Steinberg explained.

    But the farther the first photon travels, the less reliable the second photon’s report becomes. The reason is nonlocality. Because the two photons are entangled, the path that the first photon takes will affect the polarization of the second photon. By the time the first photon reaches the screen, the second photon’s polarization is equally likely to be oriented one way as the other—thus giving it “no opinion,” so to speak, as to whether the first photon took the first route or the second (the equivalent of knowing which of the two slits it went through).

    The problem isn’t that Bohm trajectories are surreal, said Steinberg. The problem is that the second photon says that Bohm trajectories are surreal—and, thanks to nonlocality, its report is not to be trusted. “There’s no real contradiction in there,” said Steinberg. “You just have to always bear in mind the nonlocality, or you miss something very important.”

    Faster Than Light

    Some physicists, unperturbed by ESSW, have embraced the Bohmian view all along and aren’t particularly surprised by what Steinberg and his team found. There have been many attacks on the Bohmian view over the years, and “they all fizzled out because they had misunderstood what the Bohm approach was actually claiming,” said Basil Hiley, a physicist at Birkbeck, University of London (formerly Birkbeck College), who collaborated with Bohm on his last book, The Undivided Universe. Owen Maroney, a physicist at the University of Oxford who was a student of Hiley’s, described ESSW as “a terrible argument” that “did not present a novel challenge to de Broglie–Bohm.” Not surprisingly, Maroney is excited by Steinberg’s experimental results, which seem to support the view he’s held all along. “It’s a very interesting experiment,” he said. “It gives a motivation for taking de Broglie–Bohm seriously.”

    On the other side of the Bohmian divide, Berthold-Georg Englert, one of the authors of ESSW (along with Marlan Scully, George Süssman and Herbert Walther), still describes their paper as a “fatal blow” to the Bohmian view. According to Englert, now at the National University of Singapore, the Bohm trajectories exist as mathematical objects but “lack physical meaning.”

    On a historical note, Einstein lived just long enough to hear about Bohm’s revival of de Broglie’s proposal—and he wasn’t impressed, dismissing it as too simplistic to be correct. In a letter to physicist Max Born, in the spring of 1952, Einstein weighed in on Bohm’s work:

    “Have you noticed that Bohm believes (as de Broglie did, by the way, 25 years ago) that he is able to interpret the quantum theory in deterministic terms? That way seems too cheap to me. But you, of course, can judge this better than I.”

    But even for those who embrace the Bohmian view, with its clearly defined particles moving along precise paths, questions remain. Topping the list is an apparent tension with special relativity, which prohibits faster-than-light communication. Of course, as physicists have long noted, nonlocality of the sort associated with quantum entanglement does not allow for faster-than-light signaling (thus incurring no risk of the grandfather paradox or other violations of causality). Even so, many physicists feel that more clarification is needed, especially given the prominent role of nonlocality in the Bohmian view. The apparent dependence of what happens here on what may be happening there cries out for an explanation.

    “The universe seems to like talking to itself faster than the speed of light,” said Steinberg. “I could understand a universe where nothing can go faster than light, but a universe where the internal workings operate faster than light, and yet we’re forbidden from ever making use of that at the macroscopic level—it’s very hard to understand.”

    *Science paper:
    Experimental nonlocal and surreal Bohmian trajectories

    **Science paper:
    Surrealistic Bohm Trajectories

    See the full article here .

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  • richardmitnick 8:45 am on May 8, 2016 Permalink | Reply
    Tags: "Physics: Material to meaning", , , , Quantum Mechanics, Robert P. Crease, Sean Carroll   

    From Nature: “Physics: Material to meaning” Book Review 

    Nature Mag
    Nature

    Published online:
    04 May 2016
    Robert P. Crease

    Robert P. Crease assesses Sean Carroll’s attempt to construct morality out of quantum field theory.

    I don’t think I have ever read anything with a bigger ambition than The Big Picture, physicist Sean Carroll’s latest book.

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    The Big Picture: On the Origins of Life, Meaning, and the Universe Itself
    Sean Carroll Dutton: 2016. ISBN: 9780525954828

    Physics, Carroll writes, gives us a complete picture of the foundations of nature. Although that view has had an enormous impact on cosmology, materials science and other scientific fields, its implications for meaning and morality have yet to be determined. “Our values,” writes Carroll, “have not yet caught up to our best ontology.” In this book, he conducts a quest to catch up.

    Carroll creates his big picture as follows. Quantum field theory provides a unified perspective on the subatomic realm. Carroll calls that the “Core Theory”, noting that its behaviour is fully captured by a formula called a Feynman path integral. Some features of the macro world can be directly tethered to it; others, including many concepts of thermodynamics, cannot. He calls these “emergent” features, ways of talking about the world that are not incompatible with Core Theory, yet cannot be grounded in it.

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    A bubble-chamber image showing the decay of a positive kaon particle. CERN.

    In the fun parts of The Big Picture, Carroll demonstrates the absurdity of adding to the Core Theory to explain the possibility of things such as an afterlife or a transcendent underlying purpose. These are easy targets. The narrative begins to get awkward when it comes to, say, conscious experiences. These, Carroll writes, are “not part of the fundamental architecture of reality”; they are emergent, a handy way of talking about what brains do. Like entropy, he argues, consciousness is a concept that “we invent to give ourselves more useful and efficient descriptions of the world”. He calls his approach “poetic naturalism”. By using “poetic”, he means to give his blessing to ways of describing the world other than through fundamental physics — ways that, he says, can be meaningful if they are useful and don’t violate the Core Theory.

    Carroll has a fluid, often engaging style, and the passages that explain science — including his appendix about the Feynman path integral — are excellent. The book brims, however, with avuncular clichés such as “Life is short, and certainty never happens”. Carroll confidently defines many concepts, including belief and consciousness, as if 2,500 years of philosophy have yielded little relevant to the subject; he dismisses the task of drawing careful distinctions and heeding subtleties as “ontologically fastidious”. All he finds in philosophical literature are a few interesting puzzles. It’s like getting a whirlwind tour of a city from a tour guide who doesn’t live there, but enthusiastically gives you capsule descriptions of favourite sites.

    It is hardly surprising, therefore, that Carroll’s philosophical conclusions sound profound but leave us with disappointingly empty propositions, such as, “Morality exists only insofar as we make it so, and other people might not pass judgments in the same way that we do.” Outlining his own moral approach, Carroll offers a poetic naturalist’s version of the Ten Commandments, the “Ten Considerations”: greetings-card-like homilies such as “It Takes All Kinds”.

    What’s fascinating about The Big Picture is that Carroll’s clarity and directness make its fundamental assumptions easy to spot, and whether you like this book will depend on whether you share them. Laboratories, as Carroll well knows, are workshops, controlled environments with unusual equipment, regulated conditions and specially trained workers. He writes from the perspective of such a worker who has come to believe that a mathematical physicist’s way of thinking is just how people think — or should think — about everything, even when they are not in a workshop or when they ponder values or the existence of God. Carroll describes deciding how to be morally good, for instance, as similar to a dinner-table conversation in which, like scientists collaborating, we “talk to others about their desires and how we can work together, and reason about how to make it happen”. Our group, he adds, “may include both vegetarians and omnivores, but with a good-faith effort”, universal satisfaction should result.

    Reality, too, is just what things look like from a physicist’s perspective — and if it looks different to others, that is an illusion. When Carroll discusses time, he means the quantity that scientists measure. Everyday experience leads us to think that time flows in one direction, but he assures us that “in reality, both directions of time are created equal”. The ontologically fastidious would say, “Not so fast!” Time as lived by humans is something else again. Both outside and even inside workshops, to be bored or expectant, to hear a melody or to plan and execute an action is not to register one moment after another, but to retain previous ones and anticipate the next in an asymmetrical flow. Determining time in the workshop is an elaborate process, and assumes that you can mark it off as you can space, and then measure the spatial movement of something, whether it is the motions of heavenly bodies in ancient times or electronic transitions in caesium atoms in ours. Yet according to Carroll, this is real time.

    If we accept the strict ontology of the workshop, as Carroll does, then we get his big picture and regard lived time, conscious experience and the rest of pre-workshop life as poetic and emergent. But there are broader ontologies in which the same things — which belong to the world described by the humanities and branches of biology, for instance — are regarded as fundamental, and as the driving force for workshop activity. Carroll’s is a naturalistic metaphysics.

    Carroll brings tremendous passion to his writing. He is sure that honest human beings who care about the world make an effort to understand it as he does. He is right that science springs from certain basic human impulses to achieve goals and ward off threats. But where do his passion and certainty about this come from? They, too, are imported from and continue to be rooted in pre-workshop life. To find a way to talk about how scientific workshops emerge from life rather than the other way around — that would be a big picture indeed.

    See the full article here .

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    Nature is a weekly international journal publishing the finest peer-reviewed research in all fields of science and technology on the basis of its originality, importance, interdisciplinary interest, timeliness, accessibility, elegance and surprising conclusions. Nature also provides rapid, authoritative, insightful and arresting news and interpretation of topical and coming trends affecting science, scientists and the wider public.

     
  • richardmitnick 7:48 am on May 8, 2016 Permalink | Reply
    Tags: , , Quantum Mechanics,   

    From Ethan Siegel: “Ask Ethan: Can we use quantum entanglement to communicate faster-than-light?” 

    Starts with a Bang

    5.7.16
    Ethan Siegel

    NanoSail-D poses after a successful laboratory deployment test  NASA
    NanoSail-D poses after a successful laboratory deployment test NASA

    Einstein called it spooky, but if we figure it out right, can we learn about distant star systems instantaneously?

    “Trying to understand the way nature works involves a most terrible test of human reasoning ability. It involves subtle trickery, beautiful tightropes of logic on which one has to walk in order not to make a mistake in predicting what will happen.”

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    -Richard Feynman

    Earlier this month, billionaire Yuri Milner and astrophysicist Stephen Hawking teamed up to announce the Breakthrough Starshot, an incredibly ambitious plan to send the first human-created spacecraft to other star systems within our galaxy. While a giant laser array could, feasibly, launch a low mass, microchip-sized spaceship towards another star at some ~20% the speed of light, it’s unclear how such an underpowered, small device like that would ever communicate across the vastness of interstellar space. But Olivier Manuel had an idea that he submitted for Ask Ethan:

    It’s a long shot, but could quantum entanglement be used for communication?

    Imagine you have two coins, where each one can turn up either heads or tails. You have one and I have one, and we’re located extremely far away from each other. We each toss them up in the air, catch them, and slap them down on the table. When we reveal the flip, we fully expect that there’s a 50/50 chance that each one of us will uncover a “heads” result and a 50/50 shot we’ll each get a “tails.” In the normal, unentangled Universe, your results and my results are completely independent of one another: if you get a “heads” result, there’s still a 50/50 shot for my coin to either display “heads” or “tails.” But under some circumstances, these results could be entangled, meaning that if we do this experiment and you get a “heads” result, you’ll know with 100% certainty that my coin is displaying “tails,” even before I told you. You’d know it instantaneously, even if we were separated by light years and not even a single second had passed.

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    The quantum mechanical Bell test for half-integer spin particles. Image credit: Wikimedia Commons user Maksim, under a c.c.a.-s.a.-3.0 license.

    In quantum physics, we normally entangle not coins but individual particles like electrons or photons, where, for example, each photon can have a spin of either +1 or -1. If you measure the spin of one of them, you instantaneously know the spin of the other, even if it’s halfway across the Universe. Until you measure the spin of either one, they both exist in an indeterminate state; but once you measure even one, you immediately know both. We’ve done an experiment on Earth where we’ve separated two entangled photons by many miles, measuring their spins within nanoseconds of one another. What we find is that if we measure one of them to be +1, we know the other to be -1 at least 10,000 times faster than the speed of light would enable us to communicate.

    A quantum optics setup. Image credit Matthew Broome
    A quantum optics setup. Image credit Matthew Broome

    So now to Olivier’s question: could we use this property — quantum entanglement — to communicate from a distant star system to our own? The answer to that is yes, if you consider making a measurement at a distant location a form of communication. But when you say communicate, typically you want to know something about your destination. You could, for example, keep an entangled particle in an indeterminate state, send it aboard a spacecraft bound for the nearest star, and tell it to look for signs of a rocky planet in that star’s habitable zone. If you see one, make a measurement that forces the particle you have to be in the +1 state, and if you don’t see one, make a measurement that forces the particle you have to be in the -1 state.

    3
    Artist’s impression of a sunset from the world Gliese 667 Cc, in a trinary star system. Image credit: ESO/L. Calçada.

    Therefore, you reason, the particle you have back on Earth will then either be in the -1 state when you measure it, telling you that your spacecraft found a rocky planet in the habitable zone, or it will be in the +1 state, telling you that it didn’t find one. If you know the measurement has been made, you should then be able to make your own measurement, and instantly know the state of the other particle, even if it’s many light years away.

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    The wave pattern for electrons passing through a double slit. If you measure “which slit” the electron goes through, you destroy the quantum interference pattern shown here. Image credit: Dr. Tonomura and Belsazar of Wikimedia Commons, under c.c.a.-s.a.-3.0.

    It’s a brilliant plan, but there’s a problem: entanglement only works if you ask a particle, “what state are you in?” If you force an entangled particle into a particular state, you break the entanglement, and the measurement you make on Earth is completely independent of the measurement at the distant star. If you had simply measured the distant particle to be +1 or -1, then your measurement, here on Earth, of either -1 or +1 (respectively) would give you information about the particle located light years away. But by forcing that distant particle to be +1 or -1, that means, no matter the outcome, your particle here on Earth has a 50/50 shot of being +1 or -1, with no bearing on the particle so many light years distant.

    5
    A quantum eraser experiment setup, where two entangled particles are separated and measured. No alterations of one particle at its destination affect the outcome of the other. Image credit: Wikimedia Commons user Patrick Edwin Moran, under c.c.a.-s.a.-3.0.

    This is one of the most confusing things about quantum physics: entanglement can be used to gain information about a component of a system when you know the full state and make a measurement of the other component(s), but not to create-and-send information from one part of an entangled system to the other. As clever of an idea as this is, Olivier, there’s still no faster-than-light communication.

    6
    Quantum teleportation, an effect (erroneously) touted as faster-than-light travel. In reality, no information is being exchanged faster than light. Image credit: American Physical Society, via http://www.csm.ornl.gov/SC99/Qwall.html.

    Quantum entanglement is a wonderful property that we can exploit for any number of purposes, such as for the ultimate lock-and-key security system. But faster-than-light communication? Understanding why that’s not possible requires us to understand this key property of quantum physics: that forcing even part of an entangled system into one state or another doesn’t allow you to gain information about that forcing from measuring the remainder of the system. As Niels Bohr once famously put it:

    If quantum mechanics hasn’t profoundly shocked you, you haven’t understood it yet.

    The Universe plays dice with us all the time, much to Einstein’s chagrin. But even our best attempts to cheat at the game are thwarted by nature itself. If only all referees and umpires were as consistent as the laws of quantum physics!

    See the full article here .

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    “Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

     
  • richardmitnick 8:24 am on May 6, 2016 Permalink | Reply
    Tags: , Quantum Mechanics, Reality and measurement,   

    From Science Alert: “Reality doesn’t exist until we measure it, quantum experiment confirms” 

    ScienceAlert

    Science Alert

    1 JUN 2015 [this cool article just appeared or re-appeared in social media.]
    FIONA MACDONALD

    1

    Australian scientists have recreated a famous experiment and confirmed quantum physics’s bizarre predictions about the nature of reality, by proving that reality doesn’t actually exist until we measure it – at least, not on the very small scale.

    That all sounds a little mind-meltingly complex, but the experiment poses a pretty simple question: if you have an object that can either act like a particle or a wave, at what point does that object ‘decide’?

    Our general logic would assume that the object is either wave-like or particle-like by its very nature, and our measurements will have nothing to do with the answer. But quantum theory predicts that the result all depends on how the object is measured at the end of its journey. And that’s exactly what a team from the Australian National University has now found.

    “It proves that measurement is everything. At the quantum level, reality does not exist if you are not looking at it,” lead researcher and physicist Andrew Truscott said in a press release.

    Known as John Wheeler’s delayed-choice thought experiment, the experiment was first proposed back in 1978 using light beams bounced by mirrors, but back then, the technology needed was pretty much impossible. Now, almost 40 years later, the Australian team has managed to recreate the experiment using helium atoms scattered by laser light.

    “Quantum physics predictions about interference seem odd enough when applied to light, which seems more like a wave, but to have done the experiment with atoms, which are complicated things that have mass and interact with electric fields and so on, adds to the weirdness,” said Roman Khakimov, a PhD student who worked on the experiment.

    To successfully recreate the experiment, the team trapped a bunch of helium atoms in a suspended state known as a Bose-Einstein condensate, and then ejected them all until there was only a single atom left.

    Bose-Einstein-condensates making waves a many-particle phenomenon
    Bose-Einstein-condensates making waves a many-particle phenomenon

    This chosen atom was then dropped through a pair of laser beams, which made a grating pattern that acted as a crossroads that would scatter the path of the atom, much like a solid grating would scatter light.

    They then randomly added a second grating that recombined the paths, but only after the atom had already passed the first grating.

    When this second grating was added, it led to constructive or destructive interference, which is what you’d expect if the atom had travelled both paths, like a wave would. But when the second grating was not added, no interference was observed, as if the atom chose only one path.

    The fact that this second grating was only added after the atom passed through the first crossroads suggests that the atom hadn’t yet determined its nature before being measured a second time.

    So if you believe that the atom did take a particular path or paths at the first crossroad, this means that a future measurement was affecting the atom’s path, explained Truscott. “The atoms did not travel from A to B. It was only when they were measured at the end of the journey that their wave-like or particle-like behaviour was brought into existence,” he said.

    Although this all sounds incredibly weird, it’s actually just a validation for the quantum theory that already governs the world of the very small. Using this theory, we’ve managed to develop things like LEDs, lasers and computer chips, but up until now, it’s been hard to confirm that it actually works with a lovely, pure demonstration such as this one.

    The full results* have been published in Nature Physics.

    *Science paper:
    Wheeler’s delayed-choice gedanken experiment with a single atom

    See the full article here .

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  • richardmitnick 2:36 pm on April 30, 2016 Permalink | Reply
    Tags: , , Quantum Mechanics,   

    From Ethan Siegel: “Can We Use Quantum Entanglement To Communicate Faster-Than-Light?” 

    Starts with a Bang

    4.30.16
    Ethan Siegel

    1
    The concept art of a solar sail (Japan’s IKAROS project) at a distant planet or star system. Image credit: Andrzej Mirecki of Wikimedia Commons, under a c.c.a.-s.a.-3.0 license.

    Earlier this month, billionaire Yuri Milner and astrophysicist Stephen Hawking teamed up to announce the Breakthrough Starshot, an incredibly ambitious plan to send the first human-created spacecraft to other star systems within our galaxy. While a giant laser array could, feasibly, launch a low mass, microchip-sized spaceship towards another star at some ~20% the speed of light, it’s unclear how such an underpowered, small device like that would ever communicate across the vastness of interstellar space. But Olivier Manuel had an idea that he submitted for Ask Ethan:

    It’s a long shot, but could quantum entanglement be used for communication?

    It’s certainly worth considering. Let’s take a look at the idea.

    1
    Two coins: one showing heads and the other showing tails. Image credit: United States Mint, public domain.

    Imagine you have two coins, where each one can turn up either heads or tails. You have one and I have one, and we’re located extremely far away from each other. We each toss them up in the air, catch them, and slap them down on the table. When we reveal the flip, we fully expect that there’s a 50/50 chance that each one of us will uncover a “heads” result and a 50/50 shot we’ll each get a “tails.” In the normal, unentangled Universe, your results and my results are completely independent of one another: if you get a “heads” result, there’s still a 50/50 shot for my coin to either display “heads” or “tails.” But under some circumstances, these results could be entangled, meaning that if we do this experiment and you get a “heads” result, you’ll know with 100% certainty that my coin is displaying “tails,” even before I told you. You’d know it instantaneously, even if we were separated by light years and not even a single second had passed.

    2
    The quantum mechanical Bell test for half-integer spin particles. Image credit: Wikimedia Commons user Maksim, under a c.c.a.-s.a.-3.0 license.

    In quantum physics, we normally entangle not coins but individual particles like electrons or photons, where, for example, each photon can have a spin of either +1 or -1. If you measure the spin of one of them, you instantaneously know the spin of the other, even if it’s halfway across the Universe. Until you measure the spin of either one, they both exist in an indeterminate state; but once you measure even one, you immediately know both. We’ve done an experiment on Earth where we’ve separated two entangled photons by many miles, measuring their spins within nanoseconds of one another. What we find is that if we measure one of them to be +1, we know the other to be -1 at least 10,000 times faster than the speed of light would enable us to communicate.

    3
    By creating two entangled photons from a pre-existing system and separating them by great distances, we can know information about the state of one by measuring the state of the other. Image credit: Melissa Meister, of laser photons through a beam splitter, under c.c.-by-2.0 generic, from https://www.flickr.com/photos/mmeister/3794835939.

    So now to Olivier’s question: could we use this property — quantum entanglement — to communicate from a distant star system to our own? The answer to that is yes, if you consider making a measurement at a distant location a form of communication. But when you say communicate, typically you want to know something about your destination. You could, for example, keep an entangled particle in an indeterminate state, send it aboard a spacecraft bound for the nearest star, and tell it to look for signs of a rocky planet in that star’s habitable zone. If you see one, make a measurement that forces the particle you have to be in the +1 state, and if you don’t see one, make a measurement that forces the particle you have to be in the -1 state.

    4
    Artist’s impression of a sunset from the world Gliese 667 Cc, in a trinary star system. Image credit: ESO/L. Calçada.

    Therefore, you reason, the particle you have back on Earth will then either be in the -1 state when you measure it, telling you that your spacecraft found a rocky planet in the habitable zone, or it will be in the +1 state, telling you that it didn’t find one. If you know the measurement has been made, you should then be able to make your own measurement, and instantly know the state of the other particle, even if it’s many light years away.

    5
    The wave pattern for electrons passing through a double slit. If you measure “which slit” the electron goes through, you destroy the quantum interference pattern shown here. Image credit: Dr. Tonomura and Belsazar of Wikimedia Commons, under c.c.a.-s.a.-3.0.

    It’s a brilliant plan, but there’s a problem: entanglement only works if you ask a particle, “what state are you in?” If you force an entangled particle into a particular state, you break the entanglement, and the measurement you make on Earth is completely independent of the measurement at the distant star. If you had simply measured the distant particle to be +1 or -1, then your measurement, here on Earth, of either -1 or +1 (respectively) would give you information about the particle located light years away. But by forcing that distant particle to be +1 or -1, that means, no matter the outcome, your particle here on Earth has a 50/50 shot of being +1 or -1, with no bearing on the particle so many light years distant.

    6
    A quantum eraser experiment setup, where two entangled particles are separated and measured. No alterations of one particle at its destination affect the outcome of the other. Image credit: Wikimedia Commons user Patrick Edwin Moran, under c.c.a.-s.a.-3.0.

    This is one of the most confusing things about quantum physics: entanglement can be used to gain information about a component of a system when you know the full state and make a measurement of the other component(s), but not to create-and-send information from one part of an entangled system to the other. As clever of an idea as this is, Olivier, there’s still no faster-than-light communication.

    7
    Quantum teleportation, an effect (erroneously) touted as faster-than-light travel. In reality, no information is being exchanged faster than light. Image credit: American Physical Society, via http://www.csm.ornl.gov/SC99/Qwall.html.

    Quantum entanglement is a wonderful property that we can exploit for any number of purposes, such as for the ultimate lock-and-key security system. But faster-than-light communication? Understanding why that’s not possible requires us to understand this key property of quantum physics: that forcing even part of an entangled system into one state or another doesn’t allow you to gain information about that forcing from measuring the remainder of the system. As Niels Bohr once famously put it:

    If quantum mechanics hasn’t profoundly shocked you, you haven’t understood it yet.

    The Universe plays dice with us all the time, much to Einstein’s chagrin. But even our best attempts to cheat at the game are thwarted by nature itself. If only all referees and umpires were as consistent as the laws of quantum physics!

    See the full article here .

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  • richardmitnick 3:38 pm on April 25, 2016 Permalink | Reply
    Tags: , , Quantum Mechanics   

    From phys.org: “Scientists take next step towards observing quantum physics in real life” 

    physdotorg
    phys.org

    April 25, 2016

    1
    An artist’s impression of the membrane coupled to a laser beam. The periodic pattern makes the device highly reflective, while the thin tethers allow for ultra-low mechanical dissipation. Credit: Felix Fricke

    Small objects like electrons and atoms behave according to quantum mechanics, with quantum effects like superposition, entanglement and teleportation. One of the most intriguing questions in modern science is if large objects – like a coffee cup – could also show this behavior. Scientists at the TU Delft have taken the next step towards observing quantum effects at everyday temperatures in large objects. They created a highly reflective membrane, visible to the naked eye, that can vibrate with hardly any energy loss at room temperature. The membrane is a promising candidate to research quantum mechanics in large objects.

    The team has reported their results* in Physical Review Letters.

    Swing

    “Imagine you’re given a single push on a playground swing. Now imagine this single push allows you to gleefully swing non-stop for nearly a decade. We have created a millimeter-sized version of such a swing on a silicon chip”, says prof. Simon Gröblacher of the Kavli Institute of Nanoscience at the TU Delft.

    Tensile stress

    “In order to do this, we deposit ultra-thin films of ceramic onto silicon chips. This allows us to engineer a million psi of tensile stress, which is the equivalent of 10,000 times the pressure in a car tire, into millimeter-sized suspended membranes that are only eight times thicker than the width of DNA”, explains dr. Richard Norte, lead author of the publication. “Their immense stored energies and ultra-thin geometry mean that these membranes can oscillate for tremendously long times by dissipating only small amounts of energy.”

    Super-mirrors

    To efficiently monitor the motion of the membranes with a laser they need to be extremely reflective. In such a thin structure, this can only be achieved by creating a meta-material through etching a microscopic pattern into the membrane. “We actually made the thinnest super-mirrors ever created, with a reflectivity exceeding 99%. In fact, these membranes are also the world’s best force sensors at room temperature, as they are sensitive enough to measure the gravitational pull between two people 100 km apart from each other”, Richard Norte says.

    Room temperture

    “The high-reflectivity, in combination with the extreme isolation, allows us to overcome a major hurdle towards observing quantum physics with massive objects, for the first time, at room temperature”, says Gröblacher. Because even a single quantum of vibration is enough to heat up and destroy the fragile quantum nature of large objects (in a process called decoherence), researchers have relied on large cryogenic systems to cool and isolate their quantum devices from the heat present in our everyday environments. Creating massive quantum oscillators which are robust to decoherence at room temperature has remained an elusive feat for physicists.

    This is extremely interesting from a fundamental theoretical point of view. One of the strangest predictions of quantum mechanics is that things can be in two places at the same time. Such quantum ‘superpositions’ have now been clearly demonstrated for tiny objects such as electrons or atoms, where we now know that quantum theory works very well.

    Coffee cup

    But quantum mechanics also tells us that the same rules should also apply for macroscopic objects: a coffee cup can be on the table and in the dishwasher at the same time, or Schrödinger’s cat can be in a quantum superposition of being dead and alive. This is however not something we see in our daily lives: the coffee cup is either clean or dirty and the cat is either dead or alive. Experimentally demonstrating a proverbial cat that is simultaneously dead and alive at ambient temperatures is still an open question in quantum mechanics. The steps taken in this research might allow to eventually observe ‘quantum cats’ on everyday life scales and temperatures.

    *Science paper:
    Mechanical Resonators for Quantum Optomechanics Experiments at Room Temperature

    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 4:58 pm on April 17, 2016 Permalink | Reply
    Tags: , , Quantum Mechanics   

    From NOVA: “Can Quantum Computing Reveal the True Meaning of Quantum Mechanics?” 

    PBS NOVA

    NOVA

    24 Jun 2015 [NOVA just put this up in social media.]
    Scott Aaronson

    Quantum mechanics says not merely that the world is probabilistic, but that it uses rules of probability that no science fiction writer would have had the imagination to invent. These rules involve complex numbers, called “amplitudes,” rather than just probabilities (which are real numbers between 0 and 1). As long as a physical object isn’t interacting with anything else, its state is a huge wave of these amplitudes, one for every configuration that the system could be found in upon measuring it. Left to itself, the wave of amplitudes evolves in a linear, deterministic way. But when you measure the object, you see some definite configuration, with a probability equal to the squared absolute value of its amplitude. The interaction with the measuring device “collapses” the object to whichever configuration you saw.

    Those, more or less, are the alien laws that explain everything from hydrogen atoms to lasers and transistors, and from which no hint of an experimental deviation has ever been found, from the 1920s until today. But could this really be how the universe operates? Is the “bedrock layer of reality” a giant wave of complex numbers encoding potentialities—until someone looks? And what do we mean by “looking,” anyway?

    1
    Could quantum computing help reveal what the laws of quantum mechanics really mean? Adapted from an image by Flickr user Politropix under a Creative Commons license.

    There are different interpretive camps within quantum mechanics, which have squabbled with each other for generations, even though, by design, they all lead to the same predictions for any experiment that anyone can imagine doing. One interpretation is Many Worlds, which says that the different possible configurations of a system (when far enough apart) are literally parallel universes, with the “weight” of each universe given by its amplitude.

    Multiverse. Image credit: public domain, retrieved from https://pixabay.com/
    Multiverse. Image credit: public domain, retrieved from https://pixabay.com/

    In this view, the whole concept of measurement—and of the amplitude waves collapsing on measurement—is a sort of illusion, playing no fundamental role in physics. All that ever happens is linear evolution of the entire universe’s amplitude wave—including a part that describes the atoms of your body, which (the math then demands) “splits” into parallel copies whenever you think you’re making a measurement. Each copy would perceive only itself and not the others. While this might surprise people, Many Worlds is seen by many (certainly by its proponents, who are growing in number) as the conservative option: the one that adds the least to the bare math.

    A second interpretation is Bohmian mechanics, which agrees with Many Worlds about the reality of the giant amplitude wave, but supplements it with a “true” configuration that a physical system is “really” in, regardless of whether or not anyone measures it. The amplitude wave pushes around the “true” configuration in a way that precisely matches the predictions of quantum mechanics. A third option is Niels Bohr’s original “Copenhagen Interpretation,” which says—but in many more words!—that the amplitude wave is just something in your head, a tool you use to make predictions. In this view, “reality” doesn’t even exist prior to your making a measurement of it—and if you don’t understand that, well, that just proves how mired you are in outdated classical ways of thinking, and how stubbornly you insist on asking illegitimate questions.

    But wait: if these interpretations (and others that I omitted) all lead to the same predictions, then how could we ever decide which one is right? More pointedly, does it even mean anything for one to be right and the others wrong, or are these just different flavors of optional verbal seasoning on the same mathematical meat? In his recent quantum mechanics textbook, the great physicist Steven Weinberg reviews the interpretive options, ultimately finding all of them wanting. He ends with the hope that new developments in physics will give us better options. But what could those new developments be?

    In the last few decades, the biggest new thing in quantum mechanics has been the field of quantum computing and information. The goal here, you might say, is to “put the giant amplitude wave to work”: rather than obsessing over its true nature, simply exploit it to do calculations faster than is possible classically, or to help with other information-processing tasks (like communication and encryption). The key insight behind quantum computing was articulated by Richard Feynman in 1982: to write down the state of n interacting particles each of which could be in either of two states, quantum mechanics says you need 2n amplitudes, one for every possible configuration of all n of the particles. Chemists and physicists have known for decades that this can make quantum systems prohibitively difficult to simulate on a classical computer, since 2n grows so rapidly as a function of n.

    But if so, then why not build computers that would themselves take advantage of giant amplitude waves? If nothing else, such computers could be useful for simulating quantum physics! What’s more, in 1994, Peter Shor discovered that such a machine would be useful for more than physical simulations: it could also be used to factor large numbers efficiently, and thereby break most of the cryptography currently used on the Internet. Genuinely useful quantum computers are still a ways away, but experimentalists have made dramatic progress, and have already demonstrated many of the basic building blocks.

    I should add that, for my money, the biggest application of quantum computers will be neither simulation nor codebreaking, but simply proving that this is possible at all! If you like, a useful quantum computer would be the most dramatic demonstration imaginable that our world really does need to be described by a gigantic amplitude wave, that there’s no way around that, no simpler classical reality behind the scenes. It would be the final nail in the coffin of the idea—which many of my colleagues still defend—that quantum mechanics, as currently understood, must be merely an approximation that works for a few particles at a time; and when systems get larger, some new principle must take over to stop the exponential explosion.

    But if quantum computers provide a new regime in which to probe quantum mechanics, that raises an even broader question: could the field of quantum computing somehow clear up the generations-old debate about the interpretation of quantum mechanics? Indeed, could it do that even before useful quantum computers are built?

    At one level, the answer seems like an obvious “no.” Quantum computing could be seen as “merely” a proposed application of quantum mechanics as that theory has existed in physics books for generations. So, to whatever extent all the interpretations make the same predictions, they also agree with each other about what a quantum computer would do. In particular, if quantum computers are built, you shouldn’t expect any of the interpretive camps I listed before to concede that its ideas were wrong. (More likely that each camp will claim its ideas were vindicated!)

    At another level, however, quantum computing makes certain aspects of quantum mechanics more salient—for example, the fact that it takes 2n amplitudes to describe n particles—and so might make some interpretations seem more natural than others. Indeed that prospect, more than any application, is why quantum computing was invented in the first place. David Deutsch, who’s considered one of the two founders of quantum computing (along with Feynman), is a diehard proponent of the Many Worlds interpretation, and saw quantum computing as a way to convince the world (at least, this world!) of the truth of Many Worlds. Here’s how Deutsch put it in his 1997 book “The Fabric of Reality”:

    “Logically, the possibility of complex quantum computations adds nothing to a case [for the Many Worlds Interpretation] that is already unanswerable. But it does add psychological impact. With Shor’s algorithm, the argument has been writ very large. To those who still cling to a single-universe world-view, I issue this challenge: explain how Shor’s algorithm works. I do not merely mean predict that it will work, which is merely a matter of solving a few uncontroversial equations. I mean provide an explanation. When Shor’s algorithm has factorized a number, using 10500 or so times the computational resources that can be seen to be present, where was the number factorized? There are only about 1080 atoms in the entire visible universe, an utterly minuscule number compared with 10500. So if the visible universe were the extent of physical reality, physical reality would not even remotely contain the resources required to factorize such a large number. Who did factorize it, then? How, and where, was the computation performed?”

    As you might imagine, not all researchers agree that a quantum computer would be “psychological evidence” for Many Worlds, or even that the two things have much to do with each other. Yes, some researchers reply, a quantum computer would take exponential resources to simulate classically (using any known algorithm), but all the interpretations agree about that. And more pointedly: thinking of the branches of a quantum computation as parallel universes might lead you to imagine that a quantum computer could solve hard problems in an instant, by simply “trying each possible solution in a different universe.” That is, indeed, how most popular articles explain quantum computing, but it’s also wrong!

    The issue is this: suppose you’re facing some arbitrary problem—like, say, the Traveling Salesman problem, of finding the shortest path that visits a collection of cities—that’s hard because of a combinatorial explosion of possible solutions. It’s easy to program your quantum computer to assign every possible solution an equal amplitude. At some point, however, you need to make a measurement, which returns a single answer. And if you haven’t done anything to boost the amplitude of the answer you want, then you’ll see merely a random answer—which, of course, you could’ve picked for yourself, with no quantum computer needed!

    For this reason, the only hope for a quantum-computing advantage comes from interference: the key aspect of amplitudes that has no classical counterpart, and indeed, that taught physicists that the world has to be described with amplitudes in the first place. Interference is customarily illustrated by the double-slit experiment, in which we shoot a photon at a screen with two slits in it, and then observe where the photon lands on a second screen behind it. What we find is that there are certain “dark patches” on the second screen where the photon never appears—and yet, if we close one of the slits, then the photon can appear in those patches. In other words, decreasing the number of ways for the photon to get somewhere can increase the probability that it gets there! According to quantum mechanics, the reason is that the amplitude for the photon to land somewhere can receive a positive contribution from the first slit, and a negative contribution from the second. In that case, if both slits are open, then the two contributions cancel each other out, and the photon never appears there at all. (Because the probability is the amplitude squared, both negative and positive amplitudes correspond to positive probabilities.)

    Likewise, when designing algorithms for quantum computers, the goal is always to choreograph things so that, for each wrong answer, some of the contributions to its amplitude are positive and others are negative, so on average they cancel out, leaving an amplitude close to zero. Meanwhile, the contributions to the right answer’s amplitude should reinforce each other (being, say, all positive, or all negative). If you can arrange this, then when you measure, you’ll see the right answer with high probability.

    It was precisely by orchestrating such a clever interference pattern that Peter Shor managed to devise his quantum algorithm for factoring large numbers. To do so, Shor had to exploit extremely specific properties of the factoring problem: it was not just a matter of “trying each possible divisor in a different parallel universe.” In fact, an important 1994 theorem of Bennett, Bernstein, Brassard, and Vazirani shows that what you might call the “naïve parallel-universe approach” never yields an exponential speed improvement. The naïve approach can reveal solutions in only the square root of the number of steps that a classical computer would need, an important phenomenon called the Grover speedup. But that square-root advantage turns out to be the limit: if you want to do better, then like Shor, you need to find something special about your problem that lets interference reveal its answer.

    What are the implications of these facts for Deutsch’s argument that only Many Worlds can explain how a quantum computer works? At the least, we should say that the “exponential cornucopia of parallel universes” almost always hides from us, revealing itself only in very special interference experiments where all the “universes” collaborate, rather than any one of them shouting above the rest. But one could go even further. One could say: To whatever extent the parallel universes do collaborate in a huge interference pattern to reveal (say) the factors of a number, to that extent they never had separate identities as “parallel universes” at all—even according to the Many Worlds interpretation! Rather, they were just one interfering, quantum-mechanical mush. And from a certain perspective, all the quantum computer did was to linearly transform the way in which we measured that mush, as if we were rotating it to see it from a more revealing angle. Conversely, whenever the branches do act like parallel universes, Many Worlds itself tells us that we only observe one of them—so from a strict empirical standpoint, we could treat the others (if we liked) as unrealized hypotheticals. That, at least, is the sort of reply a modern Copenhagenist might give, if she wanted to answer Deutsch’s argument on its own terms.

    There are other aspects of quantum information that seem more “Copenhagen-like” than “Many-Worlds-like”—or at least, for which thinking about “parallel universes” too naïvely could lead us astray. So for example, suppose Alice sends n quantum-mechanical bits (or qubits) to Bob, then Bob measures qubits in any way he likes. How many classical bits can Alice transmit to Bob that way? If you remember that n qubits require 2n amplitudes to describe, you might conjecture that Alice could achieve an incredible information compression—“storing one bit in each parallel universe.” But alas, an important result called Holevo’s Theorem says that, because of the severe limitations on what Bob learns when he measures the qubits, such compression is impossible. In fact, by sending n qubits to Bob, Alice can reliably communicate only n bits (or 2n bits, if Alice and Bob shared quantum correlations in advance), essentially no better than if she’d sent the bits classically. So for this task, you might say, the amplitude wave acts more like “something in our heads” (as the Copenhagenists always said) than like “something out there in reality” (as the Many-Worlders say).

    But the Many-Worlders don’t need to take this lying down. They could respond, for example, by pointing to other, more specialized communication problems, in which it’s been proven that Alice and Bob can solve using exponentially fewer qubits than classical bits. Here’s one example of such a problem, drawing on a 1999 theorem of Ran Raz and a 2010 theorem of Boaz Klartag and Oded Regev: Alice knows a vector in a high-dimensional space, while Bob knows two orthogonal subspaces. Promised that the vector lies in one of the two subspaces, can you figure out which one holds the vector? Quantumly, Alice can encode the components of her vector as amplitudes—in effect, squeezing n numbers into exponentially fewer qubits. And crucially, after receiving those qubits, Bob can measure them in a way that doesn’t reveal everything about Alice’s vector, but does reveal which subspace it lies in, which is the one thing Bob wanted to know.

    So, do the Many Worlds become “real” for these special problems, but retreat back to being artifacts of the math for ordinary information transmission?

    To my mind, one of the wisest replies came from the mathematician and quantum information theorist Boris Tsirelson, who said: “a quantum possibility is more real than a classical possibility, but less real than a classical reality.” In other words, this is a new ontological category, one that our pre-quantum intuitions simply don’t have a good slot for. From this perspective, the contribution of quantum computing is to delineate for which tasks the giant amplitude wave acts “real and Many-Worldish,” and for which other tasks it acts “formal and Copenhagenish.” Quantum computing can give both sides plenty of fresh ammunition, without handing an obvious victory to either.

    So then, is there any interpretation that flat-out doesn’t fare well under the lens of quantum computing? While some of my colleagues will strongly disagree, I’d put forward Bohmian mechanics as a candidate. Recall that David Bohm’s vision was of real particles, occupying definite positions in ordinary three-dimensional space, but which are jostled around by a giant amplitude wave in a way that perfectly reproduces the predictions of quantum mechanics. A key selling point of Bohm’s interpretation is that it restores the determinism of classical physics: all the uncertainty of measurement, we can say in his picture, arises from lack of knowledge of the initial conditions. I’d describe Bohm’s picture as striking and elegant—as long as we’re only talking about one or two particles at a time.

    But what happens if we try to apply Bohmian mechanics to a quantum computer—say, one that’s running Shor’s algorithm to factor a 10,000-digit number, using hundreds of thousands of particles? We can do that, but if we do, talking about the particles’ “real locations” will add spectacularly little insight. The amplitude wave, you might say, will be “doing all the real work,” with the “true” particle positions bouncing around like comically-irrelevant fluff. Nor, for that matter, will the bouncing be completely deterministic. The reason for this is technical: it has to do with the fact that, while particles’ positions in space are continuous, the 0’s and 1’s in a computer memory (which we might encode, for example, by the spins of the particles) are discrete. And one can prove that, if we want to reproduce the predictions of quantum mechanics for discrete systems, then we need to inject randomness at many times, rather than only at the beginning of the universe.

    But it gets worse. In 2005, I proved a theorem that says that, in any theory like Bohmian mechanics, if you wanted to calculate the entire trajectory of the “real” particles, you’d need to solve problems that are thought to be intractable even for quantum computers. One such problem is the so-called collision problem, where you’re given a cryptographic hash function (a function that maps a long message to a short “hash value”) and asked to find any two messages with the same hash. In 2002, I proved that, at least if you use the “naïve parallel-universe” approach, any quantum algorithm for the collision problem requires at least ~H1/5 steps, where H is the number of possible hash values. (This lower bound was subsequently improved to ~H1/3 by Yaoyun Shi, exactly matching an upper bound of Brassard, Høyer, and Tapp.) By contrast, if (with godlike superpower) you could somehow see the whole histories of Bohmian particles, you could solve the collision problem almost instantly.

    What makes this interesting is that, if you ask to see the locations of Bohmian particles at any one time, you won’t find anything that you couldn’t have easily calculated with a standard, garden-variety quantum computer. It’s only when you ask for the particles’ locations at multiple times—a question that Bohmian mechanics answers, but that ordinary quantum mechanics rejects as meaningless—that you’re able to see multiple messages with the same hash, and thereby solve the collision problem.

    My conclusion is that, if you believe in the reality of Bohmian trajectories, you believe that Nature does even more computational work than a quantum computer could efficiently simulate—but then it hides the fruits of its labor where no one can ever observe it. Now, this sits uneasily with a principle that we might call “Occam’s Razor with Computational Aftershave.” Namely: In choosing a picture of physical reality, we should be loath to posit computational effort on Nature’s part that vastly exceeds what could ever in principle be observed. (Admittedly, some people would probably argue that the Many Worlds interpretation violates my “aftershave principle” even more flagrantly than Bohmian mechanics does! But that depends, in part, on what we count as “observation”: just our observations, or also the observations of any parallel-universe doppelgängers?)

    Could future discoveries in quantum computing theory settle once and for all, to every competent physicist’s satisfaction, “which interpretation is the true one”? To me, it seems much more likely that future insights will continue to do what the previous ones did: broaden our language, strip away irrelevancies, clarify the central issues, while still leaving plenty to argue about for people who like arguing. In the end, asking how quantum computing affects the interpretation of quantum mechanics is sort of like asking how classical computing affects the debate about whether the mind is a machine. In both cases, there was a range of philosophical positions that people defended before a technology came along, and most of those positions still have articulate defenders after the technology. So, by that standard, the technology can’t be said to have “resolved” much! Yet the technology is so striking that even the idea of it—let alone the thing itself—can shift the terms of the debate, which analogies people use in thinking about it, which possibilities they find natural and which contrived. This might, more generally, be the main way technology affects philosophy.

    See the full article here .

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  • richardmitnick 9:11 am on April 17, 2016 Permalink | Reply
    Tags: , , Quantum Mechanics,   

    From Science Alert: “Physicists find a way to probe the quantum realm without wrecking everything” 

    ScienceAlert

    Science Alert

    15 APR 2016
    BRENDAN COLE

    1
    solarseven/Shutterstock.com

    In 1930, German theoretical physicist Werner Heisenberg came up with a thought experiment, now known as Heisenberg’s microscope, to try to show why it’s impossible to measure an atom’s location with unlimited precision. He imagined trying to measure the position of something like an atom by shooting light at it.

    Light travels as a wave, and Heisenberg knew that different wavelengths could give you different degrees of confidence when used to measure where something is in space. Short wavelengths can give a more precise measurement than long ones, so you’d want to use light with a tiny wavelength to measure where an atom is, since atoms are really small. But there’s a problem: light also carries momentum, and short wavelengths carry more momentum than long ones.

    That means if you use light with a short wavelength to find the atom, you’ll hit the atom with all of that momentum, and that kicks it around and risks completely changing its location (and other properties) in the process. Use longer wavelengths, and you’ll move the atom less, but you’ll also be more uncertain about your measurement.

    You’re in a bind: any measurement changes what you’re measuring, and better measurements lead to bigger changes.

    It’s also possible to prepare atoms in what’s called an entangled state, which means they act cooperatively like a single atom, no matter how far away they are from each other. If you push one, the rest move like you pushed them all individually. And if you mess up one atom by shooting some light at it, you generally mess up the whole collection.

    In the past, these two effects made it impossible to measure how entangled atoms are arranged without destroying the arrangement and the entanglement – which were presumably prepared for some specific purpose, like to make a quantum computer.

    But now, physicists led by T. J. Elliott from the University of Oxford in the UK have proposed a way to measure large-scale properties of a group of entangled atoms without messing up the entanglement. It isn’t measuring individual atoms – that’s permanently off-limits – but it’s more than physicists have managed to do before.

    Usually, when physicists entangle atoms, they have to be careful that the atoms are all more or less the same when they start out. If there are lots of different kinds of atoms in there, they become a lot harder to match up, so the entanglement becomes more fragile.

    But it’s still possible to make stable groups of entangled atoms that have some outliers among them that are unlike the main group, and the paper’s authors have shown that these outliers can be used to measure things about the main group without messing up their entanglement.

    This includes really basic information like the density of atoms – how closely they are to one another – while they’re entangled, which historically has been out of physicists’ reach in individual experiments.

    Before, physicists had to measure a whole bunch of the entangled atoms really quickly, and they’d have to accept that they were changing things around as soon as they measured that first atom. More measurements might check more atoms, but they’d be increasingly uncertain as time went on.

    Now, all they have to do is measure what the outliers are doing and they can figure out how the atoms are distributed without the chaos. Within some limits, knowledge about the atoms’ density gets better – not worse – as more measurements are made.

    Admittedly, measurements still change things a little bit (light is still being used and Heisenberg’s microscope still applies) but the measurements won’t wreck the whole system like they would have before.

    This method of measuring the outliers is a window into a new realm for physicists, who could previously only see what entangled atoms did, not what they’re doing.

    The researchers simulated a simple system as a proof-of-concept, but they showed mathematically that this should work with a wide range of quantum systems where entanglement plays a key role. And small changes to the method could make it possible to measure properties like the magnetisation of entangled atoms, instead of just their density.

    All with atoms that shouldn’t be in the group in the first place. Not bad, physicists. Not bad.

    The research has been published in the journal Physical Review A.

    Science paper:
    Nondestructive probing of means, variances, and correlations of ultracold-atomic-system densities via qubit impurities

    See the full article here .

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  • richardmitnick 8:40 pm on March 22, 2016 Permalink | Reply
    Tags: , , Quantum Mechanics,   

    From Ethan Siegel: “10 Quantum Truths About Our Universe” 

    Starts with a bang
    Starts with a Bang

    3.22.16
    Sabine Hossenfelder

    Hydrogen wave function Wikimedia Commons user PoorLeno
    Hydrogen wave function Wikimedia Commons user PoorLeno

    Even most of the pros don’t know all 10.

    This post was contributed to Starts With A Bang by Sabine Hossenfelder. Sabine is a theoretical physicist specialized in quantum gravity and high energy physics. She also freelance writes about science.

    “In fact, the mere act of opening the box will determine the state of the cat, although in this case there were three determinate states the cat could be in: these being Alive, Dead, and Bloody Furious.” -Terry Pratchett

    From the moment that it was discovered that the macroscopic, classical rules that governed electricity, magnetism and light didn’t necessarily apply to the smallest, subatomic scales, a whole new view of the Universe became accessible to humanity. This quantum picture is much larger and all-encompassing than most people realize, including many professionals. Here are ten essentials of quantum mechanics that may cause you to re-examine how you picture our Universe, on the smallest scales and beyond.

    1.) Everything is quantum.

    It’s not like some things are quantum mechanical and others are not. Everything obeys the same laws of quantum mechanics — it’s just that quantum effects of large objects are very hard to notice. This is why quantum mechanics was a latecomer to the development of theoretical physics: it wasn’t until physicists had to explain why electrons sit on shells around the atomic nucleus that quantum mechanics became necessary to make accurate predictions.

    2.) Quantization doesn’t necessarily imply discreteness.

    Quanta” are discrete chunks, by definition, but not everything becomes chunky or indivisible on short scales. Electromagnetic waves are made of quanta called “photons,” so the waves can be thought of as being discretized. And electron shells around the atomic nucleus can only have certain discrete radii. But other particle properties do not become discrete even in a quantum theory. The position of electrons in the conducting band of a metal for example is not discrete — the electron can occupy any continuous location within the band. And the energy values of the photons that make up electromagnetic waves are not discrete either. For this reason, quantizing gravity — should we finally succeed at it — does not necessarily mean that space and time have to be made discrete. (But, on the other hand, they might be.)

    3.) Entanglement not the same as superposition.

    A quantum superposition is the ability of a system to be in two different states at the same time, and yet, when measured, one always finds a particular state, never a superposition. Entanglement on the other hand is a correlation between two or more parts of a system — something entirely different. Superpositions are not fundamental: whether a state is or isn’t a superposition depends on what you want to measure. A state can for example be in a superposition of positions and not in a superposition of momenta — so the whole concept is ambiguous. Entanglement on the other hand is unambiguous: it is an intrinsic property of each system and the so-far best known measure of a system’s quantum-ness. (For more details, read What is the difference between entanglement and superposition?)

    2
    A beam splitter, one mechanism for creating entangled photons. Image credit: Wikimedia Commons user Zaereth.

    4.) There is no spooky action at a distance.

    Nowhere in quantum mechanics is information ever transmitted non-locally, so that it jumps over a stretch of space without having to go through all places in between. Entanglement is itself non-local, but it doesn’t do any action — it is a correlation that is not connected to non-local transfer of information or any other observable. When you see a study where two entangled photons are separated by a great distance and then the spin of each one is measured, there is no information being transferred faster than the speed of light. In fact, if you attempt to bring the results of two observations together (which is information transmission), that information can only travel at the speed of light, no faster! What constitutes “information” was a great source confusion in the early days of quantum mechanics, but we know today that the theory can be made perfectly compatible with Einstein’s theory of Special Relativity in which information cannot be transferred faster than the speed of light.

    A quantum optics setup. Image credit Matthew Broome
    A quantum optics setup. Image credit Matthew Broome

    5.) Quantum physics an active research area.

    It’s not like quantum mechanics is yesterday’s news. True, the theory originated more than a century ago. But many aspects of it became testable only with modern technology. Quantum optics, quantum information, quantum computing, quantum cryptography, quantum thermodynamics, and quantum metrology are all recently formed and presently very active research areas. With the new capabilities brought about by these technologies, interest in the foundations of quantum mechanics has been reignited.

    6.) Einstein didn’t deny it.

    Contrary to popular opinion, Einstein was not a quantum mechanics denier. He couldn’t possibly be — the theory was so successful early on that no serious scientist could dismiss it. (In fact, it was his Nobel-winning discovery of the photoelectric effect, proving that photons acted as particles as well as waves, that was one of the foundational discoveries of quantum mechanics.) Einstein instead argued that the theory was incomplete, and believed the inherent randomness of quantum processes must have a deeper explanation. It was not that he thought the randomness was wrong, he just thought that this wasn’t the end of the story. For an excellent clarification of Einstein’s views on quantum mechanics, I recommend George Musser’s article What Einstein Really Thought about Quantum Mechanics (paywalled, sorry).

    7.) It’s all about uncertainty.

    The central postulate of quantum mechanics is that there are pairs of observables that cannot simultaneously be measured, like for example the position and momentum of a particle. These pairs are called “conjugate variables,” and the impossibility to measure both their values precisely is what makes all the difference between a quantized and a non-quantized theory. In quantum mechanics, this uncertainty is fundamental, not due to experimental shortcomings. One of the most bizarre manifestations of this is the uncertainty between energy and time, which means that unstable particles (with a short lifetime) have inherently uncertain masses, thanks to Einstein’s E=mc2. Particles like the Higgs boson, the W-and-Z bosons and the top quarks all have masses that are intrinsically uncertain by 1–10% because of their short lifetimes.

    CERN ATLAS Higgs Event
    CERN ATLAS Higgs Event

    4
    Image credit: the LEP collaboration and various sub-collaborations, 2005, via http://arxiv.org/abs/hep-ex/0509008. Precision Electroweak Measurements on the Z Resonance. Note that the Z-particle appears with a “width” in energy.

    8.) Quantum effects are not necessarily small…

    We do not normally observe quantum effects on long distances because the necessary correlations are very fragile. Treat them carefully enough however, and quantum effects can persist over long distances. Photons have for example been entangled over separations as much as several hundreds of kilometers. In Bose-Einstein condensates, a degenerate state of matter found at cold temperatures, up to several million of atoms have been brought into one coherent quantum state. And finally, some researchers even believe that dark matter may have quantum effects which span across entire galaxies.

    9.) …but they dominate the small scales.

    In quantum mechanics, every particle is also a wave and every wave is also a particle. The effects of quantum mechanics become very pronounced once one observes a particle on distances that are comparable to the associated wavelength. This is why atomic and subatomic physics cannot be understood without quantum mechanics, whereas planetary orbits are effectively unchanged by quantum behavior.

    10.) Schrödinger’s cat is dead. Or alive. But not both.

    It was not well-understood in the early days of quantum mechanics, but the quantum behavior of macroscopic objects decays very rapidly. This “decoherence” is due to constant interactions with the environment which are, in relatively warm and dense places like those necessary for life, impossible to avoid. This explains that what we think of as a measurement doesn’t require a human; simply interacting with the environment counts. It also explains why bringing large objects into superpositions of two different states is therefore extremely difficult and the superposition fades rapidly. The heaviest object that has so far been brought into a superposition of locations is a carbon-60 molecule, while the more ambitious have proposed to do this experiment for viruses or even heavier creatures like bacteria. Thus, the paradox that Schrödinger’s cat once raised — the transfer of a quantum superposition (the decaying atom) to a large object (the cat) — has been resolved. We now understand that while small things like atoms can exist in superpositions for extended amounts of time, a large object would settle extremely rapidly in one particular state. That’s why we never see cats that are both dead and alive.

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    “Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

     
  • richardmitnick 9:59 am on February 22, 2016 Permalink | Reply
    Tags: , , Quantum Mechanics   

    From COSMOS: “A different picture of quantum surrealism” 

    Cosmos Magazine bloc

    COSMOS

    22 Feb 2016
    Cathal O’Connell

    Quantum Surrealism COSMOS
    No image credit found.

    New research supports an old, more intuitive theory of how sub-atomic particles behave. Cathal O’Connell explains.

    With its ideas of particles zipping in and out of existence, quantum mechanics is probably the kookiest-sounding theory in science. And our understanding of it is little helped by the mysterious “probability fields” most physicists say dictate the zipping.

    But a more intuitive picture may lie beneath. As new research demonstrates, beneath the shroud of probability, particles can in fact be viewed as behaving like billiard balls rolling along a table – although in surreal fashion.

    The result helps resurrect an 80-year-old picture of quantum mechanics, and provides one of the most stirring demonstrations yet of an effect [Albert] Einstein called “spooky action at a distance”.

    The work, reported in Science Advances, is a new version of the most famous experiment in quantum mechanics, in which particles of light, called photons, are fired at two slits before being detected on a screen.

    Hog-tied by Heisenberg’s uncertainty principle, for decades physicists thought they could never know which slit a particular photon went through – any attempted measurement stops it in its tracks.

    But in 2011, physicist Aephraim Steinberg at the University of Toronto achieved the seemingly impossible by tracking the trajectories of photons using a series of “weak” measurements, gentle enough not to disturb their position.

    This method showed trajectories that looked similar to classical ones – like those of balls flying through the air.

    Although it was a seemingly outstanding result, some physicists were not convinced, highlighting the experiment’s inability to deal with entanglement (where two particles, in this case photons, are intimately connected so that measurement on one instantly affects the other, no matter how far away it is).

    The critics pointed out that doing the same experiment with two entangled photons would lead to a contradiction – such as the photon’s trajectory being measured as going through the top slit, but the photon itself hitting the bottom of the detector (as if it came from the bottom slit). They coined the term “surreal trajectories” to describe this result.

    Now Steinberg’s team has achieved the experiment for entangled photons, and shown how the surreal behaviour is caused by the “spooky” influence of the other particle.

    The team first entangled two photons, then sent one of the pair through the regular two-slit apparatus, and the other through an apparatus that monitored polarisation – the plane the light waves are travelling in.

    Weirdly, the choice made by the experimenters in how to measure the polarisation determined which slit the first photon went through – as if interfering with one particle caused the other to change direction instantaneously.

    This kind of bizarre phenomenon is exactly what Einstein had in mind when he dubbed it “spooky action”. Physicists have seen evidence of it before, but never in such a direct fashion.

    The results bolster a non-standard interpretation of quantum mechanics, which throws out the notion of abstract probability fields altogether.

    First put forward by Louis de Broglie in 1927, the interpretation treats quantum objects just like classical particles, but imagines them riding like a surfer on top of a so-called pilot wave.

    The wave is still probabilistic, but the particle does take a real trajectory from source to target.

    The new work does not disprove the standard “probabilistic” view of quantum mechanics, but it does highlight that the pilot-wave interpretation is perfectly valid too. That is “something that’s not recognised by a large part of the physics community”, says Howard Wiseman, a physicist at Griffith University who proposed the experiment.

    It may be easier to visualise real trajectories, rather than abstract wave function collapses.

    “I would phrase it in terms of having different pictures,” says Steinberg. “Different pictures can be useful. They can help shape better intuitions.”

    See the full article here .

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