171. Hasancan özdenli

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172. Possibility of Cloning Quantum Information from the Past

Dec. 8, 2013 — Popular television shows such as "Doctor Who" have brought the idea of time travel into the vernacular of popular culture. But problem of time travel is even more complicated than one might think. LSU's Mark Wilde has shown that it would theoretically be possible for time travelers to copy quantum data from the past.   It all started when David Deutsch, a pioneer of quantum computing and a physicist at Oxford, came up with a simplified model of time travel to deal with the paradoxes that would occur if one could travel back in time. For example, would it be possible to travel back in time to kill one's grandfather? In the Grandfather paradox, a time traveler faces the problem that if he kills his grandfather back in time, then he himself is never born, and consequently is unable to travel through time to kill his grandfather, and so on. Some theorists have used this paradox to argue that it is actually impossible to change the past. "The question is, how would you have existed in the first place to go back in time and kill your grandfather?" said Mark Wilde, an LSU assistant professor with a joint appointment in the Department of Physics and Astronomy and with the Center for Computation and Technology, or CCT. Deutsch solved the Grandfather paradox originally using a slight change to quantum theory, proposing that you could change the past as long as you did so in a self-consistent manner. "Meaning that, if you kill your grandfather, you do it with only probability one-half," Wilde said. "Then, he's dead with probability one-half, and you are not born with probability one-half, but the opposite is a fair chance. You could have existed with probability one-half to go back and kill your grandfather." But the Grandfather paradox is not the only complication with time travel. Another problem is the no-cloning theorem, or the no "subatomic Xerox-machine" theorem, known since 1982. This theorem, which is related to the fact that one cannot copy quantum data at will, is a consequence of Heisenberg's famous Uncertainty Principle, by which one can measure either the position of a particle or its momentum, but not both with unlimited accuracy. According to the Uncertainty Principle, it is thus impossible to have a subatomic Xerox-machine that would take one particle and spit out two particles with the same position and momentum -- because then you would know too much about both particles at once. "We can always look at a paper, and then copy the words on it. That's what we call copying classical data," Wilde said. "But you can't arbitrarily copy quantum data, unless it takes the special form of classical data. This no-cloning theorem is a fundamental part of quantum mechanics -- it helps us reason how to process quantum data. If you can't copy data, then you have to think of everything in a very different way." But what if a Deutschian closed timelike curve did allow for copying of quantum data to many different points in space? According to Wilde, Deutsch suggested in his late 20th century paper that it should be possible to violate the fundamental no-cloning theorem of quantum mechanics. Now, Wilde and collaborators at the University of Southern California and the Autonomous University of Barcelona have advanced Deutsch's 1991 work with a recent paper in Physical Review Letters. The new approach allows for a particle, or a time traveler, to make multiple loops back in time -- something like Bruce Willis' travels in the Hollywood film "Looper." "That is, at certain locations in spacetime, there are wormholes such that, if you jump in, you'll emerge at some point in the past," Wilde said. "To the best of our knowledge, these time loops are not ruled out by the laws of physics. But there are strange consequences for quantum information processing if their behavior is dictated by Deutsch's model." A single looping path back in time, a time spiral of sorts, behaving according to Deutsch's model, for example, would have to allow for a particle entering the loop to remain the same each time it passed through a particular point in time. In other words, the particle would need to maintain self-consistency as it looped back in time. "In some sense, this already allows for copying of the particle's data at many different points in space," Wilde said, "because you are sending the particle back many times. It's like you have multiple versions of the particle available at the same time. You can then attempt to read out more copies of the particle, but the thing is, if you try to do so as the particle loops back in time, then you change the past." To be consistent with Deutsch's model, which holds that you can only change the past as long as you can do it in a self-consistent manner, Wilde and colleagues had to come up with a solution that would allow for a looping curve back in time, and copying of quantum data based on a time traveling particle, without disturbing the past. "That was the major breakthrough, to figure out what could happen at the beginning of this time loop to enable us to effectively read out many copies of the data without disturbing the past," Wilde said. "It just worked." However, there is still some controversy over interpretations of the new approach, Wilde said. In one instance, the new approach may actually point to problems in Deutsch's original closed timelike curve model. "If quantum mechanics gets modified in such a way that we've never observed should happen, it may be evidence that we should question Deutsch's model," Wilde said. "We really believe that quantum mechanics is true, at this point. And most people believe in a principle called Unitarity in quantum mechanics. But with our new model, we've shown that you can essentially violate something that is a direct consequence of Unitarity. To me, this is an indication that something weird is going on with Deutsch's model. However, there might be some way of modifying the model in such a way that we don't violate the no-cloning theorem." Other researchers argue that Wilde's approach wouldn't actually allow for copying quantum data from an unknown particle state entering the time loop because nature would already "know" what the particle looked like, as it had traveled back in time many times before. But whether or not the no-cloning theorem can truly be violated as Wilde's new approach suggests, the consequences of being able to copy quantum data from the past are significant. Systems for secure Internet communications, for example, will likely soon rely on quantum security protocols that could be broken or "hacked" if Wilde's looping time travel methods were correct. "If an adversary, if a malicious person, were to have access to these time loops, then they could break the security of quantum key distribution," Wilde said. "That's one way of interpreting it. But it's a very strong practical implication because the big push of quantum communication is this secure way of communicating. We believe that this is the strongest form of encryption that is out there because it's based on physical principles." Today, when you log into your Gmail or Facebook, your password and information encryption is not based on physical principles of quantum mechanical security, but rather on the computational assumption that it is very difficult for "hackers" to factor mathematical products of prime numbers, for example. But physicists and computer scientists are working on securing critical and sensitive communications using the principles of quantum mechanics. Such encryption is believed to be unbreakable -- that is, as long as hackers don't have access to Wilde's looping closed timelike curves. "This ability to copy quantum information freely would turn quantum theory into an effectively classical theory in which, for example, classical data thought to be secured by quantum cryptography would no longer be safe," Wilde said. "It seems like there should be a revision to Deutsch's model which would simultaneously resolve the various time travel paradoxes but not lead to such striking consequences for quantum information processing. However, no one yet has offered a model that meets these two requirements. This is the subject of open research."

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173. Mad Genius: Study Suggests Link Between Psychosis And Creativity

Vincent van Gogh cut off his ear. Sylvia Plath stuck her head in the oven. History teems with examples of great artists acting in very peculiar ways. Were these artists simply mad or brilliant? According to new research reported in Psychological Science, a journal of the Association for Psychological Science, maybe both.

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174. Simay Esmer

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175. Ezoterica

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176. Yaro Zamily

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177. On the nature of psi phenomena

J Parapsychol. 1946 Jun;10:107-19. On the nature of psi phenomena. THOULESS RH, WIESNER BP. PMID: 20990521 [PubMed - indexed for MEDLINE]

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178. Yerküre

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179. Ebru Yılmaz

Tarihi yüksek bir bina (cami olabilir yada bir kulede olabilir) bahçesinde kuşlar var arka tarafında deniz denizde tekneler gidiyor... ama yoğunlaştığım sırada en fazla hissettiğim şey yükseklik,.... ÇİZİM YOK. 

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180. Physicists Create Quantum Link Between Photons That Don't Exist at the Same Time

Timeless. In standard entanglement swapping (top), entanglement (blue shading) is transferred to photons 1 and 4 by making a measurement on photons 2 and 3. The new experiment (bottom) shows that the scheme still works even if photon 1 is destroyed before photon 4 is created. Now they're just messing with us. Physicists have long known that quantum mechanics allows for a subtle connection between quantum particles called entanglement, in which measuring one particle can instantly set the otherwise uncertain condition, or "state," of another particle—even if it's light years away. Now, experimenters in Israel have shown that they can entangle two photons that don't even exist at the same time. "It's really cool," says Jeremy O'Brien, an experimenter at the University of Bristol in the United Kingdom, who was not involved in the work. Such time-separated entanglement is predicted by standard quantum theory, O'Brien says, "but it's certainly not widely appreciated, and I don't know if it's been clearly articulated before." Entanglement is a kind of order that lurks within the uncertainty of quantum theory. Suppose you have a quantum particle of light, or photon. It can be polarized so that it wriggles either vertically or horizontally. The quantum realm is also hazed over with unavoidable uncertainty, and thanks to such quantum uncertainty, a photon can also be polarized vertically and horizontally at the same time. If you then measure the photon, however, you will find it either horizontally polarized or vertically polarized, as the two-ways-at-once state randomly "collapses" one way or the other. Entanglement can come in if you have two photons. Each can be put into the uncertain vertical-and-horizontal state. However, the photons can be entangled so that their polarizations are correlated even while they remain undetermined. For example, if you measure the first photon and find it horizontally polarized, you'll know that the other photon has instantaneously collapsed into the vertical state and vice versa—no matter how far away it is. Because the collapse happens instantly, Albert Einstein dubbed the effect "spooky action at a distance." It doesn't violate relativity, though: It's impossible to control the outcome of the measurement of the first photon, so the quantum link can't be used to send a message faster than light. Now Eli Megidish, Hagai Eisenberg, and colleagues at the Hebrew University of Jerusalem have entangled two photons that don't exist at the same time. They start with a scheme known as entanglement swapping. To begin, researchers zap a special crystal with laser light a couple of times to create two entangled pairs of photons, pair 1 and 2 and pair 3 and 4. At the start, photons 1 and 4 are not tangled. But they can be if physicists play the right trick with 2 and 3. The key is that a measurement "projects" a particle into a definite state -- just as the measurement of a photon collapses it into either vertical or horizontal polarization. So even though photons 2 and 3 start out unentangled, physicists can set up a "projective measurement" that asks, are the two in one of two distinct entangled states or the other? That measurement entangles the photons, even as it absorbs and destroys them. If the researchers select only the events in which photons 2 and 3 end up in, say, the first entangled state, then the measurement also entangles photons 1 and 4. (See diagram, top.) The effect is a bit like joining two pairs of gears to form a four-gear chain: Enmeshing to inner two gears establishes a link between the outer two. In recent years, physicists have played with the timing in the scheme. For example, last year a team showed that entanglement swapping still works even if they make the projective measurement after they've already measured the polarizations of photons 1 and 4. Now, Eisenberg and colleagues have shown that photons 1 and 4 don't even have to exist at the same time, as they report in a paper in press at Physical Review Letters. To do that, they first create entangled pair 1 and 2 and measure the polarization of 1 right away. Only after that do they create entangled pair 3 and 4 and perform the key projective measurement. Finally, they measure the polarization of photon 4. And even though photons 1 and 4 never coexist, the measurements show that their polarizations still end up entangled. Eisenberg emphasizes that even though in relativity, time measured differently by observers traveling at different speeds, no observer would ever see the two photons as coexisting. The experiment shows that it's not strictly logical to think of entanglement as a tangible physical property, Eisenberg says. "There is no moment in time in which the two photons coexist," he says, "so you cannot say that the system is entangled at this or that moment." Yet, the phenomenon definitely exists. Anton Zeilinger, a physicist at the University of Vienna, agrees that the experiment demonstrates just how slippery the concepts of quantum mechanics are. "It's really neat because it shows more or less that quantum events are outside our everyday notions of space and time." So what's the advance good for? Physicists hope to create quantum networks in which protocols like entanglement swapping are used to create quantum links among distant users and transmit uncrackable (but slower than light) secret communications. The new result suggests that when sharing entangled pairs of photons on such a network, a user wouldn't have to wait to see what happens to the photons sent down the line before manipulating the ones kept behind, Eisenberg says. Zeilinger says the result might have other unexpected uses: "This sort of thing opens up people's minds and suddenly somebody has an idea to use it in quantum computing or something." Correction, 23 May at 3:30 p.m.: Photon 4 at right in the upper image was incorrectly labeled as photon 2.

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