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  9/9/2019Quantum Physics May Be Even Spookier Than You Think - Scientific Americanhttps://www.scientificamerican.com/article/quantum-physics-may-be-even-spookier-than-you-think/1/8 PHYSICS Quantum Physics May Be Even Spookier Than YouThink  A new experiment hints at surprising hidden mechanics of quantum superpositions By Philip Ball on May 21, 2018  ﺔﯾﺑرﻌﻟ ﺔﻐﻠﻟﺎﺑ   ذھ   ضرﻋ Superposition—the notion that tiny objects can exist in multiple places or states simultaneously—is a cornerstone of quantumphysics. A new experiment seeks to shed light on this mysterious phenomenon. Credit: Tai GinDa Getty Images  It is the central question in quantum mechanics, and no one knows the answer: What really happens in a superposition—the peculiar circumstance in whichparticles seem to be in two or more places or states at once? Now, in a new paper a SCROLL TO TOP Subscribe SHARELATEST   We use cookies topersonalize contentand ads, to providesocial media featuresand to analyze our traffic. We also shareinformation aboutyour use of our sitewith our social media,advertising andanalytics partners.Privacy Policy ✓      Accept CookiesCookie Settings ❯  9/9/2019Quantum Physics May Be Even Spookier Than You Think - Scientific Americanhttps://www.scientificamerican.com/article/quantum-physics-may-be-even-spookier-than-you-think/2/8  finally let us say something for sure about the nature of this puzzling phenomenon.Their experiment, which the researchers say could be carried out within a few months, should enable scientists to sneak a glance at where an object—in this casea particle of light, called a photon—actually resides when it is placed in asuperposition. And the researchers predict the answer will be even stranger andmore shocking than “two places at once.”The classic example of a superposition involves firing photons at two parallel slitsin a barrier. One fundamental aspect of quantum mechanics is that tiny particlescan behave like waves, so that those passing through one slit “interfere” with thosegoing through the other, their wavy ripples either boosting or canceling oneanother to create a characteristic pattern on a detector screen. The odd thing,though, is this interference occurs even if only one  particle is fired at a time. Theparticle seems somehow to pass through both slits at once, interfering with itself.That’s a superposition. And it gets weirder: Measuring which slit such a particle goes through willinvariably indicate it only goes through one—but then the wavelike interference(the “quantumness,” if you will) vanishes. The very act of measurement seems to“collapse” the superposition. “We know something fishy is going on in asuperposition,” says physicist Avshalom Elitzur of the Israeli Institute for Advanced Research. “But you’re not allowed to measure it. This is what makesquantum mechanics so diabolical.”For decades researchers have stalled at this apparent impasse. They cannot say exactly what a superposition is without looking at it; but if they try to look at it, itdisappears. One potential solution—developed by Elitzur’s former mentor, Israeliphysicist Yakir Aharonov, now at Chapman University, and his collaborators—suggests a way to deduce something about quantum particles before  measuringthem. Aharonov’s approach is called the two-state-vector formalism (TSVF) of quantum mechanics, and postulates quantum events are in some sensedetermined by quantum states not just in the past—but also in the future. That is,the TSVF assumes quantum mechanics works the same way both forward and backward in time. From this perspective, causes can seem to propagate backwardin time, occurring after  their effects.   We use cookies topersonalize contentand ads, to providesocial media featuresand to analyze our traffic. We also shareinformation aboutyour use of our sitewith our social media,advertising andanalytics partners.Privacy Policy ✓      Accept CookiesCookie Settings ❯  9/9/2019Quantum Physics May Be Even Spookier Than You Think - Scientific Americanhttps://www.scientificamerican.com/article/quantum-physics-may-be-even-spookier-than-you-think/3/8 But one needn’t take this strange notion literally. Rather, in the TSVF one can gainretrospective knowledge of what happened in a quantum system by selecting theoutcome: Instead of simply measuring where a particle ends up, a researcherchooses a particular location in which to look for it. This is called post-selection,and it supplies more information than any unconditional peek at outcomes evercould. This is because the particle’s state at any instant is being evaluatedretrospectively in light of its entire history, up to and including measurement. Theoddness comes in because it looks  as if the researcher—simply by choosing to look for a particular outcome—then causes that outcome to happen. But this is a bit likeconcluding that if you turn on your television when your favorite program isscheduled, your action causes that program to be broadcast at that very moment.“It’s generally accepted that the TSVF is mathematically equivalent to standardquantum mechanics,” says David Wallace, a philosopher of science at theUniversity of Southern California who specializes in interpretations of quantummechanics. “But it does lead to seeing certain things one wouldn’t otherwise haveseen.”Take, for instance, a version of the double-slit experiment devised by Aharonov and co-worker Lev Vaidman in 2003, which they interpreted with the TSVF. Thepair described (but did not build) an optical system in which a single photon actsas a “shutter” that closes a slit by causing another “probe” photon approaching theslit to be reflected back the way it came. By applying post-selection to themeasurements of the probe photon, Aharonov and Vaidman showed, one coulddiscern a shutter photon in a superposition closing both (or indeed arbitrarily many) slits simultaneously. In other words, this thought experiment would intheory allow one to say with confidence the shutter photon is both “here” and“there” at once. Although this situation seems paradoxical from our everyday experience, it is one well-studied aspect of the so-called “nonlocal” properties of quantum particles, where the whole notion of a well-defined location in spacedissolves.In 2016 physicists Ryo Okamoto and Shigeki Takeuchi of Kyoto University verified Aharonov and Vaidman’s predictions experimentally  using a light-carrying circuitin which the shutter photon is created using a quantum router, a device that letsone photon control the route taken by another. “This was a pioneering experimentthat allowed one to infer the simultaneous position of a particle in two places,”says Elitzur’s colleague Eliahu Cohen of the University of Ottawa in Ontario.   We use cookies topersonalize contentand ads, to providesocial media featuresand to analyze our traffic. We also shareinformation aboutyour use of our sitewith our social media,advertising andanalytics partners.Privacy Policy ✓      Accept CookiesCookie Settings ❯  9/9/2019Quantum Physics May Be Even Spookier Than You Think - Scientific Americanhttps://www.scientificamerican.com/article/quantum-physics-may-be-even-spookier-than-you-think/4/8 ow tzur an oen ave teame up wt amoto an aeuc to concoct aneven more mind-boggling experiment. They believe it will enable researchers tosay with certainty something about the location of a particle in a superposition at aseries of different points in time—before any actual measurement has been made.This time the probe photon’s route would be split into three by partial mirrors. Along each of those paths it may interact with a shutter photon in a superposition.These interactions can be considered to take place within boxes labeled A, B and C,one of which is situated along each of the photon’s three possible routes. By looking at the self-interference of the probe photon, one can retrospectively conclude with certainty the shutter particle was in a given box at a specific time. Credit: Amanda Montañez The experiment is designed so the probe photon can only show interference if itinteracted with the shutter photon in a particular sequence of places and times:Namely, if the shutter photon was in both boxes A and C at some time ( t  1), then ata later time ( t  2) only in C, and at a still later time ( t  3) in both B and C. Sointerference in the probe photon would be a definitive sign the shutter photon   We use cookies topersonalize contentand ads, to providesocial media featuresand to analyze our traffic. We also shareinformation aboutyour use of our sitewith our social media,advertising andanalytics partners.Privacy Policy ✓      Accept CookiesCookie Settings ❯
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