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Quantum mechanics has not only revolutionized physics but also created certain puzzles as well. One such puzzle, referred to as ‘reductio ad absurdum’ by Albert Einstein, Boris Podolsky, and Nathan Rosen, implies that 'ghostly action at a distance' remains to be solved. It involves a reciprocal reaction from two atoms separated by a large distance; e.g., two atoms are placed so that their spins are always in opposite directions, but do not measure the exact spin of either particle. You shoot those particles out in different directions and wait until they are well separated before measuring them. When you measure them, you will discover that whenever one of the two has its spin pointing up, the other spin is pointing down. Or, if you measure one to be pointing to the left, the other will be pointing to the right.
It is also known as EPR paradox. In 1935, Albert Einstein, Boris Podolsky, and Nathan Rosen published an article, which remains a reference today, in which they revealed the existence of quantum entanglement. There are pairs of “entangled” photons, connected by a strange link. However far apart they are, if you measure the polarization of one of the photons, you can infer that of the other. This is hard to describe with images. On one hand, these particles do not communicate with each other because the information would have to travel faster than light. But on the other hand, their behavior cannot be predetermined: “The violation of Bell's inequalities show that, unlike twins, these photons do not have the equivalent of an identical genotype which could explain their similarities,” Alain Aspect points out. Aspect and his team discovered that under certain circumstances subatomic particles such as electrons are able to instantaneously communicate with each other regardless of the distance separating them. It doesn't matter whether they are 10 feet or 10 billion miles apart.
Einstein in 1935 stated that, the lack of an image for entanglement showed that something was missing in quantum theory. Physicist Niels Bohr, however, disagreed, claiming that adding supplementary variables analogous to twins' chromosomes, capable of predicting the values of quantum observables before being measured, would shatter the consistency of quantum mechanics. Only in 1964 did John Bell discover an inequality, which, if confirmed experimentally, would prove Einstein right but quantum mechanics wrong or, if violated, would show that quantum theory could not be completed according to Einstein's view.
A correlation between their wave functions remained, as they were once part of the same wave function that was not disturbed before one of the child particles was measured. The determination of an axis direction for the polarization measurement of one photon, forcing the wave function to 'collapse' onto that axis, will influence the measurement of its twin. This influence occurs despite any experimenters not knowing which axes have been chosen by their distant colleagues, and at distances that disallow any communication between the two photons, even at the speed of light. A French scientist, Alain Aspect's, experiments provide strong evidence that a quantum event at one location can affect an event at another location without any obvious mechanism for communication between the two locations. This has been called "spooky action at a distance" by Einstein (who doubted the physical reality of this effect). However, these experiments do not allow faster-than-light communication, as the events themselves appear to be inherently random. He then succeeded, together with Philippe Grangier, in being first to produce single photons at identifiable moments—now called “heralded” single photons. They further revealed that a photon could travel as a wave through two places at the same time, even if it could only be observed in one location, as expected for a single particle.
In 1974, Aspect began probing the subject, building upon the pioneering work of John Clauser and collaborators. He understood how to test the locality hypothesis, which is central in the controversy. He developed polarizers whose settings could be changed every ten nanoseconds and set up a source of entangled photons with an unprecedented efficiency. The key experiments, carried out at Orsay in 1982 by Aspect, Philippe Grangier, Gérard Roger, and Jean Dalibard, showed a clear violation of Bell's inequalities in conditions closely resembling the ideal “Gedanken Experiment”—the foundation for the theoretical discussions. Quantum theory was once again vindicated. “A pair of entangled photons should be considered as a global, inseparable quantum system,” Aspect concludes. Twenty years later, it appears this work has helped in launching the second quantum revolution, with promises for quantum cryptography and quantum information processing.
His group performed the first experimental tests of Bell’s Inequality, which shows that the predictions of quantum mechanics cannot be explained by a “local hidden variable” theory. Explaining that will take some space, so I’ll move it below the fold…
Experimental Realization of Einstein-Podolsky-Rosen-Bohm Gedankenexperiment: A New Violation of Bell's Inequalities, A. Aspect, P. Grangier, and G. Roger, Physical Review Letters, Vol. 49, Iss. 2, pp. 91–94 (1982) doi:10.1103/PhysRevLett.49.91
Experimental Test of Bell's Inequalities Using Time-Varying Analyzers, A. Aspect, J. Dalibard and G. Roger, Physical Review Letters, Vol. 49, Iss. 25, pp. 1804–1807 (1982) doi:10.1103/PhysRevLett.49.1804
J.S. Bell, “On the Einstein Podolsky Rosen Paradox” (1964). Paper reproduced in Speakable and unspeakable in quantum mechanics, Cambridge: Cambridge University Press, 2nd edition, 2004.
To be or not to be local, A. Aspect, Nature, Vol. 446, pp. 866–867 (2007)
It is also known as EPR paradox. In 1935, Albert Einstein, Boris Podolsky, and Nathan Rosen published an article, which remains a reference today, in which they revealed the existence of quantum entanglement. There are pairs of “entangled” photons, connected by a strange link. However far apart they are, if you measure the polarization of one of the photons, you can infer that of the other. This is hard to describe with images. On one hand, these particles do not communicate with each other because the information would have to travel faster than light. But on the other hand, their behavior cannot be predetermined: “The violation of Bell's inequalities show that, unlike twins, these photons do not have the equivalent of an identical genotype which could explain their similarities,” Alain Aspect points out. Aspect and his team discovered that under certain circumstances subatomic particles such as electrons are able to instantaneously communicate with each other regardless of the distance separating them. It doesn't matter whether they are 10 feet or 10 billion miles apart.
Einstein in 1935 stated that, the lack of an image for entanglement showed that something was missing in quantum theory. Physicist Niels Bohr, however, disagreed, claiming that adding supplementary variables analogous to twins' chromosomes, capable of predicting the values of quantum observables before being measured, would shatter the consistency of quantum mechanics. Only in 1964 did John Bell discover an inequality, which, if confirmed experimentally, would prove Einstein right but quantum mechanics wrong or, if violated, would show that quantum theory could not be completed according to Einstein's view.
A correlation between their wave functions remained, as they were once part of the same wave function that was not disturbed before one of the child particles was measured. The determination of an axis direction for the polarization measurement of one photon, forcing the wave function to 'collapse' onto that axis, will influence the measurement of its twin. This influence occurs despite any experimenters not knowing which axes have been chosen by their distant colleagues, and at distances that disallow any communication between the two photons, even at the speed of light. A French scientist, Alain Aspect's, experiments provide strong evidence that a quantum event at one location can affect an event at another location without any obvious mechanism for communication between the two locations. This has been called "spooky action at a distance" by Einstein (who doubted the physical reality of this effect). However, these experiments do not allow faster-than-light communication, as the events themselves appear to be inherently random. He then succeeded, together with Philippe Grangier, in being first to produce single photons at identifiable moments—now called “heralded” single photons. They further revealed that a photon could travel as a wave through two places at the same time, even if it could only be observed in one location, as expected for a single particle.
In 1974, Aspect began probing the subject, building upon the pioneering work of John Clauser and collaborators. He understood how to test the locality hypothesis, which is central in the controversy. He developed polarizers whose settings could be changed every ten nanoseconds and set up a source of entangled photons with an unprecedented efficiency. The key experiments, carried out at Orsay in 1982 by Aspect, Philippe Grangier, Gérard Roger, and Jean Dalibard, showed a clear violation of Bell's inequalities in conditions closely resembling the ideal “Gedanken Experiment”—the foundation for the theoretical discussions. Quantum theory was once again vindicated. “A pair of entangled photons should be considered as a global, inseparable quantum system,” Aspect concludes. Twenty years later, it appears this work has helped in launching the second quantum revolution, with promises for quantum cryptography and quantum information processing.
His group performed the first experimental tests of Bell’s Inequality, which shows that the predictions of quantum mechanics cannot be explained by a “local hidden variable” theory. Explaining that will take some space, so I’ll move it below the fold…
Experimental Realization of Einstein-Podolsky-Rosen-Bohm Gedankenexperiment: A New Violation of Bell's Inequalities, A. Aspect, P. Grangier, and G. Roger, Physical Review Letters, Vol. 49, Iss. 2, pp. 91–94 (1982) doi:10.1103/PhysRevLett.49.91
Experimental Test of Bell's Inequalities Using Time-Varying Analyzers, A. Aspect, J. Dalibard and G. Roger, Physical Review Letters, Vol. 49, Iss. 25, pp. 1804–1807 (1982) doi:10.1103/PhysRevLett.49.1804
J.S. Bell, “On the Einstein Podolsky Rosen Paradox” (1964). Paper reproduced in Speakable and unspeakable in quantum mechanics, Cambridge: Cambridge University Press, 2nd edition, 2004.
To be or not to be local, A. Aspect, Nature, Vol. 446, pp. 866–867 (2007)
