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The switches change settings very rapidly, effectively changing the detector settings for the experiment while the photons are in flight. Figure by Chad Orzel. The basic scenario for entanglement-based communication looks like this: two people, traditionally named "Alice" and "Bob" share a pair of particles that can each be measured in one of two quantum states, which we'll call "0" and "1.
That is, if Alice measures her particle in state 1 at precisely noon in Schenectady, she knows that Bob in Portland will also measure his particle to be in state 1, whether he's in Portland, Maine, Portland, Oregon, or Portland Station on one of the moons of Yavin.
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. This seems like a really obvious application, and in fact a bunch of people seized on this as a justification for ESP and various other schemes-- I recommend David Kaiser's How the Hippies Saved Physics for the fascinating history of this whole business.
And, in fact, if the situation described above were possible-- if you could measure a particle's state in a way that forced a particular outcome-- you could absolutely send information this way. But you can't do that. And this is the point where I don't quite agree with the way Ethan explains the situation. He writes:. There's a subtle shift here from the impossible operation that would allow FTL communication to a different sort of operation, and it deserves to be spelled out. That is, in the original statement, you "make a measurement that forces the particle" to be in a particular state, while in the second you "force an entangled particle into a particular state" which breaks the entanglement.
Those are not the same thing, though-- one is a measurement, the other is a change of state followed by a measurement. Image of a scheme for ion-trap quantum computing. It helps to think about a concrete implementation of this to make the distinction clear. So, imagine Alice's particle is one of the trapped ions that people regularly use to do quantum information experiments, which can be in one of two internal states.
At the same time, it prompts some of the more philosophically oriented discussions concerning quantum theory. The correlations predicted by quantum mechanics, and observed in experiment, reject the principle of local realism, which is that information about the state of a system should only be mediated by interactions in its immediate surroundings. Different views of what is actually occurring in the process of quantum entanglement can be related to different interpretations of quantum mechanics.
Reference Terms. This leads to correlations between observable physical properties of the systems. Related Stories. Researchers have now provided a much finer characterization of the distributions of entanglement in multi-qubit systems Both have been checked in rigorous experiments. In the actual experiments, people measure properties like the angular momentum of electrons rather than shapes or colors of cakes.
The EPR effect marries a specific, experimentally realizable form of quantum entanglement with complementarity. An EPR pair consists of two q-ons, each of which can be measured either for its shape or for its color but not for both. We assume that we have access to many such pairs, all identical, and that we can choose which measurements to make of their components. If we measure the shape of one member of an EPR pair, we find it is equally likely to be square or circular. If we measure the color, we find it is equally likely to be red or blue.
The interesting effects, which EPR considered paradoxical, arise when we make measurements of both members of the pair. When we measure both members for color, or both members for shape, we find that the results always agree.
Thus if we find that one is red, and later measure the color of the other, we will discover that it too is red, and so forth. On the other hand, if we measure the shape of one, and then the color of the other, there is no correlation. Thus if the first is square, the second is equally likely to be red or to be blue. We will, according to quantum theory, get those results even if great distances separate the two systems, and the measurements are performed nearly simultaneously.
The choice of measurement in one location appears to be affecting the state of the system in the other location. But does it? And any message revealing the result you measured must be transmitted in some concrete physical way, slower presumably than the speed of light. Upon deeper reflection, the paradox dissolves further. Indeed, let us consider again the state of the second system, given that the first has been measured to be red.
Thus, far from introducing a paradox, the EPR outcome is logically forced. It is, in essence, simply a repackaging of complementarity. Nor is it paradoxical to find that distant events are correlated. After all, if I put each member of a pair of gloves in boxes, and mail them to opposite sides of the earth, I should not be surprised that by looking inside one box I can determine the handedness of the glove in the other. Similarly, in all known cases the correlations between an EPR pair must be imprinted when its members are close together, though of course they can survive subsequent separation, as though they had memories.
Again, the peculiarity of EPR is not correlation as such, but its possible embodiment in complementary forms.
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