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. It helps to think about a concrete implementation of this to make the distinction clear. Image of a scheme for ion-trap quantum computing. Those are not the same thing, though- one is a measurement, the other is a change of state followed by a measurement. 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. 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. 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. 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. 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. But you can't do that.Īnd this is the point where I don't quite agree with the way Ethan explains the situation. 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. 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. 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. 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. But when you say communicate, typically you want to know something about your destination. 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. This seems like a perfect mechanism for sending information over vast distances, as Ethan notes: 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 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." These particles are prepared in an entangled state in which a measurement of the state of Alice's particle is correlated with the measured state of Bob's particle, no matter how far apart they are.
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