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According to a MinutePhysics video ("How to See Light") physicists won a Nobel Prize for showing that creating a certain superposition state for a particle and observing the particle state after it interacted with light, may allow us to prove the presence of a photon without directly observing it.

If this is true, it must mean that we are capable of create certain superposition states for particles or creating particles that are already in a superposition.

If this is possible, is it possible to force a particle currently in a superposition into a state of our choosing?

Example: Two spin 1/2 fermions are entangled and both in superpositions. Detecting either of them in the same way will result in one being measured as spin up and the other spin down. However, if you could force the first particle to become spin up, the other will instantly become spin down. This would allow faster-than-light communication in cases where the particles are far enough, because changing the state of one particle would affect the state of the second particle.
By creating 32 pairs of entangled particles one could transfer a 32-bit integer across an unlimited distance without the constraints of data transfer speed.

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    $\begingroup$ Any state is a superposition; in fact, an infinity of superpositions. $\endgroup$ Jan 28, 2015 at 2:27
  • $\begingroup$ This Youtube video? $\endgroup$
    – Qmechanic
    Jan 28, 2015 at 17:13
  • $\begingroup$ @AlfredCentauri How are all states superpositions? Do you mean that any electron in the universe has no definite spin until measured? $\endgroup$
    – Arc676
    Feb 3, 2015 at 5:32
  • $\begingroup$ You start in a state, say, UD+DU. You have some magical way to force the first photon into state U. Now you have UD+UU. Tell me again how you're going to use this to communicate? $\endgroup$
    – WillO
    Aug 27, 2019 at 16:23

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There are a couple of non-clear things in what you say. For the experiment of which you talk we can't enforce any particle into the specific superposition convenient for the experiment purpose. I didn't understand what is the type of particle was used and which exactly were the two states, but I know the principle of the experiment. Not any particle is fit for it, some particles may just absorb the photon, phenomenon not desirable here.

But the main thing which is not clear is: do you distinguish between a two-particle entanglement and single particle superposition? Here is the example of entanglement of which you speak,

$$ | \psi \rangle = \frac { |\uparrow \rangle|\downarrow \rangle - |\downarrow \rangle |\uparrow \rangle}{\sqrt {2}} $$

You don't have here each particle in a state of single particle superposition, what you have is a superposition of two-particle states, i.e. a superposition of the two-particle state $|\uparrow \rangle|\downarrow \rangle $ and the two-particle state $|\downarrow \rangle |\uparrow \rangle $.

A third issue is that an experimenter cannot enforce a specific result in the measurement of the particle arriving as his/her lab. One can perform a measurement, but one has no power on the result. Of course, an observer can do whatever pleases with his/her particle, but, which result will be obtained does not depend on the observer. There exists also the non-signaling principle, which says, for our case, that entanglements can't be used for faster-than-light communication. For the rigorous version see the article in Wikipedia.

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The latest study on this subject that I could find, "All-Optical Control of a Single Spin Employing Coherent Dark States," uses 2 lasers to keep the particle in a coherent "dark state," which I understand to be a superposition where spin direction has not been determined, and hence, not measured. My question is, could that unmeasured particle be somehow split into 2 entangled particles (or however entangled particles are created) so that the original, still controlled with lasers, would maintain that superpositional state, forcing one that gets moved to maintain that state, where they could be separated by a long distance, where the original one would then be released from the superposition, signaling the other as it also is released?

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