# Superpositions: forcing particles into a state

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.

• Any state is a superposition; in fact, an infinity of superpositions. Jan 28, 2015 at 2:27
• Jan 28, 2015 at 17:13
• @AlfredCentauri How are all states superpositions? Do you mean that any electron in the universe has no definite spin until measured? Feb 3, 2015 at 5:32
• 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? Aug 27, 2019 at 16:23

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