# Can you tell if a particle is in superposition?

This may be an easy answer for anybody. Is it possible to detect if a particle A is still in a superposition via the sending a group of particles B through a box containing particle A?

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Not really.

There is Quantum Tomography, but it requires several copies of the same quantum system (that is an apparatus that can prepare the system in the same state several times).

I understand the problem as: You give me a state, I tell you what are coefficients of expansion into eigenbasis. I quess this is not possible. If it were not for the No Cloning theorem, then you could copy the state and do qUantum tomography on the copies.

Again Quantum Nondemolition Measurement does not mean the state is unchanged and in fact this i just an almost perfect projection. There is no hope here either.

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Thanks for the answer. I'm just a physics fanatic, not much educational background except for books published articles. I was wondering if this is possible, because if so, we would be able to send comprehensible "data" by using probability if a group of particles are in superposition. I know this is recognized as impossible, but just a thought. What if we entangled particles in pairs or groups and separated them. We keep entangled particles here, and others in space. Keep track of which group of particle was entangled in some order, and selectively observe them. Then we can send bits. – TheWino Dec 19 '12 at 19:49
The bits could be 0 for having a set state and 1 for being still in superposition. Does this make ANY sense in the world of quantum physics? – TheWino Dec 19 '12 at 19:50
So it means that you do not need a state after measurement any more... nevertheless 0 for "eigenstate" and 1 for "superposition" is very inefficient, as you map virtually whole Hilbert space to 1, and measure 0 set to bit 0. Moreover I think this is also very impractical in general. – Lacek Dec 19 '12 at 22:45