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  How can you explain the 4 Bell States in Quantum Physikcs?

+ 0 like - 1 dislike
Im so confused. I'm trying to understand quantum mechanics for a school project about teleportation. I'm trying to understand how the 4 Bell states come to be. If we have two particles that can be either up or down, then, looking at both at the same time, they could be either both up/ down or one of them is up and one is down.  
The 4 Bell states are
sqrt(1/2)*(up up + down down)
sqrt(1/2)*(up up - down down)
sqrt(1/2)*(up down -down up)
sqrt(1/2)*(up down+down up)

I don't know much about this topic so sorry if this question is dumb. I am just stuck trying to imagine the 4 states. What difference does the minus make? They are still either up and up or down and down regardless of the minus or plus right? But what is the difference between the states?
asked Dec 31, 2018 in Q&A by Cassiopeia [ no revision ]

Those signs signify making superpositions of the sates. If you have a superposition $\psi=a\pm b$, then the square has an "interference term" $\psi^2=a^2 \pm ab + b^2$. Thus the observed probability $\propto \psi^2$ depends on the relative sign of the states in the superposition.

These 4 states are the conventional maximally entangled ones. For a given experiment, Alice and Bob chose and use only one of them.

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