# Understanding on quantum entanglement [duplicate]

Understanding on quantum entanglement? I am very vague on this topic and would appreciate a detailed explanation on this phenomenon. Also what are the possible applied uses for quantum entanglement? What are the problems of putting this phenomenon in practice?

## marked as duplicate by Brandon Enright, Emilio Pisanty, Qmechanic♦Jun 4 '13 at 13:01

Since it's so completely general, nearly every non-trivial quantum-mechanical situation involves entanglement. There is no problem in putting it into practice. For example, the state $$\frac{\left|\uparrow\downarrow\right\rangle - \left|\downarrow\uparrow\right\rangle}{\sqrt{2}}$$ can represent two particles with total spin $0$ where after measuring one to be spin-up, the other would always be measured to be spin-down, and vice versa. This is the situation in the EPR pseudo-paradox, but it's also a quite common situation that describes, for example, the electron spins in a bond between two hydrogen atoms in a hydrogen molecule.
In this particular example, the other measurement is determined with probability $1$, but any degree of non-independence counts as entanglement.