In quantum mechanics, when two particles are entangled, their quantum states become correlated in such a way that the measurement outcomes of certain properties are linked. However, the specific nature of the correlation depends on the type of entangled state involved.
The most commonly discussed type of entanglement is called "spin-entanglement," where two particles have their spins entangled. In this case, if the spins are measured along a particular axis, such as up/down or left/right, the entangled particles will always have opposite spin values. For example, if one particle is measured to have spin-up along a certain axis, the other particle will be found to have spin-down along the same axis.
However, it is important to note that entangled particles can have other types of entangled states where their quantum states are not necessarily opposite. For instance, there are entangled states called "singlet states" or "Bell states" where the spins are entangled in such a way that they are maximally correlated but not necessarily opposite. In these states, the outcomes of spin measurements along any axis will be correlated, but the specific values can vary. For example, both particles may have spin-up or both may have spin-down, but the correlation between their measurement outcomes will still be present.
So, while opposite spin states are commonly associated with entangled particles, it is not always the case. The specific nature of the correlation depends on the type of entangled state that the particles are in.