DennisN said:
the property
spin (quantum spin) can be in a superposition of states.
More precisely, it can be in a superposition of eigenstates of the spin operator in a particular direction. But whether or not a particular spin state is a superposition will depend on the direction you choose for the spin operator. Every spin state is an eigenstate of the spin operator in some direction, so every spin state is
not a superposition for some choice of spin operator.
thermia said:
is the superposition only a matter of definition?
It's a matter of choice of operator. See above.
thermia said:
Schrodinger's cat's life is in superposition but also our knowledge about it's life is in superposition
This depends on which interpretation of QM you adopt. On a collapse interpretation, when you observe the cat, its state collapses to either "alive" or "dead". So even if the cat was in a superposition of alive and dead before you observed it, it isn't afterwards, so your knowledge of it is knowledge that it's alive or knowledge that it's dead, never a superposition of the two.
On a no collapse interpretation, such as many worlds, when you observe the cat, your state of knowledge (which is actually a physical state, the state of your brain) gets entangled with the state of the cat, so there are two "branches" or "worlds": one in which the cat is alive and you observe it to be alive, and one in which the cat is dead and you observe it to be dead. This is often described as a superposition, but "entangled" is a better term because the entanglement between the cat's state and your state of knowledge is independent of any choice of operator (whereas, as we saw above, whether or not a particular state of a quantum system is a superposition does depend on your choice of operator).
