Entanglement = Stability?
“The first rule is that this qubit can slide smoothly and continuously between the two states in its superposition. This is sufficient to distinguish quantum theory from classical physics, where such effortless reversible transitions are not possible, but does not rule out a weirder theory.
The second rule is that whatever superposition state the qubit is in, you can only ever extract one bit of information from it- you can only measure it in one state at once.
The third rule applies only to composite systems of two or more qubits. Knowing the probabilities that the individual qubits are in a particular state plus the probabilities of correlations between them tells you the state of the whole system. This encapsulates the property of entanglement between remote quantum states that experiments show holds in the real world.
Only a theory precisely as correlated as quantum theory can obey all the axioms and produce the kind of entanglement observed in nature. Less correlated theories don’t create entanglement at all, while weirder theories produce a situation where, for example, you might measure the state of all the qubits in a system, know the correlations between them, and still not be able to say what state the whole system is in. ‘Entanglement is the unique feature, and it comes out of the three axioms,’ says Brukner.”