FTL communication has not yet been observed/discovered. The parent's last sentence explains the restrictions in this case. >the spin you observe is effectivel

FTL communication has not yet been observed/discovered. The parent's last sentence explains the restrictions in this case. >the spin you observe is effectively random, so there’s no way to know that the results match up unless you compare them with another means of communication, one that is at most the speed of light. The term for this is called "quantum teleportation" and for each qubit that is "teleported" there are two "classically communicated" bits that are required for information to be transmitted.