The principle of superposition is a cornerstone of quantum mechanics. It says that when two evolving states solve the SchröUdinger equation, any linear combination of the two is also a solution. For that reason, waves from the two slits in the double-slit experiment simply add together to create the familiar interference pattern. As it happens, the superposition principle also prohibits the arbitrary copying of quantum states.
To see why, imagine a machine that copies the state of a photon or an electron. When the original enters, two copies come out, each having the same state as the original. If such a machine were successful, it would convert the state |〉 to | 〉 and |〉 to | 〉, where the fanciful symbols |〉 and |〉 rep resent arbitrary states. The problem arises when we send a linear combination, |s = a|〉 + b...
