For each photon, the would-be counterfeiter has a probability 3 / 4 of success in duplicating it correctly. If the total number of photons on the bank note is N , a duplicate will have probability ( 3 / 4 ) N of passing the bank's verification test. If N is large, this probability becomes exponentially small. The fact that a quantum state cannot be copied is ultimately guaranteed by its proof by the no-cloning theorem, which underlies the security of this system.