It is well-known and follows from Holevo’s theorem [7] that approximately determining an arbitrary quantum state |ψ〉 of n qubits requires exponentially many (in n) copies of |ψ〉. One way of circumventing this problem is to relax the notion of what it means to determine |ψ〉 (e.g. by requiring only that we are able to predict the result of “most” measurements on |ψ〉, according to some probability distribution [1]); another way is to restrict the class of states to be determined to some class… CONTINUE READING