Fidelity of quantum states

Summary

The fidelity of two quantum states is a number between $0$ and $1$ that measures how close they are. It equals $1$ exactly when the states are equal, and it equals $0$ when the states can be told apart with certainty. For pure states it is the squared overlap of the state vectors. It is used to judge the quality of state preparation, teleportation, cloning and error correction.