Non-orthogonal States cannot be perfectly distinguished from one another Orthogonal States are perfectly distinguishable. Eve wants to get information from Alice's quantum state, while leaving it intact. So she could perform a unitary transformation on the qubit. Eve's unitary tries to imprint on a register, information about Alice's state. If the 2 states are non-orthogonal you cannot distinguish between the states. ![[Pasted image 20211212184114.png]] The idea that you can't collect information that distinguishes non-orthogonal states from one another if you don't disturb those states is the foundation of [[Quantum Cryptography]] If we use signal states that are non-orthogonal and an eavesdropper tries to learn something about those states, that will cause some disturbance, that can be detected. We can design our protocol so that we won't wind up using compromised information in our protocol to make it secure. That's not usually the case today with encryption schemes, it is possible to break them in principles. The protection is in the form of the computation to be able to do that being too hard to do in feasible time. Rather the security is based on the fundamental laws of physics, the principle that we can't distinguish non-orthogonal states without disturbing them.