  Quantum cryptography exploits principles of quantum physics for the secure
processing of information. A prominent example is secure communication, i.e.,
the task of transmitting confidential messages from one location to another.
The cryptographic requirement here is that the transmitted messages remain
inaccessible to anyone other than the designated recipients, even if the
communication channel is untrusted. In classical cryptography, this can usually
only be guaranteed under computational hardness assumptions, e.g., that
factoring large integers is infeasible. In contrast, the security of quantum
cryptography relies entirely on the laws of quantum mechanics. Here we review
this physical notion of security, focusing on quantum key distribution and
secure communication.
