Quantum cryptography is the science of exploiting
quantum mechanical properties to perform
cryptographic tasks. The best known example of quantum cryptography is
quantum key distribution which offers an
information-theoretically secure solution to the
key exchange problem. Currently used popular
public-key encryption and
signature schemes (e.g.,
RSA and
ElGamal) can be
broken by quantum adversaries. The advantage of quantum cryptography lies in the fact that it allows the completion of various cryptographic tasks that are proven or conjectured to be impossible using only classical (i.e. non-quantum) communication (see below for examples). For example, It is
impossible to copy data encoded in a quantum state and the very act of reading data encoded in a quantum state changes the state. This is used to detect eavesdropping in quantum key distribution.