Quantum cryptography:

Is an approach based on quantum physics for secure communications.

A central problem in cryptography is the key distribution problem.
One solution, that of public-key cryptography, relies on the computational
difficulty of certain hard mathematical problems (such as integer factorisation),
whereas quantum cryptography relies on the laws of quantum mechanics.
Quantum cryptographic devices typically employ individual photons of light.

How...... ?

it is possible to encode information into some quantum properties of a photon
in such a way that any effort to monitor them necessarily disturbs them in
some detectable way. The effect arises because in quantum theory, certain
pairs of physical properties are complementary in the sense that measuring
one property necessarily disturbs the other.

SO..... ?

Two different types of quantum cryptographic protocols were invented:

The first type uses the polarization of photons to encode the bits of information
and relies on quantum randomness to keep you from learning the secret key.

The second type uses entangled photon states to encode the bits and relies
on the fact that the information defining the key only "comes into being"
after measurements performed.

Cool ........ :)