In the history of cryptography, quantum cryptography is a new and important chapter. It is a recent technique that can be used to ensure the confidentiality of information transmitted between two parties, usually called Alice and Bob, by exploiting the counterintuitive behavior of elementary particles such as photons.

The physics of elementary particles is governed by the laws of quantum mechanics, which were discovered in the early twentieth century by talented physicists. Quantum mechanics fundamentally change the way we must see our world. At atomic scales, elementary particles do not have a precise location or speed, as we would intuitively expect. An observer who would want to get information on the particle's location would destroy information on its speed – and vice versa – as captured by the famous Heisenberg uncertainty principle. This is not a limitation due to the observer's technology but rather a fundamental limitation that no one can ever overcome.

The uncertainty principle has long been considered as an inconvenient limitation, until recently, when positive applications were found.

In the meantime, the mid-twentieth century was marked by the creation of a new discipline called information theory. Information theory is aimed at defining the concept of information and mathematically describing tasks such as communication, coding and encryption. Pioneered by famous scientists like Turing and von Neumann and formally laid down by Shannon, it answers two fundamental questions: what is the fundamental limit of data compression, and what is the highest possible transmission rate over a communication channel?