The 90s of the last century were marked by the invention of quantum algorithms, like the one by P. W. Shor, which is used for factorization of integers, or the algorithm for fast database searches by L. K. Grover. Those algorithms can be used to crack widespread cryptographic systems like RSA, which relies on the difficulty of the factorization of big numbers.
If classic cryptographic systems are cracked, all past and future communication using those systems is compromised. Quantum encryption solves this problem, as it can only be successfully attacked in real time. For its implementation, quantum key distribution systems are needed.
This paper surveys the state of the art of quantum key distribution systems as well as their properties.
Can I use acronyms like RSA? It stands for “Rivest, Shamir und Adleman”, which sounds kind of silly in the middle of a sentence.