## Quantum Key Distribution for Techies

Thursday 18 June 2009

**Quantum Key Distribution** [1] — also known as **Quantum Cryptography** — was invented about twenty five years ago and complements conventional cryptographic techniques to raise the security of data transmission over optical fiber links to an unprecedented level. It is used to exchange keys between two stations and exploits the laws of quantum physics to reveal the presence of an adversary, as detailed in the **Introduction to Quantum Physics and its applications**.

Quantum Key Distribution can be used to exchange keys between two remote sites. The keys are then used with so-called secret key ciphering algorithms to securely encrypt information. These algorithms combine the key with the data to be protected in such a way that it becomes unintelligible to a third party with no knowledge of the key. In **Cryptography**, the security relies on the fact that a key is kept secret during its entire life, from the time it is created, transmitted to a second party, and finally until it is discarded after use. The **Key Distribution** is a central problem in cryptography.

The principle of operation of a *Quantum Key Distribution (QKD)* system is quite straightforward: two stations (in cryptography they are often referred as Alice and Bob) are linked together with a quantum channel and a classical channel. Alice generates a random stream of qubits (quantum bits) that are sent over the quantum channel. Upon reception of the stream Bob and Alice — using the classical channel — perform classical operations to check if an eavesdroper has tried to extract information on the qubits stream. The presence of an eavesdropper is revealed by the imperfect correlation between the two lists of bits obtained after the transmission of qubits between the emitter and the receiver. If the correlation is high enough, a perfectly secure symmetric key can be created from these two lists of bits. In the opposite case — when an eavesdropper Eve has listened in on the quantum channel — the key generation process has to be aborted and started again.

The following articles give a detailed description of the Quantum Key Distribution technology:

[1] see Nicolas Gisin, Grégoire Ribordy, Wolfgang Tittel, Hugo Zbinden. Quantum cryptography. *Reviews of Modern Physics, Vol 74*, pages 145-195, 2002] for a review of quantum cryptography.