## Principle of Quantum Key Distribution

**Quantum Key Distribution** — also known as **Quantum Cryptography** — solves the Key Distribution problem by allowing the exchange of a cryptographic key between two remote parties with absolute security, guaranteed by the laws of physics. This key can then be used with conventional cryptographic algorithms.

Contrary to what one could expect, the basic principle of quantum key distribution is quite straightforward. It exploits the fact, that according to quantum physics, the mere fact of observing a quantum object perturbs it in an irreparable way. When you read this article for example, the sheet of paper must be lighted. The impact of the light particles will slightly heat it up and hence change it. This effect is very small on a piece of paper, which is a macroscopic object. However, the situation is radically different with a microscopic object. If one encodes the value of a digital bit on a single quantum object, its interception will necessarily translate into a perturbation, because the eavesdropper is forced to observe it. This perturbation causes errors in the sequence of bits exchanged by the sender and recipient. By checking for the presence of such errors, the two parties can verify whether their key was intercepted or not. It is important to stress that since this verification takes place after the exchange of bits, one finds out a posteriori whether the communication was eavesdropped or not. That is why this technology is used to exchange a key and not valuable information. Once the key is validated, it can be used to encrypt data. Quantum physics allows to prove that interception of the key without perturbation is impossible.

What does it mean in practice to encode the value of a digital bit on a quantum object? In telecommunication networks, light is routinely used to exchange information. For each bit of information, a pulse is emitted and sent through an optical fiber — a thin fiber of glass used to carry light signals — to the receiver, where it is registered and transformed back into an electronic signal. These pulses typically contain millions of particles of light, called photons. In quantum cryptography, one can follow the same approach, with the only difference that the pulses contain only one single photon. A single photon represents a very tiny amount of light (when reading this article your eyes register billions of photons every second) and follows the laws of quantum physics. In particular, it cannot be split into halves. This means that an eavesdropper cannot take half of a photon to measure the value of the bit it carries, while letting the other half continue its course. If he wants to obtain the value of the bit, he must observe the photon and will thus interrupt the communication and reveal his presence. A more clever strategy is for the eavesdropper to detect the photon, register the value of the bit and prepare a new photon according to the obtained result to send it to the receiver. In quantum cryptography, the two legitimate parties cooperate to prevent the eavesdropper from doing so, by forcing him to introduce errors. The Quantum Key Distribution protocols article details how to achieve this goal.