The ability to distribute secret keys with information-theoretic security is arguably one of the most important achievements of the field of quantum information processing and communications. The rapid progress in this field has enabled quantum key distribution in real world conditions and commercial devices are now available. We will first introduce the main ideas behind quantum cryptography and then present long-distance practical photonic implementations of quantum key distribution protocols that use as information carrier the quadratures of the electromagnetic field. We will also discuss current issues related to potential security loopholes in such implementations and to their ability to be integrated into existing telecommunication networks. Finally, we will present advances towards the optical integration of continuous-variable quantum key distribution systems using silicon photonics. This may open the way to the widespread use of this technology for high-security applications within communication networks.