  Quantum communications is the art of exchanging and manipulating information
beyond the capabilities of the conventional technologies using the laws of
quantum mechanics. With applications ranging from quantum computing to
cryptographic systems with information-theoretic security, there is strong
incentive to introduce quantum communications into many areas of the society.
However, an important challenge is to develop viable technologies meeting the
stringent requirements of low noise and high coherence for quantum state
encoding, of high bit rate and low power for the integration with classical
communication networks, and of scalable and low-cost production for a practical
wide-deployment. This tutorial presents recent advances in laser modulation
technologies that have enabled the development of efficient and versatile light
sources for quantum communications, with a particular focus on quantum key
distribution (QKD). Such approaches have been successfully used to demonstrate
several QKD protocols with state-of-the-art performance. The applications and
experimental results are reviewed and interpreted in the light of a complete
theoretical background, allowing the reader to model and simulate such sources.
