The rapid advancement of quantum computing poses a significant threat to classical cryptographic systems
that rely on the computational hardness of mathematical problems such as integer factorization and discrete
logarithm problems. In this context, quantum cryptography has emerged as a promising paradigm for secure
communication based on the fundamental principles of quantum mechanics rather than on computational
assumptions. This paper presents a comprehensive review of quantum cryptography, with a particular focus
on Quantum Key Distribution (QKD), the most mature application. The study explores the theoretical
foundations of quantum security, including superposition, entanglement, and the No-Cloning Theorem,
which collectively enable eavesdropping detection and ensure information-theoretic security. Furthermore,
the review examines major QKD protocols, such as BB84 and E91, as well as their advanced variants
designed to address practical vulnerabilities and enhance performance. Recent progress in real-world
implementations, including fiber-optic networks, free-space communication, and satellite-based systems
such as the Micius satellite, is also analyzed. In addition, the paper highlights critical challenges related to
scalability, hardware limitations, and security loopholes arising from imperfect devices. Finally, emerging
research directions, including hybrid cryptographic frameworks that integrate quantum and post-quantum
approaches, are discussed to provide insights into the future of secure communication. This review aims to
provide a structured, up-to-date understanding of quantum cryptography, bridging the gap between
theoretical developments and practical implementations, and emphasizing its crucial role in shaping next�generation cybersecurity systems.