Quantum cryptography, introduced by Gisin, Ribordy, Tittel, and Zbinden, explores the application of quantum mechanics at the individual quanta level. The review discusses recent advancements in theory and experiments, emphasizing open questions and technological challenges. It outlines the principles of quantum cryptography, including the BB84 protocol, the no-cloning theorem, and eavesdropping strategies. The article also covers other protocols like the 2-state, 6-state, and EPR protocols, highlighting their security and efficiency. Key challenges include photon sources, quantum channels, and detection technologies. The review discusses experimental implementations using weak laser pulses and photon pairs, addressing issues like error correction and privacy amplification. It emphasizes the importance of quantum mechanics in information theory and the intersection with relativity. The text explains how quantum cryptography ensures security through the fundamental principles of quantum physics, such as the no-cloning theorem and the disturbance caused by measurement. The review concludes that quantum cryptography, or quantum key distribution, is a critical tool for secure communication, complementing classical cryptographic systems.Quantum cryptography, introduced by Gisin, Ribordy, Tittel, and Zbinden, explores the application of quantum mechanics at the individual quanta level. The review discusses recent advancements in theory and experiments, emphasizing open questions and technological challenges. It outlines the principles of quantum cryptography, including the BB84 protocol, the no-cloning theorem, and eavesdropping strategies. The article also covers other protocols like the 2-state, 6-state, and EPR protocols, highlighting their security and efficiency. Key challenges include photon sources, quantum channels, and detection technologies. The review discusses experimental implementations using weak laser pulses and photon pairs, addressing issues like error correction and privacy amplification. It emphasizes the importance of quantum mechanics in information theory and the intersection with relativity. The text explains how quantum cryptography ensures security through the fundamental principles of quantum physics, such as the no-cloning theorem and the disturbance caused by measurement. The review concludes that quantum cryptography, or quantum key distribution, is a critical tool for secure communication, complementing classical cryptographic systems.