Quantum systems are physical systems that exhibit behaviors governed by the principles of quantum mechanics. These systems can include particles like electrons and photons, which can exist in multiple states simultaneously, a phenomenon known as superposition. Quantum systems are also characterized by entanglement, where the state of one particle is linked to the state of another, regardless of the distance between them. In practical applications, quantum systems are foundational to technologies such as quantum computing and quantum cryptography. Quantum computers leverage the unique properties of quantum systems to perform calculations much faster than classical computers. Meanwhile, quantum cryptography uses these principles to create secure communication channels that are theoretically immune to eavesdropping.