Quantum systems are physical systems that follow the principles of quantum mechanics, a fundamental theory in physics. These systems can include particles like electrons and photons, which exhibit behaviors that differ from classical physics, such as superposition and entanglement. In superposition, a particle can exist in multiple states at once, while entanglement describes a connection between particles, where the state of one instantly influences the state of another, regardless of distance. Quantum systems are essential for understanding the behavior of matter and energy at the smallest scales. They play a crucial role in technologies like quantum computing, which leverages the unique properties of quantum bits, or qubits, to perform complex calculations much faster than traditional computers. Additionally, quantum systems are foundational in fields such as quantum cryptography and quantum teleportation.