In physics, an open quantum system is a quantum-mechanical system which interacts with an external quantum system, the environment. In reality, no quantum system is completely isolated from its surroundings, so every quantum system is open to some extent, causing dissipation in the quantum system. Techniques developed in the context of open quantum systems have proven powerful in fields such as quantum optics, quantum measurement theory, quantum statistical mechanics, quantum information science, quantum cosmology and semi-classical approximations.