Open quantum system

In physics, an open quantum system is a quantum mechanical system that interacts with an external quantum system, known as the environment or the bath. In general, these interactions significantly change the dynamics of the system, such that the information contained in the system is lost to its environment. Because no quantum system is completely isolated from its surroundings, every real quantum system is in some ways an open quantum system. Open quantum systems theory aims to develop a theoretical framework for treating these interactions to obtain an accurate understanding of quantum systems.

Because the exact dynamics of the system and the environment are often impossible to calculate, theories of open quantum systems are often based around effective depictions that rely on sets of common assumptions about the behavior of the bath. For example, a common approximation is that the bath is Markovian, or memoryless. Taking this assumption results in a set of quantum master equations that reduce the complicated dynamics of the system to solvable differential equations.

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 thermodynamics, quantum cosmology, quantum biology, and semi-classical approximations.