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 a 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, it is important 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, such as quantum master equations, that reduce the complicated dynamics of the system under certain assumptions. A common approximation is that the bath is Markovian, or memoryless.

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.

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