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Open quantum system

In physics, an open quantum system is a quantum-mechanical system which interacts with an external quantum system, the environment or bath. In general, these interactions significantly change the dynamics of the system and result in quantum dissipation, where 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 in order to obtain an accurate understanding of quantum systems. In physics, an open quantum system is a quantum-mechanical system which interacts with an external quantum system, the environment or bath. In general, these interactions significantly change the dynamics of the system and result in quantum dissipation, where 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 in order to obtain an accurate understanding of quantum systems. 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. A complete description of a quantum system requires the inclusion of the environment. Completely describing the resulting combined system then requires the inclusion of its environment, which results in a new system that can only be completely described if its environment is included and so on. The eventual outcome of this process of embedding is the state of the whole universe described by a wavefunction Ψ {displaystyle Psi } . The fact that every quantum system has some degree of openness also means that no quantum state can ever be in a pure state. A pure state is unitary equivalent to a zero temperature ground state forbidden by the third law of thermodynamics. Even if the combined system is a pure state and can be described by a wavefunction Ψ {displaystyle Psi } , a subsystem in general cannot be described by a wavefunction. This observation motivated the formalism of density matrices, or density operators, introduced by John von Neumann in 1927 and independently, but less systematically by Lev Landau in 1927 and Felix Bloch in 1946. In general, the state of a subsystem is described by the density operator ρ {displaystyle ho } and an observable A {displaystyle A} by the scalar product ( ρ ⋅ A ) = t r { ρ A } {displaystyle ( ho cdot A)={ m {{tr}{ ho A}}}} . There is no way to know if the combined system is pure from the knowledge of the observables of the subsystem. In particular if the combined system has quantum entanglement, the system state is not a pure state.

[ "Quantum", "Quantum discord", "Quantum foam", "Cat state", "Quantum information science", "Correspondence principle" ]
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