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A pure quantum state is a state which can be described by a single ket vector, as described above. A mixed quantum state is a statistical ensemble of pure states (see quantum statistical mechanics). [3]: 73
Purity (quantum mechanics) In quantum mechanics, and especially quantum information theory, the purity of a normalized quantum state is a scalar defined as where is the density matrix of the state and is the trace operation. The purity defines a measure on quantum states, giving information on how much a state is mixed.
A quantum state for an imperfectly isolated system will generally evolve to be entangled with the quantum state for the environment. Consequently, even if the system's initial state is pure, the state at a later time, found by taking the partial trace of the joint system-environment state, will be mixed.
A pure quantum state is a state that can not be written as a probabilistic mixture, or convex combination, of other quantum states. [4] There are several equivalent characterizations of pure states in the language of density operators. [7]: 73 A density operator represents a pure state if and only if:
The von Neumann description of quantum measurement of an observable A, when the system is prepared in a pure state ψ is the following (note, however, that von Neumann's description dates back to the 1930s and is based on experiments as performed during that time – more specifically the Compton–Simon experiment; it is not applicable to most ...
Separable state. In quantum mechanics, separable states are multipartite quantum states that can be written as a convex combination of product states. Product states are multipartite quantum states that can be written as a tensor product of states in each space. The physical intuition behind these definitions is that product states have no ...
In quantum mechanics and computing, the Bloch sphere is a geometrical representation of the pure state space of a two-level quantum mechanical system (qubit), named after the physicist Felix Bloch. [1] Mathematically each quantum mechanical system is associated with a separable complex Hilbert space . A pure state of a quantum system is ...
Fidelity of quantum states. Appearance. hide. In quantum mechanics, notably in quantum information theory, fidelity quantifies the "closeness" between two density matrices. It expresses the probability that one state will pass a test to identify as the other. It is not a metric on the space of density matrices, but it can be used to define the ...