Blanket of light may give better quantum computers

Quantum mechanics is one of the most successful theories of natural science, and although its predictions are often counterintuitive, not a single experiment has been conducted to date of which the theory has not been able ...

Weaving quantum processors out of laser light

An international team of scientists from Australia, Japan and the United States has produced a prototype of a large-scale quantum processor made of laser light.

Quantum physics: Ménage à trois photon-style

Entanglement is one of the properties specific to quantum particles. When two photons become entangled, for instance, the quantum state of the first will correlate perfectly with the quantum state of the second, even if they ...

Complex energies, quantum symmetries

In a certain sense, physics is the study of the universe's symmetries. Physicists strive to understand how systems and symmetries change under various transformations.

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Quantum state

In quantum physics, a quantum state is a mathematical object that fully describes a quantum system. One typically imagines some experimental apparatus and procedure which "prepares" this quantum state; the mathematical object then reflects the setup of the apparatus. Quantum states can be statistically mixed, corresponding to an experiment involving a random change of the parameters. States obtained in this way are called mixed states, as opposed to pure states, which cannot be described as a mixture of others. When performing a certain measurement on a quantum state, the result generally described by a probability distribution, and the form that this distribution takes is completely determined by the quantum state and the observable describing the measurement. However, unlike in classical mechanics, the result of a measurement on even a pure quantum state is only determined probabilistically. This reflects a core difference between classical and quantum physics.

Mathematically, a pure quantum state is typically represented by a vector in a Hilbert space. In physics, bra-ket notation is often used to denote such vectors. Linear combinations (superpositions) of vectors can describe interference phenomena. Mixed quantum states are described by density matrices.

In a more general mathematical context, quantum states can be understood as positive normalized linear functionals on a C* algebra; see GNS construction.

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