Researchers take quantum encryption out of the lab

In a new study, researchers demonstrate an automated, easy-to-operate quantum key distribution (QKD) system using the fiber network in the city of Padua, Italy. The field test represents an important step toward implementing ...

Early endeavors on the path to reliable quantum machine learning

Anyone who collects mushrooms knows that it is better to keep the poisonous and the non-poisonous ones apart. In such "classification problems," which require distinguishing certain objects from one another and to assign ...

Researchers realize coherent storage of light over one hour

Remote quantum distribution on the ground is limited because of the loss of photons in optical fibers. One solution for remote quantum communication lies in quantum memories: photons are stored in long-lived quantum memory ...

Complex shapes of photons to boost future quantum technologies

As the digital revolution has now become mainstream, quantum computing and quantum communication are rising in the consciousness of the field. The enhanced measurement technologies enabled by quantum phenomena, and the possibility ...

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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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