Cat state

In quantum computing, the cat state, named after Schrödinger's cat, is the a quantum superposition of two macroscopically distinct states. The individual states being superposed could be classical or quantum, but their macroscopicity is an important criterion. A cat state could be of one or more modes or particles, and does not necessarily need entanglement, especially for the single-particle case. This is in contrast to the Greenberger–Horne–Zeilinger state, which by definition consists of multiple distinct particles or modes and their entanglement.

In other quantum mechanics contexts, according to The New York Times for example, physicists view the cat state as composed of two diametrically opposed conditions at the same time, such as the possibilities that a cat be alive and dead at the same time. This is sometimes connected to the many worlds hypothesis by proponents of the many worlds interpretation of quantum mechanics. More prosaically, a cat state might be the possibilities that six atoms be spin up and spin down, as published by a team led by David Wineland at NIST, December 1, 2005. Large cat states have also been experimentally created using photons by a team led by Jian-Wei Pan at University of Science and Technology of China, for instance, four-photon entanglement, five-photon entanglement, six-photon entanglement, eight-photon entanglement, and five-photon ten-qubit cat state. This spin up/down formulation was proposed by David Bohm, who conceived of spin as an observable in a version of thought experiments formulated in the 1935 EPR paradox.

In quantum optics, a cat state is defined as the coherent superposition of two coherent states with opposite phase:

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