||It has been suggested that this article be merged with Greenberger–Horne–Zeilinger state. (Discuss) Proposed since February 2013.|
In quantum computing, the cat state, named after Schrödinger's cat, is the special pure quantum state where the qubits are in an equal superposition of all being |0⟩ and all being |1⟩, i.e. (in bra–ket notation): |00⋯0⟩ + |11⋯1⟩.
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 at NIST, December 1, 2005. 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. David Bohm and Yakir Aharonov later attained recognition for the Aharonov–Bohm effect, 1959.
In quantum optics
are coherent states defined in the number (Fock) basis. Notice that if we add the two states together, the resulting cat state only contains even Fock state terms
As a result of this property, the above cat state is often referred to as an even cat state. Alternatively, we can define an odd cat state as
which only contains odd Fock states
Even and odd coherent states were first introduced by Dodonov, Malkin, and Man'ko in 1974.
Linear superposition of coherent states
A simple example of a cat state is a linear superposition of coherent states with opposite phases, when each state has the same weight:
The reading list contained in Incompleteness of quantum physics.
- John Gribbin (1984), In Search of Schrödinger's Cat, ISBN 0-552-12555-5, 22 February 1985, Transworld Publishers, Ltd, 318 pages.
- Dennis Overbye, "Quantum Trickery: Testing Einstein's Strangest Theory". New York Times Tuesday (Science Times), December 27, 2005 pages D1,D4.
- D. Leibfried, E. Knill, S. Seidelin, J. Britton, R.B. Blakestad, J. Chiaverini, D. Hume, W.M. Itano, J.D. Jost, C. Langer, R. Ozeri, R. Reichle, and D.J. Wineland. "Creation of a six atom 'Schrödinger cat' state". Nature. Dec. 1, 2005, 639–642.
- Amir D. Aczel (2001), Entanglement: the unlikely story of how scientists, mathematicians, and philosophers proved Einstein's spookiest theory. ISBN 0-452-28457-0 Penguin: paperback, 284 pages, index.
- V.V. Dodonov, I.A. Malkin, V.I. Man'ko. Physica, Volume 72, Issue 3, 15 March 1974, Pages 597–615.