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Coherent states in mathematical physics
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Coherent states in mathematical physics : ウィキペディア英語版
Coherent states in mathematical physics
Coherent states have been introduced in a physical context, first as quasi-classical states in quantum mechanics, then as the backbone of quantum optics and they are described in that spirit in the article
Coherent states (see also〔J-P. Gazeau,''Coherent States in Quantum Physics'', Wiley-VCH, Berlin, 2009.〕).
However, they have generated a huge variety of generalizations, which have led to a tremendous literature in mathematical physics.
In this article, we sketch the main directions of research on this line. For further details, we refer to several existing surveys
.〔S.T. Ali, J-P. Antoine, J-P. Gazeau, and U.A. Mueller, Coherent states and their generalizations: A mathematical overview, ''Reviews in Mathematical Physics'' 7 (1995) 1013-1104.〕〔S.T. Ali, J-P. Antoine, and J-P. Gazeau, ''Coherent States, Wavelets and Their Generalizations'', Springer-Verlag, New York, Berlin, Heidelberg, 2000.〕〔S.T. Ali, Coherent States, ''Encyclopedia of Mathematical Physics'', pp. 537-545; Elsevier, Amsterdam, 2006.〕
== A general definition ==
Let \mathfrak H\, be a complex, separable Hilbert space, X a locally compact space and d\nu a measure on X. For each x in X, denote |x\rangle a vector in \mathfrak H. Assume that this set of vectors possesses the following properties:
# The mapping x \mapsto | x \rangle is weakly continuous, i.e., for each vector |\phi\rangle in \mathfrak H, the function \Psi (x) = \langle x| \phi\rangle is continuous (in the topology of X).
# The resolution of the identity
: \int_X | x\rangle\langle x|\; d\nu (x) = I_
holds in the weak sense on the Hilbert space \mathfrak H, i.e., for any two vectors | \phi\rangle , | \psi \rangle in \mathfrak H, the following equality holds:
:\int_X \langle\phi| x\rangle\langle x|\psi\rangle\; d\nu (x) = \langle\phi|\psi\rangle\;.
A set of vectors | x\rangle satisfying the two properties above is called a family of ''generalized coherent states''.
In order to recover the previous definition (given in the article Coherent state) of canonical or standard coherent states (CCS), it suffices to take X\equiv\mathbb, the complex plane, x \equiv \alpha and d\nu (x) \equiv \frac d^2\alpha.
Sometimes the resolution of the identity condition is replaced by a weaker condition, with the vectors | x \rangle simply forming a total set in \, and the functions \Psi (x) = \langle x | \psi\rangle, as
| \psi \rangle runs through , forming a ''reproducing kernel Hilbert space''.
The objective in both cases is to ensure that an arbitrary vector | \psi \rangle be expressible as a linear (integral) combination of these vectors. Indeed, the resolution of the identity immediately implies that
:
| \psi \rangle = \int_X \Psi (x)| x\rangle\; d\nu (x)\; ,

where \Psi (x) = \langle x | \psi\rangle.
These vectors \Psi are square integrable, continuous functions on X and satisfy the ''reproducing property''
:
\int_X K (x,y )\Psi (y)\; d\nu (y) = \Psi (x)\, ,

where K (x, y) = \langle x | y \rangle is the reproducing kernel, which satisfies the following properties
: \quad K (x, y) = \overline\; , \qquad K (x, x) > 0\; ,
:\int_X K(x,z)\; K(z, y) \; d\nu (z) = K(x,y)\; .

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