= Glauber-Sudarshan representation
{c}
{title2=$\rho=\int P(\alpha)|\alpha\rangle\langle\alpha|\,d^2\alpha$}
The diagonal <coherent state> representation writes a single-mode <density matrix> in the form shown. Its weight may be a generalized distribution rather than an ordinary nonnegative probability density. A <nonclassical state of light> has no nonnegative measure of this form. This use of $P$ differs from the <Wigner distribution>; a nonnegative Wigner function does not by itself certify classicality.
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