What is the intuitive meaning of outer product of $|\psi\rangle\langle\psi |$ in Hilbert space?
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It is the projection operator of some state $|\phi\!\!>$ onto $|\psi\!\!>$.
For example, if $|\psi\!\!>$ is a state of some definite momentum $p$, and $|\phi\!\!>$ some state of uncertain momentum and position, then $|\psi\!\!><\!\!\psi| \phi\!\!>$ selects the momentum = $p$ component of $|\phi\!\!>$.
answered Aug 30, 2017 at 23:18
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2$\begingroup$ Usually
\langleand\rangleare preferred over<and<as delimiters $\ddot\smile$ $\endgroup$ Aug 30, 2017 at 23:31 -
$\begingroup$ Yes, I didn't know about those because they are not in the original LaTeX book I have. But this at least gave me the chance to use \! a few times! $\endgroup$ Sep 10, 2017 at 16:15
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$\begingroup$ Have you seen this MathJax tutorial before? I find it very helpful. $\endgroup$ Sep 10, 2017 at 17:36