How to make "double" brakets? Physics

Question

I want to make a double braket, such as this one:

Is there any package like physics that does this neatly? Thanks.

Answer 1

Here is a way to use the useful glyphs from MnSymbol, without changing all maths symbols. Incidentally, I use the math calligraphic fonts from dutchcal. An example code in normal and (partially) bold versions:

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{bm, amssymb}
\DeclareFontFamily{U} {MnSymbolE}{}
\DeclareFontShape{U}{MnSymbolE}{m}{n}{
<-6> MnSymbolE5
<6-7> MnSymbolE6
<7-8> MnSymbolE7
<8-9> MnSymbolE8
<9-10> MnSymbolE9
<10-12> MnSymbolE10
<12-> MnSymbolE12}{}
\DeclareFontShape{U}{MnSymbolE}{b}{n}{
<-6> MnSymbolE-Bold5
<6-7> MnSymbolE-Bold6
<7-8> MnSymbolE-Bold7
<8-9> MnSymbolE-Bold8
<9-10> MnSymbolE-Bold9
<10-12> MnSymbolE-Bold10
<12-> MnSymbolE-Bold12}{}

\DeclareSymbolFont{MnSyE} {U} {MnSymbolE}{m}{n}
\SetSymbolFont{MnSyE}{bold}{U}{MnSymbolE}{b}{n}

\DeclareMathSymbol{\llangle}{\mathopen}{MnSyE}{'164}
\DeclareMathSymbol{\rrangle}{\mathclose}{MnSyE}{'172}

\usepackage{dutchcal}

\begin{document}

$\widehat{\mathcal L} \bigm\vert \rho_{\mathrm{in}} \rrangle$

$\widehat{\mathbcal L} \bigm\vert \rho_{\mathrm{in}}\bm{\rrangle}$

\end{document} 

Answer 2

\documentclass{article}

\begin{document}
\[
\hat{\mathcal{L}}\vert\rho_{\mathsf{in}}\rangle\!\rangle
\]
\end{document}

or with use of the package MnSymbol:

\documentclass{article}
\usepackage{MnSymbol}

\begin{document}
\[
\hat{\mathcal{L}}\vert\rho_{\mathsf{in}}\rrangle
\]
\end{document}