[Tex/LaTex] How to write a multiline case in latex


I am trying to get the output similar to this:

First case is an equation spanning two lines.

I have written this code:

\text{RHS} \rightarrow  
\vert 1 \rangle \left[\frac{2\beta}{\sqrt{\alpha^2 + 4\beta^2}}\vert B\rangle  - \frac{\alpha}{\sqrt{\alpha^2 + 4\beta^2}}\vert A \rangle\right] , &D_1\textrm{ clicks, probability $\frac{\alpha^2}{4}$}\\
\vert 2 \rangle\vert A \rangle, &D_2\textrm{ clicks, probability $\frac{\alpha^2}{4}$}\\
\vert \text{scattered} \rangle\vert A \rangle, &\textrm{ no clicks with probability $\frac{\alpha^2}{2}$}

And the following is the output:
The first case becomes too long.

How then, should I modify the code to get the output similar to first image? If I just add a line break in the first case it gives an error.

Thank you.

Best Answer

Would this hack be sufficient?

% arara: pdflatex

\text{RHS} \rightarrow  
\vert 1 \rangle \bigg[\frac{2\beta}{\sqrt{\alpha^2 + 4\beta^2}}\vert B\rangle &D_1\textrm{ clicks, probability $\frac{\alpha^2}{4}$}\\
\quad - \frac{\alpha}{\sqrt{\alpha^2 + 4\beta^2}}\vert A \rangle\bigg], & \\
\vert 2 \rangle\vert A \rangle, &D_2\text{ clicks, probability $\frac{\alpha^2}{4}$}\\
\vert \text{scattered} \rangle\vert A \rangle, &\text{no clicks with probability $\frac{\alpha^2}{2}$}

enter image description here