Ket notation is like a kind of bracket, **one side of which is bound by single bar and the other side by a right angle bracket.**

```
\documentclass{article}
\begin{document}
\[ | a \rangle \]
\[ \vert \phi \rangle \]
\[ \vert \hat{A} \rangle \]
\end{document}
```

**Output : **

Instead of typing such a large syntax, you can define a new command. Latex has multiple methods for defining new commands.

However, in this case, the best practice is to use the `mathtools`

package.

```
\documentclass{article}
\usepackage{mathtools}
\DeclarePairedDelimiter\ket{\lvert}{\rangle}
\begin{document}
\[ \ket{\psi} \]
\[ \hat{p}\ket{p} \]
\[ U\ket{a_j}=\ket{Ua_i} \]
\[ \ket{1},\ket{2},\ldots,\ket{k} \]
\end{document}
```

**Output : **

If you use the star `*`

symbol with to this new command. The size of ket symbol will be adjustable with the argument.

```
\documentclass{article}
\usepackage{mathtools}
\DeclarePairedDelimiter\ket{\lvert}{\rangle}
\begin{document}
\[ \ket*{\Omega } \]
\[ \ket*{\Omega^\dagger} \]
\[ \ket*{\frac{\Omega^\dagger}{k}} \]
% Passing the Big Command as an optional argument
\[ \ket[\big]{\psi} \ket[\Big]{\frac{\psi}{k}} \]
\[ \ket[\bigg]{\frac{\psi_i}{n}} \ket[\Bigg]{\frac{\psi_i}{n_k}} \]
\end{document}
```

**Output : **

## Use braket package for ket notation

This package contains two commands `\ket`

and `\Ket`

. In the case of `ket`

command, size will be fixed. For example

```
\documentclass{article}
\usepackage{braket}
\begin{document}
\[ \ket{\psi} = \sum_j \sigma_j\ket{\psi_j} \]
\[ \ket{\phi} = A\ket{\psi_i}\]
\[ \ket{\frac{\psi_i}{k}} \]
\end{document}
```

**Output : **

And it would be best practice to use the `\Ket`

command because the size of the notation and argument will match dynamically.

```
\documentclass{article}
\usepackage{braket}
\begin{document}
\[ \Ket{\frac{m}{k}} = v\Ket{\frac{n}{k}}\]
\[ \Ket{\frac{\psi_i}{k}} \]
\end{document}
```

**Output : **

`\Ket`

and `\ket`

are considered two separate commands because of latex is case sensitive

## Use physics package in LaTeX

The same command is defined in the physics package as in the bracket package. For example

```
\documentclass{article}
\usepackage{physics}
\begin{document}
\[ v \to \ket{v} \]
\[ \ket{v_i} = \sum_{i}\ket{i}\alpha_i \]
\[ \ket{v} = \alpha_1\ket{a_1} + \alpha_2\ket{a_2} \]
\end{document}
```

**Output : **

Adding a star to the `\ket`

command will make the shape of the notation is constant.

```
\documentclass{article}
\usepackage{physics}
\begin{document}
\[ \ket*{v} \]
\[ \ket*{\hat{A}_i} \]
\[ \ket*{\frac{v_i}{k}} \]
\end{document}
```

**Output : **