The LaTeX command for ket is | \psi \rangle (Dirac ket (quantum mechanics)).
| \psi \rangleWriting a paper? The table generator and the BibTeX cleaner are one click away, no account.
Write | \psi \rangle inside math mode, that is between $…$ for inline math or in a \[ … \] or equation block for display math. It is part of core LaTeX math, so no extra package is required.
The physics and siunitx packages offer shortcuts for many formulas, but | \psi \rangle works in plain LaTeX.
Open this example in the live preview: it renders as you type, with no compiler and no signup.
\nabla^2\nabla \cdot\nabla \times\partial\dot{x}\ddot{x}\langle \psi |\langle \phi | \psi \rangle\rightleftharpoons\text{\AA}^\circ\mathrm{C}\infty\dagger\hbarPhysics symbols in LaTeX: the full table or search all 350+ LaTeX symbols.
Use | \psi \rangle in math mode; it needs no extra package. For example, $| \psi \rangle$.
None: | \psi \rangle is built into LaTeX, so no extra package is required.