The LaTeX command for braket is \langle \phi | \psi \rangle (inner product (quantum mechanics)).
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Write \langle \phi | \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 \langle \phi | \psi \rangle works in plain LaTeX.
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\nabla \cdot\nabla \times\partial\dot{x}\ddot{x}\langle \psi || \psi \rangle\rightleftharpoons\text{\AA}^\circ\mathrm{C}\infty\dagger\hbar\nablaPhysics symbols in LaTeX: the full table or search all 350+ LaTeX symbols.
Use \langle \phi | \psi \rangle in math mode; it needs no extra package. For example, $\langle \phi | \psi \rangle$.
None: \langle \phi | \psi \rangle is built into LaTeX, so no extra package is required.