# Quantum Phase Estimation

QPE estimates the phase of unitary time evolution.

```{math}
:label: eq-qpe-unitary
U = e^{-iHt}
```

For an eigenstate of the Hamiltonian, the unitary in {eq}`eq-qpe-unitary`
encodes the energy in a measurable phase.

```{include} ../../THEORY.md
:start-after: "# Quantum Phase Estimation"
:end-before: "# Quantum Imaginary Time Evolution"
:heading-offset: 1
```