> ## Documentation Index
> Fetch the complete documentation index at: https://docs.classiq.io/llms.txt
> Use this file to discover all available pages before exploring further.

# Qpe

Functions:

| Name           | Description                       |
| -------------- | --------------------------------- |
| `qpe_flexible` | \[Qmod Classiq-library function]. |
| `qpe`          | \[Qmod Classiq-library function]. |

### qpe\_flexible

<pre><code>qpe\_flexible(
unitary\_with\_power: QCallable\[CInt],
phase: QArray\[QBit]
) -> None</code></pre>

\[Qmod Classiq-library function]

Implements the Quantum Phase Estimation (QPE) algorithm,  which estimates the phase (eigenvalue) associated with an eigenstate of a given unitary operator $U$.
This is a flexible version that allows the user to provide a callable that generates the unitary operator $U^k$ for a given integer $k$, offering greater flexibility in handling different quantum circuits using some powering rule.

**Parameters:**

| Name                 | Type              | Description                                                                                                                                                  | Default    |
| -------------------- | ----------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------ | ---------- |
| `unitary_with_power` | `QCallable[CInt]` | A callable that returns the unitary operator $U^k$ given an integer $k$. This callable is used to control the application of powers of the unitary operator. | *required* |
| `phase`              | `QArray[QBit]`    | The quantum variable that represents the estimated phase (eigenvalue), assuming initialized to zero.                                                         | *required* |

### qpe

<pre><code>qpe(
unitary: QCallable,
phase: QNum
) -> None</code></pre>

\[Qmod Classiq-library function]

Implements the standard Quantum Phase Estimation (QPE) algorithm, which estimates the phase (eigenvalue) associated with an eigenstate of a given unitary operator $U$.

**Parameters:**

| Name      | Type        | Description                                                                                          | Default    |
| --------- | ----------- | ---------------------------------------------------------------------------------------------------- | ---------- |
| `unitary` | `QCallable` | A callable representing the unitary operator $U$, whose eigenvalue is to be estimated.               | *required* |
| `phase`   | `QNum`      | The quantum variable that represents the estimated phase (eigenvalue), assuming initialized to zero. | *required* |