# PrepareSingleQubitIdentity operation

Warning

This documentation refers to the Classic QDK, which has been replaced by the Modern QDK.

Please see https://aka.ms/qdk.api for the API documentation for the Modern QDK.

Namespace: Microsoft.Quantum.Preparation

Package: Microsoft.Quantum.Standard

Prepares a qubit in the maximally mixed state.

```
operation PrepareSingleQubitIdentity (qubit : Qubit) : Unit
```

## Description

Prepares the given qubit in the $\boldone / 2$ state by applying the depolarizing channel $$ \begin{align} \Omega(\rho) \mathrel{:=} \frac{1}{4} \sum_{\mu \in {0, 1, 2, 3}} \sigma_{\mu} \rho \sigma_{\mu}^{\dagger}, \end{align} $$ where $\sigma_i$ is the $i$th Pauli operator, and where $\rho$ is a density operator representing a mixed state.

## Input

### qubit : Qubit

A qubit whose state is to be depolarized in the manner described above.

## Output : Unit

## Remarks

The mixed state $\boldone / 2$ describing the result of
applying this operation to a state implicitly describes
an expectation value over random choices made in this operation.
Thus, for any single application, this operation maps pure states
to pure states, but it acts as described in expectation.
In particular, this operation can be used in process tomography
to measure the *non-unital* components of a channel.

## Feedback

https://aka.ms/ContentUserFeedback.

Coming soon: Throughout 2024 we will be phasing out GitHub Issues as the feedback mechanism for content and replacing it with a new feedback system. For more information see:Submit and view feedback for