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

# Bell State Preparation

<Card title="View on GitHub" icon="github" href="https://github.com/Classiq/classiq-library/blob/main/functions/qmod_library_reference/classiq_open_library/special_state_preparations/prepare_bell_state.ipynb">
  Open this notebook in GitHub to run it yourself
</Card>

The `prepare_bell_state` function creates one of the four Bell states

$$
\phi_{+} = \frac{1}{2} \left(|00\rangle + |11\rangle \right)
$$

$$
\phi_{-} = \frac{1}{2} \left(|00\rangle - |11\rangle \right)
$$

$$
\psi_{+} = \frac{1}{2} \left(|01\rangle + |10\rangle \right)
$$

$$
\psi_{-} = \frac{1}{2} \left(|01\rangle - |10\rangle \right)
$$

Function: `prepare_bell_state`

Arguments:

* `state_num: CInt`
* `q: Output[QArray[QBit]]`

## Example

The `prepare_bell_state` function creates

$$
\psi_{+} = \frac{1}{2} \left(|01\rangle + |10\rangle \right)
$$

`state_num` is set to 2

```python theme={null}
from classiq import *


@qfunc
def main(x: Output[QArray[QBit]]):
    prepare_bell_state(2, x)


qmod = create_model(main)
```

```python theme={null}

qprog = synthesize(qmod)
```