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swapped section and updated language (#2564)
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@mariaschuld
mariaschuld authored May 13, 2022
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Expand Up @@ -10,12 +10,121 @@ PennyLane offers seamless integration between classical and quantum computations
circuits in PennyLane, compute :doc:`gradients of quantum circuits <glossary/quantum_gradient>`, and
connect them easily to the top scientific computing and machine learning libraries.

Training and interfaces
-----------------------

The bridge between the quantum and classical worlds is provided in PennyLane via interfaces to
automatic differentiation libraries.
Currently, four libraries are supported: :doc:`NumPy <interfaces/numpy>`, :doc:`PyTorch
<interfaces/torch>`, :doc:`JAX <interfaces/jax>`, and :doc:`TensorFlow <interfaces/tf>`. PennyLane makes
each of these libraries quantum-aware, allowing quantum circuits to be treated just
like any other operation. Any automatic differentiation framework can be chosen with any device.

In PennyLane, an automatic differentiation framework is declared using the ``interface`` argument when creating
a :class:`QNode <pennylane.QNode>`, e.g.,

.. code-block:: python
@qml.qnode(dev, interface="tf")
def my_quantum_circuit(...):
...
.. note::
If no interface is specified, PennyLane will default to the NumPy interface (powered by the
`autograd <https://github.com/HIPS/autograd>`_ library).

This will allow native numerical objects of the specified library (NumPy arrays, JAX arrays, Torch Tensors,
or TensorFlow Tensors) to be passed as parameters to the quantum circuit. It also makes
the gradients of the quantum circuit accessible to the classical library, enabling the
optimization of arbitrary hybrid circuits.

When specifying an interface, the objects of the chosen framework are converted
into NumPy objects and are passed to a device in most cases. Exceptions include
cases when the devices support end-to-end computations in a framework. Such
devices may be referred to as backpropagation or passthru devices.

See the links below for walkthroughs of each specific interface:

.. raw:: html

<style>
#interfaces .card {
box-shadow: none!important;
}
#interfaces .card:hover {
box-shadow: none!important;
}
</style>
<div id="interfaces" class="container mt-2 mb-2">
<div class="row mt-3">
<div class="col-lg-3 mb-2 align-items-stretch">
<a href="interfaces/numpy.html">
<div class="card rounded-lg py-2" style="height:100%;">
<div class="d-flex justify-content-center align-items-center" style="height:100%;">
<img src="../_static/numpy.png" class="card-img-top" style="width:80%;"></img>
</div>
</div>
</a>
</div>
<div class="col-lg-3 mb-2 align-items-stretch">
<a href="interfaces/torch.html">
<div class="card rounded-lg py-2" style="height:100%;">
<div class="d-flex justify-content-center align-items-center" style="height:100%;">
<img src="../_static/pytorch.png" class="card-img-top"></img>
</div>
</div>
</a>
</div>
<div class="col-lg-3 mb-2 align-items-stretch">
<a href="interfaces/tf.html">
<div class="card rounded-lg py-2" style="height:100%;">
<div class="d-flex justify-content-center align-items-center" style="height:100%;">
<img src="../_static/tensorflow.png" class="card-img-top" style="width:90%;"></img>
</div>
</div>
</a>
</div>
<div class="col-lg-3 mb-2 align-items-stretch">
<a href="interfaces/jax.html">
<div class="card rounded-lg py-2" style="height:100%;">
<div class="d-flex justify-content-center align-items-center" style="height:100%;">
<img src="../_static/jax.png" class="card-img-top" style="max-width:60%;"></img>
</div>
</div>
</a>
</div>
</div>
</div>

In addition to the core automatic differentiation frameworks discussed above,
PennyLane also provides higher-level classes for converting QNodes into both Keras and ``torch.nn`` layers:


:html:`<div class="summary-table">`

.. autosummary::

pennylane.qnn.KerasLayer
pennylane.qnn.TorchLayer


.. note::

QNodes that allow for automatic differentiation will always incur a small overhead on evaluation.
If you do not need to compute quantum gradients of a QNode, specifying ``interface=None`` will remove
this overhead and result in a slightly faster evaluation. However, gradients will no
longer be available.


Gradients
---------

The interface between PennyLane and automatic differentiation libraries relies on PennyLane's ability
to compute or estimate gradients of quantum circuits. There are different strategies to do so, and they may
depend on the device used.

When creating a QNode, you can specify the :doc:`differentiation method
<glossary/quantum_differentiable_programming>` that PennyLane should use whenever the gradient of
that QNode is requested.
<glossary/quantum_differentiable_programming>` like this:

.. code-block:: python
Expand Down Expand Up @@ -127,109 +236,6 @@ support gradients of QNodes.
For more details on available gradient transforms, as well as learning how to define your own
gradient transform, please see the :mod:`qml.gradients <pennylane.gradients>` documentation.

Training and interfaces
-----------------------

The bridge between the quantum and classical worlds is provided in PennyLane via *interfaces*.
Currently, there are four built-in interfaces: :doc:`NumPy <interfaces/numpy>`, :doc:`PyTorch
<interfaces/torch>`, :doc:`JAX <interfaces/jax>`, and :doc:`TensorFlow <interfaces/tf>`. These
interfaces make each of these libraries quantum-aware, allowing quantum circuits to be treated just
like any other operation. Any interface can be chosen with any device.

In PennyLane, an interface is declared when creating a :class:`QNode <pennylane.QNode>`, e.g.,

.. code-block:: python
@qml.qnode(dev, interface="tf")
def my_quantum_circuit(...):
...
.. note::
If no interface is specified, PennyLane will default to the NumPy interface (powered by the
`autograd <https://github.com/HIPS/autograd>`_ library).

This will allow native numerical objects of the specified library (NumPy arrays, Torch Tensors,
or TensorFlow Tensors) to be passed as parameters to the quantum circuit. It also makes
the gradients of the quantum circuit accessible to the classical library, enabling the
optimization of arbitrary hybrid circuits.

When specifying an interface, the objects of the chosen framework are converted
into NumPy objects and are passed to a device in most cases. Exceptions include
cases when the devices support end-to-end computations in a framework. Such
devices may be referred to as backpropagation or passthru devices.

See the links below for walkthroughs of each specific interface:

.. raw:: html

<style>
#interfaces .card {
box-shadow: none!important;
}
#interfaces .card:hover {
box-shadow: none!important;
}
</style>
<div id="interfaces" class="container mt-2 mb-2">
<div class="row mt-3">
<div class="col-lg-3 mb-2 align-items-stretch">
<a href="interfaces/numpy.html">
<div class="card rounded-lg py-2" style="height:100%;">
<div class="d-flex justify-content-center align-items-center" style="height:100%;">
<img src="../_static/numpy.png" class="card-img-top" style="width:80%;"></img>
</div>
</div>
</a>
</div>
<div class="col-lg-3 mb-2 align-items-stretch">
<a href="interfaces/torch.html">
<div class="card rounded-lg py-2" style="height:100%;">
<div class="d-flex justify-content-center align-items-center" style="height:100%;">
<img src="../_static/pytorch.png" class="card-img-top"></img>
</div>
</div>
</a>
</div>
<div class="col-lg-3 mb-2 align-items-stretch">
<a href="interfaces/tf.html">
<div class="card rounded-lg py-2" style="height:100%;">
<div class="d-flex justify-content-center align-items-center" style="height:100%;">
<img src="../_static/tensorflow.png" class="card-img-top" style="width:90%;"></img>
</div>
</div>
</a>
</div>
<div class="col-lg-3 mb-2 align-items-stretch">
<a href="interfaces/jax.html">
<div class="card rounded-lg py-2" style="height:100%;">
<div class="d-flex justify-content-center align-items-center" style="height:100%;">
<img src="../_static/jax.png" class="card-img-top" style="max-width:60%;"></img>
</div>
</div>
</a>
</div>
</div>
</div>

In addition to the core interfaces discussed above, PennyLane also provides higher-level classes for
converting QNodes into both Keras and ``torch.nn`` layers:


:html:`<div class="summary-table">`

.. autosummary::

pennylane.qnn.KerasLayer
pennylane.qnn.TorchLayer


.. note::

QNodes with an interface will always incur a small overhead on evaluation. If you do not
need to compute quantum gradients of a QNode, specifying ``interface=None`` will remove
this overhead and result in a slightly faster evaluation. However, gradients will no
longer be available.


:html:`</div>`

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