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Document the CommutationChecker (backport #13303) #13304

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Oct 10, 2024
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Document the CommutationChecker (backport #13303) #13304

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24 changes: 24 additions & 0 deletions qiskit/circuit/__init__.py
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Expand Up @@ -772,6 +772,30 @@
Consult :ref:`the control-flow construction documentation <circuit-control-flow-methods>` for more
information on how to build circuits with control flow.

Investigating commutation relations
-----------------------------------

If two operations in a circuit commute, we can swap the order in which they are applied.
This can allow for optimizations and simplifications, for example, if it allows to merge
or cancel gates:

.. code-block:: text

┌─────────┐ ┌─────────┐ ┌─────────┐
q_0: ┤ Rz(0.5) ├──■──┤ Rz(1.2) ├──■── q_0: ┤ Rz(1.7) ├
└─────────┘┌─┴─┐└──┬───┬──┘┌─┴─┐ = └──┬───┬──┘
q_1: ───────────┤ X ├───┤ X ├───┤ X ├ q_1: ───┤ X ├───
└───┘ └───┘ └───┘ └───┘

Performing these optimizations are part of the transpiler, but the tools to investigate commutations
are available in the :class:`CommutationChecker`.

.. autosummary::
:toctree: ../stubs/

CommutationChecker


.. _circuit-custom-gates:

Creating custom instructions
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