Added explanation section to Qiskit-terra (#8685)

* Created explanation page

* Added explanation about qubit ordering

* Fix typos and minor rephrases

* changed explanation.rst to explanation/index.rst

* Removed jupyter-sphinx

* Changed header symbol from index

* Grammar suggestion

Co-authored-by: Frank Harkins <frankharkins@hotmail.co.uk>

* Add reference to YouTube video about qubit ordering

---------

Co-authored-by: Frank Harkins <frankharkins@hotmail.co.uk>
Co-authored-by: Junye Huang <h.jun.ye@gmail.com>

docs/explanation/endianness.rst

@@ -0,0 +1,47 @@
+#########################
+Order of qubits in Qiskit
+#########################
+
+While most physics textbooks represent an :math:`n`-qubit system as the tensor product :math:`Q_0\otimes Q_1 \otimes ... \otimes Q_{n-1}`, where :math:`Q_j` is the :math:`j^{\mathrm{th}}` qubit, Qiskit uses the inverse order, that is, :math:`Q_{n-1}\otimes ... \otimes Q_1 \otimes Q_{0}`. As explained in `this video <https://www.youtube.com/watch?v=EiqHj3_Avps>`_ from `Qiskit's YouTube channel <https://www.youtube.com/@qiskit>`_, this is done to follow the convention in classical computing, in which the :math:`n^{\mathrm{th}}` bit or most significant bit (MSB) is placed on the left (with index 0) while the least significant bit (LSB) is placed on the right (index :math:`n-1`). This ordering convention is called little-endian while the one from the physics textbooks is called big-endian.
+
+This means that if we have, for example, a 3-qubit system with qubit 0 in state :math:`|1\rangle` and qubits 1 and 2 in state :math:`|0\rangle`, Qiskit would represent this state as :math:`|001\rangle` while most physics textbooks would represent this state as :math:`|100\rangle`. 
+
+The matrix representation of any multi-qubit gate is also affected by this different qubit ordering. For example, if we consider the single-qubit gate
+
+.. math::
+
+    U = \begin{pmatrix} u_{00} & u_{01} \\ u_{10} & u_{11} \end{pmatrix}
+
+And we want a controlled version :math:`C_U` whose control qubit is qubit 0 and whose target is qubit 1, following Qiskit's ordering its matrix representation would be
+
+.. math::
+
+    C_U = \begin{pmatrix} 1 & 0 & 0 & 0 \\0 & u_{00} & 0 & u_{01} \\ 0 & 0 & 1 & 0 \\ 0 & u_{10} & 0& u_{11} \end{pmatrix}
+
+while in a physics textbook it would be written as 
+
+.. math::
+
+    C_U = \begin{pmatrix} 1 & 0 & 0 & 0 \\0 & 1 & 0 & 0 \\ 0 & 0 & u_{00} & u_{01} \\ 0 & 0 & u_{00} & u_{01} \end{pmatrix}
+
+
+For more details about how this ordering of MSB and LSB affects the matrix representation of any particular gate, check its entry in the circuit :mod:`~qiskit.circuit.library`.
+
+This different order can also make the circuit corresponding to an algorithm from a textbook a bit more complicated to visualize. Fortunately, Qiskit provides a way to represent a :class:`~.QuantumCircuit` with the most significant qubits on top, just like in the textbooks. This can be done by setting the ``reverse_bits`` argument of the :meth:`~.QuantumCircuit.draw` method to ``True``.
+
+Let's try this for a 3-qubit Quantum Fourier Transform (:class:`~.QFT`).
+
+.. plot::
+    :include-source:
+    :context:
+
+    from qiskit.circuit.library import QFT
+
+    qft = QFT(3)
+    qft.decompose().draw('mpl')
+
+.. plot::
+    :include-source:
+    :context: close-figs
+
+    qft.decompose().draw('mpl', reverse_bits=True)

docs/explanation/index.rst

@@ -0,0 +1,12 @@
+.. _explanation:
+
+###########
+Explanation
+###########
+
+
+
+.. toctree::
+  :maxdepth: 1
+
+  Order of qubits in Qiskit <endianness>

docs/index.rst

@@ -8,6 +8,7 @@ Qiskit Terra documentation
 
   How-to Guides <how_to/index>
   API References <apidocs/terra>
+  Explanation <explanation/index>
   Migration Guides <migration_guides/index>
   Release Notes <release_notes>