Add pytket extensions index page to pytket API docs (#1105)

* add extensions index to API docs

* remove duplicate extensions link

* Solve docs build failure caused by duplicate link warnings

* Use smaller text for sub-headings

* Put QuantinuumBackend QPU first and update devices

* some cleanup

* remove duplicate links from sidebar

pytket/docs/extensions_index.rst

@@ -0,0 +1,159 @@
+pytket extensions
+=================
+
+The pytket extensions are separate python modules which allow pytket to interface with backends from a range of providers including quantum devices from Quantinuum and IBM.
+In pytket a ``Backend`` represents a connection to a QPU (Quantum Processing Unit) or simulator for processing quantum circuits. One can also access additional quantum devices and simulators via the cloud through the extensions for `Azure <https://cqcl.github.io/pytket-qsharp/api/api.html#pytket.extensions.qsharp.AzureBackend>`_ and `Braket <https://tket.quantinuum.com/extensions/pytket-braket/api/api.html#pytket.extensions.braket.BraketBackend>`_ . 
+
+Additionally, the extensions allow pytket to cross-compile circuits from different quantum computing libraries with the extensions for `qiskit <https://tket.quantinuum.com/extensions/pytket-qiskit/api/index.html>`_, `cirq <https://tket.quantinuum.com/extensions/pytket-cirq/api/index.html>`_ and `pennylane <https://tket.quantinuum.com/extensions/pytket-pennylane/api/index.html>`_ . This enables pytket's compilation features to be used in conjunction with other software tools.
+
+The additional modules can be installed adding the extension name to the installation command for pytket. For example pytket-quantinuum can be installed by running
+
+::
+
+   pip install pytket-quantinuum
+
+The types of ``Backend`` available in pytket are the following
+
+Types of Backend
+----------------
+
+* **QPUs** - These are real quantum computers that return shots based results. E.g the `QuantinuumBackend <https://tket.quantinuum.com/extensions/pytket-quantinuum/api.html#pytket.extensions.quantinuum.QuantinuumBackend>`_ .
+* **Cloud Access** - Cloud backends allow pytket to interface with cloud platforms to access additional QPUs and simulators. E.g `BraketBackend <https://tket.quantinuum.com/extensions/pytket-braket/api.html#pytket.extensions.braket.BraketBackend>`_ .
+* **Emulators** - These classically simulate a circuit and produce shots based results. Sometimes emulators use a noise model and have connectivity constraints to emulate real QPUs. E.g. `IBMQEmulatorBackend`_ 
+* **Statevector Simulators** - Calculates the pure quantum state prepared by a circuit returning a vector/ndarray. Examples of statevector simulators are the `ForestStateBackend`_ and the `AerStateBackend`_. 
+* **Unitary Simulators** - Unitary simulators calculate the unitary operator that is applied by a circuit. A unitary matrix/ndarray is returned `AerUnitaryBackend`_ is an example of such a simulator.
+* **Density Matrix Simulators** - These simulators compute the density matrix prepared by a circuit. The result can be a statistical mixture of states in contrast to statevector simulation. E.g. `CirqDensityMatrixSampleBackend`_
+* **Other specialised simulators** - There are extensions for simulating specific types of circuit. For instance the `SimplexBackend`_ is designed to simulate Clifford circuits. 
+
+A full list of available pytket backends is shown below.
+
+QPUs
+----
+
+`QuantinuumBackend <https://tket.quantinuum.com/extensions/pytket-quantinuum/api.html#pytket.extensions.quantinuum.QuantinuumBackend>`_
+- Interface to a remote Quantinuum device or simulator. There are currently two Quantinuum devices offered (H1-1 and H2-1).
+
+`IBMQBackend <https://tket.quantinuum.com/extensions/pytket-qiskit/api.html#pytket.extensions.qiskit.IBMQBackend>`_
+- A backend for running circuits on remote IBMQ devices.
+
+`IonQBackend <https://cqcl.github.io/pytket-ionq/api/api.html#pytket.extensions.ionq.IonQBackend>`_
+- A backend for running circuits on remote IONQ devices.
+
+`ForestBackend <https://tket.quantinuum.com/extensions/pytket-pyquil/api.html#pytket.extensions.pyquil.ForestBackend>`_
+- A backend for running circuits on remote Rigetti devices.
+
+`AQTBackend <https://cqcl.github.io/pytket-aqt/api/api.html#pytket.extensions.aqt.AQTBackend>`_
+- Interface to an AQT device or simulator.
+
+`IQMBackend <https://tket.quantinuum.com/extensions/pytket-iqm/api.html#pytket.extensions.iqm.IQMBackend>`_
+- Interface to an IQM device or simulator.
+
+Cloud Access
+------------
+
+`AzureBackend <https://cqcl.github.io/pytket-qsharp/api/api.html#pytket.extensions.qsharp.AzureBackend>`_
+- Backend for running circuits remotely using Azure Quantum devices and simulators.
+
+`BraketBackend <https://tket.quantinuum.com/extensions/pytket-braket/api.html#pytket.extensions.braket.BraketBackend>`_
+- Interface to Amazon Braket service.
+
+Emulators
+---------
+
+`IBMQEmulatorBackend`_ - A backend which uses the `AerBackend <https://tket.quantinuum.com/extensions/pytket-qiskit/api.html#pytket.extensions.qiskit.AerBackend>`_ to emulate the behavior of IBMQBackend.
+
+`QuantinuumBackend <https://tket.quantinuum.com/extensions/pytket-quantinuum/api.html#pytket.extensions.quantinuum.QuantinuumBackend>`_
+- The QuantinuumBackend has two available emulators namely H1-1E and H1-2E. These are device specific emulators for the H1-1 and H1-2 devices. These emualtors run remotely on a server.
+
+Statevector Simulators
+----------------------
+
+`CirqStateSampleBackend <https://tket.quantinuum.com/extensions/pytket-cirq/api.html#pytket.extensions.cirq.CirqStateSampleBackend>`_
+- Backend for Cirq statevector simulator sampling.
+
+`CirqStateSimBackend <https://tket.quantinuum.com/extensions/pytket-cirq/api.html#pytket.extensions.cirq.CirqStateSimBackend>`_
+- Backend for Cirq statevector simulator state return.
+
+`AerStateBackend`_ - Backend for running simulations on the Qiskit Aer Statevector simulator.
+
+`ForestStateBackend`_ - State-based interface to a Rigetti device.
+
+`ProjectQBackend <https://tket.quantinuum.com/extensions/pytket-projectq/api.html#pytket.extensions.projectq.ProjectQBackend>`_
+- Backend for running statevector simulations on the ProjectQ simulator.
+
+Unitary Simulators
+------------------
+
+`AerUnitaryBackend`_ - Backend for running simulations on the Qiskit Aer unitary simulator.
+
+Density Matrix Simulators
+-------------------------
+
+`CirqDensityMatrixSampleBackend`_
+- Backend for Cirq density matrix simulator sampling.
+
+`CirqDensityMatrixSimBackend <https://tket.quantinuum.com/extensions/pytket-cirq/api.html#pytket.extensions.cirq.CirqDensityMatrixSimBackend>`_
+- Backend for Cirq density matrix simulator density_matrix return.
+
+Clifford Simulators
+-------------------
+
+`CirqCliffordSampleBackend <https://tket.quantinuum.com/extensions/pytket-cirq/api.html#pytket.extensions.cirq.CirqCliffordSampleBackend>`_
+- Backend for Cirq Clifford simulator sampling.
+
+`CirqCliffordSimBackend <https://tket.quantinuum.com/extensions/pytket-cirq/api.html#pytket.extensions.cirq.CirqCliffordSimBackend>`_
+- Backend for Cirq Clifford simulator state return.
+
+`SimplexBackend`_- Backend for simulating Clifford circuits using pysimplex.
+
+`StimBackend <https://tket.quantinuum.com/extensions/pytket-stim/api.html#pytket.extensions.stim.StimBackend>`_
+- Backend for simulating Clifford circuits using Stim.
+
+Other
+-----
+
+`AerBackend <https://tket.quantinuum.com/extensions/pytket-qiskit/api.html#pytket.extensions.qiskit.AerBackend>`_
+- Backend for running simulations on the Qiskit Aer QASM simulator. This simulator is noiseless by default but can take a user defined ``NoiseModel``.
+
+`QulacsBackend <https://tket.quantinuum.com/extensions/pytket-qulacs/api.html#pytket.extensions.qulacs.QulacsBackend>`_
+- Backend for running simulations of variational quantum circuits on the Qulacs simulator.
+
+`QsharpSimulatorBackend <https://cqcl.github.io/pytket-qsharp/api/api.html#pytket.extensions.qsharp.QsharpSimulatorBackend>`_
+- Backend for simulating a circuit using the QDK.
+
+`QsharpToffoliSimulatorBackend <https://cqcl.github.io/pytket-qsharp/api/api.html#pytket.extensions.qsharp.QsharpToffoliSimulatorBackend>`_
+- Backend for simulating a Toffoli circuit using the QDK.
+
+
+.. toctree::
+   :caption: Extensions:
+   :maxdepth: 0
+
+   pytket-aqt <https://cqcl.github.io/pytket-aqt/api/index.html>
+   pytket-braket <https://tket.quantinuum.com/extensions/pytket-braket>
+   pytket-cirq <https://tket.quantinuum.com/extensions/pytket-cirq>
+   pytket-ionq <https://tket.quantinuum.com/extensions/pytket-quantinuum>
+   pytket-iqm <https://tket.quantinuum.com/extensions/pytket-iqm>
+   pytket-pennylane <https://tket.quantinuum.com/extensions/pytket-pennylane>
+   pytket-projectq <https://tket.quantinuum.com/extensions/pytket-projectq>
+   pytket-pyquil <https://tket.quantinuum.com/extensions/pytket-pyquil>
+   pytket-pysimplex <https://tket.quantinuum.com/extensions/pytket-pysimplex>
+   pytket-pyzx <https://tket.quantinuum.com/extensions/pytket-pyzx>
+   pytket-qir <https://tket.quantinuum.com/extensions/pytket-qir>
+   pytket-qiskit <https://tket.quantinuum.com/extensions/pytket-qir>
+   pytket-qsharp <https://cqcl.github.io/pytket-qsharp/api/index.html>
+   pytket-quantinuum <https://tket.quantinuum.com/extensions/pytket-quantinuum>
+   pytket-cutensornet <https://tket.quantinuum.com/extensions/pytket-cutensornet> 
+   pytket-qulacs <https://tket.quantinuum.com/extensions/pytket-qulacs>
+   pytket-qujax <https://cqcl.github.io/pytket-qujax/api/index.html>
+   pytket-stim <https://tket.quantinuum.com/extensions/pytket-stim>
+
+
+.. _pytket: https://tket.quantinuum.com/tket/pytket/api/
+.. _Quantinuum: https://quantinuum.com
+.. _IBMQEmulatorBackend:  https://tket.quantinuum.com/extensions/pytket-qiskit/api/api.html#pytket.extensions.qiskit.IBMQEmulatorBackend
+.. _AerStateBackend: https://tket.quantinuum.com/extensions/pytket-qiskit/api.html#pytket.extensions.qiskit.AerStateBackend
+.. _ForestStateBackend: https://tket.quantinuum.com/extensions/pytket-pyquil/api/api.html#pytket.extensions.pyquil.ForestStateBackend
+.. _AerUnitaryBackend: https://tket.quantinuum.com/extensions/pytket-qiskit/api/api.html#pytket.extensions.qiskit.AerUnitaryBackend
+.. _CirqDensityMatrixSampleBackend: https://tket.quantinuum.com/extensions/pytket-cirq/api/api.html#pytket.extensions.cirq.CirqDensityMatrixSampleBackend
+.. _SimplexBackend: https://tket.quantinuum.com/extensions/pytket-simplex/api.html#pytket.extensions.pysimplex.SimplexBackend

pytket/docs/index.rst

@@ -81,14 +81,13 @@ Licensed under the `Apache 2 License <http://www.apache.org/licenses/LICENSE-2.0
     getting_started.rst
     changelog.rst
     install.rst
-    faqs.rst 
+    faqs.rst
 
 .. toctree::
     :caption: More Documentation:
-    :maxdepth: 1
 
     Manual <https://cqcl.github.io/pytket/manual/index.html>
-    Extensions <https://cqcl.github.io/pytket-extensions/api/index.html>
+    extensions_index.rst  
     Example notebooks <https://tket.quantinuum.com/examples>
 
 .. toctree::