Adds Double Excitation operations (#1123)

* Adds first verison of double excitation operations * excludes ops from reversible test * Update .github/CHANGELOG.md * Adds gate_data for testing * Adds first patch of tests * Fixes indent error * Update pennylane/devices/tf_ops.py * black * removes interface tests * fixes docstring warning * fixes docstring warning * adds interface tests for plus/minus * Apply suggestions from code review * adds grad interface tests * Apply suggestions from code review Co-authored-by: antalszava <antalszava@gmail.com> * prototype new interface tests * Apply suggestions from code review Co-authored-by: antalszava <antalszava@gmail.com> * changes from code review * Update .github/CHANGELOG.md * documentation fixes * try * this should fix it * Apply suggestions from code review * Apply suggestions from code review Co-authored-by: antalszava <antalszava@gmail.com> Co-authored-by: agran2018 <45397799+agran2018@users.noreply.github.com> * changes from code review * improves docstrings * removes useless import * black * fix indent Co-authored-by: antalszava <antalszava@gmail.com> Co-authored-by: Josh Izaac <josh146@gmail.com> Co-authored-by: agran2018 <45397799+agran2018@users.noreply.github.com>
PennyLaneAI · Mar 23, 2021 · c05978e · c05978e
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diff --git a/.github/CHANGELOG.md b/.github/CHANGELOG.md
@@ -21,8 +21,29 @@
 
   The `SingleExcitation` operation supports analytic gradients on hardware
   using only four expectation value calculations, following results from
-  [Kottmann et al.](https://arxiv.org/abs/2011.05938)
+  [Kottmann et al.](https://arxiv.org/abs/2011.05938) 
 
+  * Added the `DoubleExcitation` four-qubit operation, which is useful for quantum
+  chemistry applications. [(#1123)](https://github.com/PennyLaneAI/pennylane/pull/1123)
+
+  It can be used to perform an SO(2) rotation in the subspace 
+  spanned by the states :math:`|1100\rangle` and :math:`|0011\rangle`. 
+  For example, the following circuit performs the transformation
+  :math:`|1100\rangle\rightarrow \cos(\phi/2)|1100\rangle - \sin(\phi/2)|0011\rangle`:   
+
+    ```python
+  dev = qml.device('default.qubit', wires=2)
+
+  @qml.qnode(dev)
+  def circuit(phi):
+      qml.PauliX(wires=0)
+      qml.PauliX(wires=1)
+      qml.DoubleExcitation(phi, wires=[0, 1, 2, 3])
+  ```
+
+  The `DoubleExcitation` operation supports analytic gradients on hardware using only
+  four expectation value calculations, following results from
+  [Kottmann et al.](https://arxiv.org/abs/2011.05938).
 
 * Adds a new function ``qml.math.conj``.
   [(#1143)](https://github.com/PennyLaneAI/pennylane/pull/1143)

diff --git a/doc/introduction/operations.rst b/doc/introduction/operations.rst
@@ -88,6 +88,10 @@ Qubit gates
     ~pennylane.SingleExcitation
     ~pennylane.SingleExcitationPlus
     ~pennylane.SingleExcitationMinus
+    ~pennylane.DoubleExcitation
+    ~pennylane.DoubleExcitationPlus
+    ~pennylane.DoubleExcitationMinus
+
 
 :html:`</div>`