Add inverse method to ECRGate (#9733) (#9738)

* add inverse method to ECRGate

* add release notes

* Fix sphinx tags

* Recategorise as bugfix

---------

Co-authored-by: Julien Gacon <[email protected]>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>
Co-authored-by: Jake Lishman <[email protected]>
(cherry picked from commit 80ca1f3)

Co-authored-by: Shelly Garion <[email protected]>

Author: mergify[bot]

qiskit/circuit/library/standard_gates/ecr.py

@@ -103,6 +103,10 @@ def _define(self):
 
         self.definition = qc
 
+    def inverse(self):
+        """Return inverse ECR gate (itself)."""
+        return ECRGate()  # self-inverse
+
     def to_matrix(self):
         """Return a numpy.array for the ECR gate."""
         return (

releasenotes/notes/add-inverse-ecr-e03720252a0c9c1e.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    The method :meth:`.ECRGate.inverse` now returns another :class:`.ECRGate` instance
+    rather than a custom gate, since it is self inverse.