Fix C3SXGate to_matrix method (backport #12742) (#12745)

* Fix `C3SXGate` `to_matrix` method (#12742) * Fix c3sx matrix * Apply Jake's suggestion (cherry picked from commit d86f995) # Conflicts: # test/python/circuit/test_rust_equivalence.py * Remove rust test --------- Co-authored-by: Elena Peña Tapia <[email protected]> Co-authored-by: Elena Peña Tapia <[email protected]>
Qiskit · Jul 9, 2024 · 6b57b78 · 6b57b78
1 parent 0989210
commit 6b57b78
Show file tree

Hide file tree

Showing 3 changed files with 13 additions and 0 deletions.
diff --git a/qiskit/circuit/library/standard_gates/x.py b/qiskit/circuit/library/standard_gates/x.py
@@ -21,6 +21,7 @@
 from qiskit.circuit._utils import _ctrl_state_to_int, with_gate_array, with_controlled_gate_array
 
 _X_ARRAY = [[0, 1], [1, 0]]
+_SX_ARRAY = [[0.5 + 0.5j, 0.5 - 0.5j], [0.5 - 0.5j, 0.5 + 0.5j]]
 
 
 @with_gate_array(_X_ARRAY)
@@ -562,6 +563,7 @@ def __eq__(self, other):
         return isinstance(other, RCCXGate)
 
 
+@with_controlled_gate_array(_SX_ARRAY, num_ctrl_qubits=3, cached_states=(7,))
 class C3SXGate(SingletonControlledGate):
     """The 3-qubit controlled sqrt-X gate.
 

diff --git a/releasenotes/notes/fix-c3sx-gate-matrix-050cf9f9ac3b2b82.yaml b/releasenotes/notes/fix-c3sx-gate-matrix-050cf9f9ac3b2b82.yaml
@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    Fixed a missing decorator in :class:`.C3SXGate` that made it fail if :meth:`.Gate.to_matrix` was called.
+    The gate matrix is now return as expected.
diff --git a/test/python/circuit/test_controlled_gate.py b/test/python/circuit/test_controlled_gate.py
@@ -230,6 +230,12 @@ def test_special_cases_equivalent_to_controlled_base_gate(self):
                 # Ensure that both the array form (if the gate overrides `__array__`) and the
                 # circuit-definition form are tested.
                 self.assertTrue(Operator(special_case_gate).equiv(naive_operator))
+                if not isinstance(special_case_gate, (MCXGate, MCPhaseGate, MCU1Gate)):
+                    # Ensure that the to_matrix method yields the same result
+                    np.testing.assert_allclose(
+                        special_case_gate.to_matrix(), naive_operator.to_matrix(), atol=1e-8
+                    )
+
                 if not isinstance(special_case_gate, CXGate):
                     # CX is treated like a primitive within Terra, and doesn't have a definition.
                     self.assertTrue(Operator(special_case_gate.definition).equiv(naive_operator))