Update PhaseGate.control(n) to return MCPhaseGate (#4994)

* Fix PhaseGate.control to return an MCPhaseGate if multiple controls.

* Update test/python/circuit/test_controlled_gate.py

Co-authored-by: Julien Gacon <gaconju@gmail.com>
(cherry picked from commit 81e73da)

qiskit/circuit/library/standard_gates/p.py

@@ -98,8 +98,7 @@ def control(self, num_ctrl_qubits=1, label=None, ctrl_state=None):
         if num_ctrl_qubits == 1:
             gate = CPhaseGate(self.params[0], label=label, ctrl_state=ctrl_state)
         elif ctrl_state is None and num_ctrl_qubits > 1:
-            from .u1 import MCU1Gate
-            gate = MCU1Gate(self.params[0], num_ctrl_qubits, label=label)
+            gate = MCPhaseGate(self.params[0], num_ctrl_qubits, label=label)
         else:
             return super().control(num_ctrl_qubits=num_ctrl_qubits, label=label,
                                    ctrl_state=ctrl_state)

releasenotes/notes/4989-Fixes-MCPhaseGate-definition-cbe5e14992116e67.yaml

@@ -0,0 +1,10 @@
+---
+fixes:
+  - |
+    Previously, when creating a multiply controlled phase gate via
+    ``PhaseGate.control``, an ``MCU1Gate`` gate had been returned. This has been
+    had corrected so that an ``MCPhaseGate`` is returned.
+  - |
+    Previously, attempting to decompose a circuit containing an
+    ``MCPhaseGate`` would raise an error due to an inconsistency in the
+    definition of the ``MCPhaseGate``. This has been corrected.

test/python/circuit/test_controlled_gate.py

@@ -74,6 +74,11 @@ def test_controlled_phase(self):
         theta = 0.5
         self.assertEqual(PhaseGate(theta).control(), CPhaseGate(theta))
 
+    def test_double_controlled_phase(self):
+        """Test the creation of a controlled phase gate."""
+        theta = 0.5
+        self.assertEqual(PhaseGate(theta).control(2), MCPhaseGate(theta, 2))
+
     def test_controlled_u1(self):
         """Test the creation of a controlled U1 gate."""
         theta = 0.5