Create explicit C3XGate/C4XGate in MCXGrayCode (bp #5601)

qiskit/circuit/library/standard_gates/x.py

@@ -826,11 +826,11 @@ class MCXGate(ControlledGate):
     def __new__(cls, num_ctrl_qubits=None, label=None, ctrl_state=None):
         """Create a new MCX instance.
 
-        Depending on the number of controls, this creates an explicit CX, CCX, C3X or C4X
-        instance or a generic MCX gate.
+        Depending on the number of controls and which mode of the MCX, this creates an
+        explicit CX, CCX, C3X or C4X instance or a generic MCX gate.
         """
-        # these gates will always be implemented for all modes of the MCX if the number of control
-        # qubits matches this
+        # The CXGate and CCXGate will be implemented for all modes of the MCX, and
+        # the C3XGate and C4XGate will be implemented in the MCXGrayCode class.
         explicit = {
             1: CXGate,
             2: CCXGate
@@ -909,6 +909,23 @@ class MCXGrayCode(MCXGate):
 
     This delegates the implementation to the MCU1 gate, since :math:`X = H \cdot U1(\pi) \cdot H`.
     """
+    def __new__(cls, num_ctrl_qubits=None, label=None, ctrl_state=None):
+        """Create a new MCXGrayCode instance
+        """
+        # if 1 to 4 control qubits, create explicit gates
+        explicit = {
+            1: CXGate,
+            2: CCXGate,
+            3: C3XGate,
+            4: C4XGate
+        }
+        if num_ctrl_qubits in explicit.keys():
+            gate_class = explicit[num_ctrl_qubits]
+            gate = gate_class.__new__(gate_class, label=label, ctrl_state=ctrl_state)
+            # if __new__ does not return the same type as cls, init is not called
+            gate.__init__(label=label, ctrl_state=ctrl_state)
+            return gate
+        return super().__new__(cls)
 
     def __init__(self, num_ctrl_qubits, label=None, ctrl_state=None):
         super().__init__(num_ctrl_qubits, label=label, ctrl_state=ctrl_state, _name='mcx_gray')

releasenotes/notes/mcx_create_explicit_c3x_c4x-d4475c0bb70fae49.yaml

@@ -0,0 +1,8 @@
+---
+upgrade:
+  - |
+    The class :class:`~qiskit.library.standard_gates.x.MCXGrayCode` will now create
+    a ``C3XGate`` if ``num_ctrl_qubits`` is 3 and a ``C4XGate`` if ``num_ctrl_qubits``
+    is 4. This is in addition to the previous functionality where for any of the
+    modes of the :class:'qiskit.library.standard_gates.x.MCXGate`, if ``num_ctrl_bits``
+    is 1, a ``CXGate`` is created, and if 2, a ``CCXGate`` is created.

test/python/circuit/test_controlled_gate.py

@@ -569,6 +569,14 @@ def test_mcx_gates_yield_explicit_gates(self, num_ctrl_qubits):
         explicit = {1: CXGate, 2: CCXGate}
         self.assertEqual(cls, explicit[num_ctrl_qubits])
 
+    @data(1, 2, 3, 4)
+    def test_mcxgraycode_gates_yield_explicit_gates(self, num_ctrl_qubits):
+        """Test creating an mcx gate calls MCXGrayCode and yeilds explicit definition."""
+        qc = QuantumCircuit(num_ctrl_qubits+1)
+        qc.mcx(list(range(num_ctrl_qubits)), [num_ctrl_qubits])
+        explicit = {1: CXGate, 2: CCXGate, 3: C3XGate, 4: C4XGate}
+        self.assertEqual(type(qc[0][0]), explicit[num_ctrl_qubits])
+
     @data(3, 4, 5, 8)
     def test_mcx_gates(self, num_ctrl_qubits):
         """Test the mcx gates."""