Fix qpy for MCMTGate (#14096)

* Add MCMXGate to custom operations

* Add test and reno

---------

Co-authored-by: Shelly Garion <46566946+ShellyGarion@users.noreply.github.com>

Author: ksk0629

qiskit/qpy/binary_io/circuits.py

@@ -783,6 +783,11 @@ def _write_instruction(
         custom_operations[gate_class_name] = instruction.operation
         custom_operations_list.append(gate_class_name)
 
+    elif isinstance(instruction.operation, library.MCMTGate):
+        gate_class_name = instruction.operation.name + "_" + str(uuid.uuid4())
+        custom_operations[gate_class_name] = instruction.operation
+        custom_operations_list.append(gate_class_name)
+
     condition_type = type_keys.Condition.NONE
     condition_register = b""
     condition_value = 0

releasenotes/notes/fix-qpy-mcmtgate-ab735812998f4107.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    Fixed a bug in QPY (:mod:`qiskit.qpy`) where circuits containing gates of class
+    :class:`.MCMTGate` would fail to serialize.
+    See `#13965 <https://github.com/Qiskit/qiskit/issues/13965>`__.

test/python/circuit/test_circuit_load_from_qpy.py

@@ -30,6 +30,7 @@
 from qiskit.circuit.gate import Gate
 from qiskit.circuit.library import (
     XGate,
+    ZGate,
     CXGate,
     RYGate,
     QFT,
@@ -41,6 +42,7 @@
     MCXGrayCode,
     MCXRecursive,
     MCXVChain,
+    MCMTGate,
     UCRXGate,
     UCRYGate,
     UCRZGate,
@@ -1261,11 +1263,13 @@ def test_standard_control_gates(self):
         mcx_gray_gate = MCXGrayCode(5)
         mcx_recursive_gate = MCXRecursive(4)
         mcx_vchain_gate = MCXVChain(3)
+        mcmt_gate = MCMTGate(ZGate(), 2, 1)
         qc.append(mcu1_gate, [0, 2, 1])
         qc.append(mcx_gate, list(range(0, 6)))
         qc.append(mcx_gray_gate, list(range(0, 6)))
         qc.append(mcx_recursive_gate, list(range(0, 5)))
         qc.append(mcx_vchain_gate, list(range(0, 5)))
+        qc.append(mcmt_gate, list(range(0, 3)))
         qc.mcp(np.pi, [0, 2], 1)
         qc.mcx([0, 2], 1)
         qc.measure_all()