Fix qpy support for Cliffords (Qiskit#11495)

* clifford qpy support + test

* release notes

* cleanup: checking if instruction is of type Instruction

* review comment

* review comments: consistency + remove specialized handling

* adding test to test_qpy.py

* changing version string to 0.45.2

---------

Co-authored-by: Elena Peña Tapia <[email protected]>

qiskit/qpy/binary_io/circuits.py

@@ -37,7 +37,7 @@
 from qiskit.extensions import quantum_initializer
 from qiskit.qpy import common, formats, type_keys
 from qiskit.qpy.binary_io import value, schedules
-from qiskit.quantum_info.operators import SparsePauliOp
+from qiskit.quantum_info.operators import SparsePauliOp, Clifford
 from qiskit.synthesis import evolution as evo_synth
 from qiskit.transpiler.layout import Layout, TranspileLayout
 
@@ -290,6 +290,8 @@ def _read_instruction(
         gate_class = getattr(quantum_initializer, gate_name)
     elif hasattr(controlflow, gate_name):
         gate_class = getattr(controlflow, gate_name)
+    elif gate_name == "Clifford":
+        gate_class = Clifford
     else:
         raise AttributeError("Invalid instruction type: %s" % gate_name)
 
@@ -578,7 +580,11 @@ def _dumps_instruction_parameter(param, index_map, use_symengine):
 
 # pylint: disable=too-many-boolean-expressions
 def _write_instruction(file_obj, instruction, custom_operations, index_map, use_symengine):
-    gate_class_name = instruction.operation.base_class.__name__
+    if isinstance(instruction.operation, Instruction):
+        gate_class_name = instruction.operation.base_class.__name__
+    else:
+        gate_class_name = instruction.operation.__class__.__name__
+
     custom_operations_list = []
     if (
         (
@@ -587,6 +593,7 @@ def _write_instruction(file_obj, instruction, custom_operations, index_map, use_
             and not hasattr(extensions, gate_class_name)
             and not hasattr(quantum_initializer, gate_class_name)
             and not hasattr(controlflow, gate_class_name)
+            and gate_class_name != "Clifford"
         )
         or gate_class_name == "Gate"
         or gate_class_name == "Instruction"
@@ -628,7 +635,7 @@ def _write_instruction(file_obj, instruction, custom_operations, index_map, use_
             condition_value = int(instruction.operation.condition[1])
 
     gate_class_name = gate_class_name.encode(common.ENCODE)
-    label = getattr(instruction.operation, "label")
+    label = getattr(instruction.operation, "label", None)
     if label:
         label_raw = label.encode(common.ENCODE)
     else:
@@ -641,6 +648,8 @@ def _write_instruction(file_obj, instruction, custom_operations, index_map, use_
             instruction.operation.target,
             tuple(instruction.operation.cases_specifier()),
         ]
+    elif isinstance(instruction.operation, Clifford):
+        instruction_params = [instruction.operation.tableau]
     else:
         instruction_params = instruction.operation.params
 

releasenotes/notes/fix-clifford-qpy-2ffc8308c888e7e0.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    QPY (using :func:`.qpy.dump` and :func:`.qpy.load`) will now correctly serialize
+    and deserialize quantum circuits with Clifford operators
+    (:class:`~qiskit.quantum_info.Clifford`).

test/python/circuit/test_circuit_load_from_qpy.py

@@ -51,7 +51,7 @@
 from qiskit.synthesis import LieTrotter, SuzukiTrotter
 from qiskit.test import QiskitTestCase
 from qiskit.qpy import dump, load
-from qiskit.quantum_info import Pauli, SparsePauliOp
+from qiskit.quantum_info import Pauli, SparsePauliOp, Clifford
 from qiskit.quantum_info.random import random_unitary
 from qiskit.circuit.controlledgate import ControlledGate
 from qiskit.utils import optionals
@@ -1696,6 +1696,33 @@ def test_valid_circuit_with_initialize_instruction(self, param):
         self.assertEqual(qc, new_circuit)
         self.assertDeprecatedBitProperties(qc, new_circuit)
 
+    def test_clifford(self):
+        """Test that circuits with Clifford operations can be saved and retrieved correctly."""
+        cliff1 = Clifford.from_dict(
+            {
+                "stabilizer": ["-IZX", "+ZYZ", "+ZII"],
+                "destabilizer": ["+ZIZ", "+ZXZ", "-XIX"],
+            }
+        )
+        cliff2 = Clifford.from_dict(
+            {
+                "stabilizer": ["+YX", "+ZZ"],
+                "destabilizer": ["+IZ", "+YI"],
+            }
+        )
+
+        circuit = QuantumCircuit(6, 1)
+        circuit.cx(0, 1)
+        circuit.append(cliff1, [2, 4, 5])
+        circuit.h(4)
+        circuit.append(cliff2, [3, 0])
+
+        with io.BytesIO() as fptr:
+            dump(circuit, fptr)
+            fptr.seek(0)
+            new_circuit = load(fptr)[0]
+        self.assertEqual(circuit, new_circuit)
+
 
 class TestSymengineLoadFromQPY(QiskitTestCase):
     """Test use of symengine in qpy set of methods."""

test/qpy_compat/test_qpy.py

@@ -640,6 +640,21 @@ def generate_layout_circuits():
     return [qc]
 
 
+def generate_clifford_circuits():
+    """Test qpy circuits with Clifford operations."""
+    from qiskit.quantum_info import Clifford
+
+    cliff = Clifford.from_dict(
+        {
+            "stabilizer": ["-IZX", "+ZYZ", "+ZII"],
+            "destabilizer": ["+ZIZ", "+ZXZ", "-XIX"],
+        }
+    )
+    qc = QuantumCircuit(3)
+    qc.append(cliff, [0, 1, 2])
+    return [qc]
+
+
 def generate_control_flow_expr():
     """`IfElseOp`, `WhileLoopOp` and `SwitchCaseOp` with `Expr` nodes in their discriminators."""
     from qiskit.circuit.classical import expr, types
@@ -769,6 +784,8 @@ def generate_circuits(version_parts):
             "acquire_inst_with_kernel_and_disc.qpy"
         ] = generate_acquire_instruction_with_kernel_and_discriminator()
         output_circuits["control_flow_expr.qpy"] = generate_control_flow_expr()
+    if version_parts >= (0, 45, 2):
+        output_circuits["clifford.qpy"] = generate_clifford_circuits()
     return output_circuits