Add num_qubits to DepolarizingChannel cirq_google serialization (#7502)

- Currently, serialization fails if more than 1 qubit is given.

Author: dstrain115

cirq-google/cirq_google/serialization/circuit_serializer.py

@@ -288,6 +288,7 @@ def _serialize_gate_op(
             arg_func_langs.float_arg_to_proto(
                 gate.p, out=msg.noisechannel.depolarizingchannel.probability
             )
+            msg.noisechannel.depolarizingchannel.num_qubits = len(op.qubits)
         elif isinstance(gate, cirq.RandomGateChannel):
             arg_func_langs.float_arg_to_proto(
                 gate.probability, out=msg.noisechannel.randomgatechannel.probability
@@ -763,9 +764,14 @@ def _deserialize_gate_op(
                 )
                 if not isinstance(p, float):
                     raise ValueError(
-                        f"Depolarizing noise probability {p} " "cannot be symbol or None"
+                        f"Depolarizing noise probability {p} cannot be symbol or None"
                     )  # pragma: nocover
-                op = cirq.DepolarizingChannel(p=p)(*qubits)
+                num_qubits = operation_proto.noisechannel.depolarizingchannel.num_qubits
+                if num_qubits <= 0:
+                    raise ValueError(
+                        f"Depolarizing noise gate must have positive num_qubits: {num_qubits}"
+                    )  # pragma: nocover
+                op = cirq.DepolarizingChannel(p=p, n_qubits=num_qubits)(*qubits)
             elif which_channel_type == 'randomgatechannel':
                 p = arg_func_langs.float_arg_from_proto(
                     operation_proto.noisechannel.randomgatechannel.probability

cirq-google/cirq_google/serialization/circuit_serializer_test.py

@@ -390,11 +390,24 @@ def circuit_proto(json: dict, qubits: list[str]):
         cirq.depolarize(0.5)(Q0),
         op_proto(
             {
-                'noisechannel': {'depolarizingchannel': {'probability': {'float_value': 0.5}}},
+                'noisechannel': {
+                    'depolarizingchannel': {'probability': {'float_value': 0.5}, 'num_qubits': 1}
+                },
                 'qubit_constant_index': [0],
             }
         ),
     ),
+    (
+        cirq.depolarize(0.5, n_qubits=2)(Q0, Q1),
+        op_proto(
+            {
+                'noisechannel': {
+                    'depolarizingchannel': {'probability': {'float_value': 0.5}, 'num_qubits': 2}
+                },
+                'qubit_constant_index': [0, 1],
+            }
+        ),
+    ),
     (
         cirq.X(Q0).with_probability(0.5),
         op_proto(