Add noise gates to cirq_google serialization

- This is a nice-to-have feature for cirq_google protos,
since tests and simulations often use noisy circuits and
these could be serialized as well.
- This would primarily be used for testing and storage
since hardware does not (purposely) execute noise channels.

Author: dstrain115

cirq-google/cirq_google/api/v2/program.proto

@@ -215,8 +215,32 @@ message WaitGate {
   FloatArg duration_nanos = 1;
 }
 
+// Representation of cirq.DepolarizingChannel
+message DepolarizingChannel {
+  FloatArg probability = 1;
+  int32 num_qubits = 2;
+}
+
+// Representation of cirq.RandomGateChannel
+message RandomGateChannel {
+  FloatArg probability = 1;
+  Operation sub_gate = 2;
+}
+
+// Representation of noisy channels
+// These should only be used for serialization
+// of noisy circuits for simulation.
+// These channels would generally not be supported
+// by hardware.
+message NoiseChannel {
+  oneof channel_value {
+    DepolarizingChannel depolarizingchannel = 1;
+    RandomGateChannel randomgatechannel = 2;
+  }
+}
+
 // An operation acts on a set of qubits.
-// next available id = 27
+// next available id = 28
 message Operation {
   // Previously deprecated fields.  Do not use.
   reserved 1, 2;
@@ -241,6 +265,7 @@ message Operation {
     SingleQubitCliffordGate singlequbitcliffordgate = 21;
     ResetGate resetgate = 24;
     ISwapLikeGate iswaplikegate = 26;
+    NoiseChannel noisechannel = 27;
   }
 
   // Which qubits the operation acts on.

cirq-google/cirq_google/api/v2/program_pb2.py

@@ -275,6 +275,20 @@ def _serialize_gate_op(
             arg_func_langs.float_arg_to_proto(
                 gate.q1_detune_mhz, out=msg.couplerpulsegate.q1_detune_mhz
             )
+        elif isinstance(gate, cirq.DepolarizingChannel):
+            arg_func_langs.float_arg_to_proto(
+                gate.p, out=msg.noisechannel.depolarizingchannel.probability
+            )
+        elif isinstance(gate, cirq.RandomGateChannel):
+            arg_func_langs.float_arg_to_proto(
+                gate.probability, out=msg.noisechannel.randomgatechannel.probability
+            )
+            self._serialize_gate_op(
+                gate.sub_gate(*op.qubits),
+                msg=msg.noisechannel.randomgatechannel.sub_gate,
+                constants=constants,
+                raw_constants=raw_constants,
+            )
         else:
             raise ValueError(f'Cannot serialize op {op!r} of type {type(gate)}')
 
@@ -730,6 +744,28 @@ def _deserialize_gate_op(
                 or 0.0,
             )
             op = gate(*qubits)
+        elif which_gate_type == 'noisechannel':
+            which_channel_type = operation_proto.noisechannel.WhichOneof('channel_value')
+            if which_channel_type == 'depolarizingchannel':
+                p = arg_func_langs.float_arg_from_proto(
+                    operation_proto.noisechannel.depolarizingchannel.probability
+                )
+                op = cirq.DepolarizingChannel(p=p)(*qubits)
+            elif which_channel_type == 'randomgatechannel':
+                p = arg_func_langs.float_arg_from_proto(
+                    operation_proto.noisechannel.randomgatechannel.probability
+                )
+                sub_gate = self._deserialize_gate_op(
+                    operation_proto.noisechannel.randomgatechannel.sub_gate,
+                    constants=constants,
+                    deserialized_constants=deserialized_constants,
+                )
+                op = cirq.RandomGateChannel(probability=p, sub_gate=sub_gate.gate)(*qubits)
+            else:
+                raise ValueError(
+                    f'Unsupported serialized noise channel with type "{which_channel_type}".'
+                    f'\n\noperation_proto:\n{operation_proto}'
+                )
         else:
             raise ValueError(
                 f'Unsupported serialized gate with type "{which_gate_type}".'

cirq-google/cirq_google/api/v2/program_pb2.pyi

@@ -386,6 +386,32 @@ def circuit_proto(json: dict, qubits: list[str]):
         ),
     ),
     (cirq.I(Q0), op_proto({'identitygate': {'qid_shape': [2]}, 'qubit_constant_index': [0]})),
+    (
+        cirq.depolarize(0.5)(Q0),
+        op_proto(
+            {
+                'noisechannel': {'depolarizingchannel': {'probability': {'float_value': 0.5}}},
+                'qubit_constant_index': [0],
+            }
+        ),
+    ),
+    (
+        cirq.X(Q0).with_probability(0.5),
+        op_proto(
+            {
+                'noisechannel': {
+                    'randomgatechannel': {
+                        'probability': {'float_value': 0.5},
+                        'sub_gate': {
+                            'xpowgate': {'exponent': {'float_value': 1.0}},
+                            'qubit_constant_index': [0],
+                        },
+                    }
+                },
+                'qubit_constant_index': [0],
+            }
+        ),
+    ),
 ]