Json serialize MSGate (quantumlib#4605)

Fixes quantumlib#4603

cirq-core/cirq/ion/ion_gates.py

@@ -14,7 +14,7 @@
 
 """Operations native to iontrap systems."""
 
-from typing import Union, TYPE_CHECKING
+from typing import Any, Dict, Union, TYPE_CHECKING
 import numpy as np
 
 from cirq import ops, value
@@ -39,6 +39,7 @@ class MSGate(ops.XXPowGate):
 
     def __init__(self, *, rads: float):  # Forces keyword args.
         ops.XXPowGate.__init__(self, exponent=rads * 2 / np.pi, global_shift=-0.5)
+        self.rads = rads
 
     def _with_exponent(self: 'MSGate', exponent: value.TParamVal) -> 'MSGate':
         return type(self)(rads=exponent * np.pi / 2)
@@ -60,6 +61,13 @@ def __repr__(self) -> str:
             return 'cirq.ms(np.pi/2)'
         return f'cirq.ms({self._exponent!r}*np.pi/2)'
 
+    def _json_dict_(self) -> Dict[str, Any]:
+        return protocols.obj_to_dict_helper(self, ["rads"])
+
+    @classmethod
+    def _from_json_dict_(cls, rads: float, **kwargs: Any) -> 'MSGate':
+        return cls(rads=rads)
+
 
 # TODO(#3388) Add summary line to docstring.
 # pylint: disable=docstring-first-line-empty

cirq-core/cirq/ion/ion_gates_test.py

@@ -11,7 +11,7 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
-
+from typing import Callable, Union
 import numpy as np
 
 import cirq
@@ -66,3 +66,15 @@ def test_ms_diagrams():
 b: ───×───────────MS(π)───
 """,
     )
+
+
+def test_json_serialization():
+    def custom_resolver(cirq_type: str) -> Union[Callable[..., cirq.Gate], None]:
+        if cirq_type == "MSGate":
+            return cirq.ion.ion_gates.MSGate
+        return None
+
+    assert cirq.read_json(
+        json_text=cirq.to_json(cirq.ms(np.pi / 2)), resolvers=[custom_resolver]
+    ) == cirq.ms(np.pi / 2)
+    assert custom_resolver('X') == None