Add json serialization for PauliSum

cirq-core/cirq/json_resolver_cache.py

@@ -146,6 +146,7 @@ def _symmetricalqidpair(qids):
         'PauliString': cirq.PauliString,
         'PauliStringPhasor': cirq.PauliStringPhasor,
         'PauliStringPhasorGate': cirq.PauliStringPhasorGate,
+        'PauliSum': cirq.PauliSum,
         '_PauliX': cirq.ops.pauli_gates._PauliX,
         '_PauliY': cirq.ops.pauli_gates._PauliY,
         '_PauliZ': cirq.ops.pauli_gates._PauliZ,

cirq-core/cirq/ops/linear_combinations.py

@@ -498,6 +498,23 @@ def _unitary_(self) -> np.ndarray:
             return m
         raise ValueError(f'{self} is not unitary')
 
+    def _json_dict_(self):
+        def key_json(k):
+            # k is a frozenset, each element of frozen set is a tuple.
+            return [[x for x in e] for e in sorted(list(k))]
+
+        return {
+            'keys': [key_json(k) for k in self._linear_dict.keys()],
+            'values': list(self._linear_dict.values()),
+        }
+
+    @classmethod
+    def _from_json_dict_(cls, keys, values, **kwargs):
+        mapping = dict()
+        for k, v in zip(keys, values):
+            mapping[frozenset(tuple(x) for x in k)] = v
+        return cls(linear_dict=value.LinearDict(mapping))
+
     def expectation_from_state_vector(
         self,
         state_vector: np.ndarray,

cirq-core/cirq/protocols/json_test_data/PauliSum.json

@@ -0,0 +1,65 @@
+{
+  "cirq_type": "PauliSum",
+  "keys": [
+    [
+      [
+        {
+          "cirq_type": "LineQubit",
+          "x": 0
+        },
+        {
+          "cirq_type": "_PauliX",
+          "exponent": 1.0,
+          "global_shift": 0.0
+        }
+      ],
+      [
+        {
+          "cirq_type": "LineQubit",
+          "x": 1
+        },
+        {
+          "cirq_type": "_PauliX",
+          "exponent": 1.0,
+          "global_shift": 0.0
+        }
+      ]
+    ],
+    [
+      [
+        {
+          "cirq_type": "LineQubit",
+          "x": 0
+        },
+        {
+          "cirq_type": "_PauliY",
+          "exponent": 1.0,
+          "global_shift": 0.0
+        }
+      ],
+      [
+        {
+          "cirq_type": "LineQubit",
+          "x": 1
+        },
+        {
+          "cirq_type": "_PauliY",
+          "exponent": 1.0,
+          "global_shift": 0.0
+        }
+      ]
+    ]
+  ],
+  "values": [
+    {
+      "cirq_type": "complex",
+      "real": 1.0,
+      "imag": 0.0
+    },
+    {
+      "cirq_type": "complex",
+      "real": 0.0,
+      "imag": 1.0
+    }
+  ]
+}

cirq-core/cirq/protocols/json_test_data/PauliSum.repr

@@ -0,0 +1 @@
+cirq.PauliSum(cirq.LinearDict({frozenset({(cirq.LineQubit(1), cirq.X), (cirq.LineQubit(0), cirq.X)}): (1+0j), frozenset({(cirq.LineQubit(1), cirq.Y), (cirq.LineQubit(0), cirq.Y)}): (1j)}))

cirq-core/cirq/protocols/json_test_data/spec.py

@@ -47,7 +47,6 @@
         'DiagonalGate',
         'NeutralAtomDevice',
         'PauliInteractionGate',
-        'PauliSum',
         'PauliSumCollector',
         'PauliSumExponential',
         'PauliTransform',