Allow PauliString-like gates to be used in PauliSum

cirq-core/cirq/ops/gate_operation.py

@@ -331,6 +331,21 @@ def __mul__(self, other: Any) -> Any:
     def __rmul__(self, other: Any) -> Any:
         return self.gate._rmul_with_qubits(self._qubits, other)
 
+    def __add__(self, other):
+        return 1 * self + other
+
+    def __radd__(self, other):
+        return other + 1 * self
+
+    def __sub__(self, other):
+        return 1 * self - other
+
+    def __rsub__(self, other):
+        return other + -self
+
+    def __neg__(self):
+        return -1 * self
+
     def _qasm_(self, args: protocols.QasmArgs) -> str | None:
         if isinstance(self.gate, ops.GlobalPhaseGate):
             warnings.warn(

cirq-core/cirq/ops/linear_combinations.py

@@ -746,10 +746,6 @@ def __iadd__(self, other):
             other = PauliSum.from_pauli_strings([PauliString(coefficient=other)])
         elif isinstance(other, PauliString):
             other = PauliSum.from_pauli_strings([other])
-        elif isinstance(other, raw_types.Operation) and isinstance(
-            other.gate, identity.IdentityGate
-        ):
-            other = PauliSum.from_pauli_strings([PauliString()])
 
         if not isinstance(other, PauliSum):
             return NotImplemented
@@ -758,10 +754,9 @@ def __iadd__(self, other):
         return self
 
     def __add__(self, other):
-        if not isinstance(other, (numbers.Complex, PauliString, PauliSum, raw_types.Operation)):
-            return NotImplemented
         result = self.copy()
-        result += other
+        if result.__iadd__(other) is NotImplemented:
+            return NotImplemented
         return result
 
     def __radd__(self, other):
@@ -775,10 +770,6 @@ def __isub__(self, other):
             other = PauliSum.from_pauli_strings([PauliString(coefficient=other)])
         elif isinstance(other, PauliString):
             other = PauliSum.from_pauli_strings([other])
-        elif isinstance(other, raw_types.Operation) and isinstance(
-            other.gate, identity.IdentityGate
-        ):
-            other = PauliSum.from_pauli_strings([PauliString()])
 
         if not isinstance(other, PauliSum):
             return NotImplemented
@@ -787,10 +778,9 @@ def __isub__(self, other):
         return self
 
     def __sub__(self, other):
-        if not isinstance(other, (numbers.Complex, PauliString, PauliSum, raw_types.Operation)):
-            return NotImplemented
         result = self.copy()
-        result -= other
+        if result.__isub__(other) is NotImplemented:
+            return NotImplemented
         return result
 
     def __neg__(self):

cirq-core/cirq/ops/pauli_string_test.py

@@ -2086,12 +2086,13 @@ def test_resolve(resolve_fn) -> None:
     ids=str,
 )
 def test_pauli_ops_identity_gate_operation(gate1: cirq.Pauli, gate2: cirq.Pauli) -> None:
-    # TODO: Issue #7280 - Support addition and subtraction of identity gate operations.
-    if gate1 == gate2 == cirq.I:
-        pytest.skip('Not yet implemented per #7280')
     q = cirq.LineQubit(0)
     pauli1, pauli2 = gate1.on(q), gate2.on(q)
-    unitary1, unitary2 = cirq.unitary(gate1), cirq.unitary(gate2)
+    if gate1 == gate2 == cirq.I:
+        # PauliSum swallows I qubits, so resulting unitaries are dimensionless
+        unitary1 = unitary2 = np.array([[1]])
+    else:
+        unitary1, unitary2 = cirq.unitary(gate1), cirq.unitary(gate2)
     addition = pauli1 + pauli2
     assert isinstance(addition, cirq.PauliSum)
     assert np.array_equal(addition.matrix(), unitary1 + unitary2)