add _paulis_commute

Author: perlinm

cirq-core/cirq/ops/pauli_string.py

@@ -783,7 +783,7 @@ def _commutes_(
     ) -> Union[bool, NotImplementedType, None]:
         if not isinstance(other, PauliString):
             return NotImplemented
-        return sum(not protocols.commutes(p0, p1) for p0, p1 in self.zip_paulis(other)) % 2 == 0
+        return sum(not _paulis_commute(p0, p1) for p0, p1 in self.zip_paulis(other)) % 2 == 0
 
     def __neg__(self) -> 'PauliString':
         return PauliString(qubit_pauli_map=self._qubit_pauli_map, coefficient=-self._coefficient)
@@ -1080,6 +1080,18 @@ def _resolve_parameters_(
         return PauliString(qubit_pauli_map=self._qubit_pauli_map, coefficient=coefficient)
 
 
+def _paulis_commute(
+    p0: Union['cirq.Pauli', 'cirq.IdentityGate'], p1: Union['cirq.Pauli', 'cirq.IdentityGate']
+) -> bool:
+    """Determines whether two single-qubit Pauli operators commute.
+
+    Args:
+        p0: A single-qubit Pauli operator.
+        p1: A single-qubit Pauli operator.
+    """
+    return p0 == identity.I or p1 == identity.I or p0 == p1
+
+
 def _validate_qubit_mapping(
     qubit_map: Mapping[TKey, int], pauli_qubits: Tuple[TKey, ...], num_state_qubits: int
 ) -> None:
@@ -1395,8 +1407,8 @@ def inplace_after(self, ops: 'cirq.OP_TREE') -> 'cirq.MutablePauliString':
                     p1 = _INT_TO_PAULI_OR_IDENTITY[ps[1]]
 
                     # Kick across Paulis that anti-commute with the controls.
-                    kickback_0_to_1 = not (identity.I in [p0, gate.pauli0] or p0 == gate.pauli0)
-                    kickback_1_to_0 = not (identity.I in [p1, gate.pauli1] or p1 == gate.pauli1)
+                    kickback_0_to_1 = not _paulis_commute(p0, gate.pauli0)
+                    kickback_1_to_0 = not _paulis_commute(p1, gate.pauli1)
                     kick0 = gate.pauli1 if kickback_0_to_1 else identity.I
                     kick1 = gate.pauli0 if kickback_1_to_0 else identity.I
                     self.__imul__({q0: p0, q1: kick0})
@@ -1673,11 +1685,11 @@ def merge_and_kickback(
         return int(inv) * 2 - 1
 
     quarter_kickback = 0
-    if qubit0 in pauli_map and not protocols.commutes(pauli_map[qubit0], gate.pauli0):
+    if qubit0 in pauli_map and not _paulis_commute(pauli_map[qubit0], gate.pauli0):
         quarter_kickback += merge_and_kickback(
             qubit1, gate.pauli1, pauli_map.get(qubit1), gate.invert1
         )
-    if qubit1 in pauli_map and not protocols.commutes(pauli_map[qubit1], gate.pauli1):
+    if qubit1 in pauli_map and not _paulis_commute(pauli_map[qubit1], gate.pauli1):
         quarter_kickback += merge_and_kickback(
             qubit0, pauli_map.get(qubit0), gate.pauli0, gate.invert0
         )

cirq-core/cirq/ops/pauli_string_test.py

@@ -2023,3 +2023,11 @@ def test_resolve(resolve_fn):
     pst = cirq.PauliString({q: 'x'}, coefficient=t)
     ps1 = cirq.PauliString({q: 'x'}, coefficient=1j)
     assert resolve_fn(pst, {'t': 1j}) == ps1
+
+
+def test_paulis_commute():
+    assert all(
+        cirq.ops.pauli_string._paulis_commute(p0, p1) == cirq.commutes(p0, p1)
+        for p0 in [cirq.I, cirq.X, cirq.Y, cirq.Z]
+        for p1 in [cirq.I, cirq.X, cirq.Y, cirq.Z]
+    )