Fix quantumlib#6630: sort the qubits in the PauliStrings obtained by …

…iterating on a PauliSum elements

cirq-core/cirq/ops/linear_combinations.py

@@ -354,7 +354,7 @@ def _is_linear_dict_of_unit_pauli_string(linear_dict: value.LinearDict[UnitPauli
 
 
 def _pauli_string_from_unit(unit: UnitPauliStringT, coefficient: Union[int, float, complex] = 1):
-    return PauliString(qubit_pauli_map=dict(unit), coefficient=coefficient)
+    return PauliString(qubit_pauli_map=dict(sorted(unit)), coefficient=coefficient)
 
 
 @value.value_equality(approximate=True, unhashable=True)

cirq-core/cirq/ops/linear_combinations_test.py

@@ -1173,6 +1173,27 @@ def test_pauli_sum_pow():
     assert psum7copy == psum7
 
 
+def test_pauli_sum_consistent_qubit_order():
+    for str_1, str_2 in [("XY", "YYZ"), ("YX", "ZYZ")]:
+      ps_1 = cirq.DensePauliString(str_1).on(q0, q1)
+      ps_2 = cirq.DensePauliString(str_2).on(q2, q1, q3)
+      # This is equivalent to ps_2, but with the qubits in ascending order.
+      # We expected the term of the sum to have the qubits in this order.
+      str_2_sorted = str_2[1] + str_2[0] + str_2[2]
+      ps_2_sorted = cirq.DensePauliString(str_2_sorted).on(q1, q2, q3)
+
+      terms = list(cirq.PauliSum() + ps_1 + ps_2)
+
+      assert terms[0].dense(terms[0].qubits) == ps_1.dense(ps_1.qubits)
+      assert terms[1].dense(terms[1].qubits) == ps_2_sorted.dense(
+          ps_2_sorted.qubits)
+      assert str(ps_1), str(terms[0])
+      assert str(ps_2_sorted), str(terms[1])
+
+      assert np.allclose(cirq.unitary(ps_1), cirq.unitary(terms[0]))
+      assert np.allclose(cirq.unitary(ps_2_sorted), cirq.unitary(terms[1]))
+
+
 # Using the entries of table 1 of https://arxiv.org/abs/1804.09130 as golden values.
 @pytest.mark.parametrize(
     'boolean_expr,expected_pauli_sum',