Remove implicit broadcasting from Pauli/label construction of PauliList

qiskit/quantum_info/operators/symplectic/pauli_list.py

@@ -190,6 +190,11 @@ def _from_paulis(data):
         base_x = np.zeros((num_paulis, num_qubits), dtype=bool)
         base_phase = np.zeros(num_paulis, dtype=int)
         for i, pauli in enumerate(paulis):
+            if pauli.num_qubits != num_qubits:
+                raise ValueError(
+                    f"The {i}th Pauli is defined over {pauli.num_qubits} qubits, "
+                    f"but num_qubits == {num_qubits} was expected."
+                )
             base_z[i] = pauli._z
             base_x[i] = pauli._x
             base_phase[i] = pauli._phase

releasenotes/notes/paulilist-do-not-broadcast-from-paulis-96de3832fba21b94.yaml

@@ -0,0 +1,8 @@
+---
+fixes:
+  - |
+    A bug has been fixed which had allowed broadcasting when a
+    ``PauliList`` is initialized from ``Pauli``s or labels.  For
+    instance, the code ``PauliList(["XXX", "Z"])`` now raises a
+    ``ValueError`` rather than constructing the equivalent of
+    ``PauliList(["XXX", "ZZZ"])``.

test/python/quantum_info/operators/symplectic/test_pauli_list.py

@@ -198,6 +198,10 @@ def test_string_init(self):
             np.testing.assert_equal(pauli_list.z, z)
             np.testing.assert_equal(pauli_list.x, x)
 
+        with self.subTest(msg="str init prevent broadcasting"):
+            with self.assertRaises(ValueError):
+                PauliList(["XYZ", "I"])
+
     def test_list_init(self):
         """Test list initialization."""
         with self.subTest(msg="PauliList"):