Add label assigned check in SparsePauliOp (#8101)

* add assign check

* Update qiskit/quantum_info/operators/symplectic/sparse_pauli_op.py

Co-authored-by: Ikko Hamamura <ikkoham@users.noreply.github.com>

* fix check condition

* update reno

* update reno section header

Co-authored-by: Ikko Hamamura <ikkoham@users.noreply.github.com>
Co-authored-by: Julien Gacon <gaconju@gmail.com>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

qiskit/quantum_info/operators/symplectic/sparse_pauli_op.py

@@ -612,7 +612,7 @@ class SparsePauliOp(LinearOp):
         return SparsePauliOp(paulis, coeffs, copy=False)
 
     @staticmethod
-    def from_sparse_list(obj, num_qubits):
+    def from_sparse_list(obj, num_qubits, do_checks=True):
         """Construct from a list of local Pauli strings and coefficients.
 
         Each list element is a 3-tuple of a local Pauli string, indices where to apply it,
@@ -637,6 +637,7 @@ class SparsePauliOp(LinearOp):
         Args:
             obj (Iterable[Tuple[str, List[int], complex]]): The list 3-tuples specifying the Paulis.
             num_qubits (int): The number of qubits of the operator.
+            do_checks (bool): The flag of checking if the input indices are not duplicated.
 
         Returns:
             SparsePauliOp: The SparsePauliOp representation of the Pauli terms.
@@ -644,6 +645,7 @@ class SparsePauliOp(LinearOp):
         Raises:
             QiskitError: If the list of Paulis is empty.
             QiskitError: If the number of qubits is incompatible with the indices of the Pauli terms.
+            QiskitError: If the designated qubit is already assigned.
         """
         obj = list(obj)  # To convert zip or other iterable
 
@@ -655,6 +657,8 @@ class SparsePauliOp(LinearOp):
         labels = np.zeros(size, dtype=f"<U{num_qubits}")
 
         for i, (paulis, indices, coeff) in enumerate(obj):
+            if do_checks and len(indices) != len(set(indices)):
+                raise QiskitError("Input indices are duplicated.")
             # construct the full label based off the non-trivial Paulis and indices
             label = ["I"] * num_qubits
             for pauli, index in zip(paulis, indices):

releasenotes/notes/add_check_from_sparse_list-97f13fde87c7bcb6.yaml

@@ -0,0 +1,6 @@
+---
+features:
+  - |
+    Allow checking for duplicate qubit indices in :meth:`~qiskit.quantum_info.SparsePauliOp.from_sparse_list`.
+    The checks can be turned off by setting the new keyword argument ``do_checks`` to ``False`` 
+    (it  is ``True`` per default).

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

@@ -183,6 +183,15 @@ class TestSparsePauliOpConversions(QiskitTestCase):
         with self.assertRaises(QiskitError):
             _ = SparsePauliOp.from_sparse_list([("Z", [2], 1)], 1)
 
+    def test_from_index_list_same_index(self):
+        """Test from_list via Pauli + number of qubits raises correctly, if indices duplicate."""
+        with self.assertRaises(QiskitError):
+            _ = SparsePauliOp.from_sparse_list([("ZZ", [0, 0], 1)], 2)
+        with self.assertRaises(QiskitError):
+            _ = SparsePauliOp.from_sparse_list([("ZI", [0, 0], 1)], 2)
+        with self.assertRaises(QiskitError):
+            _ = SparsePauliOp.from_sparse_list([("IZ", [0, 0], 1)], 2)
+
     def test_from_zip(self):
         """Test from_list method for zipped input."""
         labels = ["XXZ", "IXI", "YZZ", "III"]