Fix a corner case of SparsePauliOp.apply_layout (#12375)

* fix a corner case of `SparsePauliOp.apply_layout`

* Add zero-qubit tests of Pauli.apply_layout

* use combine and apply isort

* Update releasenotes/notes/fix-sparse-pauli-op-apply-layout-zero-43b9e70f0d1536a6.yaml

---------

Co-authored-by: Matthew Treinish <mtreinish@kortar.org>

qiskit/quantum_info/operators/symplectic/sparse_pauli_op.py

@@ -1165,6 +1165,8 @@ def apply_layout(
                 raise QiskitError("Provided layout contains indices outside the number of qubits.")
             if len(set(layout)) != len(layout):
                 raise QiskitError("Provided layout contains duplicate indices.")
+        if self.num_qubits == 0:
+            return type(self)(["I" * n_qubits] * self.size, self.coeffs)
         new_op = type(self)("I" * n_qubits)
         return new_op.compose(self, qargs=layout)
 

releasenotes/notes/fix-sparse-pauli-op-apply-layout-zero-43b9e70f0d1536a6.yaml

@@ -0,0 +1,10 @@
+fixes:
+  - |
+    Fixed :meth:`.SparsePauliOp.apply_layout` to work correctly with zero-qubit operators.
+    For example, if you previously created a 0 qubit and applied a layout like::
+    
+        op = SparsePauliOp("")
+        op.apply_layout(None, 3)
+
+    this would have previously raised an error. Now this will correctly return an operator of the form:
+    ``SparsePauliOp(['III'], coeffs=[1.+0.j])``

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

@@ -14,42 +14,41 @@
 
 """Tests for Pauli operator class."""
 
+import itertools as it
 import re
 import unittest
-import itertools as it
 from functools import lru_cache
+from test import QiskitTestCase, combine
+
 import numpy as np
-from ddt import ddt, data, unpack
+from ddt import data, ddt, unpack
 
 from qiskit import QuantumCircuit
 from qiskit.circuit import Qubit
-from qiskit.exceptions import QiskitError
 from qiskit.circuit.library import (
-    IGate,
-    XGate,
-    YGate,
-    ZGate,
-    HGate,
-    SGate,
-    SdgGate,
     CXGate,
-    CZGate,
     CYGate,
-    SwapGate,
+    CZGate,
     ECRGate,
     EfficientSU2,
+    HGate,
+    IGate,
+    SdgGate,
+    SGate,
+    SwapGate,
+    XGate,
+    YGate,
+    ZGate,
 )
 from qiskit.circuit.library.generalized_gates import PauliGate
 from qiskit.compiler.transpiler import transpile
-from qiskit.providers.fake_provider import GenericBackendV2
+from qiskit.exceptions import QiskitError
 from qiskit.primitives import BackendEstimator
+from qiskit.providers.fake_provider import GenericBackendV2
+from qiskit.quantum_info.operators import Operator, Pauli, SparsePauliOp
 from qiskit.quantum_info.random import random_clifford, random_pauli
-from qiskit.quantum_info.operators import Pauli, Operator, SparsePauliOp
 from qiskit.utils import optionals
 
-from test import QiskitTestCase  # pylint: disable=wrong-import-order
-
-
 LABEL_REGEX = re.compile(r"(?P<coeff>[+-]?1?[ij]?)(?P<pauli>[IXYZ]*)")
 PHASE_MAP = {"": 0, "-i": 1, "-": 2, "i": 3}
 
@@ -618,6 +617,13 @@ def test_apply_layout_duplicate_indices(self):
         with self.assertRaises(QiskitError):
             op.apply_layout(layout=[0, 0], num_qubits=3)
 
+    @combine(phase=["", "-i", "-", "i"], layout=[None, []])
+    def test_apply_layout_zero_qubit(self, phase, layout):
+        """Test apply_layout with a zero-qubit operator"""
+        op = Pauli(phase)
+        res = op.apply_layout(layout=layout, num_qubits=5)
+        self.assertEqual(Pauli(phase + "IIIII"), res)
+
 
 if __name__ == "__main__":
     unittest.main()

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

@@ -14,23 +14,22 @@
 
 import itertools as it
 import unittest
+from test import QiskitTestCase, combine
+
 import numpy as np
-import scipy.sparse
 import rustworkx as rx
+import scipy.sparse
 from ddt import ddt
 
-
 from qiskit import QiskitError
-from qiskit.circuit import ParameterExpression, Parameter, ParameterVector
-from qiskit.circuit.parametertable import ParameterView
-from qiskit.quantum_info.operators import Operator, Pauli, PauliList, SparsePauliOp
+from qiskit.circuit import Parameter, ParameterExpression, ParameterVector
 from qiskit.circuit.library import EfficientSU2
+from qiskit.circuit.parametertable import ParameterView
+from qiskit.compiler.transpiler import transpile
 from qiskit.primitives import BackendEstimator
 from qiskit.providers.fake_provider import GenericBackendV2
-from qiskit.compiler.transpiler import transpile
+from qiskit.quantum_info.operators import Operator, Pauli, PauliList, SparsePauliOp
 from qiskit.utils import optionals
-from test import QiskitTestCase  # pylint: disable=wrong-import-order
-from test import combine  # pylint: disable=wrong-import-order
 
 
 def pauli_mat(label):
@@ -1191,6 +1190,22 @@ def test_apply_layout_duplicate_indices(self):
         with self.assertRaises(QiskitError):
             op.apply_layout(layout=[0, 0], num_qubits=3)
 
+    @combine(layout=[None, []])
+    def test_apply_layout_zero_qubit(self, layout):
+        """Test apply_layout with a zero-qubit operator"""
+        with self.subTest("default"):
+            op = SparsePauliOp("")
+            res = op.apply_layout(layout=layout, num_qubits=5)
+            self.assertEqual(SparsePauliOp("IIIII"), res)
+        with self.subTest("coeff"):
+            op = SparsePauliOp("", 2)
+            res = op.apply_layout(layout=layout, num_qubits=5)
+            self.assertEqual(SparsePauliOp("IIIII", 2), res)
+        with self.subTest("multiple ops"):
+            op = SparsePauliOp.from_list([("", 1), ("", 2)])
+            res = op.apply_layout(layout=layout, num_qubits=5)
+            self.assertEqual(SparsePauliOp.from_list([("IIIII", 1), ("IIIII", 2)]), res)
+
 
 if __name__ == "__main__":
     unittest.main()