Fix evolved operator ansatz (Qiskit#11682)

* Fix handling of empty operators list in EvolvedOperatorAnsatz

* Add test for empty operator list in EvolvedOperatorAnsatz

* add release note for fixing evolved operator ansatz

* Update releasenotes/notes/fix-evolved-operator-ansatz-empty-ops-bf8ecfae8f1e1001.yaml

Co-authored-by: Julien Gacon <gaconju@gmail.com>

* add a note to fix-evolved-operator-ansatz-empty-ops-bf8ecfae8f1e1001.yaml

* Update fix-evolved-operator-ansatz-empty-ops-bf8ecfae8f1e1001.yaml

* merge two conditions in evolved_operator_ansatz

Co-authored-by: Julien Gacon <gaconju@gmail.com>

---------

Co-authored-by: Julien Gacon <gaconju@gmail.com>

qiskit/circuit/library/n_local/evolved_operator_ansatz.py

@@ -106,7 +106,9 @@ def num_qubits(self) -> int:
         if self.operators is None:
             return 0
 
-        if isinstance(self.operators, list) and len(self.operators) > 0:
+        if isinstance(self.operators, list):
+            if len(self.operators) == 0:
+                return 0
             return self.operators[0].num_qubits
 
         return self.operators.num_qubits
@@ -152,7 +154,10 @@ def operators(self, operators=None) -> None:
         operators = _validate_operators(operators)
         self._invalidate()
         self._operators = operators
-        self.qregs = [QuantumRegister(self.num_qubits, name="q")]
+        if self.num_qubits == 0:
+            self.qregs = []
+        else:
+            self.qregs = [QuantumRegister(self.num_qubits, name="q")]
 
     # TODO: the `preferred_init_points`-implementation can (and should!) be improved!
     @property

releasenotes/notes/fix-evolved-operator-ansatz-empty-ops-bf8ecfae8f1e1001.yaml

@@ -0,0 +1,11 @@
+---
+fixes:
+  - |
+    The :class:`.EvolvedOperatorAnsatz` now correctly handles the case where the
+    `operators` argument is an empty list. Previously, this would result in an
+    error.
+  - |
+    From now on, :class:`.EvolvedOperatorAnsatz` will not have any `qregs` when
+    thera are zero qubits, instead of having a :class:`.QuantumRegister` instance
+    with zero qubits. This behavior aligns more consistently with its superclass 
+    :class:`.QuantumCircuit`.

test/python/circuit/library/test_evolved_op_ansatz.py

@@ -89,6 +89,11 @@ def test_empty_build_fails(self):
         with self.assertRaises(ValueError):
             _ = evo.draw()
 
+    def test_empty_operator_list(self):
+        """Test setting an empty list of operators to be equal to an empty circuit."""
+        evo = EvolvedOperatorAnsatz([])
+        self.assertEqual(evo, QuantumCircuit())
+
     def test_matrix_operator(self):
         """Test passing a quantum_info.Operator uses the HamiltonianGate."""
         unitary = Operator([[0, 1], [1, 0]])