Fix behavior of n_local calling circuits for num_qubits=1

qiskit/circuit/library/n_local/efficient_su2.py

@@ -114,10 +114,13 @@ def efficient_su2(
     if su2_gates is None:
         su2_gates = ["ry", "rz"]
 
+    # Set entanglement_blocks to None when num_qubits == 1
+    entanglement_blocks = ["cx"] if num_qubits > 1 else []
+
     return n_local(
         num_qubits,
         su2_gates,
-        ["cx"],
+        entanglement_blocks,
         entanglement,
         reps,
         insert_barriers,

qiskit/circuit/library/n_local/excitation_preserving.py

@@ -112,17 +112,21 @@ def excitation_preserving(
         raise ValueError(f"Unsupported mode {mode}, choose one of {supported_modes}")
 
     theta = Parameter("θ")
-    swap = QuantumCircuit(2, name="Interaction")
-    swap.rxx(theta, 0, 1)
-    swap.ryy(theta, 0, 1)
-    if mode == "fsim":
-        phi = Parameter("φ")
-        swap.cp(phi, 0, 1)
+    if num_qubits > 1:
+        swap = QuantumCircuit(2, name="Interaction")
+        swap.rxx(theta, 0, 1)
+        swap.ryy(theta, 0, 1)
+        if mode == "fsim":
+            phi = Parameter("φ")
+            swap.cp(phi, 0, 1)
+        entanglement_blocks = [swap.to_gate()]
+    else:
+        entanglement_blocks = []
 
     return n_local(
         num_qubits,
         ["rz"],
-        [swap.to_gate()],
+        entanglement_blocks,
         entanglement,
         reps,
         insert_barriers,

qiskit/circuit/library/n_local/pauli_two_design.py

@@ -88,19 +88,20 @@ def pauli_two_design(
     """
     rng = np.random.default_rng(seed)
     random_block = Block.from_callable(1, 1, lambda params: _random_pauli_builder(params, rng))
-    cz_block = Block.from_standard_gate(CZGate._standard_gate)
+    entanglement_block = [Block.from_standard_gate(CZGate._standard_gate)] if num_qubits > 1 else []
 
     data = py_n_local(
         num_qubits=num_qubits,
         reps=reps,
         rotation_blocks=[random_block],
-        entanglement_blocks=[cz_block],
+        entanglement_blocks=entanglement_block,
         entanglement=["pairwise"],
         insert_barriers=insert_barriers,
         skip_final_rotation_layer=False,
         skip_unentangled_qubits=False,
         parameter_prefix=parameter_prefix,
     )
+
     two_design = QuantumCircuit._from_circuit_data(data)
 
     circuit = QuantumCircuit(num_qubits, name=name)

qiskit/circuit/library/n_local/real_amplitudes.py

@@ -114,11 +114,13 @@ def real_amplitudes(
     Returns:
         A real-amplitudes circuit.
     """
+    # Set entanglement_blocks to None when num_qubits == 1
+    entanglement_blocks = ["cx"] if num_qubits > 1 else []
 
     return n_local(
         num_qubits,
         ["ry"],
-        ["cx"],
+        entanglement_blocks,
         entanglement,
         reps,
         insert_barriers,

releasenotes/notes/fix-efficient-su2-numqubits-issue-2b2c91c1186f82ac.yaml

@@ -0,0 +1,6 @@
+fixes:
+  - |
+    Fixed a bug that caused the circuit library functions :func:`.efficient_su2`,
+    :func:`.real_amplitudes`, :func:`.excitation_preserving` and :func:`.pauli_two_design`
+    to error out when constructed for ``num_qubits==1``. For a single qubit these 
+    circuits will not contain any 2-qubit gates.

test/python/circuit/library/test_nlocal.py

@@ -784,12 +784,24 @@ def test_real_amplitudes(self):
         expected = n_local(4, "ry", "cx", "reverse_linear", reps=3)
         self.assertEqual(expected.assign_parameters(circuit.parameters), circuit)
 
+    def test_real_amplitudes_numqubits_equal1(self):
+        """Test the real amplitudes circuit for a single qubit."""
+        circuit = real_amplitudes(1)
+        expected = n_local(1, "ry", [])
+        self.assertEqual(expected.assign_parameters(circuit.parameters), circuit)
+
     def test_efficient_su2(self):
         """Test the efficient SU(2) circuit."""
         circuit = efficient_su2(4)
         expected = n_local(4, ["ry", "rz"], "cx", "reverse_linear", reps=3)
         self.assertEqual(expected.assign_parameters(circuit.parameters), circuit)
 
+    def test_efficient_su2_numqubits_equal1(self):
+        """Test the efficient SU(2) circuit for a single qubit."""
+        circuit = efficient_su2(1)
+        expected = n_local(1, ["ry", "rz"], [])
+        self.assertEqual(expected.assign_parameters(circuit.parameters), circuit)
+
     @data("fsim", "iswap")
     def test_excitation_preserving(self, mode):
         """Test the excitation preserving circuit."""
@@ -808,6 +820,15 @@ def test_excitation_preserving(self, mode):
             expected.assign_parameters(circuit.parameters).decompose(), circuit.decompose()
         )
 
+    @data("fsim", "iswap")
+    def test_excitation_preserving_numqubits_equal1(self, mode):
+        """Test the excitation preserving circuit for a single qubit."""
+        circuit = excitation_preserving(1, mode=mode)
+        expected = n_local(1, "rz", [])
+        self.assertEqual(
+            expected.assign_parameters(circuit.parameters).decompose(), circuit.decompose()
+        )
+
     def test_excitation_preserving_invalid_mode(self):
         """Test an error is raised for an invalid mode."""
         with self.assertRaises(ValueError):
@@ -824,6 +845,14 @@ def test_two_design(self):
 
         self.assertTrue(circuit_ops.issubset(expected_ops))
 
+    def test_two_design_numqubits_equal1(self):
+        """Test the Pauli 2-design circuit for a single qubit."""
+        circuit = pauli_two_design(1)
+        expected_ops = {"rx", "ry", "rz", "id"}
+        circuit_ops = set(circuit.count_ops().keys())
+
+        self.assertTrue(circuit_ops.issubset(expected_ops))
+
     def test_two_design_seed(self):
         """Test the seed"""
         seed1 = 123