FIX: #1546 class Subgraph, minimum two-qubit gates

 The if statement that checks the number of two-qubit gates in the circuit

src/qibo/transpiler/placer.py

@@ -126,7 +126,7 @@ def __call__(self, circuit: Circuit):
         """
         assert_placement(circuit, self.connectivity)
         gates_qubits_pairs = _find_gates_qubits_pairs(circuit)
-        if len(gates_qubits_pairs) < 3:
+        if len(gates_qubits_pairs) < 2:
             raise_error(
                 ValueError,
                 "Circuit must contain at least two two-qubit gates "

tests/test_transpiler_placer.py

@@ -93,6 +93,16 @@ def test_subgraph_restricted(star_connectivity):
     assert_placement(circuit, restricted_connectivity)
 
 
+def test_subgraph_leasttwoqubitgates(star_connectivity):
+    circuit = Circuit(5)
+    circuit.add(gates.CNOT(0, 3))
+    circuit.add(gates.CNOT(1, 2))
+    connectivity = star_connectivity()
+    placer = Subgraph(connectivity=connectivity)
+    placer(circuit)
+    assert_placement(circuit, connectivity)
+
+
 @pytest.mark.parametrize("reps", [1, 10, 100])
 @pytest.mark.parametrize("names", [["A", "B", "C", "D", "E"], [0, 1, 2, 3, 4]])
 def test_random(reps, names, star_connectivity):