Transpiling with basis_gates=None disables unrolling (Qiskit#4263)

* allow basis_gates=None

* some docstring and organization

* lint

* reno

qiskit/compiler/transpile.py

@@ -273,17 +273,18 @@ def _transpile_circuit(circuit_config_tuple: Tuple[QuantumCircuit, Dict]) -> Qua
 
     pass_manager_config = transpile_config['pass_manager_config']
 
-    # Workaround for ion trap support: If basis gates includes
-    # Mølmer-Sørensen (rxx) and the circuit includes gates outside the basis,
-    # first unroll to u3, cx, then run MSBasisDecomposer to target basis.
-    basic_insts = ['measure', 'reset', 'barrier', 'snapshot']
-    device_insts = set(pass_manager_config.basis_gates).union(basic_insts)
     ms_basis_swap = None
-    if 'rxx' in pass_manager_config.basis_gates and \
-            not device_insts >= circuit.count_ops().keys():
-        ms_basis_swap = pass_manager_config.basis_gates
-        pass_manager_config.basis_gates = list(
-            set(['u3', 'cx']).union(pass_manager_config.basis_gates))
+    if pass_manager_config.basis_gates is not None:
+        # Workaround for ion trap support: If basis gates includes
+        # Mølmer-Sørensen (rxx) and the circuit includes gates outside the basis,
+        # first unroll to u3, cx, then run MSBasisDecomposer to target basis.
+        basic_insts = ['measure', 'reset', 'barrier', 'snapshot']
+        device_insts = set(pass_manager_config.basis_gates).union(basic_insts)
+        if 'rxx' in pass_manager_config.basis_gates and \
+                not device_insts >= circuit.count_ops().keys():
+            ms_basis_swap = pass_manager_config.basis_gates
+            pass_manager_config.basis_gates = list(
+                set(['u3', 'cx']).union(pass_manager_config.basis_gates))
 
     # we choose an appropriate one based on desired optimization level
     level = transpile_config['optimization_level']
@@ -370,13 +371,6 @@ def _parse_basis_gates(basis_gates, backend, circuits):
                                all(isinstance(i, str) for i in basis_gates)):
         basis_gates = [basis_gates] * len(circuits)
 
-    # no basis means don't unroll (all circuit gates are valid basis)
-    for index, circuit in enumerate(circuits):
-        basis = basis_gates[index]
-        if basis is None:
-            gates_in_circuit = {inst.name for inst, _, _ in circuit.data}
-            # Other passes might add new gates that need to be supported
-            basis_gates[index] = list(gates_in_circuit.union(['u3', 'cx']))
     return basis_gates
 
 

qiskit/transpiler/passes/basis/unroller.py

@@ -30,7 +30,8 @@ def __init__(self, basis):
         """Unroller initializer.
 
         Args:
-            basis (list[str]): Target basis names to unroll to, e.g. `['u3', 'cx']` .
+            basis (list[str] or None): Target basis names to unroll to, e.g. `['u3', 'cx']` . If
+                None, does not unroll any gate.
         """
         super().__init__()
         self.basis = basis
@@ -48,6 +49,9 @@ def run(self, dag):
         Returns:
             DAGCircuit: output unrolled dag
         """
+        if self.basis is None:
+            return dag
+
         # Walk through the DAG and expand each non-basis node
         for node in dag.op_nodes():
             basic_insts = ['measure', 'reset', 'barrier', 'snapshot']

releasenotes/notes/transpiling_basis_none-b2f1abdb3c080eca.yaml

@@ -0,0 +1,27 @@
+---
+fixes:
+  - |
+    Bug #3017 is fixed. The function ``qiskit.compiler.transpile()`` honors the parameter ``basis_gates=None`` 
+    to allow any gate in the final tranpiled circuit, including gates added by the transpilation process.
+    This might also have some unintended consequences during optimization. For example,
+    :class:`qiskit.transpiler.passes.Optimize1qGates` only optimizes the chains of u1, u2, and u3 gates:
+
+    .. code-block:: python
+
+      from qiskit import *
+      
+      q = QuantumRegister(1, name='q')
+      circuit = QuantumCircuit(q)
+      circuit.h(q[0])
+      circuit.u1(0.1, q[0])
+      circuit.u2(0.1, 0.2, q[0])
+      circuit.h(q[0])
+      circuit.u3(0.1, 0.2, 0.3, q[0])
+      
+      result = transpile(circuit, basis_gates=None, optimization_level=3)
+      result.draw()
+
+    .. parsed-literal::
+          ┌───┐┌─────────────┐┌───┐┌─────────────────┐
+     q_0: ┤ H ├┤ U2(0.1,0.3) ├┤ H ├┤ U3(0.1,0.2,0.3) ├
+          └───┘└─────────────┘└───┘└─────────────────┘

test/python/transpiler/test_preset_passmanagers.py

@@ -42,17 +42,25 @@ def circuit_2532():
 class TestPresetPassManager(QiskitTestCase):
     """Test preset passmanagers work as expected."""
 
-    @combine(level=[0, 1, 2, 3],
-             dsc='Test that coupling_map can be None (level={level})',
-             name='coupling_map_none_level{level}')
+    @combine(level=[0, 1, 2, 3], name='level{level}')
     def test_no_coupling_map(self, level):
-        """Test that coupling_map can be None"""
+        """Test that coupling_map can be None (level={level})"""
         q = QuantumRegister(2, name='q')
         circuit = QuantumCircuit(q)
         circuit.cz(q[0], q[1])
         result = transpile(circuit, basis_gates=['u1', 'u2', 'u3', 'cx'], optimization_level=level)
         self.assertIsInstance(result, QuantumCircuit)
 
+    @combine(level=[0, 1, 2, 3], name='level{level}')
+    def test_no_basis_gates(self, level):
+        """Test that basis_gates can be None (level={level})"""
+        q = QuantumRegister(2, name='q')
+        circuit = QuantumCircuit(q)
+        circuit.h(q[0])
+        circuit.cz(q[0], q[1])
+        result = transpile(circuit, basis_gates=None, optimization_level=level)
+        self.assertEqual(result, circuit)
+
 
 @ddt
 class TestTranspileLevels(QiskitTestCase):