Ignoring unsupported gates for Solovay-Kitaev transpiler pass (#13690)

* ignoring unsupported gates for SK

* minor

* making SK pass recurse over control-flow operations

* Update releasenotes/notes/sk-ignore-unsupported-ops-8d7d5f6fca255ffb.yaml

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

---------

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

qiskit/transpiler/passes/synthesis/solovay_kitaev_synthesis.py

@@ -33,7 +33,7 @@
     generate_basic_approximations,
 )
 from qiskit.transpiler.basepasses import TransformationPass
-from qiskit.transpiler.exceptions import TranspilerError
+from qiskit.transpiler.passes.utils.control_flow import trivial_recurse
 
 from .plugin import UnitarySynthesisPlugin
 
@@ -155,6 +155,7 @@ def __init__(
         self.recursion_degree = recursion_degree
         self._sk = SolovayKitaevDecomposition(basic_approximations)
 
+    @trivial_recurse
     def run(self, dag: DAGCircuit) -> DAGCircuit:
         """Run the ``SolovayKitaev`` pass on `dag`.
 
@@ -168,18 +169,19 @@ def run(self, dag: DAGCircuit) -> DAGCircuit:
             TranspilerError: if a gates does not have to_matrix
         """
         for node in dag.op_nodes():
-            if not node.op.num_qubits == 1:
-                continue  # ignore all non-single qubit gates
+
+            # ignore operations on which the algorithm cannot run
+            if (
+                (node.op.num_qubits != 1)
+                or node.is_parameterized()
+                or (not hasattr(node.op, "to_matrix"))
+            ):
+                continue
 
             # we do not check the input matrix as we know it comes from a Qiskit gate, as this
             # we know it will generate a valid SU(2) matrix
             check_input = not isinstance(node.op, Gate)
 
-            if not hasattr(node.op, "to_matrix"):
-                raise TranspilerError(
-                    f"SolovayKitaev does not support gate without to_matrix method: {node.op.name}"
-                )
-
             matrix = node.op.to_matrix()
 
             # call solovay kitaev

releasenotes/notes/sk-ignore-unsupported-ops-8d7d5f6fca255ffb.yaml

@@ -0,0 +1,7 @@
+---
+upgrade_transpiler:
+  - |
+    The :class:`.SolovayKitaev` transpiler pass no longer raises an exception on circuits
+    that contain single-qubit operations without a ``to_matrix`` method (such as measures,
+    barriers, control-flow operations) or parameterized single-qubit operations,
+    but will leave them unchanged.

test/python/transpiler/test_solovay_kitaev.py

@@ -22,8 +22,10 @@
 from ddt import ddt, data
 
 from qiskit import transpile
-from qiskit.circuit import QuantumCircuit
+from qiskit.circuit import QuantumCircuit, Parameter
+from qiskit.circuit.classicalregister import ClassicalRegister
 from qiskit.circuit.library import TGate, TdgGate, HGate, SGate, SdgGate, IGate, QFT
+from qiskit.circuit.quantumregister import QuantumRegister
 from qiskit.converters import circuit_to_dag, dag_to_circuit
 from qiskit.quantum_info import Operator
 from qiskit.synthesis.discrete_basis.generate_basis_approximations import (
@@ -32,7 +34,6 @@
 from qiskit.synthesis.discrete_basis.commutator_decompose import commutator_decompose
 from qiskit.synthesis.discrete_basis.gate_sequence import GateSequence
 from qiskit.transpiler import PassManager
-from qiskit.transpiler.exceptions import TranspilerError
 from qiskit.transpiler.passes import UnitarySynthesis, Collect1qRuns, ConsolidateBlocks
 from qiskit.transpiler.passes.synthesis import SolovayKitaev, SolovayKitaevSynthesis
 from test import QiskitTestCase  # pylint: disable=wrong-import-order
@@ -152,23 +153,6 @@ def test_exact_decomposition_acts_trivially(self):
         decomposed_circuit = dag_to_circuit(decomposed_dag)
         self.assertEqual(circuit, decomposed_circuit)
 
-    def test_fails_with_no_to_matrix(self):
-        """Test failer if gate does not have to_matrix."""
-        circuit = QuantumCircuit(1)
-        circuit.initialize("0")
-
-        synth = SolovayKitaev(3, self.basic_approx)
-
-        dag = circuit_to_dag(circuit)
-
-        with self.assertRaises(TranspilerError) as cm:
-            _ = synth.run(dag)
-
-        self.assertEqual(
-            "SolovayKitaev does not support gate without to_matrix method: initialize",
-            cm.exception.message,
-        )
-
     def test_str_basis_gates(self):
         """Test specifying the basis gates by string works."""
         circuit = QuantumCircuit(1)
@@ -261,6 +245,56 @@ def test_load_from_file(self):
 
         self.assertEqual(discretized, reference)
 
+    def test_measure(self):
+        """Test the Solovay-Kitaev transpiler pass on circuits with measure operators."""
+        qc = QuantumCircuit(1, 1)
+        qc.x(0)
+        qc.measure(0, 0)
+        transpiled = SolovayKitaev()(qc)
+        self.assertEqual(set(transpiled.count_ops()), {"h", "t", "measure"})
+
+    def test_barrier(self):
+        """Test the Solovay-Kitaev transpiler pass on circuits with barriers."""
+        qc = QuantumCircuit(1)
+        qc.x(0)
+        qc.barrier(0)
+        transpiled = SolovayKitaev()(qc)
+        self.assertEqual(set(transpiled.count_ops()), {"h", "t", "barrier"})
+
+    def test_parameterized_gates(self):
+        """Test the Solovay-Kitaev transpiler pass on circuits with parameterized gates."""
+        qc = QuantumCircuit(1)
+        qc.x(0)
+        qc.rz(Parameter("t"), 0)
+        transpiled = SolovayKitaev()(qc)
+        self.assertEqual(set(transpiled.count_ops()), {"h", "t", "rz"})
+
+    def test_control_flow_if(self):
+        """Test the Solovay-Kitaev transpiler pass on circuits with control flow ops"""
+        qr = QuantumRegister(1)
+        cr = ClassicalRegister(1)
+        qc = QuantumCircuit(qr, cr)
+
+        with qc.if_test((cr[0], 0)) as else_:
+            qc.y(0)
+        with else_:
+            qc.z(0)
+        transpiled = SolovayKitaev()(qc)
+
+        # check that we still have an if-else block and all the operations within
+        # have been recursively synthesized
+        self.assertEqual(transpiled[0].name, "if_else")
+        for block in transpiled[0].operation.blocks:
+            self.assertLessEqual(set(block.count_ops()), {"h", "t", "tdg"})
+
+    def test_no_to_matrix(self):
+        """Test the Solovay-Kitaev transpiler pass ignores gates without to_matrix."""
+        qc = QuantumCircuit(1)
+        qc.initialize("0")
+
+        transpiled = SolovayKitaev()(qc)
+        self.assertEqual(set(transpiled.count_ops()), {"initialize"})
+
 
 @ddt
 class TestGateSequence(QiskitTestCase):