Fix Rust-space TwoQubitBasisDecomposer for non-standard KAK gate (Qiskit#13014)

* Get rid of encoded assumption in TwoQubitBasisDecomposer that the given KAK gate is a Rust-space StandardGate

* Improve unit test

* Update test/python/synthesis/test_synthesis.py

Co-authored-by: Matthew Treinish <[email protected]>

---------

Co-authored-by: Matthew Treinish <[email protected]>

Author: ElePT

crates/accelerate/src/two_qubit_decompose.rs

@@ -55,6 +55,7 @@ use qiskit_circuit::circuit_data::CircuitData;
 use qiskit_circuit::circuit_instruction::OperationFromPython;
 use qiskit_circuit::gate_matrix::{CX_GATE, H_GATE, ONE_QUBIT_IDENTITY, SX_GATE, X_GATE};
 use qiskit_circuit::operations::{Param, StandardGate};
+use qiskit_circuit::packed_instruction::PackedOperation;
 use qiskit_circuit::slice::{PySequenceIndex, SequenceIndex};
 use qiskit_circuit::util::{c64, GateArray1Q, GateArray2Q, C_M_ONE, C_ONE, C_ZERO, IM, M_IM};
 use qiskit_circuit::Qubit;
@@ -2063,26 +2064,51 @@ impl TwoQubitBasisDecomposer {
     ) -> PyResult<CircuitData> {
         let kak_gate = kak_gate.extract::<OperationFromPython>(py)?;
         let sequence = self.__call__(unitary, basis_fidelity, approximate, _num_basis_uses)?;
-        CircuitData::from_standard_gates(
-            py,
-            2,
-            sequence
-                .gates
-                .into_iter()
-                .map(|(gate, params, qubits)| match gate {
-                    Some(gate) => (
-                        gate,
-                        params.into_iter().map(Param::Float).collect(),
-                        qubits.into_iter().map(|x| Qubit(x.into())).collect(),
-                    ),
-                    None => (
-                        kak_gate.operation.standard_gate(),
-                        kak_gate.params.clone(),
-                        qubits.into_iter().map(|x| Qubit(x.into())).collect(),
-                    ),
-                }),
-            Param::Float(sequence.global_phase),
-        )
+        match kak_gate.operation.try_standard_gate() {
+            Some(std_kak_gate) => CircuitData::from_standard_gates(
+                py,
+                2,
+                sequence
+                    .gates
+                    .into_iter()
+                    .map(|(gate, params, qubits)| match gate {
+                        Some(gate) => (
+                            gate,
+                            params.into_iter().map(Param::Float).collect(),
+                            qubits.into_iter().map(|x| Qubit(x.into())).collect(),
+                        ),
+                        None => (
+                            std_kak_gate,
+                            kak_gate.params.clone(),
+                            qubits.into_iter().map(|x| Qubit(x.into())).collect(),
+                        ),
+                    }),
+                Param::Float(sequence.global_phase),
+            ),
+            None => CircuitData::from_packed_operations(
+                py,
+                2,
+                0,
+                sequence
+                    .gates
+                    .into_iter()
+                    .map(|(gate, params, qubits)| match gate {
+                        Some(gate) => (
+                            PackedOperation::from_standard(gate),
+                            params.into_iter().map(Param::Float).collect(),
+                            qubits.into_iter().map(|x| Qubit(x.into())).collect(),
+                            Vec::new(),
+                        ),
+                        None => (
+                            kak_gate.operation.clone(),
+                            kak_gate.params.clone(),
+                            qubits.into_iter().map(|x| Qubit(x.into())).collect(),
+                            Vec::new(),
+                        ),
+                    }),
+                Param::Float(sequence.global_phase),
+            ),
+        }
     }
 
     fn num_basis_gates(&self, unitary: PyReadonlyArray2<Complex64>) -> usize {

releasenotes/notes/fix-2q-basis-decomposer-non-std-kak-gate-edc69ffb5d9ef302.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    Fixed a bug in :class:`.TwoQubitBasisDecomposer` where the Rust-based code
+    would panic if the given KAK gate wasn't a Rust-space :class:`StandardGate`.

test/python/synthesis/test_synthesis.py

@@ -1270,6 +1270,21 @@ def test_use_dag(self, euler_bases, kak_gates, seed):
             requested_basis = set(oneq_gates + [kak_gate_name])
             self.assertTrue(decomposition_basis.issubset(requested_basis))
 
+    def test_non_std_gate(self):
+        """Test that the TwoQubitBasisDecomposer class can be correctly instantiated with a
+        non-standard KAK gate.
+
+        Reproduce from: https://github.com/Qiskit/qiskit/issues/12998
+        """
+        # note that `CXGate(ctrl_state=0)` is not handled as a "standard" gate.
+        decomposer = TwoQubitBasisDecomposer(CXGate(ctrl_state=0))
+        unitary = SwapGate().to_matrix()
+        decomposed_unitary = decomposer(unitary)
+        self.assertEqual(Operator(unitary), Operator(decomposed_unitary))
+        self.assertNotIn("swap", decomposed_unitary.count_ops())
+        self.assertNotIn("cx", decomposed_unitary.count_ops())
+        self.assertEqual(3, decomposed_unitary.count_ops()["cx_o0"])
+
 
 @ddt
 class TestPulseOptimalDecompose(CheckDecompositions):