Fix UnitarySynthesis for 3+ qubits when compiled for a backend (Qiskit#13591)

* Fix unitary synthesis for 3+ q

* Update releasenotes/notes/fix-unitary-synthesis-3q-2b2de5305bfd11ff.yaml

Co-authored-by: Alexander Ivrii <[email protected]>

* add more tests

* use == over equiv

---------

Co-authored-by: Alexander Ivrii <[email protected]>

Author: Cryoris

crates/accelerate/src/unitary_synthesis.rs

@@ -372,7 +372,8 @@ fn py_run_main_loop(
                     None,
                     None,
                 )?;
-                out_dag = synth_dag;
+                let out_qargs = dag.get_qargs(packed_instr.qubits);
+                apply_synth_dag(py, &mut out_dag, out_qargs, &synth_dag)?;
             }
         }
     }

releasenotes/notes/fix-unitary-synthesis-3q-2b2de5305bfd11ff.yaml

@@ -0,0 +1,8 @@
+---
+fixes:
+  - |
+    Fixed a bug in the :class:`.UnitarySynthesis` transpiler pass, where blocks of
+    :class:`.UnitaryGate`\s on 3 qubits or more were not correctly synthesized.
+    This led, e.g., to the circuit being overwritten with the last processed block or
+    to internal panics when encountering measurements after such a block.
+    Fixed `#13586 <https://github.com/Qiskit/qiskit/issues/13586>`__.

test/python/transpiler/test_unitary_synthesis.py

@@ -57,7 +57,9 @@
     RYYGate,
     RZZGate,
     RXXGate,
+    PauliEvolutionGate,
 )
+from qiskit.quantum_info import SparsePauliOp
 from qiskit.circuit import Measure
 from qiskit.circuit.controlflow import IfElseOp
 from qiskit.circuit import Parameter, Gate
@@ -1050,6 +1052,55 @@ def test_qsd(self, opt):
         qc_transpiled = transpile(qc, target=target, optimization_level=opt)
         self.assertTrue(np.allclose(mat, Operator(qc_transpiled).data))
 
+    def test_3q_with_measure(self):
+        """Test 3-qubit synthesis with measurements."""
+        backend = FakeBackend5QV2()
+
+        qc = QuantumCircuit(3, 1)
+        qc.unitary(np.eye(2**3), range(3))
+        qc.measure(0, 0)
+
+        qc_transpiled = transpile(qc, backend)
+        self.assertTrue(qc_transpiled.size, 1)
+        self.assertTrue(qc_transpiled.count_ops().get("measure", 0), 1)
+
+    def test_3q_series(self):
+        """Test a series of 3-qubit blocks."""
+        backend = GenericBackendV2(5, basis_gates=["u", "cx"])
+
+        x = QuantumCircuit(3)
+        x.x(2)
+        x_mat = Operator(x)
+
+        qc = QuantumCircuit(3)
+        qc.unitary(x_mat, range(3))
+        qc.unitary(np.eye(2**3), range(3))
+
+        tqc = transpile(qc, backend, optimization_level=0, initial_layout=[0, 1, 2])
+
+        expected = np.kron(np.eye(2**2), x_mat)
+        self.assertEqual(Operator(tqc), Operator(expected))
+
+    def test_3q_measure_all(self):
+        """Regression test of #13586."""
+        hamiltonian = SparsePauliOp.from_list(
+            [("IXX", 1), ("IYY", 1), ("IZZ", 1), ("XXI", 1), ("YYI", 1), ("ZZI", 1)]
+        )
+
+        qc = QuantumCircuit(3)
+        qc.x([1, 2])
+        op = PauliEvolutionGate(hamiltonian, time=1)
+        qc.append(op.power(8), [0, 1, 2])
+        qc.measure_all()
+
+        backend = GenericBackendV2(5, basis_gates=["u", "cx"])
+        tqc = transpile(qc, backend)
+
+        ops = tqc.count_ops()
+        self.assertIn("u", ops)
+        self.assertIn("cx", ops)
+        self.assertIn("measure", ops)
+
 
 if __name__ == "__main__":
     unittest.main()