transpiler/1q_decomposition: fix circuit score for zero gate errors

qiskit/transpiler/passes/optimization/optimize_1q_decomposition.py

@@ -180,7 +180,7 @@ def _error(circuit, target, qubit):
         if isinstance(circuit, list):
             return len(circuit)
         else:
-            return len(circuit._multi_graph) - 2
+            return circuit.size()
     else:
         if isinstance(circuit, list):
             gate_fidelities = [
@@ -193,11 +193,10 @@ def _error(circuit, target, qubit):
             ]
         gate_error = 1 - np.product(gate_fidelities)
         if gate_error == 0.0:
+            # prefer shorter circuits among those with zero error
             if isinstance(circuit, list):
                 return -100 + len(circuit)
             else:
-                return -100 + len(
-                    circuit._multi_graph
-                )  # prefer shorter circuits among those with zero error
+                return -100 + circuit.size()
         else:
             return gate_error

releasenotes/notes/0.23/fix-transpiler-optimize-1q-decomposition-score-e79ea05c3cf1b6fa.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    Fixes a bug in the :class:`Optimize1qGatesDecomposition` transformation pass
+    where the score for substitutions was wrongly calculated when the gate
+    errors are zero.

test/python/transpiler/test_optimize_1q_decomposition.py

@@ -530,6 +530,41 @@ def test_euler_decomposition_zsx_2(self):
         result = passmanager.run(circuit)
         self.assertEqual(circuit, result, f"Circuit:\n{circuit}\nResult:\n{result}")
 
+    def test_optimize_run_of_u_to_single_u_on_target_no_error(self):
+        """U(pi/3, 0, 0) * U(pi/3, 0, 0) * U(pi/3, 0, 0) -> U(pi, 0, 0)."""
+        qr = QuantumRegister(1, "qr")
+        circuit = QuantumCircuit(qr)
+        for _ in range(3):
+            circuit.append(UGate(np.pi / 3, 0.0, 0.0), [qr[0]])
+
+        expected = QuantumCircuit(qr)
+        expected.append(UGate(np.pi, 0.0, 0.0), [qr[0]])
+
+        passmanager = PassManager()
+        passmanager.append(Optimize1qGatesDecomposition(target=target_rz_ry_u_noerror))
+        result = passmanager.run(circuit)
+
+        msg = f"expected:\n{expected}\nresult:\n{result}"
+        self.assertEqual(expected, result, msg=msg)
+
+    def test_optimize_run_of_u_to_single_u_no_target(self):
+        """U(pi/3, 0, 0) * U(pi/3, 0, 0) * U(pi/3, 0, 0) -> U(pi, 0, 0)."""
+        qr = QuantumRegister(1, "qr")
+        circuit = QuantumCircuit(qr)
+        for _ in range(3):
+            circuit.append(UGate(np.pi / 3, 0.0, 0.0), [qr[0]])
+
+        expected = QuantumCircuit(qr)
+        expected.append(UGate(np.pi, 0.0, 0.0), [qr[0]])
+
+        basis = ["u"]
+        passmanager = PassManager()
+        passmanager.append(Optimize1qGatesDecomposition(basis))
+        result = passmanager.run(circuit)
+
+        msg = f"expected:\n{expected}\nresult:\n{result}"
+        self.assertEqual(expected, result, msg=msg)
+
     def test_optimize_u_to_phase_gate(self):
         """U(0, 0, pi/4) ->  p(pi/4). Basis [p, sx]."""
         qr = QuantumRegister(2, "qr")