Don't substitute ideal gates in target with Optimize1qGatesDecomposition

This commit fixes an issue in Optimize1qGatesDecomposition where the
pass would defer to synthesized gates if the output from synthesis was
ideal even if the input gate was also ideal. This typically comes up in
simulators where there are no error rates for any gates and everything
is listed as ideal. This would cause the transpiler to translate gates
unnecessarily which was unexpected. This commit fixes this by adding an
additional check to the subsitution condition to ensure we're only
substituting a gate if it's not in the target (when they're all ideal).

Fixes Qiskit#10568

qiskit/transpiler/passes/optimization/optimize_1q_decomposition.py

@@ -173,10 +173,19 @@ def _substitution_checks(self, dag, old_run, new_circ, basis, qubit):
         # if we're outside of the basis set, we're obligated to logically decompose.
         # if we're outside of the set of gates for which we have physical definitions,
         #    then we _try_ to decompose, using the results if we see improvement.
+        new_error = 0.0
+        old_error = 0.0
+        if not uncalibrated_and_not_basis_p:
+            new_error = self._error(new_circ, qubit)
+            old_error = self._error(old_run, qubit)
+
         return (
             uncalibrated_and_not_basis_p
-            or (uncalibrated_p and self._error(new_circ, qubit) < self._error(old_run, qubit))
-            or math.isclose(self._error(new_circ, qubit)[0], 0)
+            or (uncalibrated_p and new_error < old_error)
+            or (
+                math.isclose(new_error[0], 0)
+                and (not uncalibrated_and_not_basis_p and not math.isclose(old_error[0], 0))
+            )
         )
 
     @control_flow.trivial_recurse

releasenotes/notes/fix-optimize-1q-sim-407b88e45e6062b6.yaml

@@ -0,0 +1,10 @@
+---
+fixes:
+  - |
+    Fixed an issue with the :class:`.Optimize1qGatesDecomposition` transpiler
+    pass where it would potential resynthesize a single ideal (meaning the
+    error rate is ``0.0``) gate which was present in the :class:`.Target`. This
+    is now fixed so the pass :class:`.Optimize1qGatesDecomposition` will defer
+    to the circuit's gate if the error rate (which includes number of gates)
+    are the same.
+    Fixed `#10568 <https://github.com/Qiskit/qiskit/issues/10568>`__

test/python/transpiler/test_optimize_1q_decomposition.py

@@ -745,6 +745,20 @@ def test_nested_control_flow(self):
         result = passmanager.run(test)
         self.assertEqual(result, expected)
 
+    def test_prefer_no_substitution_if_all_ideal(self):
+        """Test that gates are not substituted if all our ideal gates in basis."""
+        target = Target(num_qubits=1)
+        target.add_instruction(HGate(), {(0,): InstructionProperties(error=0)})
+        target.add_instruction(
+            UGate(Parameter("a"), Parameter("b"), Parameter("c")),
+            {(0,): InstructionProperties(error=0)},
+        )
+        qc = QuantumCircuit(1)
+        qc.h(0)
+        opt_pass = Optimize1qGatesDecomposition(target)
+        res = opt_pass(qc)
+        self.assertEqual(res, qc)
+
 
 if __name__ == "__main__":
     unittest.main()