Keep resets at beginning of circuits

docs/conf.py

@@ -248,6 +248,7 @@
 # Custom extensions
 # ---------------------------------------------------------------------------------------
 
+
 def add_versions_to_config(_app, config):
     """Add a list of old documentation versions that should have links generated to them into the
     context, so the theme can use them to generate a sidebar."""

qiskit/transpiler/preset_passmanagers/builtin_plugins.py

@@ -27,7 +27,6 @@
 from qiskit.transpiler.passes import NoiseAdaptiveLayout
 from qiskit.transpiler.passes import CheckMap
 from qiskit.transpiler.passes import BarrierBeforeFinalMeasurements
-from qiskit.transpiler.passes import RemoveResetInZeroState
 from qiskit.transpiler.passes import OptimizeSwapBeforeMeasure
 from qiskit.transpiler.passes import RemoveDiagonalGatesBeforeMeasure
 from qiskit.transpiler.preset_passmanagers import common
@@ -81,7 +80,6 @@ def pass_manager(self, pass_manager_config, optimization_level=None) -> PassMana
                 pass_manager_config.unitary_synthesis_plugin_config,
                 pass_manager_config.hls_config,
             )
-            init.append(RemoveResetInZeroState())
             init.append(OptimizeSwapBeforeMeasure())
             init.append(RemoveDiagonalGatesBeforeMeasure())
         else:

qiskit/transpiler/preset_passmanagers/level1.py

@@ -82,10 +82,10 @@ def level_1_pass_manager(pass_manager_config: PassManagerConfig) -> StagedPassMa
         target is not None and target.get_non_global_operation_names(strict_direction=True)
     ):
         pre_optimization = common.generate_pre_op_passmanager(
-            target, coupling_map, remove_reset_in_zero=True
+            target, coupling_map, remove_reset_in_zero=False
         )
     else:
-        pre_optimization = common.generate_pre_op_passmanager(remove_reset_in_zero=True)
+        pre_optimization = common.generate_pre_op_passmanager(remove_reset_in_zero=False)
 
     optimization = plugin_manager.get_passmanager_stage(
         "optimization", optimization_method, pass_manager_config, optimization_level=1

qiskit/transpiler/preset_passmanagers/level2.py

@@ -80,9 +80,11 @@ def level_2_pass_manager(pass_manager_config: PassManagerConfig) -> StagedPassMa
     if (coupling_map and not coupling_map.is_symmetric) or (
         target is not None and target.get_non_global_operation_names(strict_direction=True)
     ):
-        pre_optimization = common.generate_pre_op_passmanager(target, coupling_map, True)
+        pre_optimization = common.generate_pre_op_passmanager(
+            target, coupling_map, remove_reset_in_zero=False
+        )
     else:
-        pre_optimization = common.generate_pre_op_passmanager(remove_reset_in_zero=True)
+        pre_optimization = common.generate_pre_op_passmanager(remove_reset_in_zero=False)
 
     optimization = plugin_manager.get_passmanager_stage(
         "optimization", optimization_method, pass_manager_config, optimization_level=2

qiskit/transpiler/preset_passmanagers/level3.py

@@ -97,9 +97,11 @@ def level_3_pass_manager(pass_manager_config: PassManagerConfig) -> StagedPassMa
     if (coupling_map and not coupling_map.is_symmetric) or (
         target is not None and target.get_non_global_operation_names(strict_direction=True)
     ):
-        pre_optimization = common.generate_pre_op_passmanager(target, coupling_map, True)
+        pre_optimization = common.generate_pre_op_passmanager(
+            target, coupling_map, remove_reset_in_zero=False
+        )
     else:
-        pre_optimization = common.generate_pre_op_passmanager(remove_reset_in_zero=True)
+        pre_optimization = common.generate_pre_op_passmanager(remove_reset_in_zero=False)
 
     sched = plugin_manager.get_passmanager_stage(
         "scheduling", scheduling_method, pass_manager_config, optimization_level=3

releasenotes/notes/keep-initial-resets-a5f75cb16c8edb57.yaml

@@ -0,0 +1,6 @@
+---
+upgrade:
+  - |
+    Disables the use of :class:`.RemoveResetInZeroState` in the preset passmanagers.
+    This better aligns circuits to the notion of arbitrary initial states
+    unless explicitly set to zeros with resets.

test/python/compiler/test_transpiler.py

@@ -51,7 +51,6 @@
     SXGate,
     U1Gate,
     U2Gate,
-    U3Gate,
     UGate,
     XGate,
 )
@@ -802,20 +801,26 @@ def test_move_measurements(self):
             )
             self.assertTrue(is_last_measure)
 
-    def test_initialize_reset_should_be_removed(self):
-        """The reset in front of initializer should be removed when zero state"""
+    @data(0, 1, 2, 3)
+    def test_init_resets_kept_preset_passmanagers(self, optimization_level):
+        """Test initial resets kept at all preset transpilation levels"""
+        num_qubits = 5
+        qc = QuantumCircuit(num_qubits)
+        qc.reset(range(num_qubits))
+
+        num_resets = transpile(qc, optimization_level=optimization_level).count_ops()["reset"]
+        self.assertEqual(num_resets, num_qubits)
+
+    @data(0, 1, 2, 3)
+    def test_initialize_reset_is_not_removed(self, optimization_level):
+        """The reset in front of initializer should NOT be removed at beginning"""
         qr = QuantumRegister(1, "qr")
         qc = QuantumCircuit(qr)
         qc.initialize([1.0 / math.sqrt(2), 1.0 / math.sqrt(2)], [qr[0]])
         qc.initialize([1.0 / math.sqrt(2), -1.0 / math.sqrt(2)], [qr[0]])
 
-        expected = QuantumCircuit(qr)
-        expected.append(U3Gate(np.pi / 2, 0, 0), [qr[0]])
-        expected.reset(qr[0])
-        expected.append(U3Gate(np.pi / 2, -np.pi, 0), [qr[0]])
-
-        after = transpile(qc, basis_gates=["reset", "u3"], optimization_level=1)
-        self.assertEqual(after, expected, msg=f"after:\n{after}\nexpected:\n{expected}")
+        after = transpile(qc, basis_gates=["reset", "u3"], optimization_level=optimization_level)
+        self.assertEqual(after.count_ops()["reset"], 2, msg=f"{after}\n does not have 2 resets.")
 
     def test_initialize_FakeMelbourne(self):
         """Test that the zero-state resets are remove in a device not supporting them."""
@@ -828,7 +833,7 @@ def test_initialize_FakeMelbourne(self):
         out_dag = circuit_to_dag(out)
         reset_nodes = out_dag.named_nodes("reset")
 
-        self.assertEqual(reset_nodes, [])
+        self.assertEqual(len(reset_nodes), 3)
 
     def test_non_standard_basis(self):
         """Test a transpilation with a non-standard basis"""