Add RemoveFinalReset pass (#11266)

* Add RemoveFinalReset pass

* Fix lint

* Look at predecessors of `output_map`

Suggested at #11266 (comment)

Co-authored-by: Matthew Treinish <mtreinish@kortar.org>

* Remove `RemoveFinalReset` from preset passmanagers

This pass modifies the quantum output state of the circuit, so it
should not be run by default.

* Update remove-final-reset-488247c01c4e147d.yaml

It's no longer run by default at any optimization level.

* Fix `QiskitTestCase` import

* Update to use `DoWhileController`

---------

Co-authored-by: Matthew Treinish <mtreinish@kortar.org>

qiskit/transpiler/passes/__init__.py

@@ -81,6 +81,7 @@
    Optimize1qGatesSimpleCommutation
    RemoveDiagonalGatesBeforeMeasure
    RemoveResetInZeroState
+   RemoveFinalReset
    HoareOptimizer
    TemplateOptimization
    EchoRZXWeylDecomposition
@@ -225,6 +226,7 @@
 from .optimization import Optimize1qGatesSimpleCommutation
 from .optimization import OptimizeSwapBeforeMeasure
 from .optimization import RemoveResetInZeroState
+from .optimization import RemoveFinalReset
 from .optimization import RemoveDiagonalGatesBeforeMeasure
 from .optimization import HoareOptimizer
 from .optimization import TemplateOptimization

qiskit/transpiler/passes/optimization/__init__.py

@@ -24,6 +24,7 @@
 from .optimize_1q_commutation import Optimize1qGatesSimpleCommutation
 from .optimize_swap_before_measure import OptimizeSwapBeforeMeasure
 from .remove_reset_in_zero_state import RemoveResetInZeroState
+from .remove_final_reset import RemoveFinalReset
 from .remove_diagonal_gates_before_measure import RemoveDiagonalGatesBeforeMeasure
 from .hoare_opt import HoareOptimizer
 from .template_optimization import TemplateOptimization

qiskit/transpiler/passes/optimization/remove_final_reset.py

@@ -0,0 +1,37 @@
+# This code is part of Qiskit.
+#
+# (C) Copyright IBM 2017, 2023.
+#
+# This code is licensed under the Apache License, Version 2.0. You may
+# obtain a copy of this license in the LICENSE.txt file in the root directory
+# of this source tree or at http://www.apache.org/licenses/LICENSE-2.0.
+#
+# Any modifications or derivative works of this code must retain this
+# copyright notice, and modified files need to carry a notice indicating
+# that they have been altered from the originals.
+
+"""Remove reset when it is the final instruction on a qubit."""
+
+from qiskit.circuit import Reset, Qubit
+from qiskit.dagcircuit import DAGOpNode
+from qiskit.transpiler.basepasses import TransformationPass
+
+
+class RemoveFinalReset(TransformationPass):
+    """Remove reset when it is the final instruction on a qubit wire."""
+
+    def run(self, dag):
+        """Run the RemoveFinalReset pass on `dag`.
+
+        Args:
+            dag (DAGCircuit): the DAG to be optimized.
+
+        Returns:
+            DAGCircuit: the optimized DAG.
+        """
+        for output_node in dag.output_map.values():
+            if isinstance(output_node.wire, Qubit):
+                pred = next(dag.predecessors(output_node))
+                if isinstance(pred, DAGOpNode) and isinstance(pred.op, Reset):
+                    dag.remove_op_node(pred)
+        return dag

releasenotes/notes/remove-final-reset-488247c01c4e147d.yaml

@@ -0,0 +1,35 @@
+---
+features:
+  - |
+    Added a new transpiler pass, :class:`.RemoveFinalReset`, which
+    will remove any :class:`.Reset` operation which is the final
+    instruction on a qubit wire.  For example, taking a circuit with
+    final :class:`.Reset`\ s:
+
+    .. plot::
+
+       from qiskit.circuit import QuantumCircuit
+
+       qc = QuantumCircuit(3, 1)
+       qc.reset(0)
+       qc.h(range(3))
+       qc.cx(1, 0)
+       qc.measure(0, 0)
+       qc.reset(range(3))
+       qc.draw("mpl")
+
+    will remove the final resets when the pass is run:
+
+    .. plot::
+       :include-source:
+
+       from qiskit.transpiler.passes import RemoveFinalReset
+       from qiskit.circuit import QuantumCircuit
+
+       qc = QuantumCircuit(3, 1)
+       qc.reset(0)
+       qc.h(range(3))
+       qc.cx(1, 0)
+       qc.measure(0, 0)
+       qc.reset(range(3))
+       RemoveFinalReset()(qc).draw("mpl")

test/python/compiler/test_transpiler.py

@@ -925,6 +925,7 @@ def test_init_resets_kept_preset_passmanagers(self, optimization_level):
         num_qubits = 5
         qc = QuantumCircuit(num_qubits)
         qc.reset(range(num_qubits))
+        qc.h(range(num_qubits))
 
         num_resets = transpile(qc, optimization_level=optimization_level).count_ops()["reset"]
         self.assertEqual(num_resets, num_qubits)

test/python/transpiler/test_remove_final_reset.py

@@ -0,0 +1,117 @@
+# This code is part of Qiskit.
+#
+# (C) Copyright IBM 2017, 2023.
+#
+# This code is licensed under the Apache License, Version 2.0. You may
+# obtain a copy of this license in the LICENSE.txt file in the root directory
+# of this source tree or at http://www.apache.org/licenses/LICENSE-2.0.
+#
+# Any modifications or derivative works of this code must retain this
+# copyright notice, and modified files need to carry a notice indicating
+# that they have been altered from the originals.
+
+"""Test RemoveFinalReset pass"""
+
+import unittest
+
+from qiskit import QuantumRegister, QuantumCircuit
+from qiskit.passmanager.flow_controllers import DoWhileController
+from qiskit.transpiler import PassManager
+from qiskit.transpiler.passes import RemoveFinalReset, DAGFixedPoint
+from qiskit.converters import circuit_to_dag
+from test import QiskitTestCase  # pylint: disable=wrong-import-order
+
+
+class TestRemoveFinalReset(QiskitTestCase):
+    """Test remove-reset-in-zero-state optimizations."""
+
+    def test_optimize_single_reset(self):
+        """Remove a single final reset
+        qr0:--[H]--|0>--   ==>    qr0:--[H]--
+        """
+        qr = QuantumRegister(1, "qr")
+        circuit = QuantumCircuit(qr)
+        circuit.h(0)
+        circuit.reset(qr)
+        dag = circuit_to_dag(circuit)
+
+        expected = QuantumCircuit(qr)
+        expected.h(0)
+
+        pass_ = RemoveFinalReset()
+        after = pass_.run(dag)
+
+        self.assertEqual(circuit_to_dag(expected), after)
+
+    def test_dont_optimize_non_final_reset(self):
+        """Do not remove reset if not final instruction
+        qr0:--|0>--[H]--   ==>    qr0:--|0>--[H]--
+        """
+        qr = QuantumRegister(1, "qr")
+        circuit = QuantumCircuit(qr)
+        circuit.reset(qr)
+        circuit.h(qr)
+        dag = circuit_to_dag(circuit)
+
+        expected = QuantumCircuit(qr)
+        expected.reset(qr)
+        expected.h(qr)
+
+        pass_ = RemoveFinalReset()
+        after = pass_.run(dag)
+
+        self.assertEqual(circuit_to_dag(expected), after)
+
+    def test_optimize_single_reset_in_diff_qubits(self):
+        """Remove a single final reset in different qubits
+        qr0:--[H]--|0>--          qr0:--[H]--
+                      ==>
+        qr1:--[X]--|0>--          qr1:--[X]----
+        """
+        qr = QuantumRegister(2, "qr")
+        circuit = QuantumCircuit(qr)
+        circuit.h(0)
+        circuit.x(1)
+        circuit.reset(qr)
+        dag = circuit_to_dag(circuit)
+
+        expected = QuantumCircuit(qr)
+        expected.h(0)
+        expected.x(1)
+
+        pass_ = RemoveFinalReset()
+        after = pass_.run(dag)
+
+        self.assertEqual(circuit_to_dag(expected), after)
+
+
+class TestRemoveFinalResetFixedPoint(QiskitTestCase):
+    """Test RemoveFinalReset in a transpiler, using fixed point."""
+
+    def test_two_resets(self):
+        """Remove two final resets
+        qr0:--[H]-|0>-|0>--   ==>    qr0:--[H]--
+        """
+        qr = QuantumRegister(1, "qr")
+        circuit = QuantumCircuit(qr)
+        circuit.h(qr[0])
+        circuit.reset(qr[0])
+        circuit.reset(qr[0])
+
+        expected = QuantumCircuit(qr)
+        expected.h(qr[0])
+
+        pass_manager = PassManager()
+        pass_manager.append(
+            DoWhileController(
+                [RemoveFinalReset(), DAGFixedPoint()],
+                do_while=lambda property_set: not property_set["dag_fixed_point"],
+            )
+        )
+        after = pass_manager.run(circuit)
+
+        self.assertEqual(expected, after)
+
+
+if __name__ == "__main__":
+    unittest.main()

test/python/transpiler/test_remove_reset_in_zero_state.py

@@ -23,7 +23,7 @@
 
 
 class TestRemoveResetInZeroState(QiskitTestCase):
-    """Test swap-followed-by-measure optimizations."""
+    """Test remove-reset-in-zero-state optimizations."""
 
     def test_optimize_single_reset(self):
         """Remove a single reset