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barrier_before_final_measurements.py
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barrier_before_final_measurements.py
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# This code is part of Qiskit.
#
# (C) Copyright IBM 2017, 2018.
#
# 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.
"""Add a barrier before final measurements."""
from qiskit.circuit.barrier import Barrier
from qiskit.transpiler.basepasses import TransformationPass
from qiskit.dagcircuit import DAGCircuit, DAGOpNode
from .merge_adjacent_barriers import MergeAdjacentBarriers
class BarrierBeforeFinalMeasurements(TransformationPass):
"""Add a barrier before final measurements.
This pass adds a barrier before the set of final measurements. Measurements
are considered final if they are followed by no other operations (aside from
other measurements or barriers.)
"""
def __init__(self, label=None):
super().__init__()
self.label = label
def run(self, dag):
"""Run the BarrierBeforeFinalMeasurements pass on `dag`."""
# Collect DAG nodes which are followed only by barriers or other measures.
final_op_types = ["measure", "barrier"]
final_ops = []
for candidate_node in dag.named_nodes(*final_op_types):
is_final_op = True
for _, child_successors in dag.bfs_successors(candidate_node):
if any(
isinstance(suc, DAGOpNode) and suc.name not in final_op_types
for suc in child_successors
):
is_final_op = False
break
if is_final_op:
final_ops.append(candidate_node)
if not final_ops:
return dag
# Create a layer with the barrier and add both bits and registers from the original dag.
barrier_layer = DAGCircuit()
barrier_layer.add_qubits(dag.qubits)
for qreg in dag.qregs.values():
barrier_layer.add_qreg(qreg)
barrier_layer.add_clbits(dag.clbits)
for creg in dag.cregs.values():
barrier_layer.add_creg(creg)
# Add a barrier across all qubits so swap mapper doesn't add a swap
# from an unmeasured qubit after a measure.
final_qubits = dag.qubits
barrier_layer.apply_operation_back(
Barrier(len(final_qubits), label=self.label), final_qubits, (), check=False
)
# Preserve order of final ops collected earlier from the original DAG.
ordered_final_nodes = [
node for node in dag.topological_op_nodes() if node in set(final_ops)
]
# Move final ops to the new layer and append the new layer to the DAG.
for final_node in ordered_final_nodes:
barrier_layer.apply_operation_back(
final_node.op, final_node.qargs, final_node.cargs, check=False
)
for final_op in final_ops:
dag.remove_op_node(final_op)
dag.compose(barrier_layer)
if self.label is None:
# Merge the new barrier into any other barriers
adjacent_pass = MergeAdjacentBarriers()
return adjacent_pass.run(dag)
else:
return dag