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Source code for qiskit.transpiler.passes.optimization.inverse_cancellation

# This code is part of Qiskit.
#
# (C) Copyright IBM 2021.
#
# 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.

"""
A generic InverseCancellation pass for any set of gate-inverse pairs.
"""
from typing import List, Tuple, Union

from qiskit.circuit import Gate
from qiskit.dagcircuit import DAGCircuit
from qiskit.transpiler.basepasses import TransformationPass
from qiskit.transpiler.exceptions import TranspilerError


[docs]class InverseCancellation(TransformationPass): """Cancel specific Gates which are inverses of each other when they occur back-to- back.""" def __init__(self, gates_to_cancel: List[Union[Gate, Tuple[Gate, Gate]]]): """Initialize InverseCancellation pass. Args: gates_to_cancel: list of gates to cancel Raises: TranspilerError: Initalization raises an error when the input is not a self-inverse gate or a two-tuple of inverse gates. """ for gates in gates_to_cancel: if isinstance(gates, Gate): if gates != gates.inverse(): raise TranspilerError(f"Gate {gates.name} is not self-inverse") elif isinstance(gates, tuple): if len(gates) != 2: raise TranspilerError( f"Too many or too few inputs: {gates}. Only two are allowed." ) if gates[0] != gates[1].inverse(): raise TranspilerError( f"Gate {gates[0].name} and {gates[1].name} are not inverse." ) else: raise TranspilerError( "InverseCancellation pass does not take input type {}. Input must be" " a Gate.".format(type(gates)) ) self.self_inverse_gates = [] self.inverse_gate_pairs = [] for gates in gates_to_cancel: if isinstance(gates, Gate): self.self_inverse_gates.append(gates) else: self.inverse_gate_pairs.append(gates) super().__init__()
[docs] def run(self, dag: DAGCircuit): """Run the InverseCancellation pass on `dag`. Args: dag: the directed acyclic graph to run on. Returns: DAGCircuit: Transformed DAG. """ dag = self._run_on_self_inverse(dag, self.self_inverse_gates) return self._run_on_inverse_pairs(dag, self.inverse_gate_pairs)
def _run_on_self_inverse(self, dag: DAGCircuit, self_inverse_gates: List[Gate]): """ Run self-inverse gates on `dag`. Args: dag: the directed acyclic graph to run on. self_inverse_gates: list of gates who cancel themeselves in pairs Returns: DAGCircuit: Transformed DAG. """ # Sets of gate runs by name, for instance: [{(H 0, H 0), (H 1, H 1)}, {(X 0, X 0}] gate_runs_sets = [dag.collect_runs([gate.name]) for gate in self_inverse_gates] for gate_runs in gate_runs_sets: for gate_cancel_run in gate_runs: partitions = [] chunk = [] for i in range(len(gate_cancel_run) - 1): chunk.append(gate_cancel_run[i]) if gate_cancel_run[i].qargs != gate_cancel_run[i + 1].qargs: partitions.append(chunk) chunk = [] chunk.append(gate_cancel_run[-1]) partitions.append(chunk) # Remove an even number of gates from each chunk for chunk in partitions: if len(chunk) % 2 == 0: dag.remove_op_node(chunk[0]) for node in chunk[1:]: dag.remove_op_node(node) return dag def _run_on_inverse_pairs(self, dag: DAGCircuit, inverse_gate_pairs: List[Tuple[Gate, Gate]]): """ Run inverse gate pairs on `dag`. Args: dag: the directed acyclic graph to run on. inverse_gate_pairs: list of gates with inverse angles that cancel each other. Returns: DAGCircuit: Transformed DAG. """ for pair in inverse_gate_pairs: gate_cancel_runs = dag.collect_runs([pair[0].name, pair[1].name]) for dag_nodes in gate_cancel_runs: i = 0 while i < len(dag_nodes) - 1: if ( dag_nodes[i].qargs == dag_nodes[i + 1].qargs and dag_nodes[i].op == pair[0] and dag_nodes[i + 1].op == pair[1] ): dag.remove_op_node(dag_nodes[i]) dag.remove_op_node(dag_nodes[i + 1]) i = i + 2 elif ( dag_nodes[i].qargs == dag_nodes[i + 1].qargs and dag_nodes[i].op == pair[1] and dag_nodes[i + 1].op == pair[0] ): dag.remove_op_node(dag_nodes[i]) dag.remove_op_node(dag_nodes[i + 1]) i = i + 2 else: i = i + 1 return dag