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

# This code is part of Qiskit.
#
# (C) Copyright IBM 2017, 2019.
#
# 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.

# pylint: disable=cell-var-from-loop

"""Replace each block of consecutive gates by a single Unitary node."""


from qiskit.circuit import QuantumRegister, ClassicalRegister, QuantumCircuit
from qiskit.quantum_info.operators import Operator
from qiskit.quantum_info.synthesis import TwoQubitBasisDecomposer
from qiskit.extensions import UnitaryGate
from qiskit.circuit.library.standard_gates import CXGate
from qiskit.transpiler.basepasses import TransformationPass
from qiskit.transpiler.exceptions import TranspilerError
from qiskit.transpiler.passes.synthesis import unitary_synthesis


[docs]class ConsolidateBlocks(TransformationPass): """Replace each block of consecutive gates by a single Unitary node. Pass to consolidate sequences of uninterrupted gates acting on the same qubits into a Unitary node, to be resynthesized later, to a potentially more optimal subcircuit. Notes: This pass assumes that the 'blocks_list' property that it reads is given such that blocks are in topological order. The blocks are collected by a previous pass, such as `Collect2qBlocks`. """
[docs] def __init__(self, kak_basis_gate=None, force_consolidate=False, basis_gates=None): """ConsolidateBlocks initializer. Args: kak_basis_gate (Gate): Basis gate for KAK decomposition. force_consolidate (bool): Force block consolidation basis_gates (List(str)): Basis gates from which to choose a KAK gate. """ super().__init__() self.basis_gates = basis_gates self.force_consolidate = force_consolidate if kak_basis_gate is not None: self.decomposer = TwoQubitBasisDecomposer(kak_basis_gate) elif basis_gates is not None: kak_basis_gate = unitary_synthesis._choose_kak_gate(basis_gates) if kak_basis_gate is not None: self.decomposer = TwoQubitBasisDecomposer(kak_basis_gate) else: self.decomposer = None else: self.decomposer = TwoQubitBasisDecomposer(CXGate())
[docs] def run(self, dag): """Run the ConsolidateBlocks pass on `dag`. Iterate over each block and replace it with an equivalent Unitary on the same wires. """ if self.decomposer is None: return dag new_dag = dag._copy_circuit_metadata() # compute ordered indices for the global circuit wires global_index_map = {wire: idx for idx, wire in enumerate(dag.qubits)} blocks = self.property_set['block_list'] # just to make checking if a node is in any block easier all_block_nodes = {nd for bl in blocks for nd in bl} for node in dag.topological_op_nodes(): if node not in all_block_nodes: # need to add this node to find out where in the list it goes preds = [nd for nd in dag.predecessors(node) if nd.type == 'op'] block_count = 0 while preds: if block_count < len(blocks): block = blocks[block_count] # if any of the predecessors are in the block, remove them preds = [p for p in preds if p not in block] else: # should never occur as this would mean not all # nodes before this one topologically had been added # so not all predecessors were removed raise TranspilerError("Not all predecessors removed due to error" " in topological order") block_count += 1 # we have now seen all predecessors # so update the blocks list to include this block blocks = blocks[:block_count] + [[node]] + blocks[block_count:] # create the dag from the updated list of blocks basis_gate_name = self.decomposer.gate.name for block in blocks: if len(block) == 1 and (block[0].name != basis_gate_name or block[0].op.is_parameterized()): # an intermediate node that was added into the overall list new_dag.apply_operation_back(block[0].op, block[0].qargs, block[0].cargs) else: # find the qubits involved in this block block_qargs = set() block_cargs = set() for nd in block: block_qargs |= set(nd.qargs) if nd.condition: block_cargs |= set(nd.condition[0]) # convert block to a sub-circuit, then simulate unitary and add q = QuantumRegister(len(block_qargs)) # if condition in node, add clbits to circuit if len(block_cargs) > 0: c = ClassicalRegister(len(block_cargs)) subcirc = QuantumCircuit(q, c) else: subcirc = QuantumCircuit(q) block_index_map = self._block_qargs_to_indices(block_qargs, global_index_map) basis_count = 0 for nd in block: if nd.op.name == basis_gate_name: basis_count += 1 subcirc.append(nd.op, [q[block_index_map[i]] for i in nd.qargs]) unitary = UnitaryGate(Operator(subcirc)) # simulates the circuit max_2q_depth = 20 # If depth > 20, there will be 1q gates to consolidate. if ( # pylint: disable=too-many-boolean-expressions self.force_consolidate or unitary.num_qubits > 2 or self.decomposer.num_basis_gates(unitary) < basis_count or len(subcirc) > max_2q_depth or (self.basis_gates is not None and not set(subcirc.count_ops()).issubset(self.basis_gates)) ): new_dag.apply_operation_back( UnitaryGate(unitary), sorted(block_qargs, key=lambda x: block_index_map[x])) else: for nd in block: new_dag.apply_operation_back(nd.op, nd.qargs, nd.cargs) return new_dag
def _block_qargs_to_indices(self, block_qargs, global_index_map): """Map each qubit in block_qargs to its wire position among the block's wires. Args: block_qargs (list): list of qubits that a block acts on global_index_map (dict): mapping from each qubit in the circuit to its wire position within that circuit Returns: dict: mapping from qarg to position in block """ block_indices = [global_index_map[q] for q in block_qargs] ordered_block_indices = sorted(block_indices) block_positions = {q: ordered_block_indices.index(global_index_map[q]) for q in block_qargs} return block_positions

© Copyright 2020, Qiskit Development Team. Last updated on 2021/03/04.

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