Source code for pennylane.transforms.optimization.undo_swaps
# Copyright 2018-2021 Xanadu Quantum Technologies Inc.# Licensed under the Apache License, Version 2.0 (the "License");# you may not use this file except in compliance with the License.# You may obtain a copy of the License at# http://www.apache.org/licenses/LICENSE-2.0# Unless required by applicable law or agreed to in writing, software# distributed under the License is distributed on an "AS IS" BASIS,# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.# See the License for the specific language governing permissions and# limitations under the License."""Transform that eliminates the swap operators by reordering the wires."""# pylint: disable=too-many-branchesfrompennylane.tapeimportQuantumScript,QuantumScriptBatchfrompennylane.transformsimporttransformfrompennylane.typingimportPostprocessingFndefnull_postprocessing(results):"""A postprocesing function returned by a transform that only converts the batch of results into a result for a single ``QuantumTape``. """returnresults[0]
[docs]@transformdefundo_swaps(tape:QuantumScript)->tuple[QuantumScriptBatch,PostprocessingFn]:"""Quantum function transform to remove SWAP gates by running from right to left through the circuit changing the position of the qubits accordingly. Args: tape (QNode or QuantumTape or Callable): A quantum circuit. Returns: qnode (QNode) or quantum function (Callable) or tuple[List[QuantumTape], function]: The transformed circuit as described in :func:`qml.transform <pennylane.transform>`. **Example** >>> dev = qml.device('default.qubit', wires=3) You can apply the transform directly on a :class:`QNode` .. code-block:: python @undo_swaps @qml.qnode(device=dev) def circuit(): qml.Hadamard(wires=0) qml.X(1) qml.SWAP(wires=[0,1]) qml.SWAP(wires=[0,2]) qml.Y(0) return qml.expval(qml.Z(0)) The SWAP gates are removed before execution. .. details:: :title: Usage Details Consider the following quantum function: .. code-block:: python def qfunc(): qml.Hadamard(wires=0) qml.X(1) qml.SWAP(wires=[0,1]) qml.SWAP(wires=[0,2]) qml.Y(0) return qml.expval(qml.Z(0)) The circuit before optimization: >>> dev = qml.device('default.qubit', wires=3) >>> qnode = qml.QNode(qfunc, dev) >>> print(qml.draw(qnode)()) 0: ──H──╭SWAP──╭SWAP──Y──┤ ⟨Z⟩ 1: ──X──╰SWAP──│─────────┤ 2: ────────────╰SWAP─────┤ We can remove the SWAP gates by running the ``undo_swap`` transform: >>> optimized_qfunc = undo_swaps(qfunc) >>> optimized_qnode = qml.QNode(optimized_qfunc, dev) >>> print(qml.draw(optimized_qnode)()) 0: ──Y──┤ ⟨Z⟩ 1: ──H──┤ 2: ──X──┤ """wire_map={wire:wireforwireintape.wires}gates=[]forcurrent_gateinreversed(tape.operations):ifcurrent_gate.name=="SWAP":swap_wires_0,swap_wires_1=current_gate.wireswire_map[swap_wires_0],wire_map[swap_wires_1]=(wire_map[swap_wires_1],wire_map[swap_wires_0],)else:gates.append(current_gate.map_wires(wire_map))gates.reverse()new_tape=tape.copy(operations=gates)new_tape.trainable_params=tape.trainable_paramsreturn[new_tape],null_postprocessing