Source code for pennylane.transforms.optimization.undo_swaps

# Copyright 2018-2021 Xanadu Quantum Technologies Inc.

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"""Transform that eliminates the swap operators by reordering the wires."""
# pylint: disable=too-many-branches
from pennylane import apply
from pennylane.transforms import qfunc_transform
from pennylane.wires import Wires
from pennylane.queuing import QueuingManager


[docs]@qfunc_transform def undo_swaps(tape): """Quantum function transform to remove SWAP gates by running from right to left through the circuit changing the position of the qubits accordingly. Args: qfunc (function): A quantum function. Returns: function: the transformed quantum function **Example** Consider the following quantum function: .. code-block:: python def qfunc(): qml.Hadamard(wires=0) qml.PauliX(wires=1) qml.SWAP(wires=[0,1]) qml.SWAP(wires=[0,2]) qml.PauliY(wires=0) return qml.expval(qml.PauliZ(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)(1, 2)) 0: ──Y──┤ ⟨Z⟩ 1: ──H──┤ 2: ──X──┤ """ # Make a working copy of the list to traverse list_copy = tape.operations.copy() list_copy.reverse() map_wires = {wire: wire for wire in tape.wires} gates = [] def _change_wires(wires): change_wires = Wires([]) wires = wires.toarray() for wire in wires: change_wires += map_wires[wire] return change_wires with QueuingManager.stop_recording(): while len(list_copy) > 0: current_gate = list_copy[0] params = current_gate.parameters if current_gate.name != "SWAP": if len(params) == 0: gates.append(type(current_gate)(wires=_change_wires(current_gate.wires))) else: gates.append( type(current_gate)(*params, wires=_change_wires(current_gate.wires)) ) else: swap_wires_0, swap_wires_1 = current_gate.wires map_wires[swap_wires_0], map_wires[swap_wires_1] = ( map_wires[swap_wires_1], map_wires[swap_wires_0], ) list_copy.pop(0) gates.reverse() for m in tape.measurements: gates.append(m) for gate in gates: apply(gate)