qml.transforms.undo_swaps

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.

Parameters

tape (QNode or QuantumTape or Callable) – A quantum circuit.

Returns

The transformed circuit as described in qml.transform.

Return type

qnode (QNode) or quantum function (Callable) or tuple[List[QuantumTape], function]

Example

>>> dev = qml.device('default.qubit', wires=3)

You can apply the transform directly on a QNode

@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.

Usage Details

Consider the following quantum function:

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──┤

code/api/pennylane.transforms.undo_swaps

 

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