qp.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 qp.transform.

Return type:

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

Example

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

You can apply the transform directly on a QNode

@undo_swaps
@qp.qnode(device=dev)
def circuit():
    qp.Hadamard(wires=0)
    qp.X(1)
    qp.SWAP(wires=[0,1])
    qp.SWAP(wires=[0,2])
    qp.Y(0)
    return qp.expval(qp.Z(0))

The SWAP gates are removed before execution.

Usage Details

Consider the following quantum function:

def qfunc():
    qp.Hadamard(wires=0)
    qp.X(1)
    qp.SWAP(wires=[0,1])
    qp.SWAP(wires=[0,2])
    qp.Y(0)
    return qp.expval(qp.Z(0))

The circuit before optimization:

>>> dev = qp.device('default.qubit', wires=3)
>>> qnode = qp.QNode(qfunc, dev)
>>> print(qp.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 = qp.QNode(optimized_qfunc, dev)
>>> print(qp.draw(optimized_qnode)())
0: ──Y─┤  <Z>
1: ──H─┤
2: ──X─┤

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