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 (QNode or quantum function).

Returns:

The transformed circuit as described in qp.transform.

Return type:

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

Example

You can apply the transform directly on a QNode.

import pennylane as qp

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

@qp.transforms.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))

>>> print(qp.draw(circuit)())
0: ──Y─┤  <Z>
1: ──H─┤
2: ──X─┤

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))

>>> 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, where the wires involved in the SWAP gates are interchanged:

>>> optimized_qnode = undo_swaps(qnode)
>>> print(qp.draw(optimized_qnode)())
0: ──Y─┤ <Z>
1: ──H─┤
2: ──X─┤

Gates are iterated through from right to left, where non-SWAP gates are ignored. The first gate is a Y gate, which is left to act on wire 0. Next, the right-most SWAP gate acting on wires (0, 2) is removed, and the wires are manually swapped; wire 2 now becomes wire 0, and vice versa. Next, the SWAP gate acting on wires (0, 1) is removed and the wires are interchanged.

Altogether, this affects the wire labels as follows, where the operations to the left of both SWAP gates have their wire labels changed accordingly.

• wire 0 changes to wire 2 which changes to wire 1. This moves the H gate from wire 0 to wire 1.

• wire 2 changes to wire 0.

• wire 1 changes to wire 2. This moves the X gate from wire 1 to wire 2.

code/api/pennylane.transforms.undo_swaps

 

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