qml.transforms.remove_barrier

remove_barrier(tape)

Quantum transform to remove Barrier gates.

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

The transform can be applied on QNode directly.

@remove_barrier
@qml.qnode(device=dev)
def circuit(x, y):
    qml.Hadamard(wires=0)
    qml.Hadamard(wires=1)
    qml.Barrier(wires=[0,1])
    qml.PauliX(wires=0)
    return qml.expval(qml.PauliZ(0))

The barrier is then removed before execution.

Usage Details

Consider the following quantum function:

def qfunc(x, y):
    qml.Hadamard(wires=0)
    qml.Hadamard(wires=1)
    qml.Barrier(wires=[0,1])
    qml.PauliX(wires=0)
    return qml.expval(qml.PauliZ(0))

The circuit before optimization:

>>> dev = qml.device('default.qubit', wires=2)
>>> qnode = qml.QNode(qfunc, dev)
>>> print(qml.draw(qnode)(1, 2))
    0: ──H──╭||──X──┤ ⟨Z⟩
    1: ──H──╰||─────┤

We can remove the Barrier by running the remove_barrier transform:

>>> optimized_qfunc = remove_barrier(qfunc)
>>> optimized_qnode = qml.QNode(optimized_qfunc, dev)
>>> print(qml.draw(optimized_qnode)(1, 2))
   0: ──H──X──┤ ⟨Z⟩
   1: ──H─────┤

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