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  3. qml.queuing.apply

qml.queuing.apply

apply(op, context=<class 'pennylane.queuing.QueuingManager'>)[source]

Apply an instantiated operator or measurement to a queuing context.

Parameters:

  • op (.Operator or .MeasurementProcess) – the operator or measurement to apply/queue

  • context (type[.QueuingManager] | AnnotatedQueue) – The queuing context to queue the operator to. Note that if no context is specified, the operator is applied to the currently active queuing context.

Returns:

the input operator is returned for convenience

Return type:

.Operator or .MeasurementProcess

Example

In PennyLane, operations and measurements are ‘queued’ or added to a circuit when they are instantiated.

The apply function can be used to add operations that might have already been instantiated elsewhere to the QNode:

op = qml.RX(0.4, wires=0)
dev = qml.device("default.qubit", wires=2)

@qml.qnode(dev)
def circuit(x):
    qml.RY(x, wires=0)  # applied during instantiation
    qml.apply(op)  # manually applied
    return qml.expval(qml.Z(0))

>>> print(qml.draw(circuit)(0.6))
0: ──RY(0.60)──RX(0.40)─┤  <Z>

It can also be used to apply functions repeatedly:

@qml.qnode(dev)
def circuit(x):
    qml.apply(op)
    qml.RY(x, wires=0)
    qml.apply(op)
    return qml.expval(qml.Z(0))

>>> print(qml.draw(circuit)(0.6))
0: ──RX(0.40)──RY(0.60)──RX(0.40)─┤  <Z>

Warning

If you use apply on an operator that has already been queued, it will be queued for a second time. For example:

@qml.qnode(dev)
def circuit():
    op = qml.Hadamard(0)
    qml.apply(op)
    return qml.expval(qml.Z(0))

>>> print(qml.draw(circuit)())
0: ──H──H─┤  <Z>

Instantiated measurements can also be applied to queuing contexts using apply:

meas = qml.expval(qml.Z(0) @ qml.Y(1))
dev = qml.device("default.qubit", wires=2)

@qml.qnode(dev)
def circuit(x):
    qml.RY(x, wires=0)
    qml.CNOT(wires=[0, 1])
    return qml.apply(meas)

>>> print(qml.draw(circuit)(0.6))
0: ──RY(0.60)─╭●─┤ ╭<Z@Y>
1: ───────────╰X─┤ ╰<Z@Y>

By default, apply will queue operators to the currently active queuing context.

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