qml.transform¶

transform
(quantum_transform, expand_transform=None, classical_cotransform=None, is_informative=None, final_transform=False)[source]¶ Generalizes a function that transforms tapes to work with additional circuitlike objects such as a
QNode
.transform
should be applied to a function that transforms tapes. Once validated, the result will be an object that is able to transform PennyLane’s range of circuitlike objects:QuantumTape
, quantum function andQNode
. A circuitlike object can be transformed either via decoration or by passing it functionally through the created transform. Parameters
quantum_transform (Callable) –
The input quantum transform must be a function that satisfies the following requirements:
Accepts a
QuantumTape
as its first input and returns a sequence ofQuantumTape
and a processing function.The transform must have type hinting of the following form:
my_quantum_transform(tape: qml.tape.QuantumTape, ...) > ( Sequence[qml.tape.QuantumTape], Callable)
expand_transform (Callable) – An optional expand transform is applied directly before the input quantum transform. It must be a function that satisfies the same requirements as
quantum_transform
.classical_cotransform (Callable) – A classical cotransform is a function to postprocess the classical jacobian and the quantum jacobian and has the signature:
my_cotransform(qjac, cjac, tape) > tensor_like
is_informative (bool) – Whether or not a transform is informative. If true the transform is queued at the end of the transform program and the tapes or qnode aren’t executed.
final_transform (bool) – Whether or not the transform is terminal. If true the transform is queued at the end of the transform program.
is_informative
supersedesfinal_transform
.
 Returns
Returns a transform dispatcher object that that can transform any circuitlike object in PennyLane.
 Return type
Example
First define an input quantum transform with the necessary type hinting defined above. In this example we copy the tape and sum the results of the execution of the two tapes.
from typing import Sequence, Callable def my_quantum_transform(tape: qml.tape.QuantumTape) > (Sequence[qml.tape.QuantumTape], Callable): tape1 = tape tape2 = tape.copy() def post_processing_fn(results): return qml.math.sum(results) return [tape1, tape2], post_processing_fn
We want to be able to apply this transform on both a
qfunc
and apennylane.QNode
and will usetransform
to achieve this.transform
validates the signature of your input quantum transform and makes it capable of transformingqfunc
andpennylane.QNode
in addition to quantum tapes. Let’s define a circuit as apennylane.QNode
:dev = qml.device("default.qubit") @qml.qnode(device=dev) def qnode_circuit(a): qml.Hadamard(wires=0) qml.CNOT(wires=[0, 1]) qml.PauliX(wires=0) qml.RZ(a, wires=1) return qml.expval(qml.PauliZ(wires=0))
We first apply
transform
tomy_quantum_transform
:>>> dispatched_transform = transform(my_quantum_transform)
Now you can use the dispatched transform directly on a
pennylane.QNode
.For
pennylane.QNode
, the dispatched transform populates theTransformProgram
of your QNode. The transform and its processing function are applied in the execution.>>> transformed_qnode = dispatched_transform(qnode_circuit) <QNode: wires=2, device='default.qubit', interface='auto', diff_method='best'>
>>> transformed_qnode.transform_program TransformProgram(my_quantum_transform)
If we apply
dispatched_transform
a second time to thepennylane.QNode
, we would add it to the transform program again and therefore the transform would be applied twice before execution.>>> transformed_qnode = dispatched_transform(transformed_qnode) >>> transformed_qnode.transform_program TransformProgram(my_quantum_transform, my_quantum_transform)
When a transformed QNode is executed, the QNode’s transform program is applied to the generated tape and creates a sequence of tapes to be executed. The execution results are then postprocessed in the reverse order of the transform program to obtain the final results.
Signature of a transform
A dispatched transform is able to handle several PennyLane circuitlike objects:
a quantum function (callable)
For each object, the transform will be applied in a different way, but it always preserves the underlying tapebased quantum transform behaviour.
The return of a dispatched transform depends upon which of the above objects is passed as an input:
For a
QNode
input, the underlying transform is added to the QNode’sTransformProgram
and the return is the transformedQNode
. For each execution of thepennylane.QNode
, it first applies the transform program on the original captured circuit. Then the transformed circuits are executed by a device and finally the postprocessing function is applied on the results.For a quantum function (callable) input, the transform builds the tape when the quantum function is executed and then applies itself to the tape. The resulting tape is then converted back to a quantum function (callable). It therefore returns a transformed quantum function (Callable). The limitation is that the underlying transform can only return a sequence containing a single tape, because quantum functions only support a single circuit.
For a
QuantumTape
, the underlying quantum transform is directly applied on theQuantumTape
. It returns a sequence ofQuantumTape
and a processing function to be applied after execution.For a
Device
, the transform is added to the device’s transform program and a transformedpennylane.devices.Device
is returned. The transform is added to the end of the device program and will be last in the overall transform program.