set_decomposition(custom_decomps, dev, decomp_depth=10)[source]

Context manager for setting custom decompositions.

  • custom_decomps (Dict[Union(str, qml.operation.Operation), Callable]) – Custom decompositions to be applied by the device at runtime.

  • dev (pennylane.Device) – A quantum device.

  • decomp_depth – The maximum depth of the expansion.


Suppose we would like a custom expansion function that decomposes all CNOTs into CZs. We first define a decomposition function:

def custom_cnot(wires):
    return [
        qml.CZ(wires=[wires[0], wires[1]]),

This context manager can be used to temporarily change a devices expansion function to one that takes into account the custom decompositions.

dev = qml.device("default.qubit", wires=2)

@qml.qnode(dev, expansion_strategy="device")
def circuit():
    qml.CNOT(wires=[0, 1])
    return qml.expval(qml.Z(0))
>>> print(qml.draw(circuit)())
0: ─╭●─┤  <Z>
1: ─╰X─┤

Now let’s set up a context where the custom decomposition will be applied:

>>> with qml.transforms.set_decomposition({qml.CNOT : custom_cnot}, dev):
...     print(qml.draw(circuit, wire_order=[0, 1])())
0: ────╭●────┤  <Z>
1: ──H─╰Z──H─┤