Custom gate fails to run on Lightning backend about pennylane-lightning HOT 5 CLOSED

Hi @mlxd, thanks for the explanation for taking a look! :) The example came up during a discussion, rather than as something that the plugin should support. It was surprising to see the error, as the snippet seems to execute well with the latest master version (not with the released version though). There might be perhaps some new updates that allow this to work. All in all, I think this would not be something that lightning should definitely support at the moment.

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Can this be closed?

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Hey @ankit27kh and thanks for sharing!

Looking through the code you shared, the error may be linked to defining the custom MyRX gate. Please could you confirm whether the following successfully provides the gradient:

import pennylane as qml
from pennylane import numpy as np

dev = qml.device('lightning.qubit', wires=1)

@qml.qnode(dev)
def circuit(x):
    qml.RX(x, wires=0)
    return qml.expval(qml.PauliZ(0))

par = np.array(0.3)
qml.grad(circuit)(par)

Regarding the use of the custom MyRX gate, I'm curious what the use case is? It might be useful to check out the available operations in PennyLane. If you have an arbitrary unitary, you may want to consider using qml.QubitUnitary.

Hope this helps!

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Hi @trbromley, this code was created to show a particular example for when we specify diff_method='best' and use finite differences to estimate the gradient. MyRX was created as an example of an operation that doesn't support analytic gradients. See the original comment here.

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Hi @antalszava. Lightning's current design will not support the use of custom gates in this way, as we intended to allow use of the supported gate-set and anything else to require use of the explicit matrix for custom gates. This is due to the gate-set being built as const at compile time.

Is the above example something we should be supporting?

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