Hello. I cannot find the error in my implementation of the controlled sub-circuit in H6? Any help would be appreciated.
Hi @qreg, your solution is very close 
qml.ctrl is a transform that takes an operation or quantum function as input, and returns a function as output in which every gate is controlled by the specified wire:
controlled_function = qml.ctrl(original_function, control=wire)
But this is just a function, it must then be called in order to actually implement the gates.
Please let us know if there’s anything else we can clarify!
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Thank you. I have modified the code but still cannot make it work. The attached code results in "Incorrect: your circuit doesn’t give the right amplitudes.
".
Hi @qreg, I think that might be from the second line in subcircuit. If you have a numpy array x and take x ** 2, it will compute the element-wise square. For example,
>>> a = np.array([[1, 2], [3, 4]])
>>> a ** 2
array([[1, 4], [9, 16]])
So you’ll need to make some adjustments in order to get the square of the matrix itself.
(BTW you might also have to remove the pass statement)
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Yes, thank you so much, That did it.