Define decomposition for qml.ControlledQubitUnitary

[antalszava]

Feature details

Logic to decompose qml.ControlledQubitUnitary would be great to have. One potential solution could be to regard qml.ControlledQubitUnitary as a qml.QubitUnitary and use the decomposition of QubitUnitary (note: that this is only defined for the two-qubit case).

Also see #2320.

Implementation

No response

How important would you say this feature is?

1: Not important. Would be nice to have.

Additional information

No response

[MoritzWillmann]

Hey @antalszava, I just started looking into this and would like to work on this, if that's okay. I quite like the Idea of using QubitUnitary.compute_decomposition. Do you think the ability to only use it for two-qubit gates would be a big limitation to functionality?

[antalszava]

Hi @MoritzWillmann, apologies for the delayed response here! For sure, feel free to go for it. 🙂

Using QubitUnitary.compute_decomposition could be a good short-term solution. Overall having a more general approach would be desirable.

[MoritzWillmann]

Hi @antalszava, sorry for the super delayed response 😞
Just to clarify, do you mean having a more general approach just for the ControlledQubitUnitary or use the QubitUnitary.compute_decomposition and have a more general approach for that?

[antalszava]

Hi @MoritzWillmann, a bit of both 🙂 Specifically here, extending ControlledQubitUnitary would be desired, though if I recall correctly QubitUnitary too has limitations on decomposition. This is mainly due to the fact that new techniques would be required (if found by researchers) on how arbitrary unitaries can be decomposed using elementary gates.