Pennylane functionality with arbitrary operators

[joeybarreto]

Feature details

In the context of VQE, I am wondering whether in Pennylane it is a hard requirement that all unitaries must be defined over qubits, or if it's possible to optimize a cost function <U* H U> where the dimensions of U and H aren't necessarily a power of 2. Basically, I don't want to define an underlying circuit - is this a no-go? Or is it possible that the cost can still be numerically optimized by using/hacking Pennylane methods as-is?

Implementation

I don't need a proper implementation per-se, just some way of calling (potentially) internal functions to extract the necessary functionality.

How important would you say this feature is?

1: Not important. Would be nice to have.

Additional information

No response

[trbromley]

Hi @joeybarreto, this is a good question. Right now, PennyLane is quite dimension-dependent - we have support for both qubits and qutrits, but it would be great to generalize to arbitrary $d$.

Though we're always up for prioritizing features if they are of use to users. It would be great to get a few more details on what you're trying to do, perhaps we could have a brief call to get on the same page?