Theorem demonstrates that CNNs can always be trained on quantum computers
discover.lanl.gov
discover.lanl.gov
However, constraints on the general class of polynomials of 5th order and higher can result in subclasses that are solvable in closed form.
So there may be a long tail of closed form solvable classes of NN architectures waiting to be discovered.
Whether non-trivial closed-form subclasses of neural networks exist, and whether any of those classes is actually useful ... seems an area worth exploring.
If you look at how Shor's algorithm works, it's more like manipulating some kind of embeded values (that we have no direct access to) eg via Hadamard gates and eventually collapsing the entanglement into the solution.
So maybe we can have something closed-form-ish that solves NN by arranging some q gates some way
If I had a quantum computer, I could do all these wonderful things... it's interesting theoretical research but we do not have the machines yet.
(Disclaimer: I didn't Google for known results w.r.t. the expressibility of QCNNs)