Complex sentence: It is believed but has not been proven that the power of quantum computers is incomparable to that of NTMs, that is, problems likely exist that an NTM could efficiently solve that a quantum computer cannot, and vice versa.
Split into simpler sentences: It is believed but has not been proven that the power of quantum computers is incomparable to that of NTMs. That is, problems likely exist that an NTM could efficiently solve that a quantum computer cannot, and vice versa.