...what precisely do you mean by quantum mechanics or matter engineering.
I'll just take a stab here. Quantum computing means
P=NP. The basic result of that is all of the things we can't compute with classical computers will be computable with quantum computers. Imagine
psychohistory at a galactic civ scale, artificial minds (not to be confused with AI), full simulation of anything at a quantum scale, fully holistic economical/societal forcasting, etc. All the cool things you wish a computer could do, and so much more.
Matter engineering would be like nanoengineering, but at the angstrom scale. Arbitrarily precise control of molecules, atoms, and nuclear components (protons & neutrons).
Neutronium, self-assembling anythings,
utility fog, classically impossible materials, portable black holes, etc. All the cool things you wish a factory could do, and so much more.
The future's gonna be
so cool!