Question

what is quantum theory ​

Answer 1

Quantum mechanics, including quantum field theory, is a fundamental theory in physics which describes nature at the smallest scale of energy levels of atom and subatomic particles. Classical physics, the physics existing before quantum mechanics, described nature at ordinary scale.

Answer 2

The non - deleting theorem of quantum information theory is a no-go theorem which states that in general , given two copies of some arbitrary quantum state it is impossible to delete on of the copies. it is a a time - reversed dual to the no-cloning theorem, which states that arbitrary states cannot be copied.