Quantum theory is the branch of physics that deals with the study of natural objects at an atomic or sub-atomic scale.
In 1900, Max Plank while studying black body radiation stated that energy is quantized (discrete and not continuous).
In 1905, Albert Einstein explained the photoelectric effect, where he stated that light behaves as quantum particles later known as photons and each of these particles has discrete values of energies.
In 1913, Neils Bohr explained that the electrons of a hydrogen atom can revolve around the nucleus at certain defined shells. This meant that the bound electrons could only have discrete values of energy.
Hence, Planck, Einstein and Bohr were all pioneers of quantum theory and can be credited to being the inventors.