Einstein's response to the new quantum theory seems to have been mixed. On the one hand, he supported the new breakthroughs, but he was worried by the element of chance that seemed to have entered the orderly, predictable world of physics.
In 1924, before the competing versions of quantum mechanics had been reconciled, Einstein bemoaned the fact that there were now not one, but two theories of light, which seemed to have no logical connection with each other. In that same year, Einstein also was resistant to Bohr's solution to the paradox of how the electron can know when to emit radiation.
However, also in 1924, Einstein was instrumental in the successful publication of a paper by Satyendra Nath Bose (1894—1974), which had initially been rejected for publication. When Einstein received the paper from Bose, he immediately realized its importance and pushed to get it published. In his paper, Bose proposed that photons could exist in different states, and that the number of photons was not conserved. This observation led to the property of photons called “spin.”
While he was initially resistant to quantum theory, when Einstein read Schrödinger's wave mechanics formulation in 1926, Einstein wrote to him expressing how impressed he was with his work. He stated that it seemed to be the work of pure genius, and that Schrödinger's research provided a decisive advance in quantum theory.