Acceleration-induced Carrier of the Imprints of Gravitation

Does there exist a purely quantum mechanical carrier of the imprints of gravitation? The motivation for considering this question arises from the following historical scenario: Suppose the time is 1907 when Einstein had the “happiest thought of his life”, which launched him on the path toward his formulation of gravitation (general relativity). But suppose Einstein already knew relativistic quantum mechanics, and that, in fact, he accepted and appreciated it without any reservations before he started on his journey. How different would his theory of gravitation have been from what we have today? Put differently, how different would the course of history have been if Einstein had grafted relativistic quantum mechanics onto the roots of gravitation instead of its trunk or branches? Nontrivial relativistic quantum mechanics starts with the Klein-Gordon equation ∂ψ ∂t − ∂ψ ∂z + kψ = 0, (1)