On the quantum equivalence principle

According to Galilei 'All bodies fall with the same speed'. How can the idea, upon which Einstein's explanation of this fact is founded, being transferred into quantum field theory. A formulation of a quantum equivalence principle is suggested here. It is applicable to linear quantum field theories but cannot be applied directly to interacting quantum fields. Equilibrium states of free quantum fields in the Rindler wedge are analyzed and it is shown that only the state with the Hawking-Bisognano-Wichmann temperature is admitted by the quantum equivalence principle. (orig.)