Equivalence Principle and Gravitational Redshift
  We investigate leading order deviations from general relativity that violate the Einstein equivalence principle in the gravitational standard model extension. We show that redshift experiments based on matter waves and clock comparisons are equivalent to one another. Consideration of torsion balance tests, along with matter wave, microwave, optical, and M\"ossbauer clock tests, yields comprehensive limits on spin-independent Einstein equivalence principle-violating standard model extension terms at the $10^{-6}$ level. 
Michael A. Hohensee and Steven Chu and Achim Peters and Holger Mueller
gr-qc physics.atom-ph quant-ph