Equivalence principle, quantum mechanics, and atom-interferometric tests

That gravitation can be understood as purely metric phenomenon depends crucially on the validity of a number of hypotheses which are summarised by the Einstein Equivalence Principle, the least well tested part of which being the Universality of Gravitational Redshift. A recent and currently widely debated proposal (Nature 463 (2010) 926-929) to re-interpret some 10-year old experiments in atom interferometry would imply, if tenable, substantial reductions on upper bounds for possible violations of the Universality of Gravitational Redshift by four orders of magnitude. This interpretation, however, is problematic and raises various compatibility issues concerning basic principles of General Relativity and Quantum Mechanics. I review some relevant aspects of the equivalence principle and its import into quantum mechanics, and then turn to the problems raised by the mentioned proposal. I conclude that this proposal is too problematic to warrant the claims that were launched with it.