Physicists Dispute Table-Top Relativity Test: Scientific American: The debate comes down to whether a fundamental atomic oscillation, based on the rest mass of a cesium atom, can be used as a clock. The table-top setup relied on an atom interferometer, which tracked the offset in oscillations, or phase difference, of the cesium atoms as they flew on paths of marginally different heights. But Blanchet's team argue that the phase difference between any two atoms due to the fundamental oscillation will always be zero, and therefore could never be used to detect a gravitational redshift.
They say that the Berkeley researchers were instead using their interferometer as an accelerometer to measure a different aspect of general relativity: the universality of free fall. That is no less interesting in its own right, but it has already been tested to greater levels of precision.