Atomic interferometers use lasers to place atoms into superpositions of different quantum states. These states acquire different phases as they move in a gravitational field, and this phase difference can be measured by using other lasers to return the atoms to the initial quantum state. The longer the atoms spend in the superposition state, the larger the phase difference and the more accurate the measurement of g. By vertically launching caesium atoms cooled to 1.5 microkelvin in a fountain geometry, Chu and colleagues were able to maximize the length of time spent in the superposition state.

Their results prove that the gravitational force on quantum objects, such as atoms, is the same as that which acts on larger objects. Previous interferometer experiments with neutrons had found that the gravitational force experienced by the neutrons differed by several per cent from that experienced by larger objects. The Stanford team believe they can improve the accuracy of the measurements by one to two orders of magnitude.