Physicists have used an atomic interferometer to measure the acceleration of individual atoms under the force of gravity to an accuracy of three parts in a billion. This is a million times more precise than previous measurements of g with atomic interferometers, and is nearly as good as the accuracy of two parts in a billion that is possible with the best gravimeters. Steven Chu, Keng Yeow Chung and Achim Peters from Stanford University in the US used an atomic 'fountain' of laser-cooled caesium atoms to make the measurement (Nature 400 849).
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.