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FUNDAMENTAL CONSTANTS
Published results of the gravitational constant, a measure of the strength of gravity, have failed to converge. An approach that uses cold atoms provides a new data point in the quest to determine this fundamental constant. See Letter p.518
In our daily lives, we can see the effect of the gravitational force between Earth and an object, say an apple. However, the gravitational attraction between two apples cannot be observed without using a sensitive apparatus such as a torsion balance - it is just too small. On page 518 of this issue, Rosi et al.1 describe an amazing measurement: the gravitational force between a rubidium atom and a 516-kilogram mass, with a rela- tive uncertainty of just 0.015%. Their experi- ment was aimed at a precise determination of the gravitational constant, which describes the strength of the gravitational pull that bodies exert on each other, and was based on the technique of atom interferometry - a method that takes advantage of the wave nature of cold atoms to precisely measure gravitational acceleration.
In the atom interferometer described by Rosi and colleagues, a cloud of rubidium atoms at a temperature close to absolute zero is repeat- edly tossed up vertically. To understand how this cloud in free fall probes gravity, quantum mechanics is needed. For simplicity, consider that the atoms in the cloud can be in two differ- ent atomic states, A and B. At the beginning, all atoms are in state A. By exposing an atom to an appropriately shaped light pulse, the atom can transition from A to B with a certain probabil- ity, let's say 50%. While the atom is not being observed, it is simultaneously in both states (50% in A and 50% in B), a concept known as superposition. In addition to inducing the transition from A to B, the light pulse transfers vertical momentum such that the B state has a larger vertical velocity than the A state.
The relative fraction of the two different states in this superposition varies with time, and its rate of change depends on the differ- ence of the products of the momentum and the travelled vertical distance for each state. Owing to its larger momentum, state B travels higher than state...