Nature: Newton's gravitational constant G is normally measured with torsion balances, but recent measurements have not improved the precision of its value. In 2007 an experiment showed that atom interferometry could be used for the measurement. Now Guglielmo Tino of the University of Florence in Italy and his colleagues have developed the technique to the level where its precision nearly matches conventional measurements. A pulsed laser creates a superposition of matter waves in a cloud of supercooled rubidium atoms. The two waves have different velocities, so they reach different heights and feel different gravitational pulls from a nearby array of tungsten bars. When the waves recombine, they form an interference pattern. The gravitational effects of Earth, the Sun, and the Moon are removed from the system, which allows for the calculation of G between the rubidium and tungsten. The team's result had an uncertainty of 0.015%, just slightly larger than conventional measurements. System enhancements could significantly improve the precision of the measurement.
