Cavity gravimetry

You don’t weigh the same in Boston as you do in Berlin. Granted, the difference—a consequence of Earth’s oblateness, rotation, and unevenly distributed mass—is minuscule, but it’s easily detected with state-of-the-art gravimeters. In fact, so-called atom interferometers can detect Earth’s gravitational pull to better than 1 part in 10¹¹, sensitive enough to register the change in gravity due to a change in elevation of less than a meter. The interferometers exploit quantum superposition: A free-falling atom is placed into a superposition of two states, and the wavepackets receive momentum kicks—delivered with pulses of light—that steer them along separate paths through space. When the wavepackets are finally recombined, their interference reveals information about the respective distances they traveled and the gravitational forces they experienced. Now Paul Hamilton, Holger Müller , and their colleagues at the University of California, Berkeley, have improved on that design by placing the interferometer inside a half-meter-long optical cavity. The cavity can amplify certain frequencies of light by a hundredfold or more, meaning the researchers can give their atoms bigger momentum kicks using less laser power. It also narrows the pulses’ linewidths, which makes each momentum kick more precise. In a proof-of-principle experiment, the team showed that the interferometer, pictured here, could achieve precision comparable to that of conventional atom interferometers while using orders of magnitude less power. That could pave the way to compact, energy-efficient instruments that can ride aboard satellites and space probes, where they could be used to test predictions of general relativity, among other purposes. (P. Hamilton et al., Phys. Rev. Lett., in press. )