Catching atoms in a laser beam! new way to measure gravity?

By looking at how atoms behave when suspended in the air, rather than in free fall, physicists have devised a new way to measure Earth’s gravity. Traditionally, scientists have measured the influence of gravity on atoms by tracking the speed with which atoms fall down high ramps, such experiments can help test Einstein’s theory of gravity and accurately measure fundamental constants; but the meter-long tubes used in free-fall experiments can be unwieldy and difficult to shield from environmental interference, such as stray magnetic fields. With a new table setup, physicists can measure the pull of Earth’s gravity by monitoring atoms suspended a couple of millimeters in the air by laser light.

This redesign could better investigate the gravitational forces exerted by small objects. The technique could also be used to measure slight gravitational variations in different places around the world, which can help map the seafloor or find oil and minerals underground.

Physicists at the University of California began by blasting a cloud of cesium atoms into the air and using flashes of light to split each atom into a superposition state. In this strange quantum limbo, each atom exists in two places at once: one version of the atom floats a few micrometers higher than the other; the team of researchers then caught these split cesium atoms in the air with light from a laser.

Measuring the force of gravity with atoms held in place, rather than being pulled down by a gravitational field, requires taking advantage of the wave-particle duality of atoms. That quantum effect means that, just as light waves can act as particles called photons, atoms can act as waves, and for each cesium atom trapped in superposition, the upper version of the atomic wave ripples a little faster than its lower counterpart, due to the slightly different positions of the atoms in the Earth’s gravitational field. By tracking how fast the ripple of the two versions of an atom goes out of sync, physicists can calculate the strength of the Earth’s gravity at that point.