Einstein's Equivalence Principle Works for Atoms in Superposition

I came across this article describing a pretty cool experiment and I thought I'd pass it along. Here is the article:

Key Einstein principle survives quantum test

Particles with mind-bending quantum properties still follow a standard gravitational rule, at least as far as scientists can tell. The equivalence principle — one of the central tenets of Einstein’s theory of gravity — survived a quantum test, scientists report online April 7 at arXiv.org.

In Einstein’s gravity theory — the general theory of relativity — gravity and acceleration are two sides of the same coin. According to the equivalence principle, the gravitational mass of an object, which determines the strength of gravity’s pull, is the same as its inertial mass, which determines how much an object accelerates when given a push (SN: 10/17/15, p. 16). As a result, two objects dropped on Earth’s surface should accelerate at the same rate (neglecting air resistance), even if they have different masses or are made of different materials.

One of the first reported tests of the equivalence principle — well before it was understood in the framework of general relativity — was Galileo’s apocryphal experiment in which he is said to have dropped weights from the Leaning Tower of Pisa. Scientists have since adapted that test to smaller scales, swapping out the weights for atoms. In the new study, physicists went a step further, putting atoms into a quantum superposition, a kind of limbo in which an atom does not have a definite energy but occupies a combination of two energy levels.

Manipulating rubidium atoms with lasers, scientists led by researchers from Italy gave the atoms an upward kick and observed how gravity tugged them down. To compare the acceleration of normal atoms with those in a superposition, the scientists split the atoms into two clouds, put atoms in one cloud into a superposition, and measured how the clouds interacted. These clouds of atoms behave like waves, interfering similarly to merging water waves. The resulting ripples depend on the gravitational acceleration felt by the atoms.

The scientists then compared the result of this test to one where both clouds were in a normal energy state. Gravity, the researchers concluded, pulled on atoms in a superposition at the same rate as the others — at least to the level of sensitivity the scientists were able to probe, within 5 parts in 100 million.

Article Continues Here