Quantum test of the equivalence principle for atoms in coherent superposition of internal energy states

TLDR

The Einstein equivalence principle underpins our understanding of gravity and spacetime, with its weak form asserting equivalence between inertial and gravitational mass. The study aims to test this principle in a quantum regime, where the test body’s properties must be described by quantum theory. Using a Bragg atom interferometer in a gravity gradiometer, the authors compare the free‑fall of rubidium atoms in two hyperfine states and their coherent superposition, enabling a quantum‑specific test of EEP. They determine the Eötvös ratio for atoms in two hyperfine levels with a relative uncertainty of a few parts in 10⁻⁹, surpassing prior limits by nearly two orders of magnitude.

Abstract

The Einstein equivalence principle (EEP) has a central role in the understanding of gravity and space-time. In its weak form, or weak equivalence principle (WEP), it directly implies equivalence between inertial and gravitational mass. Verifying this principle in a regime where the relevant properties of the test body must be described by quantum theory has profound implications. Here we report on a novel WEP test for atoms: a Bragg atom interferometer in a gravity gradiometer configuration compares the free fall of rubidium atoms prepared in two hyperfine states and in their coherent superposition. The use of the superposition state allows testing genuine quantum aspects of EEP with no classical analogue, which have remained completely unexplored so far. In addition, we measure the Eötvös ratio of atoms in two hyperfine levels with relative uncertainty in the low 10-9, improving previous results by almost two orders of magnitude.