LAUSR

Elusive three-body matters Tunable interactions make dilute atomic gases ideal for studying the collective dynamics of many-body systems. If the gas consists of strongly interacting fermions evenly divided into two groups of opposite spin, many of its properties can be distilled to two-body correlations. Fletcher et al. show that this does not hold for a gas of bosons, where identical particles happily congregate. The researchers measured a quantity that, in a thermal resonantly interacting Bose gas, depends only on three-body correlations. This enabled them to quantify the elusive correlations and establish unambiguously their effect on the physics of the many-body state. Science, this issue p. 377 Interferometric measurements in a strongly interacting gas of 39K atoms quantify three-body correlations. In many-body systems governed by pairwise contact interactions, a wide range of observables is linked by a single parameter, the two-body contact, which quantifies two-particle correlations. This profound insight has transformed our understanding of strongly interacting Fermi gases. Using Ramsey interferometry, we studied coherent evolution of the resonantly interacting Bose gas, and we show here that it cannot be explained by only pairwise correlations. Our experiments reveal the crucial role of three-body correlations arising from Efimov physics and provide a direct measurement of the associated three-body contact.