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. Here, using Ramsey interferometry, we study coherent evolution of the resonantly interacting Bose gas, and show 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.A fundamental challenge in many-body quantum physics is to connect the macroscopic behaviour of a system to the microscopic interactions between its constituents. In ultracold atomic gases the strength of interactions is most commonly characterised by the s-wave scattering length a, which can be tuned via Feshbach resonances [1]. On resonance a diverges and one reaches the unitary regime, in which the interactions are as strong as allowed by quantum mechanics. This regime has been extensively studied in Fermi gases [2][3][4], while the unitary Bose gas represents a new experimental frontier [5][6][7][8][9][10].In these systems, universal properties of the short-range particle correlations imply universal thermodynamic relations between macroscopic observables such as the momentum distribution, energy, and the spectroscopic response [11][12][13][14][15][16][17][18][19]. In the case of (mass-balanced) two-component Fermi gases, at the heart of these relations is a single fundamental thermodynamic parameter, the two-body contact density C 2 , which measures the strength of two-particle correlations. However, the case of the Bose gas is more subtle. In this system Efimov physics gives rise to three-body bound states [20][21][22][23][24][25][26], and more generally introduces three-particle correlations that cannot be deduced from the knowledge of pairwise ones [17][18][19]27]. The implication for many-body physics is that complete understanding of the macroscopic coherent phenomena requires knowledge of both C 2 and its three-body analogue C 3 [17][18][19].The relative importance of three-particle correlations generally grows with the strength of interactions. At moderate interaction strengths C 2 was measured spectroscopically, but C 3 was not observed [24]. However, the momentum distribution of the unitary Bose gas [7] suggested deviations from two-body physics [19,28].Here we interferometrically measure both C 2 and C 3 in a resonantly interacting thermal Bose gas, and find excellent agreement with theoretical predictions. The idea of our experiment is illustrated in Fig. 1. We perform radio-frequency (RF) Ramsey interferometry on a gas of atoms with two internal (spin) states, ↑ and ↓, and use a magnetic Feshbach resonance to enhance ↑↑ interactions, while both ↑↓ and ↓↓ interactions are negligible. For a measurement at a given magneticRamsey interferometry of a many-body system. The first π/2 pulse pu...

We study the critical point for the emergence of coherence in a harmonically trapped two-dimensional Bose gas with tuneable interactions. Over a wide range of interaction strengths we find excellent agreement with the classical-field predictions for the critical point of the Berezinskii-Kosterlitz-Thouless (BKT) superfluid transition. This allows us to quantitatively show, without any free parameters, that the interaction-driven BKT transition smoothly converges onto the purely quantum-statistical Bose-Einstein condensation (BEC) transition in the limit of vanishing interactions.

We study the anisotropic, elliptic expansion of a thermal atomic Bose gas released from an anisotropic trapping potential, for a wide range of interaction strengths across a Feshbach resonance. We show that in our system this hydrodynamic phenomenon is for all interaction strengths fully described by a microscopic kinetic model with no free parameters. The success of this description crucially relies on taking into account the reduced thermalising power of elastic collisions in a strongly interacting gas, for which we derive an analytical theory. We also perform time-resolved measurements that directly reveal the dynamics of the energy transfer between the different expansion axes.

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