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Abstract We present observations of the high momentum tail in expanding Bose–Einstein condensates of metastable Helium atoms released from a harmonic trap. The far-field density profile exhibits features that support identification of the tails of the momentum distribution as originating in the in-situ quantum depletion prior to release. Thus, we corroborate recent observations of slowly-decaying tails in the far-field beyond the thermal component. This observation is in conflict with the hydrodynamic theory, which predicts that the in-situ depletion does not survive when atoms are released from a trap. Indeed, the depleted tails even appear stronger in the far-field than expected before release, and we discuss the challenges of interpreting this in terms of the Tan contact in the trapped gas. In complement to these observations, full quantum simulations of the experiment show that, under the right conditions, the depletion can persist into the far field after expansion. Moreover, the simulations provide mechanisms for survival and for the the large-momentum tails to appear stronger after expansion due to an acceleration of the depleted atoms by the mean-field potential. However, while in qualitative agreement, the final depletion observed in the experiment is much larger than in the simulation.

Entangled particles some distance apart can be used to show the strikingly nonlocal nature of quantum mechanics. Here the authors generate spatially separated pairs of helium atoms by colliding Bose-Einstein condensates and show that they are entangled by observing nonlocal correlations.

Ultracold collisions of Bose–Einstein condensates can be used to generate a large number of counter-propagating pairs of entangled atoms, which collectively form a thin spherical shell in momentum space, called a scattering halo. Here we generate a scattering halo initially composed of pairs in a symmetric entangled state in spin, and observe a coherent oscillation with an anti-symmetric state during their separation, due to the presence of an inhomogeneous magnetic field. We demonstrate a novel method of magnetic gradiometry based on the evolution of pairwise correlation, which is insensitive to common-mode fluctuations of the magnetic field. Furthermore, the highly multimode nature and narrow radial width of scattering halos enable a 3D reconstruction of the interrogated field. Based on this, we apply Ramsey interferometry to realise a 3D spatial reconstruction of the magnetic field without the need for a scanning probe. [ABSTRACT FROM AUTHOR]

The preferential heating of protons in the perpendicular direction with respect to the ambient magnetic field is a well-known phenomenon in the solar corona. One of the physical mechanisms proposed to explain such a feature is the nonresonant wave–particle interaction between protons and kinetic Alfvén waves. The present paper examines this mechanism by employing a novel analytical method based upon the Hamiltonian dynamical test particle approach. The analytical nature of the present method reveals an interesting fact that the pure inductive component of parallel electric field associated with the kinetic Alfvén wave in the limit Ti/Te ≪ 1 has little contribution. The parallel and perpendicular diffusion of protons by kinetic Alfvén waves is investigated through computing the autocorrelation function. The calculations unambiguously demonstrate that the proton heating is achieved mainly by nonresonant wave–particle interaction involving the wave magnetic field, and that the effects of the parallel electric field are minimal. It is also found that the perpendicular diffusion is orders of magnitude stronger than parallel diffusion, implying effective perpendicular heating. [ABSTRACT FROM AUTHOR]