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Entanglement-based 3D magnetic gradiometry with an ultracold atomic scattering halo

Ultracold collisions of Bose-Einstein condensates can be used to generate a large number of counterpropagating pairs of entangled atoms, which collectively form a thin spherical shell in momentum space, called a scattering halo. Here we generate a scattering halo composed almost entirely of pairs in a symmetric entangled state and observe a scattering angle-dependent mixing to the anti-symmetric state due to the presence of an inhomogeneous magnetic field. We report on a proof-of-principle application of the observed parity dynamics to demonstrate magnetic gradiometry, insensitive to common-mode fluctuations of the background magnetic field. Furthermore, the highly multimode nature and narrow radial width of the scattering halos enable 3D tomography of an interrogated field without the need for a scanning probe.