Nonlocal correlations between freely propagating pairs of atoms

Since the formulation of quantum theory in the early twentieth century, its counterintuitive description of nature has shifted dramatically from being considered its weakness, to opening up vibrant fields of research and enabling classically untenable technologies. Perhaps the most striking aspect of quantum mechanics is exhibited in the infamous Einstein-Podolsky-Rosen (EPR) paradox, as the violation of locality from distant entangled particles. Due to many stringent technical requirements in the test of quantum nonlocality however, experimentalists have only recently demonstrated the nonlocal nature of quantum mechanics. Such well-controlled physical systems were few just a decade ago. This thesis contributes to the exponentially growing diversity of physical systems exhibiting quantum nonlocality, specifically between freely propagating massive particles, realised from an elastic collision of two helium atoms. This work investigates the entanglement between internal states of the scattered atom pairs, which opens up many exciting avenues to studying entanglement in motional variables of massive particles as well, since both types of entanglement are prepared in a collision. The thesis is composed of three projects, starting with an upgrade to the existing experimental apparatus to more stably produce ultracold gases of metastable helium. Massive particles such as atoms exhibit a wave-like behaviour at ultracold temperatures, typically requiring micro-Kelvin temperatures for a dilute gas. Laser cooling and trapping techniques in ultrahigh vacuum chambers are the workhorse of achieving such temperatures in atomic gases. The master laser system for the experimental apparatus was designed on an external-cavity diode laser, and is central to preparing Bose-Einstein condensates (BEC) of the dilute gas of metastable helium. BECs exhibit coherence in interference experiments which earn the term macroscopic matter-waves. As such, replicating classical optics phenomena with matterwaves has been of great interest since the first experimental realisation and manipulation of BECs. Here we study quantum correlations arising from a collision of BECs. In the particle picture counter-propagating pairs of atoms scatter, and quantum mechanically these pairs are expected to be entangled in their momentum and spin from conservation rules. The spins of spatially separate pairs are experimentally verified to be entangled, and exhibit EPR's "spooky" nonlocal correlations. Finally, we demonstrate the spatially separated entanglement in the freely propagating pairs of atoms in an application to quantum sensing. In this task the magnetic field gradient along the pairs' trajectories causes the atomic pair's correlation to oscillate. The pairwise entanglement enables the measurement to decouple from a common noise source, such as spatial uniform fluctuations in the magnetic field, and surpass the classical limit of measurement sensitivity