A Polarized Mode-Entangled Neutron Interferometer for Neutron Quantum Optics and Gravity*

The unique properties of the neutron make them the best choice to address an intellectually coherent set of scientific questions involving the interplay of quantum entanglement, gravitation, non-inertial effects, and the twisting mode of motion of the neutron wave packet responsible for the recently-uncovered orbital angular momentum (OAM) degree of freedom of the neutron. These questions include, but are not limited to, (1) new searches for CPT/Lorentz violation in neutron spin-gravitational couplings, (2) experimental tests of macrorealistic theories in terms of Leggett-Garg inequalities for neutrons, (3) generation and certification of mode-entangled neutron beams in four variables (neutron spin, neutron energy, neutron momentum/path, neutron OAM), and (4) measurement of the neutron Sagnac effect in the presence of mode entanglement and neutron OAM. All of these subjects can be addressed experimentally by the same apparatus: a polarized, mode-entangled neutron interferometer using Wollaston prism neutron optical devices. We will outline the intellectual motivation for these different experimental efforts, some of which have been conducted recently at the Offspec and Larmor neutron spin echo instruments at the ISIS spallation source in the UK, and describe the potentential scientific reach for an instrument installed at a MW-class spallation neutron source such as the SNS at ORNL.