Entangled states (that is, quantum states that cannot be simply described as the quantum states of each individual particle) are far more sensitive to small perturbations, so can be used to make very precise measurements. For example, when the two arms of an interferometer are entangled, a much smaller change in path-length can be resolved than with unentangled particles. We will investigate how to generate quantum entanglement in ultra-cold atoms that are compatible with use in atom interferometers. Atom interferometers already provide the most sensitive measurements of the gravitational field. By adding quantum entanglement to these devices, we will open up new regimes of sensitivity which will allow for the discovery of new fundamental physics, such as resolving quantum effects in the gravitational field.