Matter-wave interferometers are sensitive probes of low energy physics and have been used for precise tests of gravity and quantum mechanics. These applications would benefit from interfering particles of higher mass, but observing the interference of a large particle is challenging due to the need to control its initial state and to avoid decoherence. Here we propose to demonstrate matter-wave interference of a microparticle with an embedded rare-earth ion. Optical transitions of the rare-earth ion will impart momentum to the microparticle's center of mass. The use of a time-symmetric interferometer geometry and a rare-earth ion state with angular momentum
J=1/2 will eliminate sensitivity to the initial conditions of the microparticle. We show that the rates of all relevant external decoherence mechanisms either decrease or remain constant as the microparticle mass is increased, allowing decoherence to be avoided by using sufficiently large particles. An apparatus at moderate vacuum levels will support microparticle interferometry with up to
103 photons per beam splitter and millisecond coherence time. This demonstration will establish microparticle interferometry as a new platform for quantum sensing, improving searches for minimal modifications of quantum mechanics by up to three orders of magnitude in the near term and laying the foundation for future gravitational tests.
This thesis explores how to create macroscopic matter-wave interferometers (MMWIs) using nanodiamonds with nitrogen-vacancy (NV) centers. MMWIs are important for testing the limits of quantum mechanics and studying the possible quantum nature of gravity. However, building MMWIs with large masses is very challenging because of decoherence and the diamagnetic properties of diamonds. This work presents several experimental schemes to increase the spatial separation of massive quantum objects. The key idea is to use the diamagnetism of diamonds not only as a limitation but also as a tool to help separate wave packets. Methods include catapulting trajectories with nonlinear magnetic fields, using diamagnetic repulsion, combining gravitational acceleration with magnetic fields, and applying inverted harmonic potentials. The results show that it is possible to achieve large spatial superpositions, up to tens of micrometers, in short timescales. While a full-scale MMWI is still difficult to realize, these findings may contribute to future efforts to observe quantum effects in macroscopic systems.