Detection of a Levitated Nanoparticle’s Position via Self-Homodyne

The efficiency with which the position of a levitated nanoparticle is detected is one factor that sets the minimum temperature of the particle’s center-of-mass motion that can be achieved by feedback cooling. Typically, the detection is based on the interference of the light scattered by the particle with a reference Gaussian beam in a homodyne setup [1] - [3] . This detection scheme is limited by the wavefront mismatch between the scattered and the reference fields and by the numerical aperture (NA) of the lens that collects the scattered light. As a result of these limitations, temperatures close to the ground state have only been achieved in experiments exploiting high NA objectives (> 0.8), namely, optical-tweezer-based experiments [1] , [2] . Here, we demonstrate how the detection efficiency can be improved in a setup based on homodyne detection, by improving the mode-matching conditions and by increasing the sensitivity of the interference pattern to the displacement of the particle.