  We describe the use of optically levitated microspheres as test masses in
experiments aimed at reaching and potentially exceeding the standard quantum
limit for position measurements. Optically levitated microspheres have low mass
and are essentially free of suspension thermal noise, making them well suited
for reaching the quantum regime. Table-top experiments using microspheres can
bridge the gap between quantum-limited position measurements of single atoms
and measurements with multi-kg test masses like those being used in
interferometric gravitational wave detectors.
