This Master’s internship focuses on optical trapping, a technique pioneered by A. Ashkin (Nobel Prize 2018) and now widely used to probe matter at short scales. The project aims to study Brownian motion, whose instantaneous velocity was long thought immeasurable until recent breakthroughs using ultra-sensitive optical setups. Our experiment combines balanced detection, high-speed acquisition, and an ultra-stable continuous laser, reaching record sensitivity down to 10⁻¹⁵ m/√Hz. We have already observed the ballistic regime of Brownian motion in water and measured particle velocity and its autocorrelation function. The internship will provide the opportunity to refine instantaneous velocity measurements, benchmark and optimize detection performance, and explore new regimes of Brownian motion near rigid and soft boundaries.