"The objective of this project is to control the dynamics of a nanoscale object with unprecedented precision and to study interactions on the mesoscale, - the grey zone between the discrete atomistic world and the continuous world of macroscopic objects.A single nanoparticle will be captured by the gradient force of a focused laser beam in ultrahigh vacuum and its center-of-mass motion will be controlled by optical back-action. To cool the nanoparticle to its quantum ground state we will explore active parametric feedback cooling in combination with passive cavity-based cooling.A laser-trapped nanoparticle is physically decoupled from its environment, which guarantees extremely long coherence times and quality factors as high as 10^11 in ultrahigh vacuum. Force sensitivities of 10^(-20) Newtons in a bandwidth of 1 Hz can be achieved, which outperforms other measurement techniques by orders of magnitude. In this project, we will use a laser-trapped nanoparticle as a local probe for measuring mesoscopic interactions, such as Casimir forces, vacuum friction, non-equilibrium dynamics and phase transitions, with unprecedented accuracy.We will also measure the dynamics of nanoparticles in double-well potentials created by two laser beams with closely spaced foci. A pair of trapped nanoparticles defines a highly controllable coupled-oscillator model, which can be used for studying strong coupling, level splitting, and adiabatic energy transfer at the quantum - classical barrier.A nanoparticle cooled to its quantum ground state opens up a plethora of fundamental studies, such as the collapse of quantum superposition states under the influence of noise and gravity-induced quantum state reduction. This project will also open up new directions for precision metrology and provide unprecedented control over the dynamics of matter on the nanometer scale."