Coulomb coupling between two nanospheres trapped in a bichromatic optical tweezer

The oscillatory motion of levitated nanoparticles has been recently cooled down to the quantum ground state. This opens the way to the realization of an ensemble of coupled, macroscopic quantum oscillators. Here, we demonstrate the trapping of two nanospheres inside a dual optical tweezer generated by two copropagating lasers operating at different wavelengths (1064 nm and 976 nm). Due to the chromatic aberration of the tweezer optics, two focal points are created approximately 9 µm apart, each acting as an optical trap for a silica nanoparticle. At this distance, surface charges on the nanospheres produce a Coulomb force that couples their motion along the tweezer axis. The strong coupling regime is achieved, as evidenced by the observed avoided crossing of the normal-mode frequencies. These results highlight the potential of our experimental scheme for future studies on systems of strongly coupled oscillators, including their implementation in optical cavities, both in the classical and quantum regimes.