Abstract
We theoretically investigate the effect of optical-force-mediated interactions onto the quantum dynamics of a pair of dielectric nanospheres optically trapped in two neighboring optical tweezers. Thanks to the interference between the tweezer beams and the elastically scattered light by the other nanosphere, the effective inter nanosphere coupling can reach the ultrastrong coupling regime. Experimentally accessible signatures of the entangled nature of the quantum ground state are highlighted, including entanglement witnesses based on position/momentum correlation functions and a sizable dynamical Casimir excitation upon a fast modulation of the tweezer's properties.