Fluctuating Vacuum and Moving Interfaces through the Lens of Nano-Optics

We present a theoretical overview of force generation induced by vacuum. Within the framework of macroscopic quantum electrodynamics, electromagnetic field correlations are determined by the material response functions and dyadic Green's functions, providing a unified description of fluctuation-induced phenomena. While conventional forces such as van der Waals and Casimir–Polder interactions arise from equilibrium fluctuations associated with dissipative channels, nonequilibrium conditions can activate gain-related channels, leading to qualitatively different effects. We highlight how such nonequilibrium fluctuations generate asymmetric momentum transfer and novel forces. As representative examples, we discuss quantum friction between moving bodies, where Doppler-shift-induced gain leads to shear forces, and Hall-like forces driven by chiral gain in current-driven low-symmetry media, where polarisation-selective amplification combined with spin–momentum locking produces transverse forces. These results illustrate that vacuum fluctuations can be actively engineered through nanoscale material responses, opening new possibilities for fluctuation-induced force control.