  Quantum mechanics predicts the occurrence of random electromagnetic field
fluctuations, or virtual photons, in vacuum. The exchange of virtual photons
between two bodies in relative motion could lead to non-contact quantum vacuum
friction or Casimir friction. Despite its theoretical significance, the
non-contact Casimir frictional force has not been observed and its theoretical
predictions have varied widely. In this work, we report the first measurement
of the non-contact Casimir frictional force between two moving bodies. By
employing two mechanical oscillators with resonant frequencies far lower than
those in Lorentz models of electrons in dielectric materials, we have amplified
the Casimir frictional force at low relative velocities by several orders of
magnitude. We directly measure the non-contact Casimir frictional force between
the two oscillators and show its linear dependence on velocity, proving the
dissipative nature of Casimir friction. This advancement marks a pivotal
contribution to the field of dissipative quantum electrodynamics and enhances
our understanding of friction at the nanoscale.
