In recent works it has been shown that wave instabilities may build up when two closely spaced
perfectly smooth parallel surfaces are sheared past one another with a velocity exceeding some threshold
value [1, 2, 3]. This phenomenon is the counterpart of Cherenkov radiation but for neutral and polarizable
matter. It occurs because of the coupling between the guided modes supported by each surface and is
accompanied by a friction force due to the conversion of kinetic energy into electromagnetic energy.
The conditions required for the emergence of the instabilities in planar or cylindrical geometries as well
as their relation with a broken PT-symmetry and negative Doppler shifted frequencies were studied in
[4, 5, 6, 1, 2, 3, 7, 8, 9, 10]. Interestingly, it was demonstrated in Ref. [3] that similar instabilities and
spontaneous light emission can also occur for an isolated classical dipole moving in the close vicinity of a
plasmonic slab. Here, we extend this problem to quantum mechanics and study the time dynamics of a
moving two-level atom interacting with the quantized field of a plasmonic surface. It is shown that for a
certain range of parameters the system is characterized by a negative spontaneous emission, meaning that
the transition rate to the excited state exceeds the transition rate to the ground state and is associated
with a quantum friction force. However, different from the classical case, the instabilities saturate due to the peculiar energy spectrum of the two-level atom [11].