  Quantum friction, the electromagnetic fluctuation-induced frictional force
decelerating an atom which moves past a macroscopic dielectric body, has so far
eluded experimental evidence despite more than three decades of theoretical
studies. Inspired by the recent finding that dynamical corrections to such an
atom's internal dynamics are enhanced by one order of magnitude for vertical
motion -- compared to the paradigmatic setup of parallel motion -- we
generalize quantum friction calculations to arbitrary angles between the atom's
direction of motion and the surface in front of which it moves. Motivated by
the disagreement between quantum friction calculations based on Markovian
quantum master equations and time-dependent perturbation theory, we carry out
our derivations of the quantum frictional force for arbitrary angles employing
both methods and compare them.
