  It is known that, when an excited atom spontaneously emits one photon, two
effects are produced. First, the atom's internal and external states are
entangled with the states of the emitted photon. Second, the atom receives a
momentum transfered from the photon. In this work, the dynamics of such an atom
in vacuum is studied. Through a specific calculation, it is demonstrated that
these effects cause the atom to experience, on average, a friction force
opposite to its initial velocity. Properties of the force are also discussed.
