  Light is known to exert a pushing force through the radiation pressure on any
surface it is incident upon, via the transfer of momentum from the light to the
surface. For an atom, the interaction with light can lead to both absorption as
well as emission of photons, leading to repulsive and attractive forces,
respectively. For classical light, these two processes occur at the same rates.
Therefore, a thermal ensemble of atoms at a finite temperature always
experiences a net pushing force. In this paper, we show that when treated
quantum mechanically the pulsed electromagnetic field interacting with the
thermal ensemble of atoms leads to unequal transition rates, again resulting in
a non-zero net force. However, the signature and the magnitude of the force
depends upon the intensity of the light, the number of atoms, and the initial
temperature of the ensemble. Thus, even at finite temperature, controlling the
parameters of the electromagnetic pulse and the number of particles in the
ensemble, the net force can be changed from repulsive to attractive, generating
negative radiation pressure in the process. Quite counterintuitively, this
negative radiation pressure arising out of pure quantum character of light gets
stronger for higher temperatures.
