  We present a scheme for obtaining entangled photons and quantum phase gates
in a room-temperature four-state tripod-type atomic system with two-mode active
Raman gain (ARG). We analyze the linear and nonlinear optical response of this
ARG system and show that the scheme is fundamentally different from those based
on electromagnetically induced transparency and hence can avoid significant
probe-field absorption as well as temperature-related Doppler effect. We
demonstrate that highly entangled photon pairs can be produced and rapidly
responding polarization qubit phase gates can be constructed based on the
unique features of enhanced cross-phase modulation and superluminal probe-field
propagation of the system.
