Coherence properties of amplified slow light by four-wave mixing

We present an experimental study of the coherence properties of amplified slow light by four-wave mixing (FWM) in a three-level electromagnetically induced transparency (EIT) system driven by one additional pump field. High energy gain (up to 19) is obtained with a weak pump field (a few mW/cm ²) using optically dense cold atomic gases. A large fraction of the amplified light is found to be phase incoherent to the input signal field. The dependence of the incoherent fraction on pump field intensity and detuning and the control field intensity is systematically studied. With the classical input pulses, our results support a recent theoretical study by Lauk et al. [Phys. Rev. A 88, 013823 (2013)], showing that the noise resulting from the atomic dipole fluctuations associated with spontaneous decay is significant in the high gain regime. This effect has to be taken into consideration in EIT-based applications in the presence of FWM.