Microwave controlled ground state coherence in an atom-based optical amplifier

We experimentally investigate and theoretically analyze the effect of microwave controlled atomic ground state coherence on the phase-dependent amplification (PDA) of an optical probe field. We use three hyperfine levels in room temperature 85 Rb atoms, which are cyclically connected by two optical and one microwave electromagnetic field. We show that a simultaneous fulfilment of a two-photon resonance condition that creates ground state coherence and a three-photon resonance condition leads to a significantly higher amplification of 7.5 dB of the optical probe field with a visibility of 98.8 %. By selectively breaking the ground state coherence using microwaves, we show that the amplification reduces with a bandwidth of 5 MHz. Nevertheless, the system shows non-zero PDA for large two-photon detunings of 15 MHz with high visibility of 66.8 %. This novel, controllable hybrid-PDA can be potentially used to trade-off amplification for bandwidth during the transmission of phase coherent classical and quantum information.