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 ⁸⁵Rb 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.