Phase-controlled unidirectional amplification in a gain-assisted cavity-QED system

Optical nonreciprocal transmission with unidirectional amplification in a gain-assisted cavity-QED system is investigated. The results demonstrated that unidirectional amplification of this system is induced by the phase difference between the atom-cavity coupling strengths associated with the optical gain and a phase-controlled unidirectional amplifier can be achieved. The optimal parameter conditions for the observation of ideal unidirectional amplification are obtained analytically, which is shown to be phases, atom-cavity, cavity-cavity coupling strength and atomic dissipation dependent. Especially, it is found that atomic dissipation is another essential condition for the realization of nonreciprocal amplification other than the time-reversal symmetry broken, and our unidirectional amplifier is robust against atom-cavity coupling mismatch. Such unidirectional amplifiers are crucial nonreciprocal devices for controlling photon transmission and may have potential applications in various quantum information processing.