We present a theoretical study of an optical cavity coupled with single four-level atoms in closed loop formed via applied control lasers. The transmitted probe field from the cavity is analyzed. We show that the electromagnetically induced transparency (EIT) in the cavity and the normal mode splitting will be very different with changing the closed interaction phase and the intensity of the free-space control laser. This coupled cavity-atom system presents a variational double-EIT that comes from modulating the splitting of the dark state, which means that we could realize the gradual transfer between one EIT peak and two EIT peaks by adjusting the applied control lasers, and the normal mode splitting sidebands will shift slightly by changing the free-space control laser. This means that we could control the output cavity probe field more freely and it is easer to realize optical switch controlled by more parameters. We also depict the angular dispersion of the intracavity probe field in different free-space control laser. The large phase shift (−π → π) of the reflected intracavity probe field will be very useful for optical temporal differentiation and quantum phase gate.

We present a scheme to realize two-direction optical switch by a single-mode optical cavity containing some four-level atoms. The high switching efficiency can be obtained through low photon loss and large third-order nonlinear susceptibility of this N-type atomic system in cavity. Without the microwave source, it can be reduced to a -type atomic system where a coupling laser is used to realize single intracavity electromagnetically induced transparency (EIT). Namely, the probe field can be transmitted almost totally at resonance. Thus a two-direction optical switch is operated and the state for forward (backward) direction is set as "open" ("closed"). When microwave source is introduced, dressed splitting of intracavity dark state happens. The probe field is reflected almost completely at resonance and the state of the optical switch at forward and backward directions (transmitted and reflected channels) is shifted as "closed" and "open", respectively. Moreover, this scheme is much advantageous to realize splitting of intracavity dark state because weak microwave field ( m ∼ 0.1γ 14 ) induces the coupling between intracavity dark state and one sublevel of ground state. While a strong pump laser ( d ≥ γ 14 ) which couples the intracavity dark state with an excited level is applied to realize this splitting in ref. [Phys. Rev. A 85 013814 (2012)].

We propose a scheme for controlling coherent photon absorption by electromagnetically induced transparency (EIT) in a three-level atom-cavity system. Coherent perfect absorption (CPA) occurs when time-reversed symmetry of lasing process is obtained with the destructive interference at the cavity interfaces. The frequency range of CPA is generally dependent on the decay rates of the cavity mirrors. The smaller decay rate of the cavity mirror causes the wider frequency range of CPA, and the needed intensity of the probe fields is larger to satisfy CPA condition for a given frequency. Although Rabi frequency of the control laser has little effect on the frequency range of CPA, with EIT-type quantum interference, the CPA mode is tunable by the control laser. In addition, with the relative phase, the probe fields can be perfectly transmitted and/or reflected. Therefore, the system can be used as a controllable coherent perfect absorber or transmitter (reflector), and our work may have practical applications in optical logic devices.

We investigate the robustness of entanglement for a multiqubit system under dephasing and bit flip channels. We exhibit the difference between the entanglement evolution of the two forms of special states, which are locally unitarily equivalent to each other and therefore possess precisely the same entanglement properties, and demonstrate that the difference increases with the number of qubits n. Moreover, those two forms of states are either the most robust genuine entangled states or the most fragile ones, which confirm that local unitary (LU) operations can greatly enhance the entanglement robustness of n-qubit states.