Generation of bright two-mode entangled light in four-level tripod atomic system

• In this work, we provide a new feasible scheme to generate bright two-mode continuous-variable entanglement.
• In our plan, the four-level tripod atomic system is used, in which both two-mode entanglement and laser oscillation can be obtained.
• When the detuning and the cooperativity parameter are tuned properly, 30% two-mode squeezing for the respective cavity mode and the largest intensity (〈I1〉=〈I2〉=132〈I1〉=〈I2〉=132) are gained simultaneously.

We show that it is possible to generate laser oscillation and two-mode entanglement via atomic coherence in a four-level tripod atomic system. On two-photon resonance, two coherent fields induce the decoupling of a coherent superposition state of two lower states, which leads to quantum beat between the cavity modes. In terms of the dressed states produced by the two coherent fields, the system is reduced to a standard electromagnetically induced transparency (EIT) configuration when the detunings of the applied fields are properly adjusted. Laser frequencies locate at a pair of Rabi sidebands and light amplification is supported by coherent coupling between one of the dressed states and an auxiliary metastable state. The laser transition, the coherent coupling, and the spontaneous decay constitute a successive population transfer pathway, through which the laser electron is regularly recycled. It is for the very mechanism that the laser oscillates and exhibits two-mode entanglement. This provides an alternative scheme as an active and bright source of two-mode entangled light.