Generation of ⁸⁷Rb resonant bright two-mode squeezed light with four-wave mixing

Squeezed states of light have found their way into a number of applications in quantum-enhanced metrology due to their reduced noise properties. In order to extend such an enhancement to metrology experiments based on atomic ensembles, an efficient light-atom interaction is required. Thus, there is a particular interest in generating narrow-band squeezed light that is on atomic resonance. This will make it possible not only to enhance the sensitivity of atomic based sensors, but also to deterministically transfer quantum correlations between two distant atomic ensembles. We generate bright two-mode squeezed states of light, or twin beams, with a non-degenerate four-wave mixing (FWM) process in hot ⁸⁵Rb in a double-lambda configuration. Given the proximity of the energy levels in the D1 line of ⁸⁵Rb and ⁸⁷Rb, we are able to operate the FWM in ⁸⁵Rb in a regime that generates two-mode squeezed states in which both modes are simultaneously on resonance with transitions in the D1 line of ⁸⁷Rb, one mode with the F = 2 to F' = 2 transition and the other one with the F = 1 to F' = 1 transition. For this configuration, we obtain an intensity difference squeezing level of 3.5 dB. Moreover, the intensity difference squeezing increases to -5.4 dB and -5.0 dB when only one of the modes of the squeezed state is resonant with the D1 F = 2 to F' =-2 or F = 1 to F' = 1 transition of ⁸⁷Rb, respectively.