Fiber-Coupled Twin Beams from Four Wave Mixing in Rubidium

The reduced noise properties of twin beams make it possible to enhance the sensitivity of measurements beyond the classical shot noise limit (SNL). Expanding such capabilities beyond proof-of-principle experiments and into practical applications will require technological advancements, such as the implementation of fiber coupled compact sources. We show that it is possible to generate fiber coupled twin beams with a large degree of quantum correlations. We generate the twin beams with a nonlinear four wave mixing (FWM) process in a double-Λ configuration in rubidium vapor. In this process the absorption of two pump photons leads to the simultaneous generation of probe and conjugate photons, which gives rise to the quantum correlations. In order to generate bright twin beams, we combine the pump beam with a seed probe beam at a slight angle at the center of a hot rubidium vapor cell. The generated bright probe and conjugate beams are then fiber coupled to two separate single mode fibers with ∼90% coupling efficiency. In doing so, special care needs to be taken due to the multi-spatial-mode nature of the twin beams. We optimize the FWM process to maximize the level of squeezing while minimizing the number of spatial modes in the generated probe and conjugate beams. We show that is possible to obtain fiber coupled twin beams with ∼ 4.5 dB of intensity difference squeezing out of an initial level of ∼ 7 dB of squeezing.