A Custom Design Stainless Steel Hot Sodium Vapor Cell with Sapphire Viewports for the Generation of Resonant Twin Beams

Four-wave mixing (FWM) is a non-linear process that can produce quantum correlated twin beams of light with noise properties below the shot noise limit. This makes them useful for quantum-enhanced sensing, as has been shown by recent experiments at the LIGO interferometer. We are interested in narrowband near or on atomic resonance squeezed light at 589 nm to enhance density measurements of our sodium Bose-Einstein condensate. One of the techniques to generate twin beams is by employing a nonlinear crystal inside a cavity, i.e. an optical parametric oscillator (OPO), but this technique leads to technical challenges, such as engineering the nonlinear crystal to operate at a desired wavelength. Therefore, we generate the twin beams with an atomic ensemble. In particular, we use FWM in a double-lambda configuration to generate twin beams of light, known as probe and conjugate, in hot sodium vapor. However, the high temperature required to obtain the necessary atomic density for an efficient FWM process also leads to complications. Owing to a strong chemical reaction between Pyrex glass and alkali atoms at high temperatures, we design a stainless-steel heat pipe that will allow temperatures of up to 400 degrees Celsius. The high temperature, enabled by employing sapphire viewports, will lead to the higher vapor densities needed to obtain large FWM gains, and thus larger levels of intensity-difference squeezing in the generated twin beams. We present the design of our home-built hot sodium vapor cell and preliminary results on the dependence of the FWM gain on temperature, pump beam intensity, detunings and other experimental parameters.