Low-light-level four-wave mixing by quantum interference

Abstract

We observed electromagnetically induced transparency-based four-wave mixing (FWM) in the pulsed regime at low light levels. The FWM conversion efficiency of $3.8(9)%$ was observed in a four-level system of cold ${}^{87}\mathrm{Rb}$ atoms using a driving laser pulse with a peak intensity of $\ensuremath{\approx}80$ $\ensuremath{\mu}\mathrm{W}/{\mathrm{cm}}^{2}$, corresponding to an energy of $\ensuremath{\approx}60$ photons per atomic cross section. Comparison between the experimental data and the theoretical predictions proposed by Harris and Hau [Phys. Rev. Lett. 82, 4611 (1999)] showed good agreement. Additionally, a high conversion efficiency of $46(2)%$ was demonstrated when applying this scheme using a driving laser intensity of $\ensuremath{\approx}1.8$ $\mathrm{mW}/{\mathrm{cm}}^{2}$. According to our theoretical predictions, this FWM scheme can achieve a conversion efficiency of nearly $100%$ when using a dense medium with an optical depth of 500.