Abstract
Electromagnetically-induced-transparency-based four-wave mixing (FWM) in a resonant four-level double-Λ system has a maximum conversion efficiency (CE) of 25% due to spontaneous emission. Herein, we demonstrate that spontaneous emission can be considerably suppressed by arranging the applied laser beams in a backward configuration. With the backward double-Λ FWM scheme, we observe a CE of 63% in cold rubidium atoms with an optical depth (OD) of 48. To the best of our knowledge, this is the first observation of a CE exceeding the conversion limit in resonant FWM processes. Furthermore, we present a theoretical model that includes the phase-mismatch effect in the backward double-Λ FWM system. According to the theoretical model, the present scheme can achieve 96% CE using a medium with a large OD of 200 under ideal conditions. Such an efficient frequency conversion scheme has potential applications in optical quantum information technology.
Highlights
Photons are suitable carriers for quantum information because of their fast propagation speed and weak interaction with the environment
Even though the interaction strength between light and matter is decreased in the non-resonant case, two research teams have demonstrated that the four-wave mixing (FWM) efficiency can overcome the conversion limit (25%) of the resonant case[33,34]
We demonstrate that spontaneous emission can be effectively suppressed under a resonant condition only by arranging the applied laser beams in a backward configuration, which is the so-called backward FWM that was theoretically studied by Kang et al.[35]
Summary
The frequency detuning of the driving field coupling to the off-resonant excited state is around 44 times the spontaneous decay rate, it causes a slight damage to the EIT condition through the far-detuning photon-switching effect[12]. This unwanted effect can be mitigated using the Rb87D1 transition with larger frequency splitting between the hyperfine energy levels compared with the D2 transition used in this experiment. If the phase-mismatch effect is not zero (∆kL = 0.273π) but the ground-state dephasing rate is neglected (γ21 = 0) in our experiment, a maximum CE of 69% can be obtained from the backward FWM scheme, according to equation (20). We note that a different scheme has been proposed recently that uses the spatially varied intensity of two laser fields to overcome the conversion limit (25%) in resonant FWM processes; this scheme has not yet been experimentally demonstrated[47]
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