  Long-distance quantum optical communications usually require efficient
wave-mixing processes to convert the wavelengths of single photons. Many
quantum applications based on electromagnetically induced transparency (EIT)
have been proposed and demonstrated at the single-photon level, such as quantum
memories, all-optical transistors, and cross-phase modulations. However,
EIT-based four-wave mixing (FWM) in a resonant double-$\Lambda$ configuration
has a maximum conversion efficiency (CE) of 25% because of absorptive loss due
to spontaneous emission. An improved scheme using spatially modulated
intensities of two control fields has been theoretically proposed to overcome
this conversion limit. In this study, we first demonstrate wavelength
conversion from 780 to 795 nm with a 43% CE by using this scheme at an optical
density (OD) of 19 in cold $^{87}$Rb atoms. According to the theoretical model,
the CE in the proposed scheme can further increase to 96% at an OD of 240 under
ideal conditions, thereby attaining an identical CE to that of the previous non
resonant double-$\Lambda$ scheme at half the OD. This
spatial-light-modulation-based FWM scheme can achieve a near-unity CE, thus
providing an easy method of implementing an efficient quantum wavelength
converter for all-optical quantum information processing.
