  Quantum frequency conversion (QFC), a critical technology in photonic quantum
information science, requires that the quantum characteristics of the
frequency-converted photon must be the same as the input photon except for the
color. In nonlinear optics, the wave mixing effect far away from the resonance
condition is often used to realize QFC because it can prevent the vacuum field
reservoir from destroying the quantum state of the converted photon
effectively. Under conditions far away from resonance, experiments typically
require strong pump light to generate large nonlinear interactions to achieve
high-efficiency QFC. However, strong pump light often generates additional
noise photons through spontaneous Raman or parametric conversion processes.
Herein, we theoretically study another efficient QFC scheme based on a resonant
four-wave mixing system. Due to the effect of electromagnetically induced
transparency (EIT), this resonant QFC scheme can greatly suppress vacuum field
noise at low light levels; consequently, the converted photon can inherit the
quantum state of the input photon with high fidelity. Our research demonstrates
that if the conversion efficiency of the EIT-based QFC is close to 100%, the
wave function and quadrature variance of the converted photon are almost the
same as the input probe photon.
