  In a fiber-based quantum network, utilizing the telecom band is crucial for
long-distance quantum information (QI) transmission between quantum nodes.
However, the near-infrared wavelength is identified as optimal for processing
and storing QI through alkaline atoms. Efficiently bridging the frequency gap
between atomic quantum devices and telecom fibers while maintaining QI carried
by photons is a challenge addressed by quantum frequency conversion (QFC) as a
pivotal quantum interface. This study explores a telecom-band QFC mechanism
using diamond-type four-wave mixing (FWM) with rubidium energy levels. The
mechanism converts photons between the near-infrared wavelength of 795 nm and
the telecom band of 1367 or 1529 nm. Applying the Heisenberg-Langevin approach,
we optimize conversion efficiency (CE) across varying optical depths while
considering quantum noises and present corresponding experimental parameters.
Unlike previous works neglecting the applied field absorption loss, our results
are more relevant to practical scenarios. Moreover, by employing the
reduced-density-operator theory, we demonstrate that this diamond-type FWM
scheme maintains quantum characteristics with high fidelity, unaffected by
vacuum field noise, enabling high-purity QFC. Another significant contribution
lies in examining how this scheme impacts QI encoded in photon-number, path,
and polarization degrees of freedom. These encoded qubits exhibit remarkable
entanglement retention under sufficiently high CE. In the case of perfect CE,
the scheme can achieve unity fidelity. This comprehensive exploration provides
theoretical support for the application of the diamond-type QFC scheme based on
atomic ensembles in quantum networks, laying the essential groundwork for
advancing the scheme in distributed quantum computing and long-distance quantum
communication.
