Wavelength conversion of ns-long pulses in cold atomic ensemble with extreme optical depth

Interfacing atoms with solid-state emitters such as quantum dots, which emit short (~ns-long) photon pulses, necessitates broadband (~GHz) operation of the atomic medium. Here, we theoretically investigate single photon wavelength conversion using atomic ensembles. In particular, we consider the conversion of photons (~895 nm) emitted by semiconductor quantum dots (InAsP/InAs) to wavelengths suitable for satellite-based QKD (794 nm) or fibre-based (1469 nm) telecommunication using Cs atoms. We show that efficient broad bandwidth photon conversion can be realized with atomic clouds of high optical depths (10³ - 10⁴) which can be created by confining the atoms inside a hollow-core optical fibre. We compare the efficiency of conversion to other platforms and discuss the role of the applied control fields, as well as of the geometry and temperature of the atomic ensemble inside the hollow-core fibre in achieving high conversion efficiencies.