We demonstrate a compact, spectrally-selective, and tunable delay line for single photons emitted by quantum dots. This is achieved by fine-tuning the wavelength of the optical transitions of such "artificial atoms" into a spectral window in which a cloud of natural atoms behaves as slow-light medium. By employing the ground-state fine-structure-split exciton confined in an InGaAs/GaAs quantum dot as a source of single photons at different frequencies and the hyperfine-structure-split D1 transition of Cs-vapors as a tunable delay-medium, we achieve a differential delay of up 2.4 ns on a 7.5 cm long path for photons that are only 60 \mu eV (14.5 GHz) apart. To quantitatively explain the experimental data we develop a theoretical model that accounts for both the inhomogeneously broadening of the quantum-dot emission lines and the Doppler-broadening of the atomic lines. The concept we proposed here may be used to implement time-reordering operations aimed at erasing the "which-path" information that deteriorates entangled-photon emission from excitons with finite fine-structure-splitting.
@article{arxiv.1505.04071,
title = {Atomic Clouds as Spectrally-Selective and Tunable Delay Lines for Single Photons from Quantum Dots},
author = {Johannes S. Wildmann and Rinaldo Trotta and Javier Martín-Sánchez and Eugenio Zallo and Oliver G. Schmidt and Armando Rastelli},
journal= {arXiv preprint arXiv:1505.04071},
year = {2015}
}