Strong delay of quantum dot single photons in cesium vapor

In forthcoming quantum networks various quantum systems might be involved to accomplish individual tasks, including storage of quantum states, quantum logic operations, error correction, or entanglement distillation. An interface between a single photon emitter and a potential photon storage could provide one fundamental building block of such a hybrid quantum system. Semiconductor quantum dots (QD) and atomic vapors have already been brought together by single photon spectroscopy of cesium and rubidium [1,2], narrow filtering of QD resonance fluorescence in cesium [3], slowing down QD photons by dispersion between the 6.8 GHz hyperfine-split rubidium D 2 transitions [4] and in the middle of the four cesium D1 transitions [5] (at around 5.0 GHz in Fig. 1 b).In our experiment the neutrally charged excitonic transition of a strain-tunable InGaAs QD is pumped either resonantly or non-resonantly by a pulsed laser. The QD emission is set between two cesium D1 transitions at 894 nm, which are hyperfine-split only by 1.2 GHz, with a very strong dispersion compared to preceding studies [4,5]. This allows for delay times of a few nanoseconds even at low optical densities, resulting in a propagation velocity at the order of 1/20 of the vacuum speed of light. Such a single photon - atom interface might lay the foundations for a low-loss quantum memory in a future hybrid quantum network.