Quantum dots interfaced with alkali atoms: Filtering, delaying and quantum interfering single photons

Hybrid quantum systems are based on the capability to interface elements from complementary fields: only the best properties from all components are utilized, overcoming the limitation of each field. Semiconductor quantum dots (QDs) are well established as sources of bright, pure and highly indistinguishable on-demand single photons. A crucial limitation is given by the relatively short coherence time. This can be overtaken by interfacing them to alkali atoms, which display a very long coherence time and will benefit from the superior properties of QD-based non-classical light sources. As one example of an efficient QD-to-atom interface, we recently demonstrated the filtering of two consecutive single photons generated from a resonantly excited QD by using a Cs-based Faraday anomalous dispersion optical filter (FADOF). The Mollow triplet sidebands were tuned on resonance with the Cs-D
₁
transition and simultaneously filtered with more than 10 % transmission. All other spectral components were suppressed below the noise level. In the present study, a Cs vapor was used to realize a variable optical delay. A resonantly excited QD is used as a source of on-demand single photons, frequency tuned in between the aforementioned Cs-D1 transition. An extensive study on the effects of vapor temperature on the achievable delay has been conducted using single photons with several different linewidth. A record delay of 28 ns was achieved for almost 100% of the transmitted photons. Autocorrelation measurements revealed that an almost ideal g
⁽²⁾
(0) ≈ 0 is preserved after the interaction with the atomic vapor.