Gigahertz-clocked Generation of Highly Indistinguishable Photons at C-band Wavelengths

High-performance single-photon sources at telecom C-band wavelentghs are key building blocks for applications in long-distance quantum communication. Here, we report the generation of highly indistinguishable, single photons at a clock-rate of 2.5 GHz. This is achieved by coherently driving the biexciton transition ($T_1^{\mathrm{X}\mathrm{X}}=64(1)$ps) of a semiconductor quantum dot embedded in a microcavity with strong asymmetric Purcell enhancement. Employing pulsed two-photon resonant excitation, strong multiphoton suppression with $g^{(2)}(0)<4\mathrm{\%}$ and high two-photon-interference visibility of $V_{\mathrm{r}\mathrm{a}\mathrm{w}}>85\mathrm{\%}$ is observed. The observed photon indistinguishability is close to the theoretical limit expected for the photonically engineered radiative cascade and matches values obtained at lower repetition rates. Our results show a substantial advancement towards interference-based quantum information protocols at unprecedented data rates in the telecom C-Band.