Gigahertz-Clocked Generation of Highly Indistinguishable Photons at C-Band Wavelengths

High-performance single-photon sources at telecom C-band wavelengths 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$^{xx}_1$ = 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% and high two-photon-interference visibility of V$_{raw}$ > 85% are 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 toward interference-based quantum information protocols at unprecedented data rates in the telecom C-band.