  Optical quantum emitters are a key component of quantum devices for metrology
and information processing. In particular, atomic defects in 2D materials can
operate as optical quantum emitters that overcome current limitations of
conventional bulk emitters, such as yielding a high single-photon generation
rate and offering surface accessibility for excitation and photon extraction.
Here we demonstrate electrically stimulated photon emission from individual
point defects in a 2D material. Specifically, by bringing a metallic tip into
close proximity to a discrete defect state in the band gap of WS2, we induce
inelastic tip-to-defect electron tunneling with an excess of transition energy
carried by the emitted photons. We gain atomic spatial control over the
emission through the position of the tip, while the spectral characteristics
are highly customizable by varying the applied tip-sample voltage. Atomically
resolved emission maps of individual sulfur vacancies and chromium substituent
defects are in excellent agreement with the electron density of their
respective defect orbitals as imaged via conventional elastic scanning
tunneling microscopy. Inelastic charge-carrier injection into localized defect
states of 2D materials thus provides a powerful platform for electrically
driven, broadly tunable, atomic-scale single-photon sources.
