Ultrashort single-walled carbon nanotubes (SWCNTs) that fluoresce brightly in the shortwave infrared shows great potentials in bioimaging, sensing and enabling room-temperature, solid-state single-photon sources. However, this novel family of materials remains largely unexplored due to the synthetic challenge. In this thesis, I will introduce a high-yield synthesis method of fluorescent ultrashort nanotubes based on a fundamental new understanding of defect-induced chemical cutting of SWCNTs. The method includes implanting fluorescent quantum defects onto the carbon nanotubes, and then cutting the nanotubes preferentially from the defect sites. Firstly, I demonstrate that this simple two-step process leads to the synthesis of ultrashort nanotubes with a narrow length distribution and fluorescence in the shortwave infrared. Then, I will further elucidate the mechanism of this defect-induced chemical cutting at the molecular level for the first time by in-situ monitoring the spectral fingerprint of the nanotube and the fluorescent quantum defect as the reaction proceeds. This thesis reveals fundamental insights in defect chemistry and makes fluorescent ultrashort nanotubes synthetically accessible for both basic and applied studies.